Revitalizing the Gravimetric Determination in Quantitative Analysis Laboratory Thomas M. Harris The University of Tulsa, Tulsa, OK 741 04

As pointed out recently by Charles Beck (I), classical analy- The Experimental Procedure ses still play an important role in analytical chemistry. None- Caution: The concentrated hydrochloric acid used in theless, some of these methods are disappearingfrom the un- the following experiment can cause severe eye and

dergraduate chemistry cumculum. Gravimetry is the prime skin irritation. Chemical warkers'goggles should be worn while handling the acid and the acidified sample solutions.

example of a methodology threatened with extinction in the Ignition offilter paper can result in acrid smoke; this part of quantitative analysis laboratory Unfortunately, performing a the procedure should be conducted in a fume hood. gravimetric analysis may be the students'only hands-on ex- The "base" procedure utilized in our laboratory is pre- ~erience with precipitation, a process of great importance in sented in the table. Deviations from this procedure that both nature and industry. With the aim of enhancing the have been tested to date are also listed in the table. All overall pedagogical value of a gravimetric analysis, we have solutions were prepared with reagent-grade chemicals and developed a gravimetry laboratory exercise that also illus- high-purity water produced with a Milli-Q water purifica- trates many of the chemical and physical principles of pre- tion system (Millipore Corp.). However, demineralized cipitation. water is suitable if high-purity water is not available.

The gravimetric analysis that we chose to expand upon Results and Discussion is the determination of barium by precipitation with sul- The results obtained by students using the base proce- fate. Several problems can be encountered with this analy- dure and variations on it are presented in the Ex- sis when i t is applied to real samples. In a typical Vant i - ..riments were conducted in du-olicate: the range of the tative analysis laboratory exercise, each student is given a balues obtained by each student ire indicated by :he black different unknown barium-containing sample and is ex- cap on each column in the figure. pected to obtain the "correct" answer. The same analytical Students No. 1 and 2 followed the base procedure exactly proced:re is used by each student. In our laboratory, all of The results in the figure indicate that, for a given student, the the students analyze a single barium standard solution. repeatability of this procedure is quite good. The difference in

H ~ ~ ~ ~ ~ ~ , most of students are forced to deviate from the the average value for each student is probably due to differ-

'8ase" analytical procedure, in a manner that does not nec- ences in the volumetric glassware used. This source of error can be eliminated through calibration.

essarily enhance the accuracy of their results. In addition, Students No. 3 and 4 used the Gooch crucible/glass fiber some of the students are asked to include Ca2+, a potential pad (variation ,ja) to separate their product interference, in their test solutions. At the end of the lab from the reaction mixture. l'his filtration method pre- period (if the lah period is limited to three hours, two peri- cludes ignition, because the glass fibers will melt when ods will be required), the instructor compiles the results of heated by a Bunsen burner (2). Water adsorbed on the sur- all the students, and differences are rationalized within face of the particles was removed by heating the samples the framework of precipitation phenomenology. to 110 "C (variation 7a). The additional mass obtained by

both of these students relative to Nos. 1 and 2 is due primarily to

Base Procedure (First Column) and Tested Variations (Second Column) occluded water. which is not re-

I . Pipet 50.0 mL of a 0.0500a M BaCh solution la. Add 10. mL of 0.050 M CaCIz solution. moved by heating the precipitate

into a 400 mL beaker. Then add an additional in an oven (2). 100 m L of high-purity water. Student No. 5 also followed the

2. Add 1 mLof concentrated HCI 2a. Exclude the HCI from the sample. base procedure; however, h e 3. Heat to approximately 90 C on a hotplate. 3a. Do not heat the reagents or the sample added CaZ+ to his samples (vari-

mixture prior to filtration. ation l a ) . Because the precipi- 4. Add 50 mL of a 0.150 M NazSOa solution to 4a. Add the NanSOn solution dropwise from a tates were ignited in this case,

the beaker all at once. buret. 5. Continue heating ("age") for 1 hour after the Sa. Age overnight at 90°C. the additional mass must be due

~reci~itate has settled out. 5b. Allow the reaction mixture to cool to room cO~reci~itated Ca2+. Thus, the . . temperature (e.g, during the time between lab base procedure does not elimi- periods). nate the possibility of positive er-

6. Wash the precipitate with warm water, then 6a. Wash the precipitate with warm water, then 'Or due cO~reci~itatiOn. The collect it on ashless filter paper. collect it in a Gooch crucible containing a glass large difference between the du-

fiber pad (Whatman 934-AH). plicate samples may he a n indi- 7. Transfer the paper and precipitate to a cruci- 7a. Dry the precipitate in the Gooch crucible in cation that the extent of copre.

ble, and heat the crucible with a Bunsen burner an oven at 110°C. cipitation is not particularly to char the paper and dehydrate the precipitate.

8. Weigh the precipitate. Reheat and reweigh un- reproducible. Thus, the use of a ti1 a constant mass is achieved. correction factor to account for T h e concentration of this solution need not be exactly 0.0500 M, but should be known to at least three significant this source of error would not

figures. prove reliable.

Volume 72 Number 4 April 1995 355

- calcium present (la) M -

Student No. Results of student trials of the standard procedure (Student No. 1 and No. 2) and variations. The caps on the columns indicate the range of values obtained by each student. The alpha-numeric codes refer to procedural variations defined in the tahle.

Student No. 6 also added Ca2+ to his samples, and then added the sodium sulfate precipitant dropwise from a hu- ret (variation 4a). The figure indicates that this procedural variation limits the coorecioitation of Ca2+. [Since one of the two results is well below the results obtained by other students. i t urohahlv suffers from a student-induced error.1 This behavibr is consistent with the well-known relation: shiv between suoersaturation and the rate of mowth of pricipitate particles (3, 4 ) . Regardless of the growth mechanism (e.g., diffusion-controlled, surface reaction- controlled) the growth rate increases with a n increase in tQe supersaturation ratio, n, defined for this system as:

ahZi as$ n = KSP (1)

Activities are denoted by the symbol a. The solubility prod- uct constant, K,,, for BaS04 is 1.1 x 10-lo (5). When the precipitant is added slowly, the supersaturation ratio only slightly exceeds a value of one throughout the entire pre- cipitation. This serves to minimize the growth rate, result- ing in less Ca2+ incorporation. The same effect may be achieved by slowly generating the precipitant in-situ, a process referred to as "precipitation from homogeneous so- lution" (6).

Student No. 7 added Ca2+ to the reaction mixture, hut did not add hydrochloric acid (variation 2a). The absence of the acid caused the precipitation reaction to proceed rap- idly, resulting in even greater coprecipitation of Ca2+. Hy- drogen ion increases the solubility of barium sulfate, and thus reduces the supersaturation of the reaction mixture. Although the value of&, in eq 1 does not vary with pH, the actual equilibrium concentrations may vary as a result of the weak base nature of sulfate ion. This effect can be con- sidered quantitatively by substituting into the denomina- tor of eq 1 an "effective" equilihrium constant (71, derived from the true equilibrium constant through the use of a pH-dependent "a-factor":

Kas is the second dissociation constant for sulfuric acid, 1.2 x (7). Accordingly, when the hydrogen ion concentra-

tion of the solution is decreased from lo-' (Student No. 5) to (Student No. 71, the supersaturation ratio increases by a factor of 10.

Student No. 8 did not heat her sample solutions during the precipitation and filtered the samples a s soon as the supernatant clarified (variation 3aj. Although the precipi- tates were not allowed to aee. the samoles still were - , washed and filtered successfully. However, gross co-pre- cioitation of Ca2+ is evident (see fieure). This behavior is consistent with the fact that the solubility of BaS04 de- creases with decreasing temperature (8). Thus, the super- saturation ratio and the rate of nucleation and growth are both higher a t room temperature relative to the near-hoil- ing tempersture speritied in the base procedure.

Studtmt No. 9 u;it:d :I prolonged abeng trwrmenr wriarion 531; n t l ~ c ~ u i ~ c , her procedure n~nvsprmdtd t u th;~t ofStudtmr No. 5 . The priman ohiecrive of agmg IS ro increase the awr- ;Igr p;lrticlo .ire ofthe prrcipitnrc- p.lrticlm. In rhr H n N : sys- tem, this occurs primarily by a process known as ripening, in which the smaller particles dissolve and the larger particles grow. As can be noted in the figure, prolonged aging also re- sults in substantial ourification of the orecioitate. This be- . . havior points out that recrystallization also occurs during rip- ening. Under these near-equilibrium conditions, the crystals that grow during ripening are more perfect (i.e., more pure) than those produced during the initial precipitation (3).

Finally, it should he noted that the solubility of CaS04 must he considered in choosing the amount of Ca2+ included in these experiments. If the room-temperature solubility is ex- ceeded, post-precipitation of CaSOd will occur if the reaction mixturek aliowed to cool to room temperature (variation 5hj. The soluhility of CaS04 can he exceeded by adding too much Ca2+ or by failing to add acid to the sample mixtures. The effect ofthe latter can he seen in the results of Student No. 10, who used procedural variations la, 2a, 5a, and 5b.

Conclusions The analytical procedure for the gravimetric determination

of barium can be modified to demonstrate several different aspects ofthe chemistry and physics of precipitation. Drying the urecioitate in a n oven fails to remove occluded water. . . When a moderate amount of Ca2+ is present in the sample, coprecipitation will occur unless the precipitant is added slowly Conducting the precipitation a t higher pH's or lower temperatures will speed up the initial oreciuitation. resultine in even greater co~;tamin;tion of theLproduct with calciu< However, prolonged aging of the precipitate can eliminate much of the coprecipitated calcium. Finally, allowing the sample to cool to room temperature before filtration may al- low post-precipitation of the Ca2+ to occur.

Unlike most mavimetry laboratories, this exercise has proven to he quite popular with the students. It lends itself well to a quantitative discussion of precipitation and acid- base equilibria, as well as a qualitative treatment of nuclea- tion, growth, and ripening. And it most certainly achieves the goal of equipping students with the fundamental knowledge required to conduct successfully the gravimetric procedures they might encounter as professionals.

Literature Cited 1. Beck. C. M.. II.Anol. Cham. 1991.63.993A-1003A. 2 D u d , C. Inorgoncc Thermopmuimetric Annhrir. 2nd ed.: EIsevler: Amsterdam,

I962 nn SO 610 . , . . , . . . . LA. G. TheFormolion andPmpertier ofPre~ipiMes: Wileylnterscienie: New

York. 19fi7.

York, 1983: Part I. Vol. 5. Lide, 0. R. CRC Handh,

Raton. FL. 1990: on R 4 6. Gordon, L.; s a lu tak~~ . I

Wlley: New York, 1959; 7. Day. H. A . 1I;Undenuoad

wood Clifk, NJ. 1986: r 8. Lanze. N. A. Handboh o.

37,8&39. ..;Willard. H. H. PrecrpiiolionfromHomogem~olis Solulion; pp 7&74. , A L. Q""lliiloli"eAnoLySia. 5th ed.: Prentiee-Hall: En& ,p 263-267. fChemirlry, 5th ed.; Handbook Publi8hers: Sandusky OH.

356 Journal of Chemical Education