.4 PRELIMINARY STUDY OF T H E POSTPRECIPITATION OF NICKEL SULFIDE WITH COPPER, MERCURIC, AND ZINC SULFIDES‘ I. M. KOLTHOFF
AND
FRANK S. GRIFFITH
School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received January $2, 1858
Baubigny (1) stated in 1882 that nickel sulfide is very slow to separate from slightly acid (0.01N hydrochloric acid) solutions. The solid, homever, rapidly changes in some manner to yield products which are much less soluble in 2 N hydrochloric acid than would be expected from its small rate of precipitation in dilute acid (3, 7, 6). The solubility of precipitated nickel sulfide decreases markedly on standing in contact with acid solution; in pure water the effect is less. Higher temperatures seem to hasten the transition. Thiel and Gessner (6) postulated the existence of different “modifications” of nickel sulfide, roughly designated as the CY-, 8-, and y-forms, corresponding to more or less definite solubilities in hydrachloric acid of a given strength. They advanced the belief that the different preparations differed in the extent of polymerization, and that differences in solubility were to be attributed to this condition. Although this may be true to a certain extent, differences in crystal structure of the different “modifications” may have to be considered. Recently Lcvi and Boroni (4)found that nickel sulfide prepared from dilute sulfuric acid medium is identical with millerite, but that the product prepared from acetic acid solutions has a different crystal structure. Since the solubility of nickel sulfide decreases considerably with increasing age, it was expected that other insoluble sulfides might induce the precipitation of nickel sulfide at acidities at which the latter is extremely slow to separate in the absence of promoting sulfides. For this reason a preliminary study was made of the postprecipitation of nickel sulfide with the sulfides of copper, mercury, and zinc, respectively. EXPERIMENTAL
All of the chemicals used were of c. P. quality. The nickel left in the filtrate after the postprecipitation experiments was determined gravimetriThis article is based on a thesis submitted by Frank S. Griffith to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy, June, 1937. 541
542
I. M. KOLTHOFF AND FRANK S. GRIFFITH
cally with dimethylglyoxime or, more rapidly, volumetrically by the cyanide method (5). In the latter method the nickel is transformed to the complex cyanide ion Ni++
+ 4CN-Ft
Ni(CN);-
and the end point is indicated by the disappearance of the turbidity of suspended silver iodide : AgI
+ 2CN- -+ Ag(CK), + I-
The location of the end point depends upon the concentrations of iodide and ammonia present. Accurate results were obtained by the following procedure: The nickel solution is diluted to 50-70 ml., 5 ml. of 6 N ammonia solution, 1 g. of ammonium sulfate, and 1 ml. of 25 per cent potassium iodide solution are added, and the mixture is titrated with standard potassium cyanide solution until the green-blue color fades. Three or four drops of standard silver nitrate are added and the titration with cyanide continued until the solution becomes clear. A correction is applied for the amount of silver added. The turbidity is best seen if the flask containing the solution is placed on a black underground with a black background behind the flask and a strong light coming from the side, Under the best lighting conditions the end point can be detected with an accuracy of 0.04 ml. of 0.02 M cyanide ( = 0.01 mg. of nickel) in a total volume of 50 ml. The ammonium sulfate is added to prevent the precipitation of nickel cyanide during the courbe of the titration. If such a precipitate forms, it is very slow to redissolve upon further addition of cyanide, thus spoiling the titration. Tartrate and citrate (2) are more effective than ammonium sulfate in preventing the precipitation of nickel cyanide. With the concentrations used in this work only in a few cases did any turbidity result from the separation of nickel cyanide. The following standard solutions were used. ( I ) 0.1034 M silver nitrate (calculated from the weight of silver nitrate used, 0.1035 X ;from titration against pure potassium thiocyanate, 0.1033 M ) . ( E ) 0.1370 M potassium cyanide. Twenty grams of potassium cyanide and 40 ml. of 6 N sodium hydroxide were diluted t o a volume of 2 liters. The solution was standardized against silver nitrate by the Liebig-DhigBs method. ( 3 ) 0,05120 M nickel aulfate. By precipitation with dimethylglyoxirne a molarity of 0.05122 was found; by the volumetric procedure described above a molarity of 0.05115. If desirable, more dilute solutions were prepared from the standard solutions. In the titration of 25 ml. of the 0.05 M nickel solution by the recommended procedure the average deviation found was 0.0 per cent (six determinations; error deviated between 0.05 and - 0.05 per cent; in the titration of 10 ml. of the nickel solution the average error was 0.13
+
543
POSTPRECIPITATION OF NICKEL SULFIDE
per cent (varying between 0 and 0.3 per cent). Even very dilute nickel solutions (50 ml. of 0.000512 M solution = 1.5 mg. of nickel) could be titrated with an accuracy of 1 per cent. Small amounts of zinc (0.5 millimole) did not interfere with the titration. TABLE 1 Postprecipitation of nickel sulfide with cupric, mercuric, and zinc sulfides, respectively (at 86°C.)
--
AMOUNT OF PBOMOTINQ METAL BWLFIDE
CONCENTRATION OF HC1
ltillimoler
N
1 1 1
0.2 0.2 0.2
0.004 0.004 0.004
3 hours 52 hours 96 hours
3.2 31 74
56
2 2 2 2 2
0.5 0.5 0.5 0.5 0.5
0.01 0.01 0.01 0.01 0.01
3 hours 12 hours 24 hours 36 hours 60 hours
0 3.0 6.5 13.5 37.0
3 3 3
0.5 0.5 0.5
0.02 0.02 0.02
23 hours 59 hours 81 hours
0.5 2.0 3.0
4 4 4
0.5 0.5 0.5
0.05 0.05 0.05
2 days 5 days 7 days
1.4 6.0 16
SET'
TIME OF BFIAHINQ
NICKEL PRECIPITATED I N PER C l N T I N T E E PRESENCEOF
ZnS Blank -
GUS per cent
per cent
per cent
1.9 77.0
0.5 23.5 85.5
0 3.5 36 54 82
0 3.0 33.0 55 88
0 0 0 0 0
4.0 46 69
1.0 ,4.5 9.5
0 0 0
0 0 0
0
per cent
6\ 0
86
1.3 11
13
0 0
+
* Set 1: 100 ml. of 0.05 M nickel chloride 4 ml. of 0.05 M cupric, or mercuric, or zinc chloride. Acid concentration after precipitation of promoting sulfide (0.2 millimole) was 0.004 N. Set 2: 100 ml. of 0.05 M nickel chloride 10 ml. of 0.05 M cupric, or mercuric, or zinc chloride. Acid concentration after precipitation of promoting sulfide (0.5 millimole) was 0.01 N . Set 3: As set 2, but with 1 ml. of 1.21 N hydrochloric acid in addition. Set 4: As set 2, but with 1.9 ml. of 2.42 N hydrochloric acid in addition.
+
Postprecipitation of nickel suljide with other sulfides Nickel sulfide is precipitated very slowly from acid medium. When a 0.04 M nickel chloride solution was kept in an atmosphere of hydrogen sulfide a first separation was noticed after 21 hours in 0.005 N hydrochloric acid, after 66 hours in 0.01 N acid, and none at all even after 20 days in 0.02 N acid. With 0.01 M nickel chloride solution a first separation of sulfide was noticed after 26 to 28 hours in 0.005 N acid, and after 5 days in 0.01 N acid. The results of the postprecipitation experiments are combined in table 1.
544
I. M. KOLTHOFF AND FRANK S. GRIFFITH
Performance of experiments The 250-ml. Erlenmeyer flasks containing the solutions were placed on a rotary shaker, and hydrogen sulfide was passed through continuously. From time to time the shaker was stopped, the precipitate allowed to settle, and 10 ml. of the liquid pipetted off for nickel analysis. The sample taken for analysis was filtered through a paper which had been previously washed with 0.02 N hydrochloric acid, the filter was washed with water, and the filtrate was analyzed for nickel by the cyanide method. Washing the filter with dilute acid appeared necessary, else a slight precipitation of nickel sulfide from the supersaturated solution would occur. The acid concentration given in table 1 corresponds to that in the solution after precipitation of the more insoluble promoting sulfide. The experiments whose results are given in the column headed “blank” had no inducing sulfide present but were of the same volume and contained the same amounts of nickel and acid as the solutions with the promoting metals. TABLE 2 Postprecipitation of nickel sulfide w’th copper sulfide at 80°C. Tots1 volume, 35 ml. Amount of copper sulfide, 0.5 millimole; amount of nickel chloride, 1.25 millimoles. Time of shaking. 19 hours Hydrochloric acid concentration ( N ) h‘ickel precipitated in per cent*
* Blanks run in the absence of
1
I
0 05 58
1
0 08
52
1
0 11 17
copper showed no nickel precipitated.
From the first set of experiments (blank) it is seen that nickel sulfide in the absence of promoting sulfides is formed very slowly in 0.004 N hydrochloric acid. The induction period is relatively long. After the precipitation has started the nickel sulfide promotes its own precipitation, the precipitation curve being typical of an autocatalytic process. At hydrochloric acid concentrations of 0.01 N or greater the induction period in a pure nickel chloride solution is so long that no precipitate of nickel sulfide occurred in the time during which the experiments were run. It is evident that nickel sulfide is postprecipitated with copper, mercuric, and zinc sulfides. Particularly from the experiments of set 2 it is seen that the induction period is relatively long in the presence of the promoting sulfides and that the postprecipitation curves are typical again of an autocatalytic process. Mercuric sulfide has a stronger effect on the postprecipitation than copper sulfide. The relative effect of zinc siilfide varies with the acid concentration. In set 1 the zinc sulfide is formed from neutral solution, the final acid concentration being 0.004 N . Under these conditions the zinc sulfide formed is extremely finely
POSTPRECIPITATION O F NICKEL SULFIDE
545
divided, and its effect is greater than that of mercuric sulfide. In set 2 the zinc sulfide is somewhat coarser (final acid concentration 0.01 N ) , and its effect equals that of mercuric sulfide. With increasing acidity of the solutions (sets 3 and 4) the zinc sulfide becomes coarser. In set 4 (0.05 N acid) the zinc sulfide formed is so coarse that it does not promote the precipitation of nickel sulfide a t that acidity. A few experiments were carried out on the postprecipitation of nickel sulfide with copper sulfide at 8OOC. The results are given in table 2. The flasks containing the solutions were placed on the gravel in the hot room (temperature about 80°C.). A slow stream of hydrogen sulfide saturated with water vapor a t this temperature was passed through the flasks for 19 hours. After this period the mixtures were filtered and the filtrates analyzed for nickel. The few results show that the rate of postprecipitation with copper sulfide is much greater at 80°C. than at room temperature. SUMMARY
I t has been shown that nickel sulfide postprecipitates with the sulfides of copper, mercury, and zinc, the effect of mercuric sulfide being greater than that of copper sulfide. The relative promoting effect of zinc sulfide greatly depends upon the acidity a t which it is separated from solution. The rate of postprecipitation with copper sulfide a t 80°C. is much greater than at room temperature. Nickel sulfide autocatalyzes its own precipitation. REFERENCES (1) BAUBIGNY, H.: Compt. rend. 94, 1183, 1251, 1417, 1473, 1715 (1882); 96, 34 (1S83). (2) HECZKO, T.: Z. anal. Chem. 78, 325 (1929). (3) HERTZ, W.: Z. anorg. allgem. Chem. 27, 390 (1901). (4) LEVI, G. R., AKD BORONI,A.: Z. Krist. 92, 210 (1936). ( 5 ) SCOTT, W. W.: Standard Methods of Technical Analysis, 4th Edition, p. 334. D. Van Sostrand and Company, Kew York (1925). (6) THIEL, A., AND GESSNER, H.: Z. anorg. allgem. Chem. 86, 1 (1914). (7) THIEL, A., A N D OHL, A . : z. anorg. allgem. Chem. 61, 396 (1909).
