THE POSTPRECIPITATION OF ZINC SULFIDE WITH BISMUTH

THE POSTPRECIPITATION OF ZINC SULFIDE WITH BISMUTH SULFIDE’ I. &I. KOLTHOFF

AND

FRANK S. GRIFFITH

School of Chemistry, University of M i n n e s o t a , M i n n e a p o l i s , Minnesota

Downloaded by STOCKHOLM UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: January 1, 1937 | doi: 10.1021/j100899a009

Received J a n u a r y $8, 1938

In previous investigations carried out in this laboratory it was shown that the induced precipitation of zinc sulfide from acid medium by copper wlfide (7) and mercuric sulfide ( 5 ) actually is a postprecipitation. The rate of precipitation of zinc sulfide from acid medium is small, but is greatly enhanced at the interphase of the copper or mercuric sulfides. Conclusive evidence was given that the promoting effect of these sulfides upon the postprecipitation of zinc sulfide 1s to be attributed to the presence of an adsorbed layer of hydrogen sulfide at the interphase, the acid being, partially a t least, ionized in the adsorbed state. The results of Caldwell and Moyer (3) substantiate the views developed in previous papers (6). In the postprecipitation of zinc sulfide induced by copper sulfide the former precipitates as a separate phase and can be readily extracted from the mixed precipitate with 3 N hydrochloric acid. The zinc sulfide postprecipitated by mercuric sulfide forms to a limited extent mixed crystals with the latter; this explains why the zinc is not easily extracted from the mixed precipitate with 3 N hydrochloric acid. From our omi work and from the extensive study by Bottger and Ahrens (1) it may be concluded that cadmium sulfide is also postprecipitated with mercuric sulfide, and that the mixed precipitates of the two consist of mixed crystals. In the present study the effect of bismuth sulfide upon the precipitation of zinc sulfide was investigated. Bismuth sulfide was chosen not only because it has a different crystal structure from zinc sulfide (in this respect it is comparable with copper sulfide), but also because it is of a different formula type. Zinc sulfide crystallizes either in the cubic or in the hexagonal system, whereas bismuth sulfide crystals belong to the orthorhombic system. EXPEHIMENThL

Materials used All chemicals used were c. P. products. The stock solutions of bismuth chloride were 1 normal with respect to hydrochloric acid in order to preThis article is based on a thesis submitted by Frank s. Griffith t o the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1937. 531

532

I. M. KOLTHOFF . I S D FRBKK S. GRIFFITH


vent the separation of bismuth oxychloride. After mixing n i t h the zinc and other solutions the acid concentration was reduced and a white precipitate of bismuth oxychloride was formed, the latter being transformed to bismuth sulfide when the solution was baturated with hydrogen sulfide. It was also desirable to investigate the effect of bismuth sulfide separated from clear solutions. In such cases the stock solution of bismuth chloride was 0.4 N in hydrochloric acid and contained 50 g. of tartaric acid per liter No precipitate separated from such a solution upon dilution.

A nul ytical After the treatment with hydrogen sulfide the zinc was determined in the filtrate by titration with potassium ferrocyanide, using diphenylamine as indicator (8).

Keproducibility of experiments In the early work great differences were found between results-of apparently identical experiments; for example, 68 per cent and 46 per cent of the zinc were found precipitated in two experiments supposedly identical, but run on different days. After a systematic search it was found that, in general, the amount of postprecipitated zinc sulfide decreased with depletion of the Kipp generator in which the hydrogen sulfide was produced by the interaction of stick iron sulfide and commercial hydrochloric acid. The iron sulfide contains frer iron which leads to a dilution of the hydrogen sulfide with hydrogen. The gas generated was tested for hydrogen by removing the hydrogen sulfide by absorption in sodium hydroxide. The amount of hydrogen present was found to vary between 2 and 18 per cent, depending upon the use and age of the generator. With fresh 3 S hydrochloric acid in the generator, 1.6 per cent hydrogen was found nhen the gas was withdrawn a t a rapid rate; when the rate was decreased to 7 5 bubbles (about 20 ml.) per minute the percentage of hydrogen increased to 10 per cent. When the generator was allowed to stand unused for a few hours, the percentage of hydrogen in the gas at the start of the next discharge was found to be unusually high (18 to 20 per cent). In order t o keep the hydrogen sulfide pressure constant it would be preferable to use gas from a tank. As the latter was not available the error due to varying composition of the gas was mainly eliminated by runiiing standard comparison experiments or identical experiments in series with the particular system investigated. Experimental procedure I n general the same method of precipitation n a ? used as dewribed in the paper of Kolthoff and Moltzau (5). The precipitate was removed by filtration and washed eight t o ten times with 0 1 S hydrochloric acid

POSTPRECIPITATION O F ZINC SULFIDE KITH BISMUTH SULFIDE

533

saturated with hydrogen sulfide. The solubility of the zinc sulfide in this wash solution was low. In some cases the precipitate was subjected to twenty-five more washings after the ten usual washings; it was found that only 1 per cent of the total zinc was removed by the twenty-five extra washings when the total amount of zinc in the system was about 1.25 millimoles.


TABLE 1 Eflecl of concenlralion of acid and lime of shaking" u p.o n postprecipitation o j zinc

s u l f d e a i 25°C. C(XCCESTRATI0N O F HC1 BEFORE PKECIPITATION OF ZISC

TIME OF S H A K I l Q

ZINC PRECIPITATED'

N

minulea

per cent

(j.15 0.15 0.15 0.15 (b1ank)t

5 15 60 00

30 75 90 2

0.20 I) 20

5 15

I).20

M

0

0.5 31 82 0.5

025 0.25

5 15 60

0.0 0.5-3 32

I).30 0.30

BO 180

2 7

0 20 (h1ank)t 0 25 a

* The average value

of two to four experiments is given.

t The blanks contained 25 ml. of water instead of 25 ml. of the bismuth solution. RESULTS

Effect of acidity upon postprecipitation In the following experiments 25 ml. of 0.05 M bismuth chloride solution, which was normal with respect to hydrochloric acid, was mixed with 25 ml. of 0.05 M zinc chloride; a measured volume of 5.9 N sodium hydroxide mas added to obtain the acid concentration (after precipitation of the bismuth as sulfide) indicated in table 1. The total volume was made up to 55 ml. The solutions were shaken continuously while hydrogen sulfide was passed through the flasks a t a constant rate. From the results, particularly in 0.25 N hydrochloric acid, it follows conclusively that we are dealing again with a postprecipitation of the zinc sulfide. From the analytical viewpoint it is of interest t o notice that a quantitative separation of bismuth and zinc is obtained when the initial


534

I. M. KOLTHOFF AKD FRASK S. GRIFFITH

hydrochloric acid concentration is 0.3 iV and the filtration is made a few minutes after the precipitation of the bismuth sulfide. In the above experiments part of the bismuth was precipitated as oxychloride before hydrogen sulfide was passed through. In order to keep all of the bismuth in solutbn, experiments were carried out in the presence of tartaric acid. A mixture was prepared from 25 ml. of 0.05 ;If bismuth chloride which was 0.4 in hydrochloric acid and 0.3 M in tartaric acid, 25 ml. of 0.05 AM zinc chloride which was 0.1 AT in hydrochloric acid, and 1 ml. of ,515 S sodium hydroxide. The final hydrochioric acid concentration (after precipitation of the bismuth) was 0.2 A'. The air mas removed by passing a rapid &ream of hydrogen sulfide through the flask for half a minute; this was continued for 2 minutes after the shaker was started, then decreased for 3 minutes to a stream which just broke into bubbles as it passed through the ivash bottlc, and finally decreased to a rate of about 150 bubbles per miriute for the remainder of the precipitation period, In theae experinients piire hydrogen sulfide prepared by heating a magnesium hydrosulfide solutio11 and stored in a gas holder was used. Under identica! conditions the experiments were well reproducible. Tlit: results are given in tablc 2. TAiBLE2 I'catprecipitntion of iiric s d j i d e from 0.2 .V hydiochlutic acid solutions conlaining tartaric acid at 25°C.

1

Time of shaking in minutes Z:ne precipitated in per cent --

'

18

3

j j

33 20

, '

18 47

Again it is seen that the amount of zinc sulfide precipitated increases with the t,irne of contact, with bismut'h sulfide when the solution i s saturated with hydrogen sulfide. A comparison of the results with those in table 1 at the same acidit,y reveals that tartaric acid inhibits the postprecipitatiox As the experiments in the two tables were carried out under different' conditions of hydrogen sulfide treatment, iz new scl. was made under identical conditions, using a shaking period of 30 minutcs arid a hydrochloric acid concentration of 0.2 N . In the absence of tartaric acid 45 per cent of the zinc was found precipitated; in the presence of tartarlc acid 18 per cent. These experiments show conclusively the inhibiting effect of tartaric acid upon the postprecipitation of zinc sulfide on bismuth sulfide. I n the absence of tartaric acid the greatest, part, of, the biamuth ~ i l f i d eis obtained by transformation of the precipitated bismuth oxychloridc into sulfide, TThereas in the presence of tartaric acid it is obtained from a clear solution. Apparently the struciurc. of the sulfide formed in the absence of tartaric acid is such that it has a greater promoting cffect upon thc postprwipiiation of zinc sulfide than n-l~ei; formed from


POSTPRECIPITATIOS OF ZINC SULFIDE WITH BISMVICTH SULFIDE

535

tartaric acid-containing solutions. Addition of tartaric acid to 0.05 M zinc chloride which was 0.1 N with hydrochloric acid did not appreciably affect the rate of precipitation when no promoting sulfide was present. From this it is inferred that the inhibiting effect of the tartaric acid is, for the most part, not due to complex formation with zinc. Incidentally, it may be mentioned that the rate of postprecipitation decreases with increasing sodium chloride content of the solution. The experiments reported in table 2 were repeated, but with the addition of 2.5 g. of sodium chloride to the mixture. After half an hour of shaking 1 per cent of the zinc was precipitated, whereas 18 per cent was found precipitated when no sodium chloride was added. A simflar inhibiting effect of sodium chloride was found by Kolthoff and Pearson (7) in the postprecipitation of zinc sulfide by copper sulfide.

Comparison of the effectiveness of the sulfides of copper, bismuth, mercury, and zinc o n the precipitation of zinc sulfide The experiments were carried out with 1.25 millimoles of the promoting sulfide and 1.25 millimoles of zinc chloride, the hydrochloric acid concentration being 0.2 N after precipitation of the promoting sulfide and the TABLE 3 C o m p a r i s o n of promoting effect of suljides of copper, bismuth, mercury, and z i n c o n precipitation of z i n c sulfide Promoting sulfide.. . . , . . . . , . , . . . . , , . , . Zinc precipitated in per c e n t . . . , . . . . . . . . . . Sodium chloride present in grams. . . . . . . . .

,

1.4

total volume 55 ml. In all cases the hydrogen sulfide was passed through the mixture of the metal sulfide and zinc salt for 30 minutes. The zinc sulfide used as promoting agent was prepared by passing hydrogen sulfide through a slightly acid (0.002 N ) solution of 0.05 M zinc chloride; the other promoting sulfides were precipitated in the presence of the zinc solution. The results are given in table 3. The results are not strictly comparable, as the sodium chloride contents of the various solutions were not identical. Still, it may be concluded that mercuric and zinc sulfides have about the same promoting effect on the precipitation of zinc sulfide, the effect of bismuth and copper being less, Such a result is probably explained by the fact that mercuric sulfide and zinc sulfide crystallize in the same system and form mixed crystals (5), whereas bismuth sulfide and copper sulfide crystallize in different systems. The induction period in the postprecipitation of zinc with mercuric sulfide may, therefore, be expected to be materially shorter than with the other sulfides. A quantitative comparison of the figures in table 3 is not per-


536

I. M. KOLTHOFF AND FR.4SK S. GRIFFITH

missible, as the promoting effect of a certain sulfide depends upon the conditions of precipitation. This is particularly true of the zinc sulfide. I n the above case it was precipitated from very dilute acid solution and was very finely divided. If prepared from stronger acid medium it would be coarser and would have less inducing effect upon the precipitation of zinc from solution. This may explain the result of Glixelli (4), who found copper sulfide to be a more efficient promotor of the precipitation of zinc sulfide than the latter; he describes the zinc sulfide used as being fresh, but does not state the conditions under which it was precipitated.

Effect of aging of bismuth suljiide In the following experiments a mixture of 25 ml. of 0.05 M bismuth chloride solution and enough 5.15 N sodium hydroxide to make the acid concentration 0.206 N after the precipitation of bismuth was treated with hydrogen sulfide and shaken, with a slow stream of hydrogen sulfide passing through it, until the sulfide had aged for a given period of time. Then 25 ml. of 0.05 M zinc chloride which was 0.1 N in hydrochloric acid was added, and the flask shaken while a slow stream of hydrogen sulfide passed through for 30 minutes. Acid had to be added to the zinc solution in order to prevent the precipitation of zinc sulfide, as the solution mas pipetted into the flask full of hydrogen sulfide. In order to get results comparable with the effect of a fresh precipitate, a mixture of the bismuth-sodium hydroxide-zinc solution was treated with hydrogen sulfide for 30 minutes in exactly the same way. The reproducibility of the experiments was poor, but all of the twenty experiments showed the same trend. The bismuth sulfide aged for 10 minutes caused the precipitation of 20 per cent less zinc than the fresh precipitate, the 15- to 20-minutes old precipitate 24 per cent less, the 60-minutes old precipitate 25 per cent lese, but a 41-hours old precipitate 10 per cent more. A precipitate aged for a day had about the same promoting effect upon the precipitation of zinc sulfide as fresh bismuth sulfide. It was thought that the separation of bismuth oxychloride before the passage of hydrogen sulfide might account for the unexpected and badly reproducible results. For this reason a great number of experiments was run in which the precipitation of the oxychloride was prevented by addition of tartaric acid. The final composition of the mixture was about the same as that used in the experiments of tabIe 2, the period of precipitation of zinc being again 30 minutes. In all cases blanks were run with the entire mixture (fresh bismuth sulfide) in series with the experiments with aged bismuth sulfide. In some instances the bismuth sulfide was aged at 85"-95'C.; the temperature dropped during the aging, as in general no further heat was supplied. After the period of aging the sample was cooled as quickly as possible, and resaturated with hydrogen sulfide, then the zinc chloride


POSTPRECIPITATION OF ZINC SULFIDE WITH BISMUTH SULFIDE

537

was added and the sample further shaken in a slow stream of hydrogen sulfide for 30 minutes. The results are given in table 4. In the presence of tartaric acid the bismuth sulfide, even upon short periods of aging, becomes more effective in the promotion of the precipitation of zinc sulfide than a fresh product. The effectiveness increases with the age of the bismuth sulfide, until after a relatively long period a maximum is reached. Thus it is seen from table 4 that bismuth sulfide aged at room temperature for 42 hours caused the precipitation of 63 per cent more zinc than the fresh precipitate; after aging for 6 months it had about the same promoting effect as a fresh precipitate. Similar results are found when the bismuth sulfide is aged a t 80"-90OC. The 10-minutes old product caused 69 per cent more of the zinc to precipitate than the fresh prodTABLE 4 Effect of aging of bismuth sulfide u p o n its promoting effect o n precipitation of zinc sulfide

1

PER CENT ZINC PRECIPITATED' AFTER TEMPERATURE OP AQINQ

AQE OF BISXDTIi BULFIDE

30 MINUTES

Aged BisSs (8)

Fres;b3i&

DIFFERENCE BETWEEN (8) AND (b)

'C.

25 25 25 25 85-95 80

* Average

10 minutes 1 hour 42 hours 6 months 10 minutes 4 days

31 45 83 15 69 10

18 18 20 16 18 16

values of three to six experiments.

uct; after aging for 4 days at 80°C. the sulfide became less effective than the fresh precipitate. No study has been made of the structural changes taking place during the short and long periods of aging of bismuth sulfide at various temperatures. From the data of table 4 it may be expected that such a studywill yield interesting results. It may be mentioned that it is immaterial whether the bismuth sulfide is shaken or allowed to stand quietly during the aging period. Entrance of air during the aging decreases the effectiveness of the bismuth sulfide somewhat; probably some sulfur formed by oxidation of hydrogen sulfide separates on the surface of the sulfide.

Postprecipitation ut higher temperatures Experiments were carried out with a mixture of bismuth and zinc chlorides of the same composition as that in the experiments reported in table

538

I. M. KOLTHOFF AND FRANK S. GRIFFITH


1. The hydrochloric acid concentration after the precipitation of bismuth was 0.2 N , and the time of shaking 60 minutes. One set of experiments was carried out at 50°C., another set at 25°C. In the former 32 per cent of the zinc was found precipitated, in the latter 80 per cent.

Extractibility of the zinc from the mixed sulfides Precipitates obtained under the same conditions as those of table 2 after a shaking period of 60 minutes, and washing, were shaken for 1 hour with 75 ml. of 3 N hydrochloric acid a t room temperature, the liquid decanted through a filter, and the residue shaken for 3 more hours with a fresh portion of the acid. The filtrates were evaporated to small volumes and diluted, and the bismuth which had dissolved was reprecipitated with hydrogen sulfide. The zinc was determined in the filtrates. The following results were obtained: per cent of zinc in the original filtrate, 30.0; in the first extraction, 68.1; in the second, 2.5; total, 100.6 per cent. It appears that the zinc is fairly easily extracted from the precipitate. It may be objected (Balarew (2)) that the easy extractibility is due to air oxidation of the zinc sulfide and to solution of some of the bismuth. In order to eliminate this objection experiments were carried out with a bismuth-zinc solution (final acidity 0.22 N , further composition as in table 1) treated for 30 minutes with hydrogen sulfide. The mixture was transferred to a volumetric flask of 100 ml and made up to volume with 0.2 N hydrochloric acid saturated with hydrogen sulfide. After mixing and allowing to settle, 50 ml of the clear solution was pipetted out and analyzed for zinc. The remainder was filtered, washed with 0.2 ‘%1 hydrochloric acid saturated n i t h hydrogen sulfide (called “remainder”), and then the precipitate with paper dropped into 50 ml. of 2 N hydrochloric acid saturated with hydrogen sulfide. The mixture was shaken for 40 minutes while hydrogen sulfide was passed through, was filtered (extraction 11, and the extraction was repeated (extraction 2). No bismuth dibsolved during this extraction process. The following results were obtained: per cent of zinc in 50 mi. pipetted out, 12.1; in remainder, 20.4; in extraction 1 , 69.0; in extraction 2, 0.2; total, 101.7 per cent. Evidently, some of the zinc is already extracted by 0 2 N hydrochloric acid saturated with hydrogen sulfide, indicating that the zinc sulfide is very finely divided and hardly aged. Well-aged zinc sulfide is insoluble in 0.2 N hydrochloric acid saturated with hydrogen sulfide (Kolthoff and Moltzau ( 5 ) ) . Furthermore, it is seen that the zinc is easily extracted with 2 N hydrochloric acid under conditions which preclude oxidation of the zinc sulfide or solution of any of the bismuth. The ready extractibility contrasts markedly with the extractibility of zinc sulfide postprecipitated with mercuric sulfide (5). In the latter case the zinc sulfide forms a solid soliltion with the mercuric sulfide, whereas mixed-crystal formation does not : m u 1 between bismuth sulfide and zinc sulfidc.

POSTPRECIPITATION OF ZINC SULFIDE WITH BISMUTH SULFIDE

539

Kolthoff and Pearson (7) found that zinc sulfide postprecipitated with copper sulfide can be easily extracted from the mixcd precipitate. Balarew (2) attributed this ready extractibility to air oxidatioil of the zinc sulfide. Some of Kolthoff and Pearson’s experiments were repeated under the above extraction conditions precluding air oxidation. Again it was found that the zinc was easily extracted, contrary to Balarew’s statements.


SUMMARY

1. Zinc sulfide is postprecipitated with bismuth sulfide. 2. Zinc can be separated quantitatively from bismuth when the hydrochloric acid concentration after precipitation of bismuth is at least 0.3 N and filtration is made within a few minutes after the bismuth has separated. 3. The rate of postprecipitation of zinc with bismuth sulfide is small during the early periods of shaking and becomes greater with longer periods of contact. I n this respect the effect of bismuth sulfide is comparable to that of cupric sulfide, but in contrast to that of mercuric sulfide. I n the latter case the rapid initial rate of postprecipitation is attributed to the similarity of the lattices of zinc sulfide and mercuric sulfide, which is responsible for the reduction of the induction period in the precipitation of zinc sulfide. 4. Mercuric and zinc sulfides are more effective in the postprecipitation of zinc sulfide than bismuth and copper sulfides. 5. Bismuth sulfide aged at room temperature or higher temperatures under specified conditions becomes markedly more effective in the postprecipitation of zinc. After long periods of aging it becomes less effective. Copper sulfide behaves in a similar way, but mercuric sulfide becomes less effective on aging. 6. The zinc can be readily extracted with 2 N hydrochloric acid from a mixed sulfide with bismuth or copper, but not from a mixed sulfide with mercury. In the latter case part of the zinc is incorporated in the precipitate as mixed crystals. 7. Addition of sodium chloride to the solution inhibits the rate of postprecipitation of zinc sulfide with bismuth (and copper) sulfide. REFERENCES (1) AHRENS,W.: Thesis, Leipsig, 1933. (2) BALAREW, D.: Z. anal. Chem. lOa, 408 (1935). (3) CALDWELL, J. R., AND MOYER,H. V.: J. Am. Chem. SOC. 69, 90 (1937). S.: Z. anorg. allgem. Chem. 66, 297 (1907). (4) GLIXELLI, (5) KOLTHOFF, I . M., AND MOLTZAU, R.: J. Phys. Chem. 40, 779 (1936). (6) KOLTHOFF, I. M., AND MOLTZAU, R. : Chem. Rev. 17,293 (1935). (7) KOLTHOFF, I. M., AND PEARSON, E. A. : J. Phys. Chem. 38,549 (1932). (8) KOLTHOFF, 1.M., AND PEARSON, E. A.: Ind.Eng. Chem., Anal. Ed. 4 , 1 8 (1932).

THE POSTPRECIPITATION OF ZINC SULFIDE WITH BISMUTH

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