COAGULATION OF COLLOIDAL SOLUTIONS O F ARSENIOUS SULPHIDE BY ELECTROLYTES BY
E. F. BURTON AND E. D. MAcINNES
I n a paper by Burton and Bishop1 on the Coagulative Power of Electrolytes for Sols of Arsenious Sulphide, Mastic and Copper in so far as this power changes with the concentration of the sol, the following results were obtained: (1) For univalent ions h e concentration of ion necessary to produce coagulation increases with decreasing concentration of the colloid-this increase being very rapid with low concentrations of the colloid. (2) For divalent ions the concentration of ion necessary to produce coagulation is almost constant and independent of the concentration of the colloid. (3) For trivalent ions the concentration of ion necessary to produce coagulation varies almost directly with the concentration of the colloid. These results are in line with those already published by Mukherjee2 and his co-workers and Kruyt and his collaborat o r ~ . Kruyt ~ explains his results by adopting ideas suggested by Smoluchowski and Burger, viz., that coagulation depends on two sets of circumstances, first the probability of particles coming into collision with one another and, secondly, the likelihood of two parbicles adhering to one another when they do meet. The first probability depends fundamentally on the concentration of the sol and the second on the potential of the charge on the particle (see also Powis4). The experiments described herewith were undertaken to get still more accurate and definite evidence on the coagulaBurton and Bishop: Jour. Phys. Chem., 24, 701 (1920). Mukherjee: Jour. Am. Chem. Soc., 37,2024 (1915); Mukherjee and Sen: Jour. Chem. Soc., 115, 462 (1919); 117, 350 (1920). Kruyt: Koll. Zeit., 22, 81 (1918); Kruyt and van der Speck: Ibid., 25, page 1 (1919); Rec. trav. chim. Pays-Bas, 39, 618 (1920). Powis: Zeit. phys.Chem., 89,186 (1915); Jour.Chem.Soc., 109,73411916).
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E. F . Burton and E . D.Maclnnes
tion of Arsenious Sulphide by using mono-, di,- tri-, and tetra-valent active ions. The results reported by Burton and Bishop have been completely confirmed.
Preparation of the Sol The sol was prepared by bubbling hydrogen sulphide gas into a solution of arsenious oxide in water. Since the solubility of the oxide increases with the temperature, comparatively strong solutions of arsenious sulphide were obtained by boiling water, adding arsenious oxide, and bubbling in hydrogen sulphide gas. By bubbling in hydrogen sulphide gas before adding the oxide and continuously afterwards, one insures that a t no time is arsenious acid present to any very great extent to act as a coagulating agent-the acid having been converted as soon as formed into arsenious sulphide. By this method colloidal solutions were prepared containing as high as .lo96 mols. arsenious sulphide per litre. The method of analysis of the solutions used was a coprecipitation method suggested by Professor Rogers. A 15 cc sample of the solution was dissolved by boiling with about 25 cc of concentrated nitric acid, a little hydrochloric acid and a pinch of potassium chlorate. About 2 grams of sodium phosphate was dissolved in a few cc of distilled water and added with a few cc of ammonia to the solution. Then all the phosphate and arsenate present was precipitated together by adding about 200 cc of magnesia mixture (the phosphate helping to bring down the arsenate). The phosphate and arsenate were precipitated as magnesium ammonium phosphate (NH, MgP04 6HzO) and magnesium ammonium 6HzO), respectively-bo th coming arsenate (NH4 MgAs04 down as fine white crystalline precipitates (Fresenius) . The precipitates were allowed to settle over night and then filtered, washing out the flask with ammonia water (lo%), the precipitates being less soluble in ammonia than in water. What remained in the flask was then dissolved in 40 cc of hydrochloric acid and 30 cc of water and the precipitate added,
+ +
Coagulatiow of Colloidal Solutions
519
including the filter paper-the whole being dissolved. The solution was then cooled and about 2 grams of potassium iodide dissolved in a few cc of water added, and the whole allowed to stand for a minute. Then 70 cc of water was added and the solution titrated against a standard solution of sodium thiosulphate, using starch as indicator. The concentration of the sol obtained was .027 grams arsenious sulphide per cc of sol; this is the concentration A in the tables and figures.
Experimental (1) Variation in the concentration of the electrolytic solution. Mukherjeel found that using “different concentrations of the same electrolyte for titration of the same preparation of solution the quantities of electrolyte required are not in inverse ratio of the concentration of the electrolyte.” This is a surprising result and certainly needed further testing. Experiments were first carried out in which the concentration of the electrolyte used was varied and the quantities of electrolyte of different strengths required to produce coagulation were determined. I n these tests 10 cc of colloid were TABLE I Electrolytic Solution, A = 492.0 millimols aluminium chloride per litre Concentration of electrolytic solution
A/1,000 A/2,500 A/5,000 A/S,OOO A/ 12,000 A/l5,000 A/20,000 A/25,000 A/28,500 A/32,000
Cc solution for coagulation
.42 1.45 2.70 3.10 4.45 5.00 7.55 9.10 10.75 13.75
Concentration volume
.00042 .00058 .00054 .00039 .00037 .00033 ,00038 .00036 .00038 .00043
‘Mukerjee: Jour. Am. Chem. SOC.,37, 2024 (1915); Mukherjee and Sen: Jour. Chem. SOC.,115, 462 (1912); 117, 360 (1920).
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E . F . Burton and E . D . MacInnes
taken, water and electrolyte added in the order named so that in each test the final volume was made up to 25 cc. As is shown in Table I and the curves of Figure 1, the volume of a given
Fig. 1
electrolytic solution required for coagulation of a given amount of the disperse phase varies inversely as the concentration of the electrolytic solution, provided the final concentration of the colloid is kept constant. (2) Coagulative power and concentration of the sol. I n making the following tests 10 cc of colloidal sol of determined concentration were put into a 75 cc test-tube. After preliminary tests, the strength of electrolyte used was such that about 10 cc of it caused coagulation within 10 hours. If b cc of electrolytic solution of strength x grams per cc were required to produce coagulation in a sample of 10 cc of sol, the final volume of the mixture would be (10 b) cc. If the concentration of the sol is y grams per cc then 10Y the final concentration of the colloid would be
+
wb
grams per cc and the final concentration of the electrolyte would be
b. x grams 10 + b
per cc.
Coagulation of Colloidal Solutions
521
Experiments were carried out with the following electrolytes : Univalent cations : Divalent cations : Trivalent cations : Tetravalent cations :
Potassium Chloride Lithium Chloride Magnesium chloride Barium chloride Aluminium chloride Lanthanum sulphate Cerium nitrate Zirconium chloride
The results are given in Tables 2 to 5 and illustrated in Figures 2, 3, and 4. I n the tables and figures A means the maximum concentration of arsenious sulphide per cc of final solution. New samples of sol generally of concentraof A were used in successive experition 3/4, 1/2, 1/4, and ments.
TABLE2 Univalent Ions Potassium chloride conc. 487.5 millimols per litre
Final conc. sol.
A 3A/4 AI2
A/4 A/8
electrolyte required For coagulation I
4.7 4.4 5.2 6.9 7.2
Conc. electrolyte in final mixture, millimols per litre
91.6 85.8 101.4 134.5 140.4
Final conc. sol.
A 3A/4 A12.3 AI4
AI8
Conc. elecCc electrolyte re- trolyte in final mixture, quired for coagula- millimols per litre tion
6.2 6.5 7.4 9.3 11.5
95.2 99.8 113.7 142.9 176.6
d
E. F . Burton and E. D. MacInnes
522
Fig. 2
TABLE 3 Divalent Ions Magnesium chloride conc. 4.74 millimols per litre Final conc. sol.
CC electrolyte O' n'' mixture, lyte in final milrequired for limols per litre coagulation
Barium chloride 4.23 millimols per litre Final cone. sol.
I
A 3A/4 A/2.5 A/4 A/8
9.3 9.2 8.8 8.7 9.8
1.76 1.74 1.67 1.65 1.86
A 3A/4 A/2.4 A/4 A/8
Cc electrolyte re- Cone. in quired for final mixture, coagula- millimo's per litre tion
10.0 9.3 8.1 8.3 8.2
1.69 1.57 1.37 1.41 1.39
Coagulation of Colloidal Solutiom
523
Fig. 3
TABLE 4 Trivalent Ions Aluminium chloride conc. ,0615 millimols per litre Conc. electroCc electrolyte in final lyte required mixture, milfor coagulation limols per litre
Final conc. sol.
A 3A/4 A/2.1 A/4 A/8
'
11.9 9.7 6.3 4.1 2.8
.0293 .0239 ,0155 .0101 ,0069
Lanthanum sulphate .134 millimois per litre Final cone. sol.
A 3A/4 A/2.2 A/4 A/8
Cc elec- Conc. electrotrolyte re- lyte in final quired for mixture, milcoagulalimols per tion litre
11.5 9.5 7.1 6.0 5.8
1
.Of321 .Os13 ,0383 .0324 ,0313
524
Fig. 4
TABLE 5 Tetravalent Ions Zirconium chloride conc. .0378 Millimols per litre
A 3A/4 A/2.2 A/4 A/8
E.5* 12.0 7.1 3.75 3.0
Cerium nitrate
.OX6 millimols per litre
.0234
.OM .0107 .0057 .0045
.065 .062 .057
* This represents the first coagulation produced by these electrolytes. Addition of greater amounts produced a stabilizing effect: this shows the existence of what has been called a coagulation zone.
Coagulation of Colloidal Solutions
525
Conclusion These results bear out exactly the former experiments performed by Burton and Bishop already referred to in the first paragraph of this paper. It may be worth noting that the curve for barium chloride shows a tendency toward the trivalent curve represented by aluminium chloride ; the barium ion is noted as one of the most powerful of the divalent ions. I n addition we have the curves representing tetravalent ion, Zr showing the characteristics which we should have expected, while cerium seems to act similarly to trivalent ions. I n conclusion we wish to express our thanks to Professor Rogers of the and the Department of Chemistry for advice and'gift rarer salts.
