The free energy of auric oxide as determinedfrom electromotive force measurements

Item Type text; Thesis-Reproduction (electronic)

Authors Roseveare, William Earl, 1904-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/553383

EBB mEE EBERGY OF AURIO OZIDE AS IBTEHMniED IBOM

BI®G$ROH(XDIVB P(BOB m SIEBI52TT3

By '

m%HAM EAB1 BOSETEABE

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Submittod in p a rtia l fulfillm ent of the ; requirements for the degree of

in the Oollege of le tte rs , Arta and Soienoes of the

1986

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TABLE OF COBTEETS

Introauo t lo n » . . .....................

TheorotioaL O ozisidorations

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Experimental Brooe&we,.. . .............................. . . 6Preparation of Pure Aurio O xl& e.... . . . . . . . . . 7Final Method of Preparing Aurio O x i & e , . 9 Re-purifloation of Aurio O z i d e . 10 Analysis of the Oxide hy Means of Combustion

T r a i n . . . . . . . . . . . . . . . * . . # . . . ............ 13Procedure of Analysis by the Decomposition

Method**................... ......................... * * . . . . . . 14Sources of Error by the Decomposition Method..** 1GCarbon Dioxide for Combustion Train ................. 16Analysis by the Displacement of Water.......... 18Procedure of Analysis by the Displacement Method Sources of Error in the Displacement Method*....Volumetric Method of Analysis Using Ferrous

Sulphate».*♦•♦•♦.«•»*••••»*•*•►••*••••••••♦•*Analysis of the Auric Oxide by Titanous Chloride Details of the Analysis by Titanous Chloride**.. Preparation of pure C o ld .* ...* .* preparation of Pure Sulphuric A c i d . . . . . . . . . .Equilibration of Solutions*..**.**.*..*.***........Hydrogen for the Cells. ............... ...........................purification of the H y d ro g en * ...# ........* .** ...Thermoregulation» * . . . • * • • • » . .The E lectrow tire Force C ells .................................Electromotive Force Measurements of C e lls .. . . . .*Suzroary of Condi t i one Affeo ting the Constancy of

Electrode p o te n tia l..* ...* .* ..* .........Summary and Discussion of R e s u l t s . . . . . . . . . . * . . . .Calculations of Thermodynamio Data for Auric

Oxide................................................... *.............. .Conclusions.. . . . . . . . . ....................B ib liog raphy .................

59193

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The writer wishes to express his sincere appreciation to Dr. 5?. F. Buehrer, under whose direction th is investigation was oarrirtl f « him valuable advioe and aseiatanoo, and for the many suggestions

he ham offered.

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they are a measure of the tonaenoleB of sutatanoeB to tmdergo ehemloal reaction, ana of the extent to vihloh suoh reactions m ill proceed* Although the principle of free energy has been known and i t s importance realized ever since 1883, when i t mas discovered hy Helmholtz, i t was not until 1913 that lewis, in hie paper on rt$he Free Energy of Chemical Substances*0 sot forth the methods on the basis of tdhioh free energy calcula­tions might readily be made* Since that time lewis and Ban- d a ll8 have systematized the free energy data for many com­pounds, but the table of free energies is s t i l l far from com­p le te . Only a few of the metallic oxides have been studied, and among those for which free energy data have not yet been obtained la auric oxide, which lias been made the subject of the present investigation.

2HE0RE2I0AL GOHSIDERATIOHS

There are three iaipertaat oetheds of aeter- mining free energy ea^erimentally, eaoh of which Involves a tree eqmlilhrlnm process•

!• from equlllh rlm oonstanteeIn many oases I t Is Imposelhle to determine

experiments!ly the pressures or oonoentratlons of the equlllh* rlum mixture. Irreversible reactions, of uhloh there are many, do net give a true equilibrium; hence, another method must be employed. When the equilibrium constant can be obtained, the free energy Is obtained from the equation; '

4 P = * ETinK

where R Is the gae constant, T the absolute temperature, and K the equilibrium constant.

8. from thermal data.for th is method the change In heat content

and entropy of reaction must be known, the value of the free energy being given by the equation;

4F ==dE » TASAH being the Change In heat content, $ the absolute teiaperature and 48 the entropy of reaction. This method requires Involved apparatus to obtain trustworthy re su lts , since accurate calor­imetric and specific heat data qt liquid a ir temperatures are necessary to obtain the value of 4 s.

3. Prom electromotive force data.I t i s possible to obtain the free energy

change for the reaction by measuring the electromotive force of suitable galvanic ce lls in which a very delicate equilib­rium can be established between the reacting substances. The free energy is then given by the relation :

4PX -T Owhere If is the number of equivalents undergoing Change, B the electromotive force, and P the value of the faraday. This method Is experimentally applicable to a greater number of re ­actions occurring in liquid phase than either of the two meth­ods mentioned above and a suitable potentiometer is available in most laberaterles.

The free energy of reaction so found is then combined with other accurately known free energy quanti­ties and the standard free energy of formation of the desired compound from i t s elements Is calculated. Applying th is reason lug te auric oxide, whose equation of formation is ;

2 Au (s) + 3/8 02(g) • Au803 (s)the standard free energy change of th is reaction can be found by combining the two free energy equations:

1. 3 H 2 (g) +Au203 ( s ) • 3 H 20 (1) e Au (s)MPlr-HEF

2. 3 Eg (g) + 3/2 Og (g? s 3 HgQ ( l ) ; -169,700 oal.

Whenoe 2 Au (s) + 3/2 0g (g) = Aug03 (s); - JP*.

lewis ana BancLall8 with great aoouraoy and fotmd to be -66,660 oal* per mol of liquid water. I f the value of 4 S'* oan he de­termined, the free energy of auric oxide can a t once %e calcu­lated . This has been accomplished in the present investigation by measuring the B.M.F. of the ce ll:

Hg (g), H2 S° 4 (xm), AU2 O3 (s), £u (s) .A careful search of the lite ra tu re showed that very l i t t l e work has been dene cm the reaction shloh th is cell represents.

«3jJlrea and Jolinek arrived a t an approximation to the poten­t i a l of th is ce ll in their work on the anodic oxidation of gold, fheir value was 1,36 volts in which the third figure is in dcubt. I t is questionable whether their method would give pure auric oa&dt* ■■■■■" : '

In the oell Chosen for this study the ox­ide had the advantage in m at i t avoided liquid potentials. Sulphuric acid appeared to be the best electrolyte, as i t docs not form complex ions and can be prepared in a high state ofpurity . I t also successfully prevents colloidal formation of

a : •the oxide which occurs in alkaline or neutral solutions.

From the ce ll reaction, we conclude that since the electrolyte is neither formed nor used up, the ocm- centration of the electrolyte w ill have l i t t l e or no effect on the B.M.F. of the ce ll, except in so far as the activ ity of the water was changed. Several concentrations of sulphuric acid were used to determine the magnitude of the effect pre-

&T1Q8& by bo varying the activ ity of the water.From the ©leotrOBOtivo force data and the

ehange in heat content, i t is possible to calculate ttte three Important thermodynamic quantities relating to auric oxide $ namely, the free energy, the dissociation pressure and the entropy-' change when i t is formed from i t s elements a t 298°A.

EraRIMBmi HtOOEDTBB

the problem involved the preparation of pure materials for use in the ce lls , construction of the necess­ary apparatus, and fina lly the measurement of the electromotive forces* The experimental work w ill be outlined as fellows*.

1. Preparation of cell materials* a* Pure auric oxide.

Auric oxide was prepared by precipitation . - • ; .

from chloraurio acid solution with pstaeeitua hydroxide and re -purified by re-solution and re-precip itation . The purity of the product was determined by various te sts and snalysee.

b. Pure gold*Various methods of preparing gold were tried

and the one giving the product that appeared to be nearest the; " : ' . ■' ■ ■ ■ ■■ - . V ■ ' ■ . ■ . •stable form was adopted*

o. Pure sttlpherle acid.Highly pure sulphurie aeld was obtained by

dletllllng the “dhemloally pure" aeld of commerce f i r s t with

BBngimege dioxide and then by i ts e lf , Testa were made to de­termine i t s purity , and i t was fina lly diluted with conductivity water to molel, tenth-molal, end hundredth-racial concentrations for use in the c e lls .

d. Equilibration of solutions.Sulphuric acid of each strength was equil­

ibrated with oxide by shaking in a suitable rotating apparatus*e. Hydrogen gas.

Hydrogen gas of high purity was obtained by electrolysing potassium hydroxide in a suitable vessel and purified to remove foreign gases and alkali girsy.

2* Apparatus* _ ' •• '.. a . Thermregul&tltta* - - . .. .

A glass mercury regulator and relay was constructed to regulate the thermostat to 25°, 4 .04°.

b. Cells.4Cells of the type used by lewis and Rupert

were modified by making the gold ce lls in trip lica te and the hydrogen electrodes in duplicate.

3* Measurement of the c e lls .A number of cells were measured several

times a day un til they came to equilibrium or indicated that they would not approach true equilibrium.

Preparation of Pure Aralo Oxide

In measuring the potential of gold oelle, i t is essential to choose materials whleh do not form ooaplex ions, for in saoh esses the reactions are oomplex and i t i s met definitely known what reaction is Veing studied• The oxide or hydroxide gave the te s t prospect of fu lf illin g s&oh condi­tions. In the lite ra tu re there is l i t t l e d istinction between the oxide and i t s hydrates* In th is discussion i t is assumed that the fu lly hydrated oxide or hydroxide was obtained by the method of preparation used.

There are various methods of preparing aerie oxide given in the l ite ra tu re , but they do net give a pure pro­duct or the purity of the product can not be proved, the methods given are as follows:

1* The precipitation of the hydroxide from auric chlor­ide by belling with magnesium hydroxide,5 magnesium oxide, simo oxide or barium hydroxide5, 6#

8. Anodic oxidation5** of gold in potassium sulphate or sulphuric sold solutions*

5. Heating of finely divided gold with sodium perox­ide8 ► 9 and leaching out the aurate with water and precipitating

'■ vy • 'the hydroxide with sulphuric aoid.

4, Precipitation of the hydroxide from aurlo chloride with potassium hydroxide4,5 rediesolving the preelpitate in an excess of potassium hydroxide and re-preeipitating by making faintly aoid with sulphuric mold.

Hone of these methods gives satisfactory purity , fhe method of boiling gold ehleride solution with mag­nesium hydroxide, magnesium oxide, or zinc oxide a l l Involve the d ifflen ity of using a solid reagent which can only be com­pletely removed by the addition of an acid. This, however,

’ ' / ' ' ' .

would also dissolve the gold oxide, since there i s chloride ion' : ' . . 'present, Barium hydroxide forms a precipitate of barium aurate which yields the oxide only on decomposing with n itr ic sold.

When potassium sulphate is eleetrelysed with a geld anode a film of oxide is formed which contains potassium, probably as aurate. I f sulphuric acid is used at the electrolyte the oxide contains basic aurate, which ean net be removed with eeld water and questionably with hot water.

The method of using sodium peroxide gives hydrogen peroxide. Which w ill reduce gold, giving the possi­b ility of am*ous compounds.

Auric oxide prepared according to the fourthmethod always contains a small amount of chloride and, accord-

Bing to Heller, some potassium which ean not be removed by wash­ing. A method of re-purifying this product was devised that gave a product of high purity, so the la s t method was adopted as the f i r s t stop in the preparation.

glnal Ifethod of Preparing Itirio Oxide

Potaesixun hydroxide irbb added to boiling •elution of auric chloride containing only hydrochloric and n itr ic acid as impurities, un til the precipitate f i r s t formed dissolved, giving a solution of potassium aurate. Sulphuric acid was added u n til the solution was acid to litmus, and the mixture boiled for some time to coagulate the fine suspension of hydroxide. I t was impossible to wash cut a l l the soluble ehleridee on a f i l t e r , and decantation was unsatisfactory be­cause the precipitate was so fine that i t required a day to se ttle out completely.

lo overcome these d ifficu ltie s the pre­cip ita te was f ilte red by: suction, using an alundura thimble, and the precipitate loosed from the thimble between each addi­tion of wash water by fereing d is tille d water through in the reverse direction. The precipitate was s tirred up with a fresh portion of wash water and filte red as before. I t was washed in th is manner un til the f i l t r a te gave no te s t for chlor­ides, A much quicker and more effective method was la te r used for washing the precip itate . This consisted in centrifuging and pouring off the supernatant liqu id . The precipitate was then washed into a small glass-steppered bottle end placed in the shaking apparatus and shaken with water for half an hour.The suspension was transferred to the centrifuge tube, acidified with a few drops of dilu te sulphuric acid, end placed in the

centrifugee The acid tms added to coagulate the colloid which forma when the hydroxide i s shaken with water# Potassium sul­phate will not cause the colloid to settle out, even with the aid of the centrifuge* This process of shaking and centrifug­ing was repeated until the washings did not give any test for chlorides. This method obviateb the danger from dust and in­sures thorough washing. However, the solid product s t i l l con­tained some chloride that would not wash out#

He-purification of Aurio: Pride

To obtain oxide of the highest purity, the literature® gives the method of re*purifleatlon in which the hydroxide prepared above is dissolved in concentrated n itr ic acid and the hydroxide re-precipitated by diluting With water. When th is method was tried out, some free gold was produced while boiling# This may have been reduction by nitrous sold or purely thermal decomposition# This solution was decanted from the gold, and the hydroxide precipitated with water. The only possible impurities that could he present here would he gold, whiOh was not trivalen t, n itr ic acid, and basic nitrates# Tests were attempted Se determine whether there was any n itra te present# The ferrous sulphate ring test was not sufficiently sensitive# The diphenylamene and phenyl-disulfonic acid tests' are extremely sensitive, hut they could not be used in the presence of aurio gold since i t yields a precipitate which obscured the tests# An attempt was made to remove the gold

11*

and to s t the resulting solution for nitrate* Metals nhich mere active enough to reduce the gold in a moderate, length of time also would reduce the n itr ic acid, and the less active metals such as eepper did not completely reduce the geld. All reducing agents which completely reduced the gold also destroyed the n itr ic aoid, the gold could not he precipitated as an insoluble compound as the so lub ilities of so-called insoluble gold com­pounds have net been extensively investigated.

Since the preduet purified by the above method could not he proved pure, another method of purification was devised not involving n itr ic aoid* The oxide was helled with normal potassium hydroxide for half an hour, giving a solu­tion of the aurate and on insoluble blaofc oxide of unknown com­position, tfiiloh was filte red off through a hardened f i l t e r .This oxide was not ordinary auric oxide, as i t was insoluble In boiling potassium hydroxide. The aurate solution, containing only potassium hydroxide and a trace of chloride, was made acid by adding 3 H sulphur!e aoid* free from sulphur dioxide, un til no more precipitate formed and in addition a considerable excess. The excess of acid aided in removing a l l the potassium hydrox­ide from the precipitate, which is insoluble in sulphurlo acid. The precipitate was then washed as before by repeatedly shaking with water and centrifuging. Hhen i t was free from chloride®, the washing was continued, but without the sulphuric aoid, to coagulate the colloid u n til the washings gave no test for su l­phate.

The prepared product m s preserved under eater to keep i t homogeneous and In a fine state of subdivision, and also to prevent p a rtia l dehydration* I t was protected from lig h t to prevent phetodeewpeeition* The purity of the pro­duct was determined by testing i t for univalent gold, sulphates, dhlecidee, and for petaesium salts* Portions of the hydroxide dissolved in hydrochloric acid gave a perfectly clear solution ultiiout any residue, Chasing the absence of insoluble materials as /well as free gold or aurous compounds* Gold in the aureus form gives free gold and auric chloride with hydrochloric acid. T h is solution was tested with barium chloride, which did not even show a turbidity, thus indieating the absenee of eulphatee* To tost for chlorides some of the hydroxide was suspended in d is tilled water, reduced to free gold with sulphur dioxide, and filtered* The f i l t r a te was then tested with silver n itra te , which did net produce any change, shewing the absence of chlor­ide* When a portion of th is f i l t r a te from which the gold had been removed by sulphur dioxide was evaporated, i t gave a r e s i­due of the same magnitude as that produced by an equal volume of distilled water* I f there wee any potassium present, i t would be found here as potassium sulphate*

The oxide was analyzed te determine the ra tio of gold to oxygen* Since there is probability of the oxide being partia lly decomposed when i t is dried, i t was necessary to use a wet sample• That made i t impossible to determine the weight of the sample so that i t would have quan-

3

Figure 2.

t i ts t lv e eignifloanee. For this reason i t was possible to de­termine only the ra tio of gold to oxygon* the extent of hy­dration does not appear to be determinable without possible decomposition. I t is d iff icu lt to choose a sample of commient alee in the wet fera. Since only a small amount of gold oxide was available for use, i t was necessary to use a method that would determine accurately about 10 mg* of oxygen.

Analysis of the Oxide by Means of Combust ion Train

The f i r s t method of analysis consisted in heating the oxide, measuring the volume of gas evolved, and weighing the geM residue• The apparatus used was a modified form of the combustion tra in used for determining organic n i­trogen.9 The s ilic a combustion tube contained only the boat with the sample* and was connected with the nitrometer tube by way of a three-way stopcock* The la t te r permitted the tra in to be washed out without bubbling the carbon dioxide through the f if ty per cent potassium hydroxide which would have been eonsumed to no advantage* A special nitrometer tube (A) shown in figure 1 was constructed of 15 mm. tubing and was 60 cm. / long, A capillary entrance tube (B) was sealed through the side about 6 cm, from the bottom, Tho small and of th is cap­illa ry was drawn down to a small tip and hent toward in the center of the nitrometer tube. The leveling hulb (0) was con­nected to the lower end. With the small samples the volume of gas evolved could not be measured accurately in a tube of such

14-

large aiameter and the potaBBim hydroxide would soon he de­pleted in a smell ttibe hy the carbon dioxide# So measure th is volume aoourately* the gas was run over into a small gas bur­ette (U). A three-way oapillary stopcock (E) was sealed to the top of the nitrometer as shown in the figure* Above the etepeoek was placed a bulb {?). 5he horizontal arm of the stop­cock was oonneoted by a capillary tube to the bulb (G) attached to the upper end of the graduated portion of the burette# The tube i t s e l f did not have any graduations, so an ordinary 50 o.o. burette was slipped over i t and the zero mark kept even with a mark on the small tube. A piece of larger tubing was sealed to the upper end of the 50 o#o* burette, together forming a water jacket through whloh water a t constant temperature was run so that the temperature of the gas being measured would be accurately known* The bulb was placed above the burette to out down the drainage error. Which is large in small tubes#I t had a volume of about 5 cubic centimeter# and the graduated portion 6 cubic centimeters, making the to ta l capacity 11 o.o#

Procedure of Analysis by the Decomposition Method

The determination was carried out in the following manner: The gas In the nitrometer was run over inte the burette by opening the three-way stopcock and lowering the bulb (H) while raising the bulb (0). I f the bulb and burette w ill hold a l l the gas, the stopcock is closed when the potassium hydroxide, which follows the gas, just reaches the mark "J"

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on the oapillary. 9?he gas Is le f t a short time un til I t assumes the temperature of the water In the jacket. The tmih (H) Is leveled with the solution in Ifce burette and the vol­ume is read# I f there happened to he more than 11 o*o*, half of the gas oould he run over, and the three-way stopoook turned so that the potassium hydroxide in the bnlh (E) was connected with the burette and solution drawn in from i t un til i t reached the mark on the oapillary• After reading the volume, the gas is expelled through the potassium hydroxide bulb and the remainder of the gas measured in the manner just described.A sample that would give less than B o.o* would yield too small a quantity of gold to be weighed accurately, so that there would have been no advantage to be gained from being able to measure volumes less than 5 c .o . I f too large a sample were taken, the volume of gas oould always be divided up so as to f i l l the bulb each time, making i t possible to measure any volume greater than 5 o.o.

Souroes of Error hy the Peoeiapeeitlon Method

The burette had 500 graduations; therefore, when measuring a 5 o.o. volume, the aoeurary i s the same as that obtained when measuring a 40 o.o. volume in an ordinary 60 o.o. burette. The aoouraoy w ill be twice as great when a 10 o.o. volume is measured• The possible souroes of error by th is method are drainage, incomplete decomposition, formation of ozone, solubility of oxygen in potassium hydroxide, and use

*.16.

of itapure carbon aioxide, fhe drainage error was sufficiently reduced by the use ef the bulb above the burette so that i t was negligible. The possib ility of oxygen dissolving in the potassium hydroxide could be avoided by previously saturating i t with oxygen. Complete deeompeeition could be assured, as the sample could be heated 30Q® above the temperature a t which the oxide decomposes. The formation of omens warn not known until in a la te r experiment when i t was accidently discovered. This d ifficu lty could he overcome by passing the gas over some platinised asbestos a t the end of the combustion tube, as p la t-

0Zo*e- .inum black deoomposes^at room temperature. I t is necessary to use carbon dicxids that has Ices than one one*thousandth of tme per cent of inert gases in i t to reduce th is error to ene-tenth of one per sent, i f only one l i t e r of carbon dioxide was used te wash out the tra in .

Carbon Dioxide for Combustion Train .

A Kipp generator was tried out with the precaution given by Pregl10 who olaims to have obtained the necessary purity . All attempts with different o.p. acids end different samples of marble failed to give the desired purity . I t was thought that the impurity came from the marble, so a generator shown in figure 2 was constructed which employed a

solution of sodium bicarbonate in place of the marble. The flask (A) was f il le d fu ll to the stepper with a sodium bicar­bonate solution that had been boiled to remove dissolved a i r .

A boiling eolution of d ilu te hydroohlorlo sold v?as poured Into- ' -v ..... ■ ■■' : 'the reservoir (B)« fhe ez lt tube was then opened end the hy­drochloric Bold f ille d the bottle (D) end ment over into the fistic, xfliere i t reacted with the hbdlum bicarbonate solution, having displaced ©11 the a ir from the apparatus# When (0) was closed the acid was forced back into the bottle (B), prevent­ing the solutions from further reacting# Half of the solution in the flask was drawn off through the siphon (E) to make room for the acid, th is space was f ille d by carbon dioxide generated by the acid which attempted to f i l l i t . This apparatus had the advantage over the Kipp generator in that a l l the a ir is displaced a t tiie beginniz^, and dhemioally pure emrbonate can be used in place of the marble# The a ir which might bo adsorbed on the walls of the apparatus may be removed by boiling the solutions used in them before oonneeting them together.

Even with these precautions, the carbon dioxide contained a very small amount of gas that was not ab­sorbed by the potassium hydroxide# th is was about the same amount of impurity as was present In the Kipp generator gas a fte r i t had been in operation for throe days. Then the only advantage actually secured over the Kipp generator was that i t was not necessary to wait several days before obtaining the purest possible produet# As sufficiently pure oarbon dioxide could not be obtained, i t was impossible to analyse the gold oxide by th is method.

Figure 3.

Analysis fry the Plsplaoeaeat of Water

Another method of analysis was devised whereby the volume of oxygen oould he determined without the use of carbon dioxide* The apparatus shown in figure 3 was designed to accomplish th is result* The bulb (a) had the shape of a pipet bulb and held about 35 o.o. This was connected to the three-way stopcock (B) by a thick-walled capillary tube*The tube (0) extending below the stopeeek was drawn down like a burette tip while the other tube (D) from the stopcock went to the bottom of the large tube (B) and was rlngrsealed through i t a t the top* The tube (?) was a capillary manometer, #11# (0) was a pyrex te s t tube connected to the tube (B). She boat was made of pyrex glass with only an 8 mm. hole in the top in ­to whloh a email pyrex lid was f itted to prevent loss by spattering. The tube (J) was an svaouatsd bulb and was used to out down the volume of the gas g?ace in the apparatus. I t contained a conical reflector made of aluminum, which was in­tended to prevent heat radiation to the rubber stopper in the end of the tube (Q).

Procedure of Analysis by the Displacement Method

The inside walls of the apparatus were f i r s t wet to insure saturation of a l l the gas present. The wet sample was placed in a weighed boat and inserted into the tube (G) and the vacuum tube inserted a fte r i t . A rubber stopper was placed in the end of the tube end sealed with

paraffin . The tmlb (A) was fiU#& with water, Wbioh was run down to f i l l (0) and to f i l l (B) just below the side tube.Water was placed in the manometer whose atopoook was kept closed un til the pressure was very nearly adjusted and then opened to the a i r , ae an epen-tube manometer is more sensitive than a olosed-tube manometer. The apparatus was immersed in the thermo­sta t at 26°0. with the etepeeeke ju st above the surface and le f t un til the manometer showed the pressure inside to be oon- stan t. The pressure of the gas was adjusted by applying suction Se the top of (A) and opening the stepoeek u n til the manometer columns were level. The stopcock was thm changed to le t the water run out of (A) u n til i t reached the mark in the capillary, then i t was turned back to connect (A) to (B). The apparatus was then removed from the thermostat and the end of the tub# containing the boat was Inserted in a horizontal dleotrio tube fumaoe. I t was then heated slowly a t f i r s t un til the water was a l l evaporated from the sample end then to 460*0., and kept there for 16 minutes while the oxide decomposed. The tube (0) was cooled in loe water to condense the water vaporized from the sample* About one o.o. of water was present with the sample, whieh would occupy about 1800 o.o. when vaporized. After de­composition, i t was removed from the fumaoe and allowed to oool to room temperature and then placed in the thermostat. When equilibrium was established the pressure in the apparatus was adjusted to atmospheric as before and the stopcock turned off. The water in (A) was run off into a weighed weighing bottle

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down to the original level in the oapillary or to the mark on the oapillary* The weighing hot t ie was again weighed and the difference in i t s weights divided by the density of water at 28* gives the volume of water displaced, which was equal to the volume of gas evolved under i t s p artia l pressure. The weight of oxygen was calculated from i t s density a t this temperature and pressure. The boat was removed and dried, as water con­denses in i t when the apparatus is cooled. The difference be­tween the in i t ia l and fina l weight of the boat gives the weight of gold.

Sources of Error in the Displacement Method

The only foreseen d ifficu ltie s were the solubility of oxygen in water, and drainage. The error due to the solubility of the oxygen would be quite small and could be en tirely overcome by saturating the water in it ia l ly with the gas and f il l in g the apparatus with oxygen before the analysis. The drainage error would bo about one-half that in any ordinary burette, as the diameter of the bulb is larger in proportion to i t s length.

When analyses were run, the resu lts were a l l between 9.7 and 10.0 per cent, whereas the theoretical per­centage of oxygen in auric oxide is 10.85. During one deter­mination the apparatus broke while being heated in the furnace, and a pungent odor was noticed. A piece of potassium iodide paper was held in the apparatus and iodine was liberated.

-SI-

Oz one was the only possihle gas that would give th is te s t, since11chlorides were entirely atsent* A reference was la te r lo ­

cated which stated that auric aside gives ozone a t 250*0. th is would account for low resu lts , as ozone has a greater density than oaygen. low resu lts would also resu lt from the fact that the density of gold is several times that of i t s oxide. The following possible sources of error w ill give low resu lts :

1* incomplete decomposition.2. Solubility of oaygon.3* leakage.4. Drainage.5. Difference in density of auric oxide and gold.6. Formation of ozone.7. Presence of impurities in oxide.8. The presence of univalent gold.

The only reason why the resu lts oould be high would be due to the gas net being saturated water vapor a t the beginning.

Volume tr io Method of Analysis Using Ferrous Sulphate

A volume trio method was devised in which the oxygen was determined volumetrioally end the gold gravi- metrioally. A sample of the oxide suspended in dilu te sulphuric aoid in a 260 o.o. Brlenpeyer flask was titra ted with an excess of ferrous sulphate. Sodium carbonate was added to form car­bon dioxide to displace the oxygen of the a ir and the mixture boiled for five minutes, with a Bunsen valve to prevent a ir

oxidation* • The excess of ferrous sulphate v?as hack titra ted with permanganate, and the precipitated gold filte red off. The f i l te r papers were dried in the oven, burnt on a platinum wire, ignited in weighed crucibles, and weighed again* Since only a very small amount of the oxide was available for analysis, i t was necessary to use small samples. Which decreased the accuracy ef the methods. . v

TABLE 1 — AMT3IS Of AIBIO OXIDE

1* By the diaplaoement of water Dy the oxygon

{Density 8 of Og a t i S.O.

I Wt. On : liberated : sms.

: Wt. An: ***

0£per eemt

t 1.4891 :0.00298 : 0.0276 9.74! 1.4892 $0,017

888

: 0.1694 10.0s ' 1 10.86=! : theoretical

tf t . HgO :Bar*Preea gms. $ mm.

■. :

8.650 i s

15.188 !!8 S

699.6900.6

2. By titra tio n of eolld oxide with FeSO* and xmQ^

l e t 8 formal. : 8 l s Og as AUjfFeSO* F0SO4 8 : wt.Oa 8 Wt.Golds

8 \°ur21.09

$.1008 1 .01662

88 .1368

810.84

32.72 8 .1018 8 .02668 I .2211 t 10.7681.84 8 .1018 s .0869© .8319 8 10.8918.28 .1008 8 .01473 8 .1208 8 10.91

6.76 8 .1018 8 00662 t .0489 * 5 ■. 10.748.76 .1018 8 .00716 .0583 S 10.90

8JL

i ' 8■JL '’ml8* 10.84

10.868. By titra tio n of xide in Hoi sol’n . , with FlOlg and FeOlg

jfet 8 8 ' 5 8 8 ITT2 as iuxTIOls •formality: Wt Og sWt.Gold 8 8°7 * per

8 8 : ‘ 1 : $ ;sent7.27

8-8' .1023

10.00560

8$ 0.1039 :

8 ■ 6.0816.57 : : •1023 * 0.01257 8 0.1028 8 20.8921.65 ... $ .1026 8 0.01747 8 0.1434 8 . 10.8625.09 8 .2026 0.01906 * 0.1563 I / 8 10.8623.74 8 .1026 0.1801 8 0*1479 l : 8 10.8512.96 $ .1026 %0.00983 8 0.0801 : 8 10.94

8 8 ■ 8 I : ' I .\: 8 8 8 ' I : Mean % Oft- 10.868 8 8 iTheo. % a 10.86: 8 8 8 8

Figure 4

The resu lts of th is metbo& of analysis are liven In Table 1. Their agreement la ereellent, eoneldering the faot that a solia was titra ted to give another eolld xshioh might precipitate on the surface of the f i r s t and prevent the Inside portion from re&otlng. This would give low results for ozygen while on the other hand, the ferrous sulphate solution la liab le to bo oxidised during the boiling, thereby giving high values.

Analysis of the Auric Oxide by Titanous Chloride

Another method of analysis was designed Which would give a value without the errors inherent in the la s t method. In th is method the oxide was dissolved in hydro* oblorio sold and the gold reduced by titra tin g the hot chlor­ide solution with an excess of standard titanous chloride and titra tin g the excess with standard fe rric chloride. The gold was determined gravimetrically. As titanous chloride i s oxidised by a i r , the titra tio n was carried out in a oarbon dioxide a t ­mosphere by the use of the apparatus shown In figure 4. The burettes had three-wey stopcock#, eaoh being connected with i t s respective reagent b o ttle . The other side of eaoh one was bent to f i t through the sopper (A) on dileh an ordinary wide­mouthed, ISO o.o. extraction flask oontalning the sample was

f it te d . Oarbon dioxide, purified by passing i t through tltan- eus chloride solution, came in through the tube (B) which was broken in the middle Of the stopper so that the lower part oould

- 26-

%# remoreA with the flask i f any gold adhered to i t . The oar- hon dioxide passed through the tube (0) which also served to intredmee the indieatmp. A three-way etopoook was sealed to the top of the titaneue burette oozmeoting a bubbler and a rub­ber tube to the burette. The burette was oonneoted to the a ir through the bubbler tube, containing titaneus chloride, a l l the time except during the f il l in g then the burette was opened to the rubber tube to Which suction was applied to draw up the solution. I t was not necessary to protect the fe rric chloride, as i t i s not affected hy a i r .

Details of the Analysis by Titaneus Ohlorlde

In carrying out the analysis a sample was placed in the reaction fla££ with 6 o.o. of concentrated hydro­chloric acid, and the flask attached to the stopper. Oarbon d i­oxide was bubbled through to remove both gaseous and dissolved oxygen, and the solution was heated to boiling before adding any of the titaneue, which was added in excess. The excess was back titra ted with fe rrie chloride from the other burette, using potassium thiocyanate as the Indicator. The flask was detached from the apparatus, and the gold was filte red on a weighed alun- dum cone which was dried two hours a t 150° before each weighing. The indicator was not added u n til an excess of titanoue had been added, as potassium thiocyanate reacts .with auric chloride, forming an orange-red precipitate which dissolves on stirring in the presence of an acid. The f i r s t result by th is method.

given In table 1, show e the effeot of adding the indi eat or be­fore the tltanoue. The value, 5.08 per cent, so obtained shons that the thiocyanate must have reduced most of the gold to the aureus form. In the remainder of the analyses, the results of ehleb are given in the same table, the indicator was not added un til a fte r the tltanous solution*

The resu lts of these la s t tuo methods shois positively that the gold is in the triva len t fern, and oinoeimpurities have been Shown to be absent, i t may be stated-with

. ' •considerable assurance that the product was auric oxide and that i t was in a very high sta te of purity . The s ta te of hydration of the oxide has net been determined, and according to lewis and Bandall,12 th is quantity i s not necessary for the determina­tion of the free energy of the oxide.

Preparation of pure Gold.

Several methods of preparing pure gold fs r use in the ce lls were considered. When spongy gold from any source is heated in a porcelain dish to a temperature just be­low that a t which porcelain becomes red, i t coalesces into large partic les and the color becomes bright yellow. This gold appears to be entirely homogeneous, but is likely to be a d iffe rcut modification from the stable form in sulphuric sold a t 26o0. Sold precipitated by passing sulphur dioxide through an aquaregia solution of gold gives a product which in part appears tu lte crystalline, but there is always a considerable quantity

*•27—

in an extremely finely divided sta te . The nature of the gold precip itate! by sulphur aioxide ie very much affected by She eeneentratlon of hydrochloric and n itr ic acids. This method #»# not used, as i t is not easily reproducible and does not give a homogeneous product. Gold precipitated from auric chlor­ide by means of ferrous sulphate with constant shaking is very uniform. The partic les are a ll about 0.88 mm. in diameter and have a reddiah-brewn color and are not very dense. I t contains traoos of chloride* rhlch were f i r s t removed by repeated bo il­ing with d is tille d water. The boiling with either d is tilled water or moial sulphuric acid changed the appearance of the gold noticeably and as th is might be another form of gold stable at 100*» but not at 25°, i t was decided to wash i t a t room temperature. The ferrous sulphate geld was placed in a glass- stoppered bottle with sulphuric asid of the concentration to be used in the ce ll, and the bottle placed in the shaking appa­ratus for some time* The liquid was then decanted off and a fresh portion added. This was repeated un til the washing# failed to give a te s t for Chloride. The gold changed consider­ably in form 'and color while being shaken with sulphurlo a d d . The la s t geld prepared was shaken for about ss hours with sold of the same concentration as that to be used in the ce ll. The geld then appeared quite crystal line and was yellow in color.I t was thought that th is form must be more stable form of gold in sulphuric acid a t 26°0.

preparation of Pwe Sulphurlo Apia

Sulphuric sold m s f i r s t prepared hy the electrolysis of reorystalllzed copper sulphate, Taut I t was im­practical to remove the la s t traces of copper by th is method, so I t had to be d is tille d . The d is t i l la te contained sulphur dioxide from cathode reduction, which could he removed by d is­ti l la t io n with manganese dioxide. Sufficient acid could not be prepared by th is method, as only a small amount of copper sulphate was available*

The "Chemically pure" sulphuric acid of commerce was repurified by d is tillin g twice with manganese dioxide and once by i t s e l f . The d is tillin g flash and condenser were both constructed of pyrex glass, and to avoid contamination a l l stoppers were eliminated from the apparatus* The d i s t i l l ­ing flash had the side arm bent up a t an angle of 46° for 6 cm, and then down a t the usual angle* The condenser was about a meter long and the enlarged part a t the upper end was drawn down in the middle to f i t very nearly the side arm of the dis­til lin g flags. The space above th is constriction was f ille d with glass wool. This gave fle x ib ility without the use of steppers and yet did not allow the vapors to escape a t that end of the condenser* The loser end of the condenser was bent down to f i t into the entrance tube of a gas watih bottle in vSiich the entrance tube was broken off just below the ground glass stopper, the top ef the d is tillin g flask was sealed off a fte r placing the acid in i t . The f i r s t and la s t third of the

d is t i l l s to was disoarded, as thoeo portions would contain the iBgpuritloe* The icpuritiee present in the o*p. sold are» su l- phur dioxide, nitrogen dioxide, organic matter# iron# arsenic# and lead. The sulphur dioxide would fee oxidised to sulphuric sold fey the manganese dioxide while the nitrogen dioxide and

. organic matter would fee d is tille d off in tho f i r s t fraction as n itr ic a d d end carton dioxide. Iron# arsenic and lead would

. remain In the d is tillin g flash*........ When a portion of th is concentrated acid

■ was .evaporated I t gate a residue of the same magnitude as that . produced fey an equal volume.;of d is tille d water. When 1 - 8

o*o., diluted with water# was warmed with very d ilu te potassium permanganate solution, the color remained unchanged for several

■' hews# I t was diluted with oojcduotivity water to give molal*1/10, . and l/lOO molal solutions# which were used in the oe lle .

EQUlliferatlon of Solutions

Auric oxide was equilibrated with each con*- , centration of culphurio sold fey plaoing some solid oxide to- . gethor with the acid in a glass-stoppered bottle and Shaking . the mixture in a medhanioal Shaking device. Only the ce lls in

which the solution used had been equilibrated over 18 hours oaoe to equilibrium within two weeks* All the others gave vary­ing end its? rest&tc. fh le shows the necessity for equilibrating the solutions for a long time*

Figure 5

Hydrogen for the Pells

a?he hydrogen was f i r s t obtained from a cylinder of the ess, bat the flow of gas was not constant, dm# to the change in pressor# within the cylinder, and a fte r about 4 hears the flow would again have to be adjusted. When the Cylinder of hydrogen was exhausted, i t was prepared by else* tro lysis of potassium hydroxide. The current was f i r s t obtained from a motor generator, but i t was Impossible to use i t con* tinueuSly as i t was not in good working order and was relatively espensivs to operate* A storage battery was next used, but i t was found necessary to use the storage battery as the working battery for the potentiometer. E lectrolytic re c tif ie rs were considered, but they do not entirely prevent the reversal of the current and would give some oxygen with the hydrogen.Finally, a two-element vacuum tube of twe-e®pere capacity was used* A suitable transformer was not available, so the tube was used on 110 volts end the current out doun with a resistance. Although only 0.2 ampere was used, the filament d is tille d away in two days * An old transformer was reconstructed to deliver about 25 volts for the plate and 1.5 volts for the filament.With a Reotigon tube th is delivered an intermittent current, of * amperes a t an effective B.M.E. of 10 volts*

The electrolysis vessel diown in the figure 6 was constructed of pyrex glace* The W-tube (A) was 4 era. in diameter, and the anode side was connected to the enlargement (B) by a 2-cm* tube, 60 cm. long* The other side was joined

to a 8-om. tube, 13 oou long, to Atoh the trap (0) was sealed near the top# The electrodes of platinum fo il were sealed on the ends of glass tabes which reached nearly to the bottom of the U*tube and were held in place by stoppers# E lectrical con* sections were made by running wires down the tubes to meroury oozmeotions a t the bottom. In the f i r s t vessel constructed the 9*tube was only 1*8 cm. in diameter, which made the r e s is t ­ance of the ce ll high, so that about 50 volts were necessary to send a current of 1 ampere through i t . This voltage could be ebtalned from the motor generator or by.the use of a vacuum tube. The motor generator was out o f , the question and the vac­uum tube had to bo used on a low voltage for half on hour be­fore i t oodd be used to operate to deliver this voltage or i t would burn,out so i t could not be used* The electro lysis vessel was reconstructed so the .resistance would bo less* The 9-tub# was made 3*5 om. in diameter so the resietonce was only about one-ninth of the former tube. Ten volts would send a current of about 8 amperes through it* The trap (0) was used because when the electro lysis was stepped, the hydrogen would diffuse out through the rubber connections while the a ir would diffuse In extremely slowly and a p a rtia l vacuum would be produced which would draw the potassium hydroxide ever into the p u r if i­cation train* The cathode was placed near the bottom and the amede higher xq> because gases tend to diffuse upward in liquids, aud i t was desirable to keep the oxygen out of the cathode chamber* with the re c tif ie r tube I t was necessary to use a

Mntln««UB ourrent of a t leas t 1 ampere or the tube xiould be- oome inactive so the eathode had to be below the upper side of the horizontal part of the U-tube to permit the excess of hy­drogen to escape through the anode chamber. Otherwise, the electrolysis would be interm ittent and the re c tif ie r tube would net s ta r t up when contact was made again. When the electrolysis was run continuously the electrolyte would concentrate around the cathode and become so dilu te around the anode that i t would net conduct enough current to operate the re c tif ie r tube* How­ever, when an excess of hydrogen was generated, the excess bubbled up through the anode chamber and s tir re d the solution so that there would not be sufficient polarisation to cat.down the current noticeably. I f potassium sulphate was used as the electrolyte there would not be any difficu lty due to transfer­ence* as sulphur is acid would be formed a t the anode and potass­ium hydroxide a t the cathode* However, i t could not be used without a soda-lime tube to remove sulphur dioxide produced by cathode reduction*

Purification of the Hydrogen

fhe gas was bubbled through a small wash bottle containing sulphuric acid to remove any potassium hy­droxide spray* Terieus methods of removing the spray were tried* In the f i r s t one the gas was passed through a s ille a tube containing a coil of nlehrome wire heated to redness by an e lec tric current• A 40-om* tube was the longest one on hand

that was large enough to take the ooll of Hire Inal&e of i t , and this get hot a t the ends so rubber oonneetions oould net be used and no suitable cement Has to be found. Bezt, a fine German silver Hire waa stretched in a pyres tube by means of a small spring# This worked for a short time but soon burnt out, probably due to a trace of oxygen in the hydrogen# After th is a platinum loop was sealed in a pyres bulb, but the small­est wire was too large in that i t required a current of about 10 amperes to heat i t to incandescence. I t was out of the ques­tion to use 110 volts and we did not have a transferwer that would deliver suoh a large current. ' I t was tried passing the gas through a carbon lamp, but the hydrogen was suoh a good conductor of heat that the filament would not heat to Incan­descence on 110 volts, and when heated hotter with a higher voltage i t burnt out after several hours. The carbon probably combined with the hydrogen or perhaps small traces of oxygen.A tungsten lamp in which the filament had been shortened was substituted for the earbon lamp. The filament was Shortened so that i t would heat to incandescence in the presence of hy­drogen. However, th is tube burnt out a fte r several days* op­eration. Finally, the hydrogen was purified by passing i t through a small s ilic a tube f il le d tightly with fine copper

wire running lengthwise with the tube, tfcioh was heated to bright redness in the center by a 6-lnsh tube furnace. This tube was 60 cm. long, so i t never got hot at the ends and rub­ber eomeetions could be used.

A pyres glass thermoregulator T/as oonetruete*.as shown In figure 6. The tuhing was 6 cm. inside diameter and

' . ' ' ' - ■ ' ' ' . ' ' •1 mm. in thickness. At (B) the tube was thickened and drawndam to a capillary of 1 mm* inside diameter, the capillary was mad# tapering slightly with the email end up to prevent the mercury from separating in the capillary even though i t were slightly d irty due to sparking. The contact was a platinum wire soldered to a brass' screw that was threaded into the metal cap (A), which was fastened to the glass with sealing war. The other e lec trica l connection was a platinum wire sealed through a t (0) with a mercury connecting cup above. The stopcock (B) permits f illin g and regulating the mercury level In the c ap ill­ary. The pitch of the screw was small enough that i t required two turn# to Change the temperature one •tenth of a degree centi­grade. The open cone true t i on of this regulator permits good circulation of the water* so there w ill not be any stagnation around i t .

A home-made relay was available that re­quired 6 vo lts. The only 6-volt current available was a lte r-

‘ ' ' ' ■ ' .. . - ' ' " ' - ■nating our rent, and when th is relay was tried out i t s actionwas found to be uncertain. This was thought to be due to the poor pivoting of the armature contact system. The armature■ ' . ' . . - X ■ ?: - - ' ' - ' :

was then mounted on a thin 20-em. aluminum rod pivoted a t thecenter similar to the balance wheel of a watch and the platinum contact a t the opposite end. This system responded to very

email force at the ends, hut when an alternating current was passe! through the magnet* the armature would vibrate with the 60-cyole current* making considerable noise and making and breaking the contact continuously when the primary current was ocntlnueue# Shis was because the electro-magnet end armature were not laminated, and were also of iron that had a high re - luotanoe* I t would have worked well on direct current* but no soures of direct current was available that would deliver su ffi­cient current continuously* However* the regulator eontaot would spark oonsiderably with the current neoeseary to operate this relay and the mercury would become corroded* giving poor regulation*

I t was then decided to search for a high- resistance relay that would work in conjunction with pyrex- meroury regulator described above* Ho high-re si stance relays were available so one was made out of a Ford circuit-breaker, fhe low-resistance coil was removed and since the eontaot points already there made connection when a current was sent through the primary, i t was necessary to place eontaot points en the opposite side of the armature. A tungsten induction coll point was soldered to the upper side of the armature a t the free end and another supported above th is , insulated from the remainder of the relay* This relay had a resistance of about 76 ohms and worked best on three volts* D.O* Ttoder these conditions the current would be 0.04 amperes* and the spark a t the meroury contact was so small that i t could only be seen In the dark*

With th is arrargament Iho temperature of the thermostat &1& not vary more than 0.01°0. A 3-volt battery vzaa not available a l l the time and in place of i t an 8-volt battery* Wiiqh could not be 'tapped .at a lower voltage, was used with a realstanoe in series to out down the cu rren t.. Although the etorrent wao the same, the eparking was several times as great, with the resu lt that the mercury contact soon became dirty and the tem­perature varied as much as 0.04*0. from the desired value*This regulation was sufficient for the work in progress*

The Eleotremotive Force Pells

fwe pyrez glass ce lls of the type used by Lewis and Rupert*4 were obtained with the hydrogen h a lf-ce ll and saturator a l l ready constructed. Three gold h a lf-ce lls were constructed for each one and sealed to the hydrogen ha lf- cell* This gave a ce ll with duplicate hydrogen electrodes and trip lica te gold electrodes, permitting aim different combina­tions ef hydrogen against gold to be measured in theveame solu­tion#. : ' \... ... : V : ■ . ." ,

The oolls were carefully cleaned with ohromio aoid before use and ringed thoroughly f i r s t with dis­til le d water and four times with aoid of the concentration to be used in the cell* A layer of wet g©M, which had been

shaken with aoid of the s e ll eon®miration, was placed in the bottom of the gold compartment to a depth of 4 mm. covering the platinum wire. Then a 2-mm. layer of the oxide which had

~37**

been eauillbxated e tth aold «as placet above the gold* " gold compartments were f i l l e t up to the oonneotlug tube with some of the equilibrated solutions end the remainder of the oell f il le d by way of the hydrogen compartment with pure acid of the same concentration.

Electromotive force Measurement e of Pells

A leeds-Borthrup, type K, potentiometer was used to measure the ce ll* . Great oare was used in the wiring system to insure good insulation. Both the potentio­meter end galvanometer were mounted on omnipotent!al p lates, which were connected to the metallio container of the thermo­s ta t. The potentiometer system was sensitive to O.OQGM. v e il . Both i t and the standard o e ll had been calibrated by the Bureau of Standards.

The ce lls were kept in the thermostat a t *5* with a very small stream of hydrogen bubbling through them. They were measured two or three times a day un til they came to a constant value, or i f they did not come to equilibrium in two weeks, they were taken out and re filled . The barometric pressure was taken a t the time of each reading so that the E.H.SV could be corrected to 1 atmosphere. The resu lts of the measurements are given in tables 2, 3, 4, and 5.

TABLE 8 - - mstEBHEEraS OF ELEOTROMOTIVB FOROB OF THE CELLS

H g(fi), HsS04 ( 1 .0 8 4 -m o la l) , AUgOg ( a ) , An ( a ) . 2 5 .0 0 0 .

Age of ce ll

hoursBar.

press. X

9 696.8 674.6IS 697.0 675.4S3 696.7 675.1se 697.6 676*032 700.0 677.0

37 697.2 675.6m 696.8 675.2

1

Oorm. $ Eleotremotive * Electromotive l.M.F. :f©roea observed: force a oorr. to

:______ _______ : 1 ato .press..0015 :1.5695 :1.3695 :1*3710 ':1.3710.0015 a.3696 :1.3696 !1.3711 :1.3711.0015 :1.3687 :1.3686 a.3702 ;*1.3703.0015 :1^3689 :1«3689 :1.3704 |l.3704.0015 il.3686 il.3686 a.3701 :1.3701.0015 :1.3684 $*1.3684 *1.3699 *1.3699*0015 :1.3687 *1.3686 :1.3708 *1.3701

•.Mean v a lu e s . . . .h .3704 !l.3704i i i i

Aerie oxide used in th is c e ll teas equilibrated n ith 1.024»molal sulphurio acid for 18 hours. The gold was washed with the same acid three times by decantation. Vapor pressure of the sulphuric mold solution was 21.6 mm.

1ABIE S — HEAStBEtiBIS OF B1B0TBOJ2OTIVB FOB OB OF THE OE1M H2 (6), H^04 (Oa-BOlal), Au^lg (s)» Au (a ) . 25.0°0.

Age of oell

hourst Bar* t press.

-I-_- LL\i r

960 ; — i

101 : 700*2 67S.7106 l 701.8 678.3117 t 701.7

i670.2

122 l 701.7i$ 678.2

127 : 700.6ii$

677*0i:

. . $ :Oorm. :Bleo taroootlTe iBleotromotlve

forces slieerveaiforoes let r 1 atm.oresB.1: ' I tl 1.3609$ 1*3625;l: l; '■ $'•t 1*3681; 1*3663$I V i ' $•■

'$ .

:$$■$

*0016$ 1.3676; 1*3678; 1*3##: 1*3695*0016; l.S680r l,S67t| 1.3696; 1.3694.0016; 1.3602: 1.36891 1.3697: 1.3704

; : ; ■ ' :.0016; 1.3607; 1.3689: 1.3702; 1.5704*0015% 1.5666: 1.3689: 1*3701: 1.5704

; s •: - ; ;: Mean value a* ... * 1.3697; 1.3700

' :______ : : :

She auric oxide eqelllhrated 24 hours wtth. tenth-molb! sul­phuric acid, and the gold shaken for the same length of time with acid of the same strength. Tap or pressure of the solution is the same as that of water a t th is temperature, 23.5 mm.She oxide was s tirred vigorously tuloe dally*

(Continued)

- 4 0

fm B 3 (Oontinuod) — HBASIBBMEBTS OF EEBCEROKOTIVB ECHOS OPTHE OEMS

H2 (g), H2S04 (0.1-molal), Au203 (a ), An (s ) . B6*0oC.

Age of c e l l

h o u rs

1 Bar*:p r e s s .

5 P«P*Hg

iG o rro .:B.K*P.

# 1 696 .8 : 672.7 .0016

13 l 697 .0$$ 8 7 3 .8

#.0016

83 ! 697.71i 6 73 .2

t#0016

88 $ 6 9 7 . 61: 674 .1

1* *0015

38 : 7 0 0 .0$1 677 .0

*% .0015

57 ! 697 .811 673.7

%; .0015

68 $ 896 .81

6 7 3 .5 *0016

Bleotr ©motive ::Eleotroootiv0

oorr. to

1*3707:1*5707:1*3697;1*3704;1*3705:1*3703:1.3704;

1

1.3707; 1*3783; 1.3783 1*3707! 1.3783: 1.37851.3697; 1.3718; 1.3718

1 ■' ■■ i '1.3704; 1.3719; 1.37191.5706: 1.3719; 1.37801.3703; 1.3718: 1.3718

: :1.3704: 1.3720; 1*3780

: :--------- v$

The ourio oxldo xma equilibrated for 12 hours with tenth-molal eulphurlo aold* and the gold shaken three times by decantation. Vapor pressure of solution, 83.5 mm. These measurements repre­sent electromotive forces of the same gold against two hydrogen electrode# immersed in the same c e ll, no s tirrin g of gold c e ll .

- 41*-

TAB1B 4 — HBASUREMBIES OF ElECIROliOnVB FCBCB 0? THE OEMS H2 (b), HBS0 4 (.01-molal), Au203 (s ) , Au (e ). 8B.0°0.

t i : .' s-Age o fs B a r . $ p .p . iB le o tr o m o tlT e f o r c e s sE le o tr o m o tlv e f o r c e s

c e l l t p r e e e . ; Hg i o b serv ed i o o r r . to 1 atm .p r e s s .h o u r s i_______ s __________ i ___________________ .

: ; $ ■ i • : ; ; '1 6 i 700* : 6 7 7 . i l . 3 6 S - s l . B 8 — - s — s **•

' t ‘ i ' ■■■*'■. : ' : s ; s - s -4 0 $ 7 0 0 . % 6 7 7 . s l .3 4 6 6 s l .3 6 4 7 t l .S 2 6 $ — i — . t —

■ s ’-. ' • S "• ■ • * s' ■ S '• s :88 s 7 0 0 . % 6 7 7 . s i . 3 6 8 8 s i . 3 6 9 7 s i . 0687 s l .3 7 0 3 s l .3 7 0 2 s l .3 7 0 3

90 s 7 0 0 . ! 6 7 7 .2 s l .3 6 8 6 s l .3 6 9 5 s l .3 6 8 8 t l . 3 7 0 1 s l . 3 7 1 0 a .3 7 0 0

93 s 7 0 1 .8 s 6 7 8 .3 s l .3 6 9 3 s l .3 6 9 7 a .S 6 8 9 I i .5 7 0 8 a .3 7 1 8 s l . 3 7 0 4

105 : 7 0 1 .7 s 6 7 8 . 2 s l . 3 6 9 8 a .3 6 9 7 a .3 6 9 5 i l . 3 7 1 3 a . 3 7 1 2 a . 3 7 1 0

110 $ 7 0 1 .7 s 6 7 8 .B s l .3 6 9 9 i l .3 6 9 7 a .3 6 9 8 i l . 3 7 1 4 i l . 3 7 1 2 t l . 3 7 1 5

118 ! 7 0 0 .8 s 6 7 7 .3 i i . 3 6 9 7 i l . 3 6 9 7 a .3 6 9 7 i l . 3 7 1 2 i l . 3 7 1 2 i l . 3 7 l 2 ' t s • i -x s i

s s sMean values........... .........1.3708sl.3712$1.3707$ i sMean of three value ........... . . . . . . . .1 .3 7 0 9

_____s______%_____J _____ s s : s s volt a

The auric oxide %as equilibrated 24 hours v?lth m/lOO sulphuric acid, and the »e ta lllc gold treated llkeslee . Taper pressure of the solution » 23.5 ran. Correction of E JI.F. for pressure ©f hydrogen to 760 mm. * 0.0016 vo lt. The oxide was stirred twice a day .

• 48 -

Sinoe an extremely delicate equilibrium exists In. a galvanlo ee ll acting under tru ly reversible condi­tions, and. slnoe the E.M.ff. measurements do not have any moan­ing unless the resu lts are reproducible, certain preeaatlene must be Rigorously observed. The conditions which could pro­duce varying re su lts are enumerated below with explanations, where necessary e

Summary of Conditions Affecting the Constancy of ElectrodePotential

1» Teisperatee.2. pressure o£ hydrogen.2 * Purity of hydrogen.4 . Solid constituents of c e ll .

a . Purity.b. Physical state*

(1) State of subdivision, with varying solubility(2) Allotropio modifioations or degree of hydra-, t Uen.(3) Imperfect wetting of solid by electrolyte.(4) Colloid formation.

o. Chemical properties.(1) Basic sulphate formation*(2) Actual decomposition of oxide.(3) Possibility of more than one oxide present.(4) Peroxide formation.(8) Effect of l ig h t.

6* Oell electrolyte* a. Oonoentration.1>* nature of electrolyte. o« liga ia potential.

(1) Evaporation of solution in hydrogen ohaahor*

Many of these have already been dlBonseed, so only a few w ill he considered here. She pressure of the hydrogen is oorreoted to one atmosphere by the equations

B° * B * 1 1 ^ In (2 |2 )e

where "2" is the p a rtia l preseure of the hydrogen, v&loh i s equal to the barometrio pressure le ss the vapor pressure of the solution in the c e ll. The oorreoted E.M.E. values given in the tables were calculated in th is way. When the ooils wore le t stand without s tirrin g , they would decrease in electromotive force, but were restored by s tirr in g . This was evidently polar­isation , perhaps due to basic sulphate formation on tfce surface of the 0Ed.de* I t is quite possible that gold forms peroxides analogous to silver, but we do not have any evidence as yet from which to decide Whether i t exists or not* The electromotive forces ean be oorreoted for any concentration of sulphuric acid, knowing the activ ity-oeeffielents for sulphuric acid. The following table of vapor pressures of sulphuric acid solutions■' ■ 1 4 • • • 'was taken from Broensted.

Hol f r a c t i o n . . , . , . . . $:0*OOOS7 : 0.001 ! 0.0050 : 0.0801*: ' " t i : '

Pressure (mm* Hg)«..$ 84.0 $ 85.6 : 85*5 : 17.1' 1 1 -r ■■■• ' ■ i - :• r ' ■ ■

The taper pressure of tenth ana hundredth molal solutions can be aeeuae& to be that of pure water. The value of the vapor pressure for 1.084 molal was obtained by plotting the above data and was found to be 81*6 mm.

TABLE 5 — VAPOR PRESStBBS OP SUIPHTRIO ACID SOLDTIOHS AT 2 5 °C V

Summary and Discussion of Results

The measurements made on the foregoing gold se lls are summarised in the fina l table, as follows:

TABLE 6 m m m t op eleotrqudtxvb force msw:» • • • - -

ITS AT 25°0.

Cell 50* 1 Molality (H8304) ■ B. M. P. (Gorreotod to 1 atm.)

1 1.084 1.5704 volts

.',1 0,1 ; 1.3697 volts1 0*1 1.3700 voltsl 0.1 1.3719 volts1 0.01 1.3708 volts

1.3718 volts1 0.01t 0.01 1.3707 volts:Mean of a l l v a lu e s ..! ....1.071 to.OOl volts

tlen of the sulphuric acid makes very l i t t l e difference in the electromotive force of th is c e ll, as would ho expected from thereactlent

3 Hg 4* AugOg at Zkn + 3 HgO,

as the electrolyte does not enter into the reaction* A oalou* lation was made of the magnitude of the effect of concentration on the E.H.F. of th is cell* In th is calculation i t was assumed that the ac tiv ity of the solvent was proportional to i t s vapor pressure* At tenth and hundredth-molal the vapor pressures are the same within the lim its of experimental error and equal to that of pure water* Therefore* they should give the same 2.M.F. unless* the oxide is more soluble in one concentration than in the other* For tenth and hundredth-molal the E.H.F. difference due to concentration of acid was oaloulated to he of the order ®f one millivolt* Since the ee lls are not reproducible to betterthan a millivolt* we may consider the value obtained as inde*' . • >pendent of the concentration of the electrolyte*

The average value* 1*371 * 0*001 vo lts, then/ •

represents the potential of the gold* auric oxide half ce ll,since the potential of the hydrogen electrode is taken as zero*- ' ̂ ̂ 'Therefore* Au, AugOg (s ) f B°« - 1.371 t 0.001 vo lts.

Qaloulatlona of ghermodynamlo Data for Aurio Oadda

Using the two equationa previously mentioned ne are no® in a position to oalcralate the free energy of forma­tion of aurio oxide from i t s elements.

1. 3 Eg (g) + Au20S (0) * 3 EgO (1) .+ Att ( s ) ;^ p '= -SEP2. 3 Eg (g) > 3/2 02 (g) « 3 EgO ( l ) ; JP"= -169,700 cal.

Whence: 2 Au + 3/2 Og (g) = AugOg (a)i / IF ~/i**= AF'= P°

AugOg (298)

Per equation: 4** •

* -6 Z1 . 371 x 23074

m -189,800 oal.Whence: F°2g8 e -169,700 - (-189,800)

a + 20,100 ca l.

This high positive free energy of foreetion of aurio oxide from i t s elements indicates that i t i s very un- stable with probably a high dissociation pressure.

The dissociation pressure can be calculated from the free energy, using the thermodynamic equation:

dP m m in If I 1 ° z l

In Bo * ^ * 20.100

F * 10.6 atmospheres.°2

This high dissociation pressure explains shy auric oxide decomposes merely on being dried over phosphor* one pentoxide a t room temperature*

I t is also possible to calculate the entropy change, As when one mol of auric oxide is formed from i t s e le­ments, by combining the free energy and heat content by the equation! ' ■ . : : ; .

48 = ̂ H - 3MS

According to Thomsen3,6 the value of ATL Is -13*200 cal; for fAUgOg (Aq)*

mienoei AS * AU +A* - 13800 * 801007" ■ 2^8 ■

- • - 112 cal / mol / degrees'.

TABSB 7 OOMPABI301 OF THERKODTOliMIO M IA FCE SEVERAI, 10B I8

METAX OSIPSS

• .''Oxide 4 F° t

:; %

HgO -13,808$l : -21,600 -26 •! !l.96xlOw7atm.

w *8iS95 | -6*900i

-11*7 :4,9 x lO^4

Au2°3 +20,100 ! -13,200• i

ft:

-112 : 10.8 :

In table 7 arc assembled certain thermo­dynamic data for mercuric and silver oxides for comparison with auric oxide, These values are strikingly consistent with

their re la tive s ta b il i t ie s , the resu lts obtained for gold are tihat one would aspect from the general trend of cuoh data for azide o»

OOHCLUSIOBS

The resu lts of the investigatito of aurio oxide may be summarised as follows:

1. Pure auric oxide has been prepared and analyzed by several new methods whose accuracy has been discussed. The oxide was shewn to be highly pure and the gold in i t was proved trlva len t.

2» Electromotive force measurements on the cell:Eg (g), HgS04 (xm), AUgOg (s), Au (a)

have been measured in three m olalities of sulphuric acid* I t was tihown that the E.H.P. did not change more than one m illi­vo lt, due to change of concentration from hundredth to unit molal. The potential of the electrode: An (s), AubOs (s) , H4*, i s found to be -.1571 =t .001 volte a t 25.0°0.

S. The molal free energy and entropy of formation ofauric oxide from i t s elementsand i t s dissociation pressure togive i t s elements have been calculated for auric oxide in i t sstandard state a t 26.0*0. from the experimental dat0. The

- -+ Vo, ioosJL:values oalou3ated are: P 298* *10 r̂DOO-c^ i 8*298**112 cal. per mol per degree, the dissociation pressure of auric oxide is calculated to be 10.6 atmospheres. These re su lts are con­sisten t with similar data for the oxides of suoh noble metals as mercury and silver.

BIBHOGERAPHZ• . - '

* 4 9 -

1* Xewi®, 0. H.i£2l»»

3# Jeraa and Jelindc, "Anodic Oxidation of Gold," II*Propertiea of Aurio Sulphate, Ohem. Slaty 18, 1 (1924).

4* lewis end Rupert, "potential of the Chlorine Electrode,"Jour. Amer. Ohem. Soci., 33, 829 (1911).

5. Keller, "A OomprehendLve Treatise on Inorganic and Theo­re tic a l Chemistry," Songmane, 1983, Tol. I l l , 680.

6. Senher, Jour. Amer. Ohem. Soci., 86, 1137 (1903).

7. Hlxter, W.G., Jour. Amer. Ohem. Soci.. 33, 688 (1911).8. Meyer, Oocut. Bend. , 146, 807, (1907).9. Mixter, "Heat of Oomhination of Auric Oxide," Amor. Jour.

of Sol., (4), Tol. 32, ^)6 ( 1 9 n ) . *“ ~ w

10. Pregl., Quantitative Organic moroanalysis.11. Hodgoan and Sange, "Handhook of Chemistry and Physios,"

Tenth Ed,, 1986, p* 106. ;12. Bef. 2, p. 296.13. Ahegg: Bandhuch der Anorg. Ohemie. I I , P t. I , Group I , p.778.14. Broenated, "Studien sur ohemiachen a ff in ita e t," Zeit. -

Physik. Ohemie 68, 704 (1910) . ' ~15. Thomsen, "Thermochemistry," Songman a. 1908, p. SOI.

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