~) ',','

Indi an Journal of Pure & Applied Physics Vol 40, October 2002, pp. 683-689

, , ..)'

Effect of ammonium halides on the temperature of sound velocity maximum of water .

I I . - \ L G Venkata Raman~:, _E I Rajagopa l & ~ r;vta nohara Murthy

Department of Physics, Sri Kri shn adevaraya Uni vers ity, Anantapur 5 15003 - , . *Sri Lakshmi Srinivasa Degree Coll ege, Pullareddy Peta 5 16 175

Received I February 2002: revised 2 Jul y 2002 ; accepted 18 Jul y 2002

LEffect of ammonium halides on the temperature of sound veloc ity maximum (TSYM ) of water has been studi ed by determining the ult rasonic velocity with an accuracy of ± 0.003 % using ~ ing le- crys ta l variable-path interferometer working at 3 MH z. The veloc ity measuremen ts were carri ed out at = 2 °C interval s over a range of 5 °C to either side of the TSYM of the solutions. The accuracy in fi xi ng the TSYM is ± 0.2 0c. The shift in TSYM of water due to the add iti on of the NH4C\, />,. 7;,hs' is found to be positi ve up to IV = 3.4 x 10-2 and negative thereafter, where IV represents weight frac ti on of the solute. The shifts in TSYM of water due to the addition of the NH4Br and NH41 are found to be negati ve throughout the concentration range. The struClural contributi on to the shift in TSYM of water, />,.T"" is found to be posi ti ve fo r NH 4C\ and increases wi th increase in the concentration nonlinearly. />"T"r for NH4Br is almost zero up to w = 3 X 10-2 and thereafter becomes negati ve. />" 7~tr for NH 41 is found to be negative throughout the concentration range. The results are ex pl ained in terms of the structure. making and breaking nature of a ni o~s and c3.!io!}s present in the solutions. ~ j

1 Introduction

Water ex hibits a max imum in its sound veloc ity versus temperature plot at 74 °C, which can be expla ined on the bas is of two-state mode l for water, common to many theories 's . At any g iven temperature, water can be considered to be an equilibrium mi xture of structured (hydrogen-bonded and voluminous) and no n-structured (non-hydrogen bonded and dense) spec ies, At low temperatures , the structured form is predominant, while at hi gh temperatures it is the non-struc tured form. A ri se in temperature produces vol ume expansion of both spec ies, which corresponds to a negati ve temperature coeffic ient of ve loc ity. Super-posed over thi s will be an equilibrium shift between the two spec ies, resulting in an increase in the population of the non-hydrogen bonded , dense compone nt , which correspond s to a POSitI ve temperature coefficient of sound ve locity. The max imum in ultrasonic velocity versus temperature at 74 °C thus corresponds to the balance between these two oppos ing effec ts . The presence of a solute (e lectrolyte or non-e lectrolyte) changes the structure of water, affec ting the te mperature of the sound velocity maximum (TSV M). Solutes may be classified as structure make rs or struc ture breake rs,

de pending o n whether the shift in TSVM is towards higher or lower temperatures.

Tamm & Haddenhorst6, Mikhailov et al .7-9 ,

Marks 'o, Pancholy & Singal " and Gnanamba & Ramachandra Rao ' 2 have studi ed the effec t of diffe rent e lectrolytes on TSVM. In these invest igations, the acc uracy in fixing TSVM is less and ranged from ± 2 to ± 5 0c. Because of these large errors in fi xing TSVM, there is ambiguity in de lineating the structural properties of the ions. Subrahmanyam & Rag havan 13 studied for the f irst time, the effect of LiCl , LiBr, LiI, NaCl , KCI and RbCl on the TSVM of water by measuring ultrasonic ve locity with a higher degree of accuracy (± 0.003 %) and the accuracy in fixin g TSVM was ±0,2 0c. This enabled them to de lineate the structure, promoting and di srupting nature o f the ions, unambiguous ly. Subrahmanyam & Sivakumar '4 studied the effec t of LiOH on the TSVM of water and Achari et af. 15 studied the effect of lithium carbonate, ammonium phosphate, ammonium acetate and strontium phosphate on TSVM of wate r. The effect of a lkali flu orides, iodides, acetates , formates and nitrates on TSVM of wa ter have been studied by Sivakumar '6 T he effec t of sulphates of lithium, sod ium, potassium,

684 INDIAN J PURE & APPL PHYS , VOL 40, OCTOBER 2002

ammonium and magnesium on the TSVM of water have been studied by Vcnkata Ramana et (II. 17

In the present work , the autho rs studied the e ffect of a mmonium ha lides on T SVM of water with improved accuracy in fixin g TSVM , so as to de lineate more prec isely the structura l prope rti es of ammonium and halide ions, s ince the re exi sts ambiguity due to large errors in fix ing TSVM for these soluti ons I8

.19

• The results are presented in thi s paper.

1 5 5 S.5~-------------'----,

:::1 :::::t

1

15 77-0

1576·5 G

L.....-.l.---L----L--::L1----::--:::-1 ~ 1576.066 68 70 72 74 76 78 M

dCI--Fig. I - Ultrasonic velocity (u ) versus temperature (I) in aqueous ammonium chloride at di fferent weight fracti ons IA . Pure water; B. IV = 0.0092; C. w = 0.0 150; D. w = 0.0232; E. w = 0.037 1: F. w = 0.0482; G: w = 0.0658)

2 Experimental Details

U ltrasoni c veloc ities in water and dilute aqueous so lutions of NH.jCl, N H4Br a nd NH41 we re determined us ing a s ing le c rysta l variable-path inte rfero me te r designed a nd fa bricated in o ur la bo rato ry. A tri -te t c rysta l-contro lled osc ill ator with freque ncy sta bility ± 1 Hz was used to excite the quartz transduce r. T he transducer, whose fund a me ntal freque ncy is I MHz, is exc ited a t its third harmonic. T he freque ncy was measured w ith a dig ita l freque ncy mete r w ith an accuracy of o ne part

per million. The vo ltage vari ations across the transduce r were observed using a diffe re nce­amplifie r fo ll owed by a n e lectronic vo ltme te r.

The mechanica l assembl y of the inte rfero me ter (des igned and fabricated in the laborato ry) was imme rsed in a the rmostati c water bath whose te mpe rature can be controlled up to ± 0.01 °C , using suitable permanent heate rs fo llowed by o n-off low wattage heate rs. Stirring it pe riodica lly, minimj zed the te mpe rature g radie nts in s ide the expe rime nta l liquid . The te mpe rature of the interfero me tri c liqu id , meas ured us ing a bead type thermostat whic h fo rms one arm of a consta nt curre nt Wheat -tone bridge fo ll owed by a c hopper-s tabili zed ope rationa l a mpli f ie r as null detec to r, was found to be consta nt w ithin ± 0.005 0c. The the rmostat and the e lec tronic assembl y were housed in an a ir-cond itioned room , whose te mpe rature was ma inta ined at 20 ± 1 0c. Thi s resulted in improved stability of the crysta l osc ill a tor and the effici e ncy of the tempe rature contro l. The refl ec to r o f the interferomete r is moved by a spec ia lly des igned screw , incorporated in the inte rfe rometer, whic h can be read to an accuracy of ± I x 10.3 mm. The path length was measured fo r 50 dips and thi s resulted in measure me nt of wavelength of the ultrasonic beam with an acc uracy of ±0.0025%. The high degree of acc uracy w ith whic h the freque ncy of the transducer was measured , and the tempe rature stabi I ity of the inte rfero me tric liquid a nd accuracy in the measure ment of the path le ngth resulted in measure ment of ultrasonic ve loc ity in the expe rime ntal so luti on w ith an acc urac y of ±0.003 %. The interfero mete r was calibrated by measuring ultrasonic ve loc ity in tr iple di stil led de-gassed water. A t eac h tempe rature, s ix to e ight measure me nts were ta ken and the ave rage is c hosen as the ult rasonic ve loc ity, which is fo und to be accurate to ± 0.05 ms· I

., The standard deviation of ultrasonic data at any g iven temperature estimated is fo und to be ± 0.04 ms· l

. Thi s acc urac y could be rea lized due to the fac t that, the te mperature coeffic ient of ultrasonic ve loc ity in di lute aqueous solu tion of e lectro lytes is extreme ly sma ll apart fro m the accuracies in vol ved in the measure me nt of path length a nd frequency of the tra nsd ucer.

3 Results and Discussion

The meas ured ultrasonic ve loc ities in aqueous solut ions of NH 4C I, NH4Br a nd N H4I, as a function of temperature , are prese nted in F igs 1-3 ,

RAMANA et al.:SOUND VELOCITY 685

respectively . The velocity versus temperature curves for a ll the solutions studied have the same shape as the curve for pure water and hence a transparent template of the ultrasonic ve locity-temperature curve for pure water was used to fix the TSVM of the solutions. The accuracy in fixing TSYM is ±O.2°C.

155l.·0r--------------..,

1553-5 155l,·5

1551.·0 i 73-7

1553-0

1553·5 ) 1555·0

'fill

1554·5 .§

1554·5 ~

.[155'0/ .~ .§ 15 53-5 ~ J J

1554·0

1555·5

1555·0

67 69 71 73 75 77 o

He) --+

Fig. 2 - Ultrasonic ve locity (Il ) versus temperature (I) in aqueous ammonium bromide at different weight fractions [A. IV = 0.0015; B. \V = 0.0208; C. IV = 0.0293 ; D. IV = 0.0449 ; E. IV = 0.0553 ; F. It' = 0.0636]

The shift observed in the tempe rature of sound velocity maximum of water caused by the addition of an e lec trolyte of certain weight is g iven by:

... ( I )

where T, and Tw represent the TSYM of an aqueous electrolyte solution and water, respec tively.

This shift may be thought of as arising due to two effects, namely, the dilution effect and the structural effect. The structural effect arises as a result of interaction between so lute and solve nt. It

w ill be positive if the hydrogen-bonded structure of water is sta bilized by the solute and negative if the hydrogen-bonded structure is destabili zed by the presence of solute , creating monomers of water molecules. The dilution effect arises mere ly due to dissolution of the electrolyte in water and is always towards shifting TSYM of water to lower temperatures. To separate these two effects, it is necessary to eva luate the effect of dilution on the TSVM of water by the addition of the solute. This can be done by combining the expressions representing the temperature dependence of sound velocity in water and in the solute.

1552·Sr----_____ ---.

1552'0 A

1551 '5 1552'0

~15515 8

1551·0

1550·5 C

11550,0

J5510 1

1550·5

o 1550{)

151.9·5

E

1550 .0j

15'~5 ~E 154 9·0 ~l _-,--_-"--_,-! _-'--_-L-----i

66 70 72 71. 76 78 o t(c)--

J

Fig . 3 - Ultrasonic ve locity (1I ) versus temperature (I) in aqueous ammonium iodide at di ffe rent weight fractions [A. IV = 0.0056; B. IV = 0.0 120; C. IV = 0.0179: D. IV = 0.0220; E. IV = 0.0322]

686 INDIAN J PURE & APPL PHYS , VOL 40, OCTOBER 2002

lof

1 Oir I 0.6r 'u ~ fE 0'4 ~~ I

"6 +-

0·(

0·0 0

3400

3000

2600

I ll OO

of IBOO ~

o· J

14 00 +-

,~Hli

NH[,8r

20 W 60 80 100 120

NH,,! "-

"-

140

NHI 4

160

1 000 L _-:l::---+_+-;:J,:' ;---;;b' ;---;-:;\;, -'7!;;.l..~ ----:::. o 80 100 120 IW 160 u ______

Fig, 4 - Variation of 1/ ; and au with Illolecular weight (M ) of

H.jC1. NH4Br and NH.j 1

According to Wi ll ard 20 the dependence of sound velocity In

conforms to the re lation:

ti l = 1557 - 0.0245 (74 - f )l

temperature pure water

" .(2)

where t represents the temperature in DC. The temperature dependence of sound velocity in the solute can be represented by:

" .(3)

where 1I ; and <Xu represent the ve locity of sound in

the solute at 0 DC and temperature coeffici ent of sound velocity in the solu te, respectively.

r !

2 . ~

I Wx 102 ----. I

o~~ I~ "~~A !

B : -2 • ~''''~. " I

I :c1 -4 I ~ c ! ~ I

-6 ,-----------.-------- J

Fi g_ 5 - Variation of !1T;,hs wi th weight fraction of alllilloniulll halides [A -t NH 4C1 , B -t NH4Br. C --., 1H41]

Assuming additivity of inte rnal e nergy, the velocity of sound in an e lectrolyte solution can be expressed as21

:

o 2' u- = (l - HI) U I + HILI 2 . . . (4)

where HI represents the we ight fraction of the solute in the solution.

Substituting Egs (2) and (3) in Eg. (4), one can get:

. __ (5)

Differentiating Eg . (5) with respect to temperature and making use of the condition that:

(dll ) = 0 dt I =T,

one can get:

r 1557 - 0.0245 (74 - TJ2J 0.049 (74 -- 7~)

whereb= (~) l l- HI

... (6)

RAMANA ef al.:SOUND VELOCITY 687

1

Fig. 6 - Variation of /';. 7 ~". with we igh t fraction of ammonium halides IA ---7 H.jCI, B ---7 NH.jBr. C ---7 NH4 1J

The term within the square brackets in Eq. (6) represents the veloc ity of sound in water at temperature of sound ve loc ity maximum in solution, and it can be repl aced by 1557 m S· I if T, did not differ appreciably from 74 °C. Hence:

(1557) (0.049) (74 - TJ = b (Ll ~ - aJJ ... (7)

Simplifying, one can get:

5646 - ball u; T, = ------"::--=--

76 . 3 - ba~ . .. (8)

If there is no change in the structure of water due to the addition of an electrolyte, then, T, given by Eq. (8) represents the idea l va lue of the temperature of sound velocity maximum in the solution . That is:

... (9)

If the hydrogen-bonded structure of water is modified by presence of the solute, then, the TSVM observed ex perimentally (TJexr will differ from that of the idea l va lue (TJ,d' Hence, the structural contribution to the shift in TSVM of water due to addition of the solute is given by:

... ( 10)

The magnitude and sign of f..T,,, represent the strength and nature of structural interactions between the solute (electrolyte) and water.

To calculate (T')iJ , one needs II ; and au, whi ch

can be evaluated from data on the temperature dependence of the elastic constants. Since the solutes used are solid e lectrolytes, one can have three velocities associated with sound wave propaga ti on. one longitudinal and two transve rse. Since there is no reason to prefer anyone of these velocities, one may use the mean veloc ity 11 m fo r 11 2

in the electrolytes, given by:

... ( II )

where 11 1 and II , represent the longitudinal and transverse wave velocities in the polycrystalline aggregates of the electrolytes . It may be menti oned here that, to eva luate the Debye characteri stic temperature of polycrystalline samples, the average velocity normall y used is that given by Eq. (11 ) and the authors' preference to use li m in stead of either 1I1

or U, to eva luate (TJid is reasonable.

The mean veloc ities of sound ( ll! ) in the solu tes (N H.jCl and NH.jBr) at different temperatures have been evaluated making use of the data of e lastic constants and their temperature dependence22 The method followed to ca lculate U I and It , from elastic constant data , necessary to eva luate Ul , making use of Eq. ( 11 ), is detail ed in an earlier communicationB

. By least square-fitting of U 1 versus

temperature data to Eq. (3), u; and au have been

eva luated. lI ; and au for NH.jCI and NH.jBr are 30 II ms·l , 0.82 III S· I K I and 2189 m S· I, 0.57 m S l K I .

respectively . It was not poss ibl e to eva luate u; and

au for NH4I,since the data on temperature dependence of its e lastic constants are not avai lable

in literature. However, considering that u; and au

vary linearl y with molec ular weight, these values for NH4I have been eva luated by extrapolating the

curves as shown in Fig. 4. The va lues of Lt ; and au

obtained graphica ll y for NH.jI are 1320 III S l and 0.30 m S· I K I, respecti vely. These va lues are onl y a rough estimate, in the absence of experimental data and the authors have used these va lues to calculate (TJiJ' to get an idea about the structural contribution

688 [NOlAN J PURE & APPL PHYS, VOL 40, OCTOBER 2002

to the shift in TSVM of water. The values of structural contribution to the shift in TSVM of

water, f.,T,,, , for NH.CI, NH.Br and NHJ are ca lculated according to the procedure described

above. The va lues of (7~)"p, !1T"hs> (7~) 'd and f.,T,,, for the halides of ammonium are presented in Table I .

Tablc 1- Valucs of U s)" p' (7 ~)iJ . 6.1'. ,hs' and 6.T,,, al dilTerenl wei ght fract ions of ammonium halides

1\ ' (TJ"p COc) ( 7s) id 6. 7~'hs (OC ) 6.T,,,.(OC) (oq

0.0000 74.0 ± 0.2 74.0 0.0 00

NH.Ci

0.0092 74 .3 ± 0.2 73. 7 +0.3 ± 0.2 +0.6 ± 0.2 0.0150 74.2 ± 02 735 +0.2 ± 0.2 +0.7 ± 0.2 0.0232 74.3 ± 0.2 732 +0.3 ± 0.2 + 1.1 ± 0.2 0037 1 73.9 ± 0.2 72.8 -0. 1 ± 0.2 + 1.1 ±0.2 0.0482 739 ± 0.2 72.3 -0.1 ± 0.2 + 1.6 ± 0.2 00658 737 ± 0.2 71.8 -0.3 ± 0.2 + 1.9±0.2

NH. Br

0.0015 739 ± 0.2 74.0 -0. 1 ± 0.2 -0. 1 ± 0.2 0.0208 73 .7 ± 0.2 73 .7 -0.3 ± 0.2 00 ± 0.2 00293 73.5 ± 0.2 73.5 -0.5 ± 0.2 0.0 ± 0.2 0.0449 730 ± 0.2 73 .2 - 1.0 ± 0.2 -0.2 ± 0.2 0.0553 72.9 ± 0.2 73 1 - 1.1 ± 0.2 -0.2 ± 0.2 0.0636 72.5 ± 0.2 72.9 - 1.5 ± 0.2 -0.4 ± 0.2

H.j l

0.0056 738 ± 0.2 740 -02 ± 0.2 -0.2 ± 0.2 0.0120 735 ± 0.2 739 -0.5 ± 0.2 -0.4 ± 0.2 0.0179 733 ± 0.2 73.9 -0.7 ± 0.2 -0.6 ± 0.2 0.0220 730 ± 0.2 73 .9 - 1.0 ± 0.2 -0.9 ± 0.2 0.0322 72.3 ± 0.2 73.8 - U±0.2 -1.5 ± 0.2

The values of f.,T" hs as a function of weight fraction (w) of NH.1CI, NH.jBr and NHJ are presented graphically in Fig. 5. An exa mination of

the data presented in Fig. 5 indicates that, f., T" bs for

NH.jCl is positi ve up to w == 3.4 X 10.2 and nega tive,

thereafter. Value of f.,T" hs is negative for NH.jBr and NH.jI and the negative shift increases with increase in the concentration of these electrolytes. The values

of f.,T"r for NH.jCl , NH.Br and NH.jI are presented graphicall y in Fig. 6, !1Tstr is positive for NH.jCl and increases with increase in the concentration non­

linearly. Value of f.,Tstl for NH4Br is almost zero, up to )t. == 3 X to·2 and thereafter become negat ive.

Value of !1Tstr for NH41 is negative throughout the

concentration range and f.,Ts" versus w is parabolic '<tture.

Namoto & Endo l R.19 have studied the va riation of

ultrasonic velocity as a function of temperature in aqueous solutions of NH4C1, NH. Br and NH41 in the te mperature range 55-85 °C They reported that, the ve locity-peak temperature remains sensibl y constant up to 4.5 mole %, independent of the kinds of anions for all the ammonium hal ides and the behaviour for high concentrations beyond 4.5 mole% is quite diffe rent according to the kinds of halogen ions. Beyond 4.5 mol e % the ve locity-peak te mperature (Tp) increases w ith increasi ng concentration for NH.jCl increas ing up to 86 °C, a behaviour not observed for any of th e lec trol yte solutions. Beyond 4.5 mol e % the veloc ity-peak te mperature re mains sensibly constant for ammonium bromide at 74 °C and in the case of ammonium iodide , it decreases initially and subseq uentl y increase with concentration . They stated that the ionic radius is smallet: fo r CI ( 1.8 1 A), la rger for T (2. 16 A) and intermediate for Br ( 1.95 A), corresponding to the difference in the tendenci es of velocity-peak temperature versus concentration curves for these substances.

In the present work, the ultrasonic ve locities in aqueous solutions of ammonium ha lides were de termined with an accuracy of ± O. 05 ms·1 and the velocity measurements were confined to a narrow temperature range on ei ther s ide of TSVM (i ,e. 64-80 °C). The ultrasonic velocities in the solutions

were determined at == 2 °C intervals in the te mperature range 68-80 °C The accuracy in fixing TSVM is ± 0.2 °C In view of t e improved accuracy in fixing TSVM, the present data of TSVM at different concentrations of halides of ammonium may be considered to depict correctly,

the structural features of these solutions. Whil e f.,T" bS

is positive at low concentrations and becoming

negative at high concentrations for NH4C1, f.,T" hs is negative throughout the concentration range studied

for NH4Brand NH). Value of f.,T,tr is observed to be positive, increasing with concentration of NH.jCl in

the concentration range studied. Value of !1Tqr is constant and almost zero up to w == 3 X 10.2 and the reafter, it is slightly negative in case of NH4Br.

For NH4I, f.,Tstr is found to be negati ve throughout the concentration range studied. These observations indicate that in solutions of aqueous NH4C1 the structure-promoting nature of NH/ dominates the structure-breaking nature of CI. In aqueous solutions of NH.jBr, the structural propensities of

RAMANA el al.:SOUND VELOCITY 689

NH/ and B( are comparati ve ly equal and hence, one can observe almost zero or a very small negative shift. In aqueous solutions of NH41, the structure-breaking nature of ). dominates over the structure-promoting nature of NH/ and hence, one can observe 6.T", to be negati ve and increas ing with increase in the concentration of NH.j1. The stabili zation of the hydrogen structure of water by NH/ may be considered to be due to fillin g up of cavities in network sites of water as is evidenced from X-ray scattering data2.j.25

At any given concentration , 6.T,,, for NH.jCl > H.jBr > NH.jI indicating the structure-breaking

nature of I' > Br > CI '. It may be mentioned here that, thi s is the order found for structure-breaking nature of these ions from viscos ity B-coefficient26

and act ivat ion energy of viscous flow datan . The viscos ity B-coefficient for CI , Br' and I', respectivel y are, -0.007, -0.042 and -0.068. The negative va lues indicate their structure-breaking character and the magnitudes indicate the strength of structure-brea king property of CI , Br and I .

4 Conclusions

The present study indicates that, NH/ is a structure-promoting ion while CI , Br and r' are structure-disrupting ions.

2 The structure-disrupting property of CI ', B( and llies in the order CI ' < Br' < L

3 The observed velocity-peak temperature is slightly higher than 74 DC up to W = 4 X to'2 for NH4C1 and less than 74 DC for NH.jBr and NH.j1. This observat ion is quite contrary to the observation made by Namoto & End02

1.22 .

4 Eva luation of structural contribution to the shift in TSVM of water enabled the delineation of structural propensities of NH/ , CI ', Br' and I' in aq ueous solutions of halides of ammonium unambi guously, as compared to the studies made by Namoto & End021.22 .

Acknowledgements

One of the authors (GVR) is thankful to the University Grants Commiss ion , New Delhi , for the award of FTP research fell owship for ca lTyi ng out the research programme.

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