T H E MECHANISM OF T H E COAGULATION OF SOLS BY ELECTROLYTES IV. Arsenic Trisulfide Sol BY HARRY B. W E I S E R A S D GEORGE R . GRAY
In earlier papers, the displacement of chloride ions from colloidal particles by the stepwise addition of electrolytes to the positive hydrous oxide sols of iron, chromium, and aluminum, has been followed potentiometrically and the adsorption isotherms for the precipitating ions have been determined.' These observations have led to a proposed mechanism of the electrolyte coagulation process. The present paper deals with similar potentiometric titration and adsorption studies on negative As2S3 sol. This type of investigation on AszSs sol should prove fruitful since Whitney and Oberjz in their classical observations on adsorption during the coagulation of this sol, pointed out that, the adsorption of cations was accompanied by the setting free of an equivalent amount of acid.
Historical Following Linder and Picton's3 observations that barium but not chlorine is carried down during t,he coagulation of AszS3sol by BaC12, Whitney and Ober investigated quantitatively the adsorption of barium, strontium, calcium, and potassium during coagulation by the respective chlorides. As a result of these studies it was concluded that equivalent amounts of the respective cat,ions were carried down during the coagulation process. The filtrate from the coagulum contained free acid which was found by titration with alkali to be equivalent in quantity to the cation adsorbed. The phenomenon was believed by both Linder and Picton, and Whitney and Ober to be a case of hydrolytic adsorption in which equivalent amounts of cation and hydroxyl were carried down with the precipitate leaving HC1 in solution. Rabinowitsch4showed, however, that the adsorption does not cause hydrolytic cleavage of the salt. For example, the adsorption of barium was determined ( I ) from the difference in concentration of Ba" ion before and after coagulation; ( 2 ) from the difference in weight of the dry AS& precipitate after coagulation with BaClz and HNO,, respectively. The adsorption values by the two methods agreed quite well, showing that the barium was not carried down as Ba(OH)2. Moreover, the precipitate was free from chloride. The process would, therefore, appear to involve exchange adsorption in which hydrogen ions on the particles are exchanged for barium ions. 'Weiser: J. Phys. Chem., 35, I , 1368 (1931); U'eiser and Gray: 36,2178 (1932). * J. Am. Chem. SOC., 23, 842 (1901). J. Chem. SOC., 67, 63 (1895). il 2. physik. Chem., 116,97 (1925).
T H E COAQULATION O F SOLS BY ELECTROLYTES
2797
Contrary to the belief of Whitney and Ober and of Freundlich: Weiser2 pointed out that the amounts of several cations carried down during the coagulation of As& sol are not necessarily equivalent. Moreover, the amount of adsorption of a given cation varies widely with different sols, depending as it does on the method of preparation, age, and concentration of the sol. Since the platinum and quinhydrone electrodes can not be used successfully with As2& sol, Pauli and SemleP used conductometric methods to determine the H' ion concentration of a dialyzed sol and of the filtrate after coagulation of the sol by electrolytes. They found the ratio between [H'] in the filtrate and adsorbed [Ba"] ion to be 4 : I instead of I : I as reported by Whitney and Ober. Pauli and Semler considered the sol to be a strongly dissociated complex acid to which they assigned the formula ( X A S ~ S ~ , A S ~ S ~ H ~ . AszS4H)' H'. This formula was made to fit the specific case where but one of four H' ions are displaced by Ba", the remaining three appearing in solution only after coagulation. Rabinowitsch4 likewise determined the H' ion concentration of As2Ss sol before and after coagulation using conductometric methods. He found the total H' ion concentration of the filtrate after coagulation to be equal to the Ba"/2 adsorbed, confirming the earlier observations of Whitney and Ober. The sol was therefore assigned the general formula [(As&),SH]' H', first suggested by Linder and Picton. Since the ratios of H' ion in the filtrate to Ba" adsorbed with different sols have been found to be I and 4, using similar methods of analysis, it is altogether probable that an indefinite number of different ratios could be obtained by suitable modifications in the method of preparing the sol. The formulas of Pauli and Semler and of Rabinowitsch are, therefore, without any general significance. Rabinowitsch points out that he used an undialyzed sol while Pauli and Semler used one that had been dialyzed. To the extent that dialysis influences the ratio under consideration, it is obvious that one can get considerable variation with the same preparation simply by varying the method and time of dialysis. The conductometric method of determining the H' ion concentration in sol-electrolyte mixtures is of doubtful accuracy, at least in certain cases. Rabinowitsch5 first used a graphic method to evaluate the H' ion concentration from conductometric data but the results did not agree with glasselectrode measurements.6 Fairly good agreement was obtained if the conIO00 x,, centration C was calculated from the expression C = , where uH
+
+
+
_ I _
UH
Uc
and ue are the mobilities of H' and the added cation, respectively, and 1
Kolloid-Z., 1, 32 (1907).
* J. Phys. Chem., 29, 955 (1925).
e
Kolloid-Z., 34, 145 (1924). 2. physik. Chem., 116, 97 (1925). Z. physik. Chem., 116, 97 (1925); Rabinowitsch and Dorfmann: 131, 313 (1928). Rabinowitsch and Kargan: Z. physik. Chem., 143, ZI (1929).
2798
HARRY B. W E I S E R AND GEORGE R. GRAY
x,, = x - x, - L Ax, - , x and x, being the conductivity before and after the adAL dition of electrolyte, L , the number of cc of electrolyte added, and Ax,/AL, the change in conductivity with change in the amount of electrolyte added after the conductivity curve assumes a straight course. For salts wit,h uniAx valent cations L 2 has to be replaced by Ax from Kohlrausch's law, in AL order to get results which agree at all well with glass electrode measurements. These formulas fail to take into account bot,h the change in activity of the added salt in the sol and the mobilit'y of the colloidal particles. Since both of these have an effect, the method can, a t best, yield only approximately accurate results. Fortunately, there is no apparent need of using a conduct,ometric method to follow the change in hydrogen ion concentration. The glass electrode has been found satisfact,oryin many cases where the platinum and quinbydrone electrodes are not suitable and it would seem to be well adapted to the case at hand. Rabinowitsch and Kargan used it in three experjments to determine the change in hydrogen ion concentration during the coagulation of As& sol, with KCl, BaClz and AlCI,, respectively; but unfortunately their data cannot be compared. The implication is that they used the same sol for all three cases but, if they did, there was something radically wrong with their procedure as the difference in pH of the original samples used with KCl and BaClz was almost as great as the total change in pH on adding the electrolyte. In the subsequent experiments it will be shown that the glass elect,rode gives consistent, reproducible results with As& solelectrolyte mixtures. Experimental A . General Method of Procedure. Preparation of Sols. The Ask38 sols were prepared according t o the procedure of Freundlich and Nathansohn.' Saturated AS203 solution was diluted with a solution containing I cc of sat'urated HZS solution per I O O cc. After the appearance of a light-yellow coloration, a solution of HzS ten times as strong as the above was added. The solution was then saturated with H2S gas and the excess washed out with Hz. After standing overnight, the sol was filtered and the titrations were carried out. The sol was protected at all times from the action of light. All the experiments on a given sol were completed within a few days to avoid changes brought about by ageing. Method o j Titration. To I O cc of A s ~ S sol~ contained in a weighing bottle were added varying amounts of water and precipitating electrolyte, bringing the final volume to I j cc. A definite procedure of mixing was followed in each case. Preliminary experiments with dilute sols using 0.004 '2' Cas04 as coagulating electrolyte showed a decrease in pH for small amounts added, followed by an increase in pH a t higher concentrations of electrolyte. This effect was found to be due to the slight alkalinity of the electrolyte (probably a trace of Ca(OH)2). In all of the experiments herein recorded, the pH of the sol was first determined and then the pH of t,hewater and of the electrolyte 1
Kolloid-Z., 28, 258 (192~).
THE COAGULATION OF SOLS BY ELECTROLYTES
2799
were brought to this value by the addition of H2S04in the case of CaS04and of HC1 with the several chlorides. Thus the observed displacement of H’ ions was real, being due to the added cations and not to dilution effects. After the mixtures were made up, they were stoppered and allowed to stand in the dark for 18 hours before making the potentiometric measurements. Potentiometric Measurements. The glass electrode was employed in the determination of H’ ion concentration. A bulb was blown on the end of a piece of glass tubing of suitable composition.1 The bulb was filled with I h’ HC1 and allowed to stand in distilled water for several days before using. I n making the e.m.f. measurements a vacuum tube potentiometer constructed by Mr. M. F. Roy according to the specifications of Stadie2 was employed in conjunction with a type K Leeds and Northrup potentiometer and a Hartman and Braun moving-coil galvanometer. The glass bulb containing I N HCl was suspended in the weighing bottle containing the sol-electrolyte mixture. The tip of the salt bridge from the saturated calomel electrode making contact with the HCl, was drawn out to a fine point and plugged with cotton. To minimize the coagulation of the sol by the saturated KC1, a piece of glass tubing 3 cm long having one end covered with cellophane and containing distilled water, was attached to the tip of the bridge arm making contact with the liquid in the weighing bottle. This tube was removed and flushed out after each measurement. To calculate the pH from the e.m.f. measurements the glass electrode was standardized with M / z o potassium acid phthalate, which has a pH of 3.97.3 The solution was placed in a weighing bottle and all contacts were made in the manner outlined above. The pH of the sol-electrolyte mixture is given by e.m.f. of phthalate-e.m.f. of mixture. the formula: pH of mixture = 3.97 0.0001 o84T , . In subsequent tables the e.m.f. of the potassium acid phthalate solution is designated “E,”. This method of standardization eliminates errors due to inequalities between the calomel electrodes and to liquid-liquid contact potentials. Other standard buffers of lower pH were frequently run as controls.
*
B.
Variation in Hydrogen Displacement with Concentration of Sol. Sols of different concentrations were investigated to find a satisfactory one for simultaneous measurements of hydrogen displacement and cation adsorption. It would be expected that with sols prepared under similar conditions, the most concentrated sol would show the greatest hydrogen ion displacement. This view was confirmed by the following experiment: As2S8 sols I, 11, and 111, containing 1 . 7 0 , 4.38, and 10.80 g/l, respectively, were titrated with 0.004 N CaSOa. Most of the data are recorded in Table I, but to conserve space a portion of the observations are omitted after the p H value attained a constant value. All of the H‘ ion displacement data are shown graphically in Fig. I. McInnes and Dole: J. Am. Chem. SOC., 52, 29 (1930). Glass used was from Corning Glass Co., their No. 015. * J. Biol. Chem., 83, 477 (1929). a Clark and Lubs: J. Biol. Chem., 25, 506 (1916); Clark: “The Determination of Hydrogen Ions,” 3rd. ed. 485 (1928).
HARRY B. WEISER A S D GEORGE R. GRAY
2x00
TABLE I Titration of As& Sols with 0.004 N Cas04 ( I O cc of sol. Total volume 15 cc. 30'C) Cc of 0.004 Ai CaSOa added
[HI X IO^ in solution
7r
millivolts
PH
Sol I 170.2
164.1 164.3 163.6 164.9 163.9 164. I 164,~
1.70
g/l E,,
3.98 3.88 3.88 3 .87 3.89 3.88 3.88 3.88
IH] X
106
displaced
169.6 mv 10.5 13.2 13 . z 13.5 12.9 13.2 13.2 13.2
W [Ca],X
105
added
=
0.0
2.7 2 , f
3.0 2.4
0.0
13.3 26,; 40.0
2.7
53.3 66.7 80.0
2.7
133.3
2.7
Sol I1 4.38 g/l E, = 182.5 mv 154.9 151.5 149.5 150.1
149.7 148.9 I49 2 149.0 '
3.51 3.45 3.42 3.43 3.42 3,41 3.42 3.41
Sol I11 10.80 g/l E, 0.0
125.4
0.5
3.0
119.; 118.1 116.9 116.2 115.9 115.4
j .O
1IS.j
I .o
I .5 2 . 0
2.5
3.30 3.21 3.18 3.16 3.15 3 , I4 3 I4 3.14 '
30.9 35.5 38.0 37.2 38.0 38.9 38.0 38.9
0.0
0.0
4.6 7.1 6.3 7.1 8 .o 7.1 8 .o
13.3 26.;
133.3
= 16j.4 mv 0.0 50.1 61 .; 11 . 6 66. I 16 . o 69.2 19.I
13.3 26.7 40.0
40.0
53.3 66.7 80.0
0.0
fO.8
20.7
72.5
22.4
72.5
22
.4
80.0
72.5
22.4
133.3
53.3 66.7
The displacement follows a smooth course similar to t'he usual adsorption isotherm. I t is of interest to note that the maximum displacement of hydrogen ions occurs before the precipitation concentration (approximate value indicated by a vertical line cutting the curve) is reached. A similar behavior was reported with BaCI2 by Rabinowitsch and Fodimann,' who confirmed Briggs" 2. physik. Chem., 154 A, 255 (1931). 2
J. Phys. Chem., 34, 1326 (1930).
THE COAGULATION OF SOLS BY ELECTROLYTES
2801
Cc 0.004 N Cas04 Added FIG.I Hydrogen Ion Displacement Curves for As& Concentrations
Sols of Different
observation that the cataphoretic migration velocity attains a minimum value well below the precipitation concentration of BaClz but not of ALCls. Except with low concentrations of electrolyte and concentrated sols, the amount of hydrogen displaced is distinctly less than the amount of calcium added. The total displaced hydrogen is 20%, 2 1% and 3 1y0of the total hydrogen in the filtrate after coagulation of sols I, I1 and 111, respectively.
C. Displacement of Hgdrogen Ions and Adsorption of Cations I . Observations on As283 Sol I V Titration Experiments. The above experiments having indicated that a sol containing IO g/l would show a satisfactory displacement of hydrogen ions, a sol of approximately this composition (10.64g As& per 1) was prepared and titrated with the chlorides of barium, strontium, calcium, aluminum, and ammonium, employing the method previously described. Most of the titration data for the salts with multivalent cations are given in Table I1 and all of the data are shown graphically in Fig. 2 .
2 802
HARRT B. W E I S E R AND GEORGE R. G R A l
TABLE I1 Titration of Aspsg Sol IV with 0.004 it' Solutions of BaClz, SrCI2, CaClz a n d A1C13 ( I O cc. of sol. Total volume I j cc. 3oOC) R Equivalents of Solution [Hl,XIO^ IH] x 1 0 3 added cc
millivolts
PH
in
solution
displaced
X
electrolyte IO:, added
I. BaClz Eo = I 5 7 4 m v 0.0
120.8
0.5
IIj,S
I .o 1.5
113 .0 110.7 108.3
2 .O
.o
2.5 3.0
107
5 .O
106.3
0 0
2 .o
118.0 113, I 110.7 107,j 105.8
107.j
11. SrClz E, 0.5 I .o
1.5 2.5
105. I
3.0 5 .o
104.8 103.8
0.0
112.6 I08 .0 105.2 102.3
1.5
2
.o
100.5
2 . j
TOO. 0
3.0 5 .O
99 5 98.4
0.0
125. x 119.0 115.9 113.9 113.2 112.8 113.4 113.1 112.3 111.6 111.3
'
3.37 3.29 3.25 3.20 3.17 3,16 3.15 3.14
IV. A1C13 E, 0
. .5
I .o 1.5 2 .O
2.5
3 .o 3.5 4.0 4.5 5.0
'
57.5 63. I 69.2 70.8 72.5 74.1
3.36 3,25 3.20 3.17 3.16 3,15 3.16 3.15 3.I4 3.13 3 ' I3
0.0
8.8 13 .8 19.4 25.5 27.1
28.8 30,4
0.0
13.3 26.7 40.0 53.3 66.7 80.0 133.3
= 154.j m v
3.36 3.28 3.24 3.19 3.16 3,15 3.14 3.13
111. CaCll E, 0.5 I .o
43 7 52.5
3.36 3.28 3.24 3.20 3.16 3.15 3 ' '4 3 ' I3
43.7 52.5
57.5
8.8 13 .8
64.6 69.2
25.5
70,8 72.5
74, I =
0.0
148.6 m v 42 7 51.3 56,2 63.I 67.6 69.2 jo.8 72.5 '
20.9
27.1
28.8 30.4 0.0
0.0
13.3 26.7 40.0 53.3 66.j 80.0 133.3 0.0
8.6 13.5
'3.3 26.7
20.4
40.0
24.9 26.5
53.3 66.7 80.0 133.3
28. I
29.8
= 162.0mv
43.7 56.2 63.I 67.6 69.2 70.8 69.2 70.8 72.5 74.1 74.1
0.0
0.0
12.5
13.3 26.7 40.0 53.3 66.7 80.0 93.3 106.7
19.4 23.9 25.5 27.1
2 5 , s 27.1
28.8 30.4 30.4
120.0
133.3
2803
THE COAGULATION OF SOLS BY ELECTROLYTES
1
I
I 2
Cc 0.004
4.
6
N f l e c t r o l y t e Added FIQ.2
Titration Curves of AstSa Sol IV. A with BaC12, B with SrCla, C with CaCls, and D with AlCl3
Since 5 cc of 0.004 N NH&l did not bring about the coagulation of I O cc of sol, the experiments were continued with 0.01 N , 0.04 N , and 0.5 N solutions. Coagulation occurred on the addition of 1.5 cc of 0.5 N solution. The abridged data are given in Table I11 and Fig. 3. For the same concentration of 0.004 N solution the displacement is only one-half as great as with electrolytes having multivalent cations. The increase in the amount displaced after the first z cc is qilite gradual.
2804
HARRY B. WEISER AND GEORGE R. GRAY
TABLE I11 Titration of As& Sol I V with N H X l ( I O cc. of sol. Total volume 15 cc. jo°C) Cc of NHaCl added
&-
volts
IH1.x io5 PH
in
solution
[HI X io5 displaced
],"I
x 105
added
Series I. E, = 150.5 mv 0.0
114.6
3.37
42.7
0.0
0.0
0.004N 0.5 I .o
114.2
3.36 3.33 3.30 3.27 3.36 3.23
43.7 46.8 50.1 53,7 54.9
I
.o
13.3 26.7 40.0 66.7 80.0 133.3
112.0
1.5
110.1
2 , s
108.5 108.2 106. I
3.0 5 .o
58.9
4.1 7.4 I1
.o
12.2
16.2
Series 11. E, = 157.4 mv .o
120.7
3.36
43.7
0.0
0.0
2.0
113.2 111.6
3.23 3.21
58.9 61.7
15.2 I8 . o
133.3
109, I 108.7
3.17 3.16 3.15
67.6 69.2 70.8
23.9
0
0.0IiV
3.0
0.04iV
2.0
o.gN
1.5 5 .o
108.I
25.j
27.1
200.0
533.3 jooo.0 16666.7
The discussion of these results will be delayed until the adsorption results are given.
Cc 0.004 N NH4CI Added FIG.3 Titration Ciirves of Ae2Sa Sol IV with NH&I
THE COAGULATION OF SOLS BY ELECTROLYTES
2805
Adsorption Experiments. The measurement of the adsorption of cations above the coagulation point was carried out as follows: To IOO cc of sol contained in a wide-mouth bottle, varying amounts of water and 0.01N electrolyte were added, the final volume being 150 co. After standing stoppered in the dark for 18 hours, the precipitate was matted down by centrifuging and the supernatant liquid filtered through a small filter paper to remove A 100 cc portion was taken for analysis. The barium and traces of As& strontium were determined as sulfates and the calcium was precipitated as oxalate and weighed as oxide. For standardization, a sample in which water was substituted for the sol was subjected to the same treatment. The results are given in Table IV and shown on the respective titration curves in Fig. 2. The amounts adsorbed are not quite equivalent, the Ba“ ion being
TABLE IV Adsorption of Ba, Sr, and Ca during the Coagulation of AszSs Sol IV (100 cc of sol. Total Volume 150 cc) Co 0.01 N solution added
mg adsorbed
BaClz
BaSOr
milliequivalents per g As&
Ba
20.0
11.3
0.091
22 .o
11.3
24.0
11.3
0.091 0.091
SrCln
Sr
Equivalents 105 adaorbed
X
Ba 64 ’ 5 64.5 64.5
Sr
Equivalents
X IO^ added
Ba 133.3 146 ’ 7 160.0
Sr
24.0
SrSOc 8 .o 7.9 7.9
CaClz
CaO
Ca
21
.o 22 .o
2.4
0.081
57.1
140.0
2.4 2.4
0.081
57.1
146.7
24.0
0.081
57.1
160.0
21 . o 22
.o
140.0
0.081
58 .o 57.3
0.081
57.3
160.0
0.082
Ca
146.7
Ca
taken up in slightly greater amount’ than the Sr” and Ca”. The sol was exhausted before the observations with A1 could be made. Later results with another sol show that the adsorption of A1 is about the same as that of Ba at a concentration 47% lower than the Ba concentration. Discussion of Results. A comparison of the H ion displacement curves with multivalent ions, Fig. 2, reveals a marked similarity in form. The curves with Ba and Sr are almost identical while the displacement with Ca is distinctly less and with A1 greater a t low concentrations, than that of Ba. With the divalent ions the displacement curve flattens out before complete coagulation takes place, and the cataphoretic velocity reaches a constant minimum value below the precipitation value of the electrolyte? This may be con1
Cf. Weiser: J. Phya. Chem., 29, 955 (1925).
* Brims: J. Phya. Chem., 34, 1326 (1930).
2806
HARRY B. WEIGER AND GEORGE R. GRAY
nected with the relatively slow course of the coagulation in the immediate region of the precipitation value. If sufficient time were allowed, coagulation would probably take place at electrolyte concentrations which give a constant minimum cataphoretic velocity and a constant maximum displacement of H'ion. With A1C13, which precipitates in appreciably lower concentration than the chlorides of the divalent metals, the maximum displacement of hydrogen ion occurs slightly above the precipitation value while the cataphoretic velocity is still on the decrease.'
Cc 0.004 N E- lec t r o lyte Added FIG.4 Hydrogen Ion Displacement Curves of As& Sol IV with Cations of Varying Valence
The displacement curves for cations of varying valence are shown in Fig. 4. l h e marked displacing action of the Al, especially at low concentrations, is in line with the higher precipitating power of A1C13, while the relatively weak displacing action of NH, is in accord with the relatively low precipitating action of NH1C1. An analogous behavior was observed with positive sols.2 The curves in Fig. z show that in the so1 under investigation, somewhat more than one-third of the total H in solution after coagulation was displaced from the particles, t,he remainder being measurable potentiometrically in the original sol. In all cases a portion of the H was derived from H,S in the intermicellar solution; but even if the solution were saturated with HZS, this would account for only about 10% of t,he H'ion concentration. The adsorption of the several cations was approximately twice as great as the hydrogen displaced. This means that a portion of the adsorbed cation corresponds to H measurable potentiometrically in the original sol. The adsorption was from 14% to zzyo less than the total H in solution after coagulation instead of being equal to the H'ion concentration, as found by Whitney and Ober. 1
Loc. cit. Weiser: J. Phys. Chem., 35, I , 1368 (1931).
2807
T H E COAGULATION OF SOLS BY ELECTROLYTES
11. Observations on As& Sol V As a check on the observations recorded in the previous section, some experiments were carried out with a sol containing 20 g As&&per liter. Tztration Experiments. Titrations were made exactly as previously deA portion of the results are scribed with the chlorides of Al, Ba and "4. given in Table IV and all of them, except for high concentrations of NH4Cl, are shown graphically in Fig. 5. TABLE IV Titration of As& Sol V with AlCla, BaC12 and NH4Cl ( I O cc of sol. Total volume I j cc. 3oOC) Cc of solution added
I. 0.0
0.004 N 0 . 5 I
.o
1.5 2 .O
2.5 3.0 4.5
5 .O
[Hi X
7r
millivolts
94 1 90. I 87.4 859 85.3 84,4 83.5 82.4 82.7 '
E,
10;
in solution
PH
= 148.0
3.07 3 .OI 2.96 2.94 2.92 2.91 2.90 2.88 2.88
mv 8.5 9.8
I1
.o
11.5 I2
.o
12.3 12.6 13.2 13.2
Equivalent of electrolyte displaced X 1 0 4 added
[HI x io4
0.0
1.3 2.5 3.0 3.5 3.8 4.1 4.7 4.7
0.0
1.3 2.7
4.0 5.3 6.7
8 .o I 2 .o
13.3
11. BaClz Eo = I47.4mv 0.0
0.004N
0 .j I
.o
1.5 2 .o
2.5
85.5
3 0 4.0
84.2 82.9 82.2 81.4
5 .O 0 OIX
93.4 90.8 87.9 87.3 87. I
3.0 3.5
81.5
3.07 3.03 2.98 2.97 2.96 2.94 2.92 2.90 2.88 2.87 2.87
8.5 9.3 10.5 10.7
0.0
0.8 2
.o
0.0
1.3 2.7
.o
2.5
11.5
3.0 3.5 4.1 4.7
4.0 5.3 6.7 8 .o 10.7 13.3
5 .O
20.0
5.0
23.3
0.0
0.0
I1
12 .o
12.6 13.2 13.5 13.5
2 . 2
111. NH&l E, = 148.3 mv 0.0
o.004N 1.0 2 .O
3.0 j .O
o .04 N
I
o.jN
3.0 0.75 3 .o
.o
94.5 92.3 90.8 89.2 88.3 85.9 84.1 83.6 83.4
3.07 3.04 3 .OI 2.99 2.96 2.93 2.90
2.89 2.88
8.5 9.1 9.8 10.2 I1
.o
11.7
12.6 12.9 13.2
0.6 7.3 1.7 2.5 3.2 4.1 4.4 4.7
2.7
5.3 8.0 13.3 26.7 80.0 250.0 IOO0,O
2808
HARRY B. WEISER AND GEORGE R. GRAY
Adsorption Experiments. The adsorption of A1 and Ba was determined by the procedure previously described except that in the case of 81, 2 5 0 cc of sol were used and the final volume was 375 cc. A 3 2 j cc sample was taken for analysis and after evaporation to j o cc, the A1 was precipitated with NHdOH and weighed as A1203. It seemed impractical t o attempt to determine the adsorption of NHa' ion from change in concentration since the percentage
CC 0.004 N € lectrolyte Added FIG.5 Titration Curves of As2S3 Sol V. A with AlC13, B with BaCh, and C with NH&l
change is so slight compared with the amount required for coagulation. The adsorption data are given in Table V and in Fig. j. Discussion of Results. As would be expected, the titration and adsorption data on the stronger sol V exhibit relationships which are quite similar to those with the more dilute sol IV. The H' ion concentration of the stronger sol is approximately twice that of the weaker but the total hydrogen ions displaced during the coagulation of the stronger sol is only approximately 1.7 times as great as with the weaker sol. This means that relatively more of the H' ions are measurable potentiometrically in the stronger sol than in the weaker. Of the total H' ion concentration in the supernatant liquid after coagulation with
2809
THE COAGULATION OF SOLS BY ELECTROLYTES
TABLE V Adsorption of A1 and Ba during the Coagulation of As& Sol V Cc 0.01 N solution added
AlC13 45 .O
mg adsorbed
(With AhOa
250
milliequivalents per g AszSa
Equivalents Equivalents 104adsorbed X 1 0 4 added
X
cc sol. Total volume 375 cc) A1 A1
A1
7.1
0.083
11.1
I2 .o
50.0
7.2
0.085
11.3
55 .O
7.0
0.082
I1
13.3 14.7
.o
(With BaC12 IOO cc of sol. Total volume Ba Ba BaS04 BaC12 11.1 19.4 0.083 33 .o 19.4 0.083 11.1 35 .o 36.0 18.7 0.080 10.9
150 cc)
Ba 2 2 .o
23.3 24 .o
Cc 0.004N E lectrolyte Added FIQ.6 Hydrogen Ion Displacement Curves of AszSs Sol V with Cations of Varying Valence
divalent ions, approximately 36% was displaced from the colloidal particles as compared with 20% from sol I, 21’7~ from sol 11, 31% from sol I11 and 41% from sol IV. Obviously the amount of displaceable H on the colloidal particle is not determined exclusively by the concentration of the sol but depends on the particle size and the conditions of formation. The displacement curves for the three types of salts are shown together in Fig. 6. Again it will be noted that at lower concentrations aluminum has a greater displacing power than Ba, and NH4 a much weaker displacing power than the multivalent ions.
2810
H A R R Y B. WEISER A S D G E O R G E R . ORA1
The adsorption of A1 and Ba in equivalents is of the same order of magnitude in the region of the precipitation value for the respective salts, but the precipitation value of the Ba is 87% greater than that of AI. From the slope of the H' ion displacement curves it appears that the adsorption of Ba is distinctlyless than the adsorption of AI at a concentration of BaCIzcorresponding to the precipitation value of AIC13. Indeed, the adsorption of aluminum is practically complete up to, and including, the precipitation concentration of AlCI,, whereas only one-half the precipitation concentration of Ba is adsorbed. Constitution of Arsenic Trisulfide Sol and the Mechanism of the Coagulation Process Since precipitated As& is quite hydrous, the composition of the body of the particles may be represented by the general formula Z A ~ Z S B . Y Hthe ~O, value of z and li being determined by the condition of formation and the age of the sol. The particles are negatively charged and since the chief electrolyte in the intermicellar solution is HZS, it is probable that the inner portion of the double layer surrounding the particles is chiefly the HS' ion derived from the primary dissociation of this acid. The anions of arsenious acid or of a thioacid may constitute a portion of this inner layer. The outer portion of the double layer is a diffuse layer of H' ions, many of which have sufficient osmotic pressure that they influence the H electrode and can be measured potentiometrically. The constitution of a colloidal particle is represented diagrammatically in Fig. 7 . The hydrogen ions within the dotted circle are so firmly held by the attraction of the adsorbed HS' layer that they do not influence the H electrode. Others with a higher kinetic energy, shown beyond the dotted circle, are measurable potentiometrically with the glass electrode. A small portion of the H' ions :\re derived from the dissociation of HZS in the intermicellar solution. On adding an electrolyte such as BaClZto the sol the stronglyadsorbed Ba" ions enter the outer layer, displace H' ions, and take up a position relatively closer to the inner layer than the H'ions, as shown diagrammatically in Fig. 8. This decrease in thickness of the double layer and increase in charge density of the outer layer, result in a lowering of the charge on the particles to the point where collisions result in partial coalescence and agglomeration. Because of the stronger adsorption of the AI"' ions the thickness of the double layer is less Than with divalent Ba. Accordingly, less AI needs to be adsorbed to reduce the chnrgc to the coagulation point and precipitation takes place with lower concentrations of AI salts than of Ba salts. With the weakly adsorbed ions such as XH4', a relatively high concentration is necessary to increase the charge density in the outer layer and so lower the charge to the coagulation point. Because of the weaker adsorption of NHI than of AI and Ba, the displacement of H by S H afrom the portion of the outer layer represented within the dotted circle is much less than with a n equivalent amount of the multivalent ions.
THE COAGULATION OF
aoLs BY
ELECTROLYTES
H5’
tr H’ H H
H’
FIG.7 Diagrammatic Representation of the Structure of the Micelle in the original AssS3 Sol
” FIG.8 Diagrammatic Representation of the Structure of the Micelle in As& Sol after adding some BaClg
2811
2812
HARRY B . WEISER AND GEORGE R. GRAY
The adsorption of multivalent cations at their precipitation value is much greater than the hydrogen displaced. The reason is obvious since a part of the adsorbed ion takes the place of hydrogen in the outer portion of the double layer and so increases the charge density of the outer layer. The adsorption is in general less than the total concentration in the filtrate after coagulation. I t was a fortuitous combination of circumstances that gave Whitney and Ober a filtrate having the same amount of H' ions as the amount of metallic ions adsorbed. Summary The following is a brief summary of the results of this investigation. I . Supplementing the classical observations of Whitney and Ober on adsorption during the coagulation of As&& sol, the glass electrode has been employed in a study of the changes in H' ion concentration during the stepwise addition of precipitating electrolytes to the sol. 2. The H' ion displacement curve follows a smooth course, similar in form to an adsorption curve. The displacement is relatively greater at lower concentrations and attains a maximum at or below the so-called precipitation value of the electrolytes. 3 . In the sols investigated the total amount of displaced H' ion varies from 20% to 4070 of the total H' ion concentration of the supernatant liquid after coagulation of the sol. The amount of displaced H' is not determined exclusively by the concentration of the sol but depends on the conditions of formation of the sol as they influence the size and form of the particles. 4. In all cases, the adsorption of precipitating ions is appreciably less than the total H' ion concentration of the filtrate after coagulation. The two values will be equal only under a fortuitous combination of conditions. 5. The amount of displaced H' is less than the amount of precipitating ion adsorbed. A part of the adsorption corresponds to H' in the diffuse outer layer of the particles, which is measurable potentiometrically in the original sol. 6. The order of displacing power of the several chlorides is: A1 > Ba, Sr > Ca >NHa. This is likewise the order of the precipitating power of the several electrolytes and the order of adsorption of the several cations at concentrations below the precipitation value. 7. A diagrammatic representation of the constitution of the colloidal particles is given to account for the above results; and an adsorption mechanism of the neutralization process leading up to coagulation, is outlined. The Race Instatute, Housfon, Texas.
