ADSORPTION BY CADMIUM SULFIDE AND I T S IMPORTAXCE I N T H E ESTIMATION OF CADMIUM BY HARRY B. WEISER AND EDWARD J . DURHAY
Precipitated cadmium sulfide is always contaminated by anions in the solution from which it separates. The determination of cadmium as sulfide, especially in the presence of sufficient hydrochloric acid to ensure its separation from zinc, has long been regarded as a useless method because of the chloride carried down with the precipitate. Treadwell‘ attributes the contamination to the formation of a double salt, CdS ’ CdCl2, in varying amounts depending on the acid concentration and the temperature and pressure at which the precipitation is carried out. Egerton and Raleigh2 believe the precipitate to have a constant composition when thrown down a t 80’ from a solution containing four cubic centimeters of concentrated hydrochloric acid in IOO cubic centimeters, and washed with an unspecified, definite quantity of water. Cnder these conditions, the precipitate is said to contain 3.57 percent of chlorine as the alleged salt, CdS . CdC12; hence, in determining cadmium quantitatively by weighing the sulfide, the molecular weight of the precipitate is taken to be 147.4 instead of 144.47 for pure cadmium sulfide. Since conclusive evidence of the existence of a definite double salt CdS CdClz is lacking, it is probable that the contamination of the sulfide gel is due to adsorption of cadmium chloride in varying amounts depending on
TABLE I Adsorption of Chloride by Cadmium Sulfide precipitated at (Fig. I , Curve A) Cubic centimeters of S/IO CdCl, Final normality Total volume 75 cubic of HC1 centimeters
Chlorine adsorbed Grams
25’.
C1
Cds* x
25 25
0.033
0.00361
2.00
0.068
0.00467
25
0.121
0.00644 0.00764 0.00815 0 00947 0.00996 0.00945 0.00923 0.00674
2.59 3.51 4.23 4.jI 5.24
0.156 0.209 25 0,349 25 0.419 25 0,407 25 0.596 25 0.664 * Corresponding to 2 5 cc. of N / I O CdCI?. 25
25
*
’
Treadwell-Hall: “Analytical Chemistry,” 2, 190 (1924). J. Chem. Soc., 123,3019 (1923).
5.52
5.23 5.11 3.73
100
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HARRY B . WEISER AND EDWARD J. DURHAM
conditions. T o test this hypothesis, hydrogen sulfide a t room temperature was passed into a definite volume of solution containing a constant amount of cadmium chloride and varying amounts of hydrochloric acid. The precipitate was filtered on a Gooch crucible, washed until the wash water gave no test for chloride and then analyzed for the chloride content. This was accomplished by dissolving the precipitate in nitric acid, adding an approximately constant excess of silver nitrate, filtering through a Gooch crucible and titrating the excess silver nitrate with ammonium thiocyanate according to Volhard’s method. The results are given in Table I and shown graphically by curve A, Fig. I. The filtrates were tested for completeness of precipitation
FIG.I
by rendering them alkaline with ammonia and saturating with hydrogen sulfide. With the most acid solution a faint yellow coloration was noted but no precipitate. The amount of cadmium required to produce this coloration was found to be negligible as compared with the total amount of precipitate. An attempt was made to repeat the above observations a t 80’ as recommended by Egerton and Raleigh, but precipitation was found to be far from complete. Accordingly, the several solutions were heated at 80°, removed from the source of heat, and hydrogen sulfide conducted through continuously while they cooled down to room temperature. The precipitates were washed and analyzed for chloride with the results given in Table 11 and shown graphically in curve B of Fig. I . Referring to curve A, it will be seen that the amount of chlorine in the precipitate does not increase continuously with the concentration of hydrochloric acid but exhibits a maximum. This maximum is due to a visible change in the physical character of the precipitate from a flocculent to a distinctly granular structure. The curve is thus a typical adsorption curve showing a maximum due to a physical change in the adsorbent.’ Curve B for the second series of the experiments lies under curve A as would be expected since the temperature at which the precipitation starts is Cf. Lottermoser and Rothe: Z. physik. Chem., 62, 359 (1908); Weiser: “The Hydrous Oxides,” 330-332 (1926).
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ADSORPTION BY CADMIUM SULFIDE
TABLE I1 Adsorption of Chloride by Cadmium Sulfide precipitated between 80' and 2 5'. (Fig. I, Curve B) Cubic centimeters of N/IO CdC12 Final normality of HC1 Total volume 75 cubic centimeters 25
0,033
25
0.121
25
0.209 0,279 0,349 0.419 0.507 0.664
25
25 25 25 25
* Corresponding to 25 cc. of
Chlorine adsorbed Grams
0.00-88 0.00452 0.00506 0.00364 0.00287 o.ooj38 0.00558
0.00459
c1 mF x
100
1.59 2.50
2.80 2.02
1.59 2.98 3.09 2.54
N/IO CdClz.
higher, hence the precipitate is more granular and the adsorption less. With increasing concentration of acid, the adsorption of chloride increases to a maximum so that the first part of curve B is very similar to that of curve A, and for the same reason. But when the acid reaches a concentration in the neighborhood of 0.3 normal, the amount of cadmium sulfide precipitated in th3 hot decreases with a corresponding increase in the amount precipitated a t lower temperatures. This means a larger amount of more finely divided particles and a correspondingly greater adsorption which rises to a second maximum as the concentration of acid is further increased. This behavior would be difficult to explain on the basis of double salt formation but is readily accounted for by considering the contamination as a case of adsorption. The dotted curve would probably represent the adsorption if the amount of salt precipitated a t each temperature during cooling were constant. The final concentration of acid recommended by Egerton and Raleigh was approximately 0.5 normal. This might appear to account for the constancy of composition of their precipitates since curve B is relatively flat when the hydrochloric acid concentration is in the neighborhood of 0.5 normal. However, the chlorine content under these conditions corresponds to 12.2 percent of the alleged double salt instead of to 16.5percent as calculated from Egerton and Raleigh's results.' The difference is, of course, in the conditions of the precipitation. Although the above authors do not say, it is probable that they carried out the precipitation as far as possible a t 80' and completed the process a t room temperature. If so, they should have obtained a greater proportion of relatively larger particles and a correspondingly smaller adsorption than was obtained by the procedure employed by us. The larger Egerton and Raleigh claim that 3.57 percent of chlorine in sav 0.1435 gram of the cadmium sulfide precipitate corresponds to 8.16 percent of the allege3 double salt, but this appears to be incorrect. 3.57 percent of 0.1 35 gram is 0.00512 gram of chlorine which is equivalent to 0.0238 grams of the alleged 6dSCdCla. This amount is 16.6 percent of 0.1435 gram, the total weight of the precipitate.
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HARRY B . WEISER AND EDWARD J. DURHAM
percentage of chlorine in their precipitates may have resulted from incomplete washing, from the presence of ammonium chloride together with hydrochloric acid in the solution from which the sulfide separated, or from both. I n any event, the procedure outlined by Egerton and Raleigh is not sufficiently definite t o be of value under the varying conditions which are encountered in technical practice. S-arY The results of this investigation may be summarized as follows: I. Cadmium sulfide thrown down from hydrochloric acid solution carries down chloride in varying amounts depending on the acid concentration, the temperature, and the pressure at which the precipitation is carried out. The contamination is due to adsorption and not to the formation of a definite double salt, CdS . CdC12. 2. Determinations were made of the adsorption of chloride by cadmium sulfide precipitated from solutions containing varying amounts of hydrochloric acid, (A) a t a temperature of 2 5 ' ; and (B) by conducting H2S continuously into the solutions while they cool from 80" to 2 5 ' . The precipitation of cadmium sulfide is incomplete a t 80". 3. Continuous curves are obtained by plotting the amount of chloride adsorbed by a definite weight of cadmium sulfide against the hydrochloric acid concentration of the solution. Curve A ( 2 5 ' ) is a typical adsorption curve with a maximum due to a physical change in the adsorbent from a flocculent t o a distinctly granular structure as the acid concentration is inlies below that obtained a t 25' since the precreased. Curve B (80'-25') cipitate formed at the higher temperature is more granular and the adsorption correspondingly less. The first part of the curve is similar to curve A and for the same reason: but when the acid concentration is in the neighborhood of 0.3 normal, the amount of salt precipitated in the hot decreases with a corresponding increase in the amount precipitated a t lower temperatures. This gives a larger amount of more finely divided particles and a correspondingly greater adsorption which rises to a second broad maximum as the concentration of the acid is further increased. 4. The determination of cadmium as sulfide under technical conditions is of doubtful value since it is questionable whether the conditions of precipitation can be maintained sufficiently constant to obtain a cadmium sulfide precipitate with a constant amount of adsorbed chloride. Department of Chemzalry, T h e Rice I?i.ddiile,
H o u s t m , Texas
