T H E PROMOTING ACTIOTU’ OF COPPER SULFIDE ON THE SPEED OF PRECIPITATION OF ZINC SULFIDE (The So-called Coprecipitation of Zinc with Copper Sulfide) BY I. M. KOLTHOFF AND E . A. PEARSON’
Introduction The “carrying down” of zinc sulfide by copper sulfide when the latter is precipitated by hydrogen sulfide a t an acidity such that the zinc solution does not react with the precipitant has been known for a long time. The phenomenon has been called vaguely “induced precipitation” and has been a subject of considerable dispute as to its extent and nature. Rivot and Bouquet2 as well as Calvert3 claimed that the separation of copper and zinc in acid medium does not yield accurate results as some zinc is found in the copper sulfide. Spirgatus’ on the other hand recommended the method provided that the copper sulfide were precipitated from strongly acid medium; whereas Grundmann5 advocated a double precipitation. This was confirmed by Fresenius (comp. Grundmann5) and by Baubigny.6 The latter made extensive studies and found that even in acid medium, copper sulfide has a tendency to carry down metals of the third group of the qualitative system. At the suggestion of Fresenius, Larsen’ made a practical study of the separation of copper and zinc and concluded that good results were obtained if the precipitation were made from relatively strong acid medium,the copper sulfide filtered immediately after the precipitation and washed with 0.5 N hydrochloric acid containing little hydrogen sulfide followed by dilute hydrogen sulfide alone. The copper sulfide was zinc free. Berglund,* in a more accurate investigation, was able to confirm Larsen’s results. Glixellis was the first to attack the carrying down of zinc by copper sulfide in a more general way and gave in an excellent paper a clear statement of the facts. From the known solubility product of zinc sulfide, he calculated that in dilute zinc solutions, zinc sulfide should precipitate with hydrogen sulfide even a t an acidity as high as I N. This he showed to be true; however, months were required for the precipitation to occur, this time being called period of induction. Kolthoff and van Dyk’O 1 From a thesis submitted by E. A. Pearsou to the Graduate School of the University of Minnesota in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Rivot and Bouquet: J. prakt. Chem., 51, 203 (1851). SCalvert: J. prakt. Chem., 71, 155 (1855). Spirgatus: J. prakt. Chem., 57, 184 (1852). ‘Grundmann: J. prakt. Chem., 73,241 (1858). Baubigny: Compt. rend., 94,1183,1251,1473, 1595 (1882); 95, 34 (1883);105, 751, 805 (1888);107, 1148(1888); 108,236,450 (1889). Larsen: Z.anal. Chem., 17, 312 (1878). 8 Berglund: Z.anal. Chem., 22, 184(1883). Glixelli: Z. anorg. Chem., 65,297 (1907). ’OKolthoff and van Dyk: Pharm. Weekblad, 59, 1351(1922).
’
5 50
I. M. KOLTHOFF AND E. A. PEARSON
showed that the latter is proportional to the square of the hydrogen ion concentration and inversely proportional to the zinc ion concentration. From a study of S. KrishnamurtP on the speed of precipitation of cadmium sulfide it follows that activities should be read here instead of concentrations. I n the case of zinc sulfide, the induction period decreases with increasing temperature (Kolthoff and van Dyk). Glixelli showed that the precipitation of zinc sulfide is an autocatalytic process; not only solid zinc sulfide but also copper sulfide and cadmium sulfide promote the precipitation of zinc sulfide in acid medium by hydrogen sulfide. Kolthoff and van Dyk10from a great number of experiments concluded that no zinc is precipitated with copper sulfide if hydrogen sulfide is passed through the equimolecular copper-zinc sulfate mixture a t room temperature a t an acidity of a t least 0.5 N sulfuric or hydrochloric acid. The filtration has to be made immediately after the precipitation of the copper as on longer standing zinc appears to be present in the copper sulfide. If the precipitation were carried out under the above conditions in a boiling solution, I to 2 % zinc was found in the filtrate. Therefore, if the precipitation is made at higher temperature, a higher acidity is required than at room temperature. This work was confirmed by W. Bottger and Druschke.'* So far only experimental facts have been described. I n recent years the interpretation of the facts observed in the promoted precipitation of metals of the third group by hydrogen sulfide in the presence of metals of the second group has been the subject of many investigations and theories. It is emphasized that in the following discussion we confine ourselves to the copper-zinc problem only; other combinations will be studied more systematically in the future. K. Scheringala advanced the peculiar interpretation that zinc sulfate (not zinc sulfide) forms a solid solution with copper sulfide, the former being distributed between solution and precipitate according to the partition law. I n a later paper,14 however, he is doubtful whether his experimental work has been correct and he rejects his statement without contributing anything new to the problem. I n the work described in the present paper, it is definitely shown that zinc sulfide neither forms a solid solution in copper sulfide nor is it adsorbed by the latter. In agreement with Gli~elli,~ Kolthoff and van Dykl" concluded that copper sulfide promotes the precipitation of zinc sulfide. Their paper is mainly of practical character, though they stateI6 that "in acid medium zinc sulfide is extremely slightly soluble and that it is not surprising that copper sulfide promotes the precipitation of zinc sulfide." Bottger and Druschkel* concluded that zinc sulfide is coprecipitated with copper sulfide, but their experimental data show definitely that the speed of precipitation of zinc s a d e is increased in presence of copper sulfide. They also found that on long treatment of the copper sulfide containing some zinc sulfide with I N hydrochloric acid all the S. Krishnamurti: J. Chem. SOC., 1926, 1549. Bottger and Druschke: Ann., 453, 315 (1927). laK. Scheringa: Pharm. Weekblad, 55, 431 (1918). I* K. Scheringa: Pharm. Weekblad, (57, 1294 (1920). 16 Pharm. Weekblad, 59, 1353 (1922). II
1z
PROMOTING ACTION O F COPPER SULFIDE O S ZINC SULFIDE
*
551
zinc could be extracted from the precipitate. This is in harmony with our results, but contrary to the statements of D. Balarew.16 In recent years, the latter has contributed interesting data to the problem of coprecipitation. He claims that the carrying down of zinc sulfide by copper sulfide is explained by his theory of “inner adsorption.” The “coprecipitation” should be entirely a capillary phenomenon like the coprecipitation of potassium permanganate and other salts with barium sulfate. I t will be shown below that the carrying down of zinc sulfide by copper sulfide is not a coprecipitation. Balarew’s experimental work seems to be of doubtful character as he was unable to extract zinc sulfide from copper sulfide even with concentrated hydrochloric acid, whereas neither Bottger and Druschkel* nor the present authors experienced particular difficulties in extracting the zinc from the precipitate. F. Feigll’ attempted to clear up the entire problem of the carrying down of sulfides of group three with those of group two by his co-ordination theory. He rejects the ionic theory and the law of mass action insofar as the precipitation of sulfides is concerned and proposes the idea that the residual valence of the sulfur in the sulfides may lead to the following types of co-ordination compounds (hence Feigl assumes a copreaipitation to take place): I. MS -SHz Hydrosulfide 2. MS -S Polysulfide 3. MiS-SM1 Isopolymer-ordinary sulfide, polymerized 4. MIS-SMZ Heteropolymer-mixed sulfide The Cu-Zn case then is represented by type 4, This theory and the experimental evidence has been vigorously attacked by 0. Ruff and Hirsch.’* I n their studies of fractional precipitation these investigator^'^ had found that the mass action law (and the solubility product principle) are the decisive factors in separations and that occlusions, adsorption or mixed crystal formation are of minor importance. In polemical papers, FeigP as well as RuffZ1defended their antagonistic view points without changing any of their former statements. Feigl in his generalization is undoubtedly wrong as will be shown in this paper on the Cu-Zn combination. It must be admitted, however, that Ruff and Hirsch are not entirely clear and consistent as may be inferred from a few citations from their interesting papers: “Die scheinbaren Wiederspruche zum Massenwirkungsgesetze welche den Anlass zur Aufstellung der Hypothese von der Bildung komplexer Sulfide gegeben hatten (Feigl), haben sich auf die Anwendung falscher Zahlen und BegrifTe zuriickfuhren lassen.22 Still, in a later paper,*3 Ruff claims that zinc 1 -
D. Balarew: S . anorg. allgem. Chem., 165, 192 (1927);Kolloidchem. Beihefte,, 249 (1930). The latter paper gives a complete review of Balarew’s work on inner adsorption. 1’ F. Feigl: Z. anal. Chem., 65,25 (1924). l8 Ruff and Hirsch: Z. anorg. allgem. Chem., 151,81 (1925). Ruff and Hirsch: Z. anorg. allgem. Chem., 146, 388 (1925);150, 84 (1926). 2o Feigl: Z.anorg. allgem. Chem., 157,251, 269 (1926). *’0.Ruff and B. Hirsch: Z. anorg. allgem. Chem., 151, 81 (1926); 0. Ruff and E. Ascher: 185,369 (1929);Ruff: 185,387 (1929). 22 Z.anorg. allgem. Chem., 151, 95 (1926). 23 Z.anorg. allgem. Chem., 185,395 (1929). I’
552
I. M. KOLTHOFF AND E. A . PEARSOPI’
sulfide can be coprecipitated with copper sulfide in the presence of an excess of hydrogen sulfide or sulfide. By molecular attraction between copper sulfide and zinc sulfide a kind of mixed crystal can be formed: “Die sogenannten induzierten Fallungen lassen sich in der Hauptsache auf Veranderung der primaren Sulfide, d. h. die Anlagerung oder Adsorption von Schwefelwasserstoff an diesen oder auf die Bildung einer undurchlassigen Hulle aus dem primaren Sulfid mit dem kleineren Loslichkeitsprodukte und das sekundare mit dem grosseren zuriickfuhren und so befriedigend erklaren.”24 In rejecting the applicability of the mass action law, Feigl overlooks one important factor, namely, that it can be applied only in the state of equilibrium. Therefore, his objections to the failure of this law cannot be accepted. It is a well-known fact that the precipitation of the metals of the third group by hydrogen sulfide in relatively weakly acid medium is a slow process. At the beginning of this paper, this “induction period” for zinc sulfide has already been mentioned. The velocity of the zinc sulfide formation in acid medium is small as the concentration of one of the reacting ions (sulfide ion) is extremely small. Therefore, in studying the precipitation of the metals of the third group in presence of those of the second group by hydrogen sulfide, it would be a great mistake to predict and explain the facts on the basis of the ratios of the solubility products. The latter principle can be applied only when the two stable solid phases are in equilibrium. The authors have made an extensive study of the so-called coprecipitation of zinc sulfide with copper sulfide and have arrived a t the conclusion that there is here neither a coprecipitation nor a mixed crystal formation. Whether this conclusion holds for all other metal combinations seems doubtful considering the various facts described in the literature (cf. esp. Baubigny,6 Feig1;17,20 Ruff and H i r s ~ h 1 ~ and J ~ ~Bottger *~ and Druschke12); this subject will be a problem of further study. That our interpretation of the copper-zinc hydrogen sulfide behavior is of primary importance for other combinations as-well, is without question, considering its general character. It is rather peculiar that the precipitation of zinc sulfide in acid medium in the presence of copper sulfide is usually interpreted as a coprecipitation or mixed crystal formation. A simple statement of the facts observed reveals immediately that neither of the two “explanations” is correct. If a mixture of copper and zinc sulfate is treated at room temperature in about 0.36 N sulfuric acid with hydrogen sulfide and filtered immediately after the quantitative precipitation of the copper all zinc is found in the filtrate. If the mixture is allowed to stand before filtration, some zinc separates on the precipitate as zinc sulfide, its amount increasing with time. From the fact that in the first stage only copper sulfide is precipitated can be inferred that there is neither a coprecipitation nor an adsorption. The copper sulfide promotes (induces) the precipitation of zinc from its supereaturated solutions and the problem, therefore, is reduced to the following question: Why is it that the precipitation of zinc sulfide is favored by the presence of copper sulfide? I n answering this problem two points were con-
*’2. anorg. allgem. Chem., 151, 95 (1926).
PROMOTING ACTION O F COPPER SULFIDE ON ZINC SULFIDE
553
sidered to be of primary importance: a. Copper sulfide as other sulfides adsorbs hydrogen sulfide, or sulfide ions at its surface (see also experimental part); therefore, the concentration of the reacting ion (S=)will be greater at the surface of the sulfide than in the bulk of the solution. Since the velocity of formation of zinc sulfide in the solution is relatively small it may be expected that its formation will be accelerated a t the surface of copper sulfide. This interpretation was confirmed by experiments in which adsorbed hydrogen sulfide or part of it mae replaced by prganic compounds containing polar sulfur. The promoting effect of copper sulfide on the precipitation of zinc sulfide was inhibited by this decrease of the adsorbed hydrogen sulfide. b. Copper sulfide like all finely divided substances, will favor the separation of a substance from its supersaturated solution; the surface acts as a center of crystallization or nucleus formation. If this interpretation were right, it could be expected that copper sulfide does not exert a specific inducing effect upon the precipitation of zinc sulfide from acid medium, but that other finely divided substances like barium sulfate, alumina, charcoal, talcum, silica, filter paper, etc., would show a similar effect. Some of these experiments have already been made by Glixelli; in the experimental part of this paper, quite a few substances are mentioned which qualitatively exert the same influence upon the precipitation of zinc sulfide as copper sulfide does.
Experimental Part Materials used: Copper sulfate penta hydrate was obtained in a pure state by recrystallizing a C.P. product three times from water, traces of ferrous iron being removed in the first crystallization by oxidation with bromine water. The salt was kept in a desiccator over deliquescent sodium bromide. Qualitative and quantitative tests indicated its purity. Zinc sulfate heptahydrate was obtained in a pure state by recrystallizing a C.P. product three times from water. The salt was kept in a desiccator with a relative vapor tension of 70%. IVater: ordinary distilled water from the laboratory supply or in some instances conductivity water was used. liydropm sulfide was prepared in a Kipp generator from commercial iron sulfide and passed through a wash bottle with sodium bicarbonate and another containing water. .]lethod of Precipitatim: The method of treating the acid zinc-copper mixture with hydrogen sulfide is of primary importance. Previous workers have repeatedly noticed that it is very hard to get reproducible results in the amounts of zinc precipitated. This amount depends mainly upon the surface action of the copper sulfide and, therefore, fairly reproducible results can be expected only if the mixture is constantly shaken after the precipitation of the copper sulfide under a constant pressure of hydrogen sulfide. I n the beginning of this work this important point was not realized and mahy experiments have been carried out according to procedure A (see below). With regard to the reproducibility of the results it may also be mentioned that in many
554
I. M. KOLTHOFF AND E. A. PEARSON
cases a distinct wall effect is noticed. I n some instances (also in the absence of copper sulfide) the zinc sulfide separates as a thin, strongly adherent layer on the surface of the flask. Great attention was paid to a careful cleaning of the flasks with dichromate and sulfuric acid before they were used for the experiments. The wall effect seems to be more or less accidental. Special experiments by treating flasks with solutions of various types of ions in order to impart to the wall a positive or negative charge did not yield conclusive results. Even in flasks lined with paraffin, there is a noticeable wall effect. Procedure A : The purified hydrogen sulfide gas was allowed to enter the space above the zinc-copper solution for about one minute. When all the air was displaced, the gas inlet tube (length 18 to 20 cm; inside diameter 5 to 6 mm.) was plunged into the solution and the gas allowed to bubble through the solution for a few minutes with shaking. The flasks after precipitation were tightly stoppered with rubber stoppers and allowed to stand or were mechanically shaken for various lengths of time. The concentration of hydrogen sulfide decreases on standing due to its diffusion from the flasks, its possible oxidation and the precipitation of zinc sulfide. In all cases sufficient experiments and blanks were run in order to justify the conclusions drawn from the results. As has been discussed before the following Procedure B gives more reproducible results and is recommended for future work. After all of the air had been driven out of the system by a brisk stream of hydrogen sulfide (2-3 minutes) the inlet tube was pushed into the copper-zinc solution and precipitation was carried out for various periods of time with mechanical shaking. The flask was kept under constant hydrogen sulfide pressure in a mechanical shaker. The latter was driven by a motor which imparted a rotary motion in the horizontal plane to the solutions shaken. The eccentric displacement was sufficient to produce a decided movement without any splashing on to the stoppers. Method of analysis: After filtration and washing (see below), the filtrates were boiled until all hydrogen sulfide was removed, cooled and titrated with potassium ferrocyanide using diphenylamine as an internal indicator. The details of this procedure will be described elsewhere (Ind. Eng. Chem., Anal. Ed. Jan. 1932). I n many cases the precipitated and washed copper sulfide was extracted with hot or cold hydrochloric acid (3 X to 4 N), most of acid evaporated and the residue titrated with ferrocyanide. Results: I n the first part of the work, the carrying down of zinc sulfide with copper sulfide was studied under various conditions. (time, acidity, temperature). Since most of the results are in harmony with those of previous authors, it will suffice to summarize the most important conclusions which in part are new. The extensive experimental evidence is to be found in the thesis of E. A. P e a r s ~ n . ~ ~ Zinc sulfide’is not coprecipitated with copper sulfide but is “post-precipitated,” the amount increasing with time of standing, decreasing with increas25
E. A. Pearson: Thesis, Minnesota 1931 p. 44-60, 97-108.
555
PROMOTING ACTION O F COPPER SULFIDE ON ZINC SULFIDE
ing acidity of the solution (sulfuric acid or hydrochloric acid) and increasing with increasing temperature. Shaking favors the precipitation of zinc. The speed of passing through hydrogen sulfide during the precipitation of the copper sulfide also has a decided influence upon the amount of zinc postprecipitated, the latter increasing with rapid precipitation of the copper. Probably the total surface area of the copper sulfide increases with more rapid precipitation; this is another factor which has to be considered in the interpretation of the fact why it is so hard to obtain strictly reproducible results. The effect of the presence of neutral salts (NaCl, KCI, NHnCI, LiCl, CaC12, MgC12) upon the precipitation of zinc sulfide generally is very small; the presence of sulfate (0.3 N) favors the precipitation but this is explained by a decrease of the hydrogen ion activity of the solution.
Effect of the Age of the Copper Sulfide It has been mentioned already that more zinc is precipitated if the experiment is carried on at a high temperature than a t room temperature. A few results given in Table I illustrate the large influence of the temperature.
TABLE I Effect of the Age of the Copper Sulfide 2 5 cc. 0.05 molar copper sulfate, 2 5 cc. 0.05 molar zinc sulfate, 5 cc. 2.38 N sulfuric acid. Procedure B. Air was driven from system for seven minutes by hydrogen sulfide, then the gas was passed through the solution fairly rapidly with mechanical shaking for ten minutes. The precipitates were washed six times with warm water and then extracted with warm 2 N hydrochloric acid. Temp. a t Temp. a t % zp 70 Zn
beginning of precipitation
end.of precipitation
23'
24
23O
24 O
O
left in filtrate
extracted from precipitate
92.3 91.0
7.1
24'
2I
O
92.5
24
2 Io
93.7 4.8
O
I ooo
5 5 O
IOOO
5 so 55O
IOOO
5 5 O
I ooo
7.1 6.4
5.0
Copper sulfide black appearance I,
!I
jJ
>I
3,
I,
19
I,
,,
f,
I,
,I
Copper sulfide green slimy appearance, hard to filter
5.1
5.7
Remarks
91 . o 91.5
An acid zinc solution alone precipitates more quickly at higher than a t room temperature. The above experiments were repeated but instead of 2 j cc. copper solution, 2 5 cc. of water were added. At the end of the ten minutes, the solution showed only a slight turbidity a t room temperature, whereas 7.1 to 7.37c precipitated if the hydrogen sulfide were passed in at 100'. Comparing these figures with those in Table I, shows definitely that the copper
556
I. M. KOLTHOFF AND E. A. PEARSOPI'
sulfide precipitated at higher temperature favors the post-precipitation of zinc much more than if precipitated a t room temperature. The copper sulfide obtained a t room temperature has a dark, black appearance and is easy to filter, whereas the sulfide formed a t 100' is slimy; it has a greenish color and is hard to filter. That the latter favors the precipitation of zinc much more than copper sulfide formed a t room temperature was definitely shown by the following experiments. (Compare with those in Table I). 2 5 cc. 0.05 molar copper sulfate, 5 cc. 2.38 N sulfuric acid. The mixture was heated to boiling, the air driven out with hydrogen sulfide for four minutes and again heated to 100'. The copper sulfide was precipitated in four minutes and the mixture cooled under hydrogen sulfide pressure to room temperature. Then z j cc. 0.05 molar zinc sulfate were pipetted in and the mixture shaken for ten minutes under hydrogen sulfide (procedure B). As an average of four experiments, it was found that 13.570 A 0.5% zinc was in the filtrate and 86% zinc in the precipitate. If the copper-zinc mixture was treated from the start with hydrogen sulfide at room temperature 91 to 9370 of the zinc was found in the filtrate. The last experiments and those described in Table I were repeated at, various acidities and also under conditions in which the air had been removed by purified nitrogen before the treatment with hydrogen sulfide. Under all circumstances the same effect was noticed as described above. If copper sulfide is precipitated at room temperature and allowed to age (under hydrogen sulfide) before the zinc solution is added its promoting effect upon the precipitation of zinc sulfide increases. Even after an hour standing the effect is quite pronounced, but it reaches a maximum after a day or more. The appearance of the copper sulfide changes from black to greenish; it becomes rather slimy and is hard to filter. It assumes the same appearance as the copper sulfide precipitated a t 100'. If the latter is allowed to stand under hydrogen sulfide pressure its promoting effect upon the precipitation of zinc decreases and it becomes materially the same as that of the aged sulfide precipitated a t room temperature (comp. Table 11). X-ray pictures (made by Professor N. H. Taylor) did not show a difference in crystal structure of the various copper sulfides, although the outer appearance is quite different. It was thought then that the cupric sulfide undergoes a decomposition on aging into cuprous sulfide and sulfur. A special investigation which will be described in a subsequent paper, however, showed that the amount of cuprous sulfide increases only very slightly on aging. Copper sulfide precipitated a t room temperature contained only about 0.5% cuprous sulfide, after seven days aging zYc. Precipitates formed at roo' contained about 5% cuprous sulfide and after standing for nine days 7%. Moreover, it was shown by special experiments that the presence of cuprous sulfide in the copper sulfide does not affect its action upon the precipitation of zinc sulfide. Therefore, it seems that the temperature and aging effect are entirely functions of the surface, although we are led to the peculiar conclusion that copper sulfide precipitated a t room temperature has a smaller surface than the same formed a t 100' and that the extent of the surface in-
PROMOTING ACTION O F COPPER SULFIDE ON ZINC SULFIDE
557
creases on standing. The surface of the sulfide precipitated a t 100' decreases on standing and becomes about the same as that of the aged copper sulfide formed at room temperature in acid medium.
Table II. Effect of Aging of Copper Sulfide 2 5 cc. 0.05 molar copper sulfate and 5 cc. 2.88 N sulfuric acid. Air driven out with hydrogen sulfide, precipitated rapidly for two minutes with shaking. Flask tightly closed with rubber stopper and allowed to stand for some days. Then placed in series with the same mixture containing a fresh precipitate of copper sulfide prepared under exactly the same conditions. After saturating with hydrogen sulfide 2 s cc. 0.05 molar zinc sulfide were added to each flask and treated fairly rapidly with hydrogen sulfide for ten minutes with shaking (Method B). After filtration the precipitates were washed four to six times with hot water (fresh precipitates) or 0.1 N ammonium sulfate (aged precipitates; to prevent passing through filter), and extracted with hot 4 N HCl.
Temp. of precipitation of copper sulfide 24' 24
O
24O
24' 2S0 2S0
2 5' 25'
90'
90° 90' 90'
Age of
3,: fresh I day fresh 5 days fresh 6 days fresh 8 days fresh 5 days fresh 6 days
Appeuance precipitate
black green black green black green black green green green green green
% zinc in Filtrate
89.9 36.5 90.8 35.0 91.1 40.4
% zinc in Precipitate IO. I
65.7
9.0 59.2
89.6 37.1 23.2
35.5
67.5 64.
25.0
75.0
40.2
59.2
Incidentally, it may be mentioned that a slight oxidation of copper sulfide on aging could not be responsible for the effect observed. All experiments were repeated in such a way that the copper sulfide was allowed to age in flasks with a hole in the stopper. The results obtained were materially the same as those given in Table 11.
The Effect of Adsorbed Hydrogen Sulfide I n the introduction it has been mentioned that part of the promoting action of copper sulfide upon the precipitation of zinc sulfide is attributed by the authors to hydrogen sulfide adsorbed on the surface of copper sulfide. Therefore, it could be expected that substances which displace hydrogen sulfide from the surface will inhibit the precipitation of zinc sulfide. From colloidal chemical investigations, it is well known that metal sulfide sols owe their stability to adsorbed hydrogen sulfide. But also in
558
I. M. KOLTHOFF AND E. A. PEARSON
the flocculated state, these sulfides keep some hydrogen sulfide in the adsorbed state. This was proved for copper sulfide by a great number of experiments. Solutions of copper sulfate of known strength were treated under various conditions of acidity with standard solutions of sodium sulfide or hydrogen sulfide. Precautions were taken to limit the volatilization of the latter. An aliquot part of the supernatant liquid was pipetted out and added to an excess of an acid iodine solution which was titrated back with thiosulfate. Under various conditions, one mol copper sulfide adsorbed one to five mol per cent hydrogen sulfide, or alkali sulfide, this number increasing with the final concentration of sulfide or hydrogen sulfide in the solution and being only slightly dependent upon the acidity of the solution. In the experiments with copper sulfate and sodium sulfide, magnesium chloride had to be added in order to flocculate the copper sulfide. Scheringal4 claims that acid is strongly adsorbed by copper sulfide. The authors were not able to confirm this statement. Standard solutions of copper sulfate were treated with hydrogen sulfide, the mixture made up with water in a volumetric flask to the mark and in an aliquot part of the filtrate the acidity determined by titration with standard base after boiling off the hydrogen sulfide. No difference in titration figure was obtained between methyl orange or phenolphthalein as indicators. It was found that one millimol copper sulfide only adsorbs o.oo3-0.01q millimoles sulfuric acid. Finally, it was shown that in shaking a fresh precipitate of copper sulfide with very dilute zinc solutions, none of the latter ion was adsorbed, which proves definitely that no solid solution is formed. I n studying the effect of substances which could displace hydrogen sulfide from the surface, it was expected that organic compounds containing a polar sulfur group would exert such an action. Actually it was shown that such substances are strongly adsorbed by copper sulfide. In the interpretation of the effect of these polar substances upon the carrying down of zinc sulfide by copper sulfide certain difficulties arise which will be shortly discussed: a. The organic substances used form complexes with cupric ions or reduce cupric copper to the cuprous state. Therefore, in studying their effect upon the after precipitation, the copper sulfide was first precipitated and thereafter the organic substance and the zinc added. This is not the most favorable experimental condition as the substance added has to displace the hydrogen sulfide which already is on the surface of the copper sulfide. b. Zinc forms complexes with some of the substances used and, therefore, the latter may retard the precipitation of zinc sulfide alone in the absence of copper. For this reason in blank experiments in the absence of copper the influence of the added substance had to be studied on the precipitation of zinc alone. Some cases were found where the added substance retarded the precipitation of zinc sulfide alone, which may be attributed to complex formation or inhibition of the autocatalysis in the precipitation of the zinc (displacing hydrogen sulfide from the surface). Therefore, the theory was not conclusively supported if such a substance exerted an inhibiting effect upon
PROMOTIKG ACTION O F COPPER SULFIDE ON ZINC SULFIDE
559
the precipitation of the zinc in presence of copper sulfide. Fortunately cases mere met in which the added substance (cysteine, thiophenol) increased the speed of precipitation of zinc sulfide alone, whereas it inhibited its precipitation in the presence of copper sulfide. Effect of Thiourea The thiourea used was prepared from ammonium thiocyanate by the method of Reynoldsz6and purified by recrystallization. I n later experiments an Eastman product was used which was found t o be pure. All other compounds used ( v i ) were Eastman products with the exception of cysteine and glutathione. These latter compounds were kindly supplied by Dr. E. KendallZ7of the Mayo Clinic and were of high degree of purity. Table 111. Adsorption of Thiourea by Copper Sulfide 2 5 cc. 0 . 2 5 molar copper sulfate, o to 1 5 cc. 4.07 N sulfuric acid. Copper sulfide precipitated moderately rapidly at room temperature. Allowed to stand with occasional shaking for two hours, after 2 5 cc. of B standard thiourea solution had been added. An aliquot part of the supernatant solution was pipetted out and after boiling off the hydrogen sulfide titrated with iodine. Blanks were run, in which the copper sulfate was replaced by an equal volume of water.
Molarity thiourea
CC. 4.07 N sulfuric acid added
Millimoles thiourea adsorbed by 6.25 millimoles CuS
thiourea adsorbed from solution
10.8 13. I
0.05
0
0 . I37
0.05
5
0.05
IO
0.165 0.109
0
0 . IO1
0.01
8.7 40.5
0.01
I
0.104
41.3
0.01
5
0 . IO0
40.0
0.01
IO
0.086
0
0.033
34.2 27.7 39 ' 2 39.2 36.9
0.005
0.005
5
0.047
0.005
10
0.047
0.ooj
15
0.045
Effect of Thiourea on the Precipitation of Zinc Sulfide In the following table, it will be shown that thiourea retards the precipitation of zinc sulfide in the absence of copper sulfide. I n the analysis the zinc was precipitated as carbonate and then titrated, in order to make the thiourea harmless. Reynolds: J. Chem. SOC.,1903, 7 . The authors wish to express their thanks to Dr. E. Kendall for his kindness in supplying these valuable products. 26
1. M. KOLTHOFF AND E. A. PEARSON
560
Table IV. Effect of Thiourea on Precipitation of Zinc Sulfide cc. 0.05 molar zinc sulfate, 2 5 cc. 0.05 molar thiourea (or 2 5 cc. water in blank), 5 cc. 2.21 N sulfuric acid and 2 5 cc. water. Air driven out for one minute then precipitated for four minutes under shaking. Tightly stoppered and mechanically shaken (Procedure A). Room temperature 28’. 25
Time of shaking
% zinc precipitated
% zinc precipitated
trace on wall 0.3 (on wall) trace
in absence of thiourea
120
minutes
41.7
I20
,,
I20
lJ
50.6 38.6
in presence of thiourea
The experiments were repeated but with 2 5 cc. 0.005 molar thiourea instead of 2 5 cc. 0.05 molar. Under these conditions an effect of the thiourea was hardly noticeable. Probably, thiourea forms a fairly stable complex with zinc ions. Table V. Effect of Thiourea on Post-Precipitation of Zinc Sulfide in Presence of Copper Sulfide at Low Acidity 2 5 cc. 0.05 molar copper sulfate, 2 5 cc. 0.05 molar zinc sulfate and 5 cc. 4.07 N sulfuric acid. Precipitation at room temperature, then 2 5 cc. 0.05 molar thiourea added and 2.1 cc. 4.07 N sulfuric acid. I n blanks, conditions were the same except 2 5 cc. of water were added after precipitation of copper sulfide, making the acidity in experiments and blanks the same (0.36 N). Flasks tightly stoppered and mechanically shaken for different periods of time. All precipitates washed four times with 0.36 N sulfuric acid saturated with hydrogen sulfide. Temperature precipitation
Time of shaking
22O
IO
32O
15
min.
Presence thiourea L/oZnin % Znin filtrate precipitate
98.4
Blank
Lib Znin filtrate
90.0
11.0
45.7 50.3 85
3 oo
20
,’
77.8
5.5 I3 0
3 oo
30
”
51.8
46
0
48.0 43 7 I4 2
31’
30
”
51.5
42
7
I1 I
”
93 ‘ 2
./,+in precipitate
89. I 0
The experiments were repeated a t other acidities arid varying acid concentrations. All showed that thiourea inhibits the precipitation of zinc sulfide in presence of copper sulfide. The results, however, do not prove anything definite about the influence of thiourea upon the mechanism of the reaction, since it exerts qualitatively the same influence upon the precipitation of zinc in the absence of copper sulfide. Thiosemicarbazide behaves in the same way as thiourea. I t is strongly adsorbed by copper sulfide and it retards the precipitation of zinc sulfide although not nearly as much as thiourea does and inh:bits the precipitation in presence of copper sulfide.
os
PROMOTING ACTION OF COPPER'SULFIDE
ZISC SULFIDE
561
Cysteine is strongly adsorbed by copper sulfide. No data are reported as no special precautions were taken to protect the cysteine hydrochloride solution from oxidation. It accelerates the speed of precipitation of zinc sulfide as is shown by the figures in Table VI. Table VI. Effect of Cysteine on Speed of Precipitation of Zinc Sulfide 2 5 cc. 0.05 molar zinc sulfate, 2 j cc. water and the indicated volume of sulfuric acid. Air was driven out with hydrogen sulfide for one minute and the gas passed through the solution for three minutes. Then 2 5 cc. 0.05 molar cysteine hydrochloride (or 2 5 cc. of water in the blank) were pipetted in and hydrogen sulfide passed in for one minute longer. Flask tightly stoppered and mechanically shaken. Precipitate washed four times with water.
vel. Sulfuric acid added
5 cc. 1,
j
2.21
T
1,
,,
J )
1)
~ ~i~~~
. Presence of cysteine
of shaking
7 'Zn in
390 33O 32' 32' 26'
30min. 60min. 60 min. 125 min. 180min.
34.3 43.8 76.8
N
Jl
cc. 3 . 9 1
~
during prec.
pu'
filtrate
C Zn in
precipitate 65.2
69.2 86.8
56.2 23.2 30.8 13.2
Absence of cysteine Zn in filtrate precipitate 68.2 31.8
';+ Zn in 5; 68.8 98.0 94.1 97.1
31.0
94.5
5.7
2.0
j.7 2.9
From the results in Table VI1 it is evident that cysteine inhibits the precipitation of zinc sulfide in the presence of copper sulfide although it accelerates the precipitation of .zinc sulfide alone. This behavior is in harmony with our theory based upon the displacement of adsorbed hydrogen sulfide by cysteine. Table VII. Effect of Cysteine on Precipitation of Zinc in Presence of Copper Sulfide 2 5 cc. 0.05 molar copper sulfate, 2 j cc. 0.0; niolar zinc sulfate and 5 cc. 2.21 N sulfuric acid. Hydrogen sulfide passed through for three minutes and then 2 j cc. 0.0 j molar cysteine hydrochloride (or in blank 2 5 cc. water) added; the gas passed through for one to two minutes longer. The cysteine is made harmless in the titration of zinc by oxidation with an excess of bromine water, the latter being removed by boiling. Time Temp. during precipitation
of
shaking
min.
25O
Ij
25O
30
"
39O
20
"
Presence of cysteine %Znin filtrate Precipitate
% Znin 88.4 80.0 13.0
Znin filtrate
Blank %Znin precipitate
20.0
83.4 52.8
18.0 48.0
82.2
5.2
94.0
11.5
With 5 cc. 3 . 9 1 N sulfuric acid instead of 5 cc. 2 . 2 1 N 3 Io I O min. 96.8 2.0 85.7 11.4 31° 2 0 ') 93.0 6. I 71.3 28.1
I. M. KOLTHOFF AND E. A . PEARSOS
562
Glutathione retards the precipitation of zinc sulfide alone and behaves similarly to thiourea and thiosemicarbazide. I t also inhibits the aging of copper sulfide a t room temperature. Thiophenol on the other hand, accelerates the precipitation of zinc sulfide alone, but has an inhibiting effect in the presence of copper sulfide. For example in the precipitation of zinc sulfate alone (conditions somewhat similar to those in Tables IV and VI), 64% was precipitated after one hour in the presence of thiophenol and only 7.6% in the blank. The results in Table VI11 show its inhibiting effect in the presence of copper sulfide.
Table VIII. Effect of Thiophenol on Precipitation of Zinc in Presence of Copper Sulfide (30”) Time of shaking 11
min.
20
”
30
I’
80
”
Presence of thiophenol yo Zn in filtrate precipitate
yo Zn in
Blank
5; Zn in filtrate
yo Zn in precipitate I7
88.2 85.8
11.0 12.0
80.3 73.6
22
67.8 45.3
30.9
61.0
35
53.0
32.5
63
Thiobarbituric acid inhibits the precipitation of zinc alone although its effect is very small. However, it has a pronounced inhibiting effect upon the speed of precipitation of zinc in the presence of copper sulfide. Summarizing, it appears that cysteine, thiophenol and thiobarbituric acid, which are strongly adsorbed by copper sulfide, accelerate the precipitation of zinc sulfide alone, whereas they inhibit, the speed in the presence of copper sulfide. This behavior is a good support for our theory that adsorbed hydrogen sulfide on the surface of copper sulfide is partly responsible for its promoting effect upon the precipitation of zinc sulfide, although it is not quite conclusive as the polar substances may change the surface of the copper sulfide in a more radical way.
Influence of Various Substances upon the Speed of Precipitation of Zinc Sulfide It has been mentioned in the introduction that the promoting effect of copper sulfide on the precipitation of zinc sulfide is not specific for this substance, but that all finely divided substances will exert more or less the same influence. Experimental evidence for this conclusion is given in the following pages. In order to show the promoting effect of the substances used it was necessary to work a t a lower acidity than was done in the experiments with copper sulfide. With regard to the “wall effect” it was of interest to study the influence of powdered Pyrex glass, silica gel and the difference in behavior between a paraffined wall and a flask etched with hydrogen fluoride.
PROMOTING ACTION O F COPPER SULFIDE ON ZINC SULFIDE
563
Table IX. Influence of Powdered Pyrex Glass on Precipitation of Zinc Sulfide 2 5 cc. 0.05 molar zinc sulfate and the indicated volume of sulfuric acid, air driven out for two minutes then treated with hydrogen sulfide (procedure B). Temperature 22’. cc. sulfuric acid
shaking
Time
cc. 2.88 N
I O min.
of
7*Zn in filtrate
without pyrex Pyrex powder powder added
% Zn in filtrate in presence of pyrex powder
g. 27.9 g. 37.8 3 cc. 2.88 N 32 I ’ 3 g. 86.2 5 cc. 2.88 N 60 I’ 5 g. 99.7 I n the experiments with silica gel a product was used with a high adsorbent power which had been purified by electrodialysis. The adsorption of mineral acid by this silica gel was negligibly small, hence, the promoting effect upon the precipitation of zinc sulfide is not due to a decrease of the acidity of the solution. The effect of silica gel is given in Table X. I
>I
,,
>I
57.2
1
58.3 98.9 99.5
I
Table X. Influence of Silica Gel on Precipitation of Zinc Sulfide cc. 0.05 molar zinc sulfate, 50 cc. water and the indicated amount of sulfuric acid. Air driven out for one minute then hydrogen sulfide passed through solution rapidly for three minutes. Flask tightly stoppered and mechanically shaken. One gram of silica gel was used. Temperature 28’-29”. 25
cc. sulfuric acid added
5
CC. 2.21 ,I 11
5
CC.
N
71 11
3.91 N
Time of shaking
15 min.
I
g. silica gel 70 Zn in precipitate
6.1
88.5
11.0
1.8
75.5
22.8 33.3
”
93.9 98.2 89.0
11.0
”
100.0
0.0
20 ”
30 30
Absence of silica gel Presence % Zn in % Zn in filtrate precipitate filtrate
70 Zinc in
63.1 99.4
0.6
It has been mentioned before that in many cases a definite “wall effect” was noticed in the precipitation of zinc sulfide. Since finely divided glass favors the precipitation of zinc, it may be expected that quantitatively this wall effect will depend upon the smoothness of the glass. A coarse surface will favor the precipitation more than an etched surface. A mixture of 50 cc. 0.025 molar zinc sulfate and 2 cc. 2.88 N sulfuric acid was treated in paraffined and etched (with HF) flasks respectively with hydrogen sulfide (Procedure B; mechanical shaking). In all cases more zinc was precipitated in the etched than in the paraffined flask. However, it was found that some of the zinc also precipitated on the paraffined wall. Some figures show that the effect of etching is relatively small: % zinc precipitated: 89.5% (paraffined); 86.9% (etched); 87.7% (paraffined;) 83.3% (etched); 90.0% (paraffined); 82.6% (etched); 92.0% (paraffined); 90% (etched). Effect of Barium Sulfate It was found that barium sulfate powder promotes the precipitation of zinc sulfide.
I. M. KOLTHOFF AND E. A. PEARSON
564
Table XI. Effect of Barium Sulfate cc. 0.05 molar zinc sulfate and 3 cc. 2.88 N sulfuric acid. Procedure B. Temperature 24’. 3 g. barium sulfate C.P. added. 25
Time of shaking
3 1 min. I45 ”
Absence of barium sulfate % Zn in filtrate precipitate
% Zn in 95.8 97.4
4.2 2.4
Presence of barium sulfate yo Zn in % Zn in filtrate precipitate
86.5 38.8
13,2 61.1
Effect of Sulfur A large number of experiments were carried out to study the effect of colloidal sulfur obtained by reaction of iodine with hydrogen sulfide or according to Oden’s method. I n most cases the sol flocculated very rapidly in the electrolyte solution and, therefore, a relatively small surface was exposed to the liquid. I n all cases, however, it was found that sulfur promotes the precipitation of zinc sulfide. The next table shows that sublimed sulfur also exerts a distinct effect. Table XII. Effect of Resublimed Sulfur 50 cc. 0.025 molar zinc sulfate and 2 cc. 2.88 N sulfuric acid. One g. of sulfur added. Method B. Temperature 21’. Time of shaking
Absence of sulfur % Zn in filtrate
Presence of I g. of sulfur % Zn in a t r a t e
I O min. 1 5 min.
87.5 80.0
51.3 42.4
Effect of Charcoal Table XI11 shows the promoting effect of charcoal (Merck product, ashfree, highly activatedj. Table XIII. Effect of Activated Ash-Free Charcoal molar zinc sulfate, 50 cc. of water and 5 cc. 2.21 N sulfuric acid. 300 mg. charcoal. After precipitation flask tightly stoppered and mechanically shaken. Temperature 28’. 25
cc.
0.05
Time of shaking IO
min.
I5
l7
20
30
”
Absence of clprcoal yo Zn in filtrate precipitate 95.0 5.0
70Zn in 94.6 93.5 88.5
5.4 6.5 11.5
Presence of charcoal Zn in yo Zn in filtrate precipitate
14.5
85.4 78.4 68.9
31.1
70.0
29.5
20.3
Ignited Aluminum Oxide Ignited aluminum oxide promotes the precipitation of zinc sulfide very strongly. Part of this, however, is due to a strong adsorption of sulfuric acid on the oxide by which the acidity of the solution is greatly decreased.
PROMOTING ACTION O F COPPER SULFIDE ON ZINC SULFIDE
565
I n special experiments, the amount of acid adsorbed after a certain period of time was determined and correspondingly more acid was added to the zinc solution to make up for the amount disappearing by the adsorption. A description of all experiments would require too much space, therefore, the statement will do that ignited aluminum oxide promotes the precipitation of zinc sulfide very strongly. Effect of Talc The talc used (U.S.P.) was purified by digesting with boiling strong hydrochloric acid and washing free from acid. Table XIV shows the distinct promoting effect of talc. Table XIV. Effect of Talc 50 cc. 0 . 0 2 5 molar zinc sulfate and indicated amount of sulfuric acid. Precipitated fairly rapidly with mechanical shaking. Procedure B. Temperature 24’. Time
2
grams talc added
of
cc. sulfuric acid added
shaking
cc. 2 . 8 8 K
IO
min.
I
IO
”
2
30
”
2
>)
JI
5 cc. 2.88 N
g. g. g.
Absence of talc
Presence of talc
% Zn in % Zn in
% Zn in % Zn in
filtrate precipitate
90.7 92.0 100.0
filtrate precipitate
9.0
71.6
28.0
8.0
42.7 98.8
57.3
0.0
1.0
Effect of Filter Paper Ash-free filter paper was finely divided and vigorously shaken with water. The suspension was added to the zinc sulfate solution. Table XV. Effect of Filter Paper 2 j cc. 0.05 molar zinc sulfate, 2 5 cc. filter paper suspension (or 2 5 cc. water in blank) and indicated amount of zinc. Method B. Time of standing 30 minutes. Temperature 23’-2 jo.
cc. sulfuric acid added
5 cc. 2.88 N JJ
11
2,
Jf
Speed of passing through hydrogen sulfide
Slow Mod. rapidly JI
J )
Absence filter paper Zn in filtrate precipitate
% !An in 97.6 75.9 85.7
24.1 14.3
Presence a t e r paper Zn in % Zn in filtrate precipitate
97.5 67.6 68.9
32.4 31.0
It is seen that even filter paper promotes the precipitation of zinc sulfide slightly. summary I. The “carrying down” of zinc sulfide by copper sulfide cannot be attributed to a coprecipitat,ion (occlusion), a mixed crystal formation, or distribution of the zinc between solution and the solid. The zinc sulfide precipitates after the copper sulfide has been quantitatively formed. The
566
I . M. KOLTHOFF AND E . A. PEARSON
copper sulfide promotes the precipitation of zinc sulfide by virtue of its finely divided state and the presence of an adsorbed layer of hydrogen sulfide on its surface. 2. Copper sulfide precipitated a t room temperature has a smaller promoting effect than that formed a t boiling temperature. On standing the copper sulfide formed at room temperature shows an increased promoting effect, that precipitated at high temperature a decreasing effect. After aging for a few days a t a certain acidity both precipitates have the same effect. On standing the precipitate formed in the cold assumes the same appearance as that obtained a t high temperature. 3. Cysteine and thiophenol accelerate the precipitation of zinc sulfide alone, but inhibit its precipitation in the presence of copper sulfide. This was expected on the basis of the adsorption theory. 4. Finely divided substances, such as glass powder, silica gel, barium sulfate, charcoal, sulfur, talcum, aluminum oxide and filter paper promote the precipitation of zinc sulfide, showing the general character of the surface effect upon the speed of formation of a precipitate from a supersaturated solution. School of Chemistry, Uniuesity of Minnesota. Minneapolis, Minnesota. 1931.
