MECHANISM OF TYE COAGULATION OF SOLS BY ELECTBOLYTES. V
SULFUR SOL HARRY B. WEISER
AND
GEORGE R. GRAY
Department of Chemistry, The Rice Institute, Houston, Texas Received December 18, 1984
A mechanism of the coagulation of sols by ele.ctrolytes based on adsorption and potentiometric measurements on hydrous oxide sols and arsenic trisulfide sol has been proposed in earlier papers (11, 13). While these papers were being prepared Bassett and Durrant (1) carried out experiments on Raffo’s sulfur sol which indicated that the precipitating cation carried down by the coagulum was always exactly equivalent to the stabilizing polythionate ion originally held by the sulfur micelles. From this it was concluded that the coagulation is the result of a stoichiometric reaction between the soluble sodium salt of a sulfur polythionate complex and the precipitating electrolyte. Somewhat later Bolam and Bowden (2) and Bolam and Muir (4) determined the change in hydrogen-ion concentration on adding electrolytes to sulfur sol, using the same procedure as we employed with arsenic trisulfide sol. These observations led to the conclusion that the same fraction of hydrogen ion is liberated from the micelles of a given sulfur sol a t the coagulation point, whatever the nature of the cation of the coagulating salt. Since neither of the above conclusions reported by the English workers would be predicted from the mechanism of the coagnlation process proposed by us, we have extended our observations on the nature of electrolyte coagulation to sulfur sols. ADSORPTION EXPERIMENTS
Since the observations of Bassett and Durrant with the various cations were carried out on different sols, it was not possible to compare directly the amounts of the several cations taken up at the precipitation concentration. In order t o make such a comparison the adsorption of a series of cations of varying valence was determined on samples of the same sol.
Preparation of sol Raffo (8) sulfur sols were prepared according to the procedure of Bassett and Durrant: 83.6 g. of sodium thiosulfate crystals was dissolved in 50 cc. 1163 T E E J O U R N A L OF PHYSICAL CEEMIETRY, VOL. X X X I X , N O .
9
1164
HARRY B. WEISER AND GEORGE R. GRAY
of water and added to 65.5 cc. of concentrated sulfuric acid, keeping the temperature below 25°C. After adding 50 cc. of water, the mixture was heated to 85°C. until all sulfur dioxide was driven off. The mixture was allowed to stand a t room temperature for three hours, the coagulated sulfur collected on a Buchner funnel, repeptized in cold water, and coagulated with 5 N sodium nitrate solution; the repeptization and coagulation were repeated three times. Since two or three liters of sol were required for each series of experiments, several portions were prepared and mixed after the first coagulation with sodium nitrate, subsequent coagulations and peptizations being carried out on the whole sol. After the second precipitation with sodium nitrate, the coagulated sulfur was peptized in a small amount of water and about one-fifth of this sol was used to wash the TABLE 1 Adsorption by sulfur sol of cations at the precipitation concentration
1
N O , O F SOL
CC. OF SOL
Final 1 -Origins
Sr Bs,
[ 150 ' 11. 2.75 g. per liter; age 60 days
~
~
200 175 175 400 400 400
Nd
Ba Sr Ca
AI Nd Th
5 . 8 7 2.00 5.32 1.66 3.68 0.20
0.73 0.71 0.68
0.45 1.30 3.73 0.15
0.74 0.72 0.64 0.59 0.52 0.41
2.30 3.07 5.28 1.60 1.34 1.39
0.08
0.38
coagulum obtained by the next precipitation with sodium nitrate. I n this way the amount of sodium nitrate retained by the sulfur was reduced. Four samples of sol of varying concentrations and ages were used in the following experiments :
Adsorption at the precipitation concentration The precipitation concentration of electrolyte for the sulfur sol was taken as that concentration which was just sufficient to give a clear supernatant solution after the mixture of sol and electrolyte had stood for twenty-four hours. For each trial a 10-cc. portion of sol was employed. The electrolytes were the chlorides of the respective metals. The precipitation concentrations in milliequivalents per liter are given in the fourth column of table 1.
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COAGULATION O F SOLS BY ELECTROLYTES
The adsorption measurements were made as follows: To a definite volume of sol contained in a wide-mouthed bottle, varying amounts of water and electrolyte were added to bring the mixture to some constant volume. The mixtures were stoppered, shaken, and allowed to stand for twenty-four hours. The coagulated sulfur was matted down by centrifuging, and the supernatant liquid filtered through a small filter paper to remove any suspended sulfur. The concentration of the cation in the filtrate was then determined. For standardization, samples in which water was substituted for the sol were subjected to the same treatment. Barium and strontium were determined as sulfates; the calcium was precipitated as oxalate and weighed as oxide; and the neodymium, aluminum, iron, and TABLE 2 Adsorption bv sulfur sol of cations at concentrations above the . precipitation values . __
CC. OF SOL
NO. OF SOL
:ATIOP 4DDEt
CATION CONCENTRATIONS MILLIEQ. P E R LITER
higins
__ -
111. 7.3 g . per liter; age 5 days
I
I :: '
100 100
IV. 2.5 g. per liter; age 80 days
,
100 100 100 100
IDSORPTION MILLIEQ. P E R QRAM OF S U L F U R
Final -
Sr A1 Nd Th
.o ,12
6.12 7.73 3.31 8 . 3 2 3.72 9.46 4.72
0.56 0.66 0.68 0.74
Ca Sr Ba ,AI Nd Th
4.00 4.00 4.00 4.00 4.00 4.00
0.48 0.58 0.71 0.77 1.16 1.21
2.68 2.42 2.04 1.88 0.82 0.68
---
thorium were determined as oxides. All experiments were carried out in duplicate and the average is reported. The adsorption data a t the precipitation values of the respective electrolytes are summarized in table 1.
Adsorption above the precipitation concentration Observations similar t o those described above were made (1) on a freshly formed sol with electrolyte concentrations approximately twice the respective precipitation values, and (2) on an aged sol with the respective electrolytes in equal concentrations well above the precipitation value. The results are summarized in table 2. Discussion of results Contrary to the belief of Bassett and Durrant, it is quite evident from the above results that the amounts of the several cations taken up by the
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HARRY B. WEISER AND GEORGE R. GRAY
particles of sulfur sol are not equivalent either at, or above, the respective precipitation concentrations. The order of precipitating power for sol I was found to be Nd > Ba > Sr, and the adsorption (at the precipitation value) in milliequivalents per gram of sulfur is in the order Sr > Ba > Nd. However, if we compare the relative amounts adsorbed we find that 94 per cent of the added neodymium is adsorbed as compared with 69 per cent of barium and 65 per cent of strontium. In the case of sol I1 we find the order of precipitation to be Nd > T h > A1 > Ba > Sr > Ca. The occurrence of the lyotropic sequence in the coagulation of sulfur sols has been pointed out by a number of investigators (12,2,5). The adsorption is in the order Ba > Sr > Ca > A1 > Nd > Th. Again if we compare the percentage amounts of the added cations adsorbed, we find the order to be the same as that of the precipitating power, namely, T h > Nd > A1 > Ba > Sr > Ca. As will be observed, tetravalent thorium does not precipitate in lower concentration than trivalent neodymium and the relative adsorption is less. Thorium chloride solution is hydrolyzed to a considerable degree, and we are dealing in this case with mutual coagulation of thorium oxide sol and sulfur sol as well as ionic interchange. The order of adsorption well above the precipitation concentration, as shown in table 2, is the same as the order of precipitating power, namely, T h > Nd > A1 > Ba > Sr > Ca. As will be shown in the subsequent experiments on hydrogen-ion displacement, cationic interchange is completed at or somewhat above the coagulation concentration. Further adsorption occurs, however, and may even lead to a reversal of charge and peptization in the case of thorium. The percentage adsorption from mixtures having the same electrolyte concentration is in the same order as the precipitating power. Thus in sol IV the relative amounts taken up are: thorium, 83 per cent; neodymium, 80 per cent; aluminum, 53 per cent ; barium, 49 per cent ; strontium, 40 per cent; and calcium, 33 per cent. Similar observations were made on a Selmi (9) sol prepared by the interaction of hydrogen sulfide and sulfur dioxide (see page 1168); but the adsorption values with the more hydrophobic Selmi sol were smaller than with the Raffo sol. Summarizing briefly, we note that the adsorption values of the precipitating ions are not equivalent a t the precipitation concentration. The relatively more marked adsorption of the cations of higher valence brings about precipitation a t lower concentrations. TITRATION EXPERIMENTS
Preparation of sots The displacement of hydrogen ion from the colloidal particles by the stepwise addition of various metallic chlorides was observed with both a
'
1167
COAGULATION OF SOLS BY ELECTROLYTES
Raffo and a Selmi sol. The former was prepared by a method similar to that used by Bolam and Bowden: 25 cc. of concentrated sulfuric acid was added slowly from a dropping funnel to 100 cc. of saturated sodium thiosulfate. The mixture was stirred vigorously with a mechanical stirrer and the temperature was maintained between 20 and 25°C. by surrounding with ice water. After the reaction was complete the mixture was cooled to 0°C. and treated with 100 cc. of saturated sodium chloride solution. The coagulated sulfur was separated by centrifuging and heating to 85°C. with 100 cc. of water. Coagulation and repeptization were then repeated TABLE 3 Titration of a Raffo sulfur sol with alkali halides AMOUNT ADDED I N EQUIVA,ENTEI 104
ELECTROLYTE
.'
NaCl.. ........................
LiC1.. .......................
.
Na > Li. A similar lyotropic sequence is observed in the case of the divalent ions (figure 1B). Although the displacing powers of barium and strontium are almost the same as are the precipitation values, that of calcium is distinctly lower. Comparing the displacement curves with the straight line representing the cation added, it will be noted that a t low concentrations about 60 per cent of the cation added is effective in displacing hydrogen ion, the effectiveness falling off with increasing con“centration until the coagulation value is reached, above which no further displacement occurs. With the cations of higher valence, figure lC, the order of displacing power and coagulating power is the same: Nd )r T h > A1 > Cr. The reason thorium is less effective than neodymium has been discussed in connection with the adsorption experiments. I n contrast to the behavior of the alkaline earth cations, thorium, neodymium, and aluminum coagulate the sol before the maximum displacement of hydrogen ion is attained, indicating further ionic interchange after the charge on the particles has been reduced to the point of coagulation. Contrary to the view of Bolam and Muir, the displacement of hydrogen ion is not the same a t the coagulation point. This is emphasized in figure l D , which shows that much less hydrogen is displaced a t the precipitation value of neodymium than a t the precipitation value of barium. Less neodymium ion needs to be adsorbed to lower the charge to the critical coagulation value and accordingly less hydrogen is displaced at the precipitation value of neodymium than of barium.
Potentiometric titrations on the Selmi sol Observations were made on the more hydrophobic Selmi sol similar to those on the Raffo sol. To conserve space the tables of titration data are omitted, the results being shown graphically in figure 2. The displacement curves with the monovalent ions, figure 2A, are the same in form as those obtained with the Raffo sol. With the divalent ions
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HARRY B . WEISER A N D GEORGE R. GRAY
the order of displacing power and coagulating power is: Ba > Sr > Ca. The difference in displacing power is slight at lower concentrations, becoming more marked as the precipitation concentration is approached. With the ions of higher valence the curves are quite similar in form, the order of displacing power being: Th > Nd > Cr > Al. This is the same as the order of precipitating power, with the exception that the chlorides of chromium and aluminum precipitate in almost the same concentration. It will be noted that at low concentrations of thorium and neodymium the hydrogen ion displaced is almost equivalent to the cation added
ELECTROLVTE
ELECTROLYTE
FIQ.2.
ADDED
ADOED
TITRaTION O F SELMI SOL WITH
CLECTROLYTE
ELECTROLYTE
ADDED
ADDED
ELECTROLYTES
(straight line). As in the case of the Raffo sol, the displacement of hydrogen ion is far from complete a t the precipitation concentration of the strongly adsorbed multivalent ions. For these ions which precipitate in low concentrations that are not far apart, the displacement of hydrogen at the precipitation value of the several ions approaches equivalence in accord with the observations of Bolam and Muir. On the other hand, with ions whose precipitation values are not close together, such as barium and neodymium, the displacement of hydrogen a t the precipitation value of the former is much greater than at the precipitation value of the latter
COAGULATION O F SOLS BY ELECTROLYTES
1173
(cf. figure 2B), just as was found in the case of the Raffo sol, and for the same reason. Comparing the observations on the two sols we find the hydrogen-ion concentration of the original Selmi sol to be over four times that of the Raffo sol, whereas after coagulation the ratio in the supernatant solution was reduced to 5 : 2 . Moreover, the effect of the valence of the cation is more marked in the aged Selmi sol than in the Raffo sol. The aging of the former causes it to assume the properties of a typical hydrophobic sol in which the valence and lyotropic influence of the cations is more marked than in the more hydrophilic Raffo sol.
3. DIAQRAMMATIC REPRESENTATION OF THE CONSTITUTION OF A PARTICLE OF COLLOIDAL SULFUR BEFORE AND AFTER THE ADDITIONOF BARIUM CHLORIDE
FIQ.
MECHANISM OF THE COAGULATION
In sulfur sols prepared by the decomposition of sulfur compounds, the stabilizing electrolytes are polythionic acids, such as pentathionic and hexathionic, which may be represented by the general formula HzS,Oe. The colloidal particles consist essentially of a nucleus of colloidal sulfur which adsorbs the polythionate ions so strongly that the latter constitute the inner portion of the double layer surrounding the particles and give the particles a negative charge. The outer layer is a diffuse layer of hydrogen ions. The constitution may be represented diagrammatically as shown in figure 3A. The hydrogen ions of the outer layer are osmotically active and have a range of movement determined by the electrostatic field due to the residual SxOe- - on the surface. Some of the hydrogen ions, shown beyond the dotted line, have sufficient osmotic pressure so that they can be measured potentiometrically. Others are so firmly held by the SxOe- layer that they do not influence the hydrogen electrode.
1174
HARRY B. WEISER AND GEORGE R . GRAY
On the addition of an electrolyte, an exchange takes place between some of the hydrogen ions held by the S,Os- - and the cations in the external liquid. The theory of the diffuse double layer indicates that coagulation occurs when the potential on the particles has fallen below a critical value, as a consequence of the contraction of the double layer by the coagulating ion. The more strongly adsorbed metal ions displace hydrogen ions and take up a position relatively closer to the inner layer, as indicated with barium ion in figure 3B. Monovalent cations are weakly adsorbed and bring about a much smaller displacement of hydrogen ions than an equivalent amount of multivalent ions. Trivalent cations are more effective than divalent, for example, less Nd+++ than Ba++ needs to be adsorbed to reduce the potential to the coagulation point, and precipitation takes place with lower concentration of neodymium salts than with barium salts. Since less neodymium than barium needs to be adsorbed to reduce the potential to the coagulation point, less hydrogen is displaced from the outer layer at the precipitation value of neodymium than of barium ion. The displacement of hydrogen by cations a t the precipitation value approaches equivalence only in case the respective precipitation values are relatively close together. For cations of the same valence, a definite sequence is shown in the flocculating power; thus the order is K > Na > Li, and Ba > Sr > Ca. Voet and Balkema (10) showed that when different ions of the same valence have the same concentration in the medium, the smaller ions have the smaller concentration in the double layer. As the magnitude of the contraction of the double layer depends on the concentration of the cations, it is evident that the larger ions have the greater effect and show therefore the greater decrease in the electrokinetic potential and the greater flocculating power. SUMMARY
The following is a brief summary of the results of this investigation: 1. The adsorption of thorium, neodymium, aluminum, barium, strontium, and calcium during the coagulation of Raffo sulfur sols has been determined at the coagulation concentration and at higher concentrations. Contrary to the results of Bassett and Durrant, the adsorption values of the various cations are not equivalent a t the respective precipitation concentrations. 2. At the precipitation concentration, the order of adsorption of the several cations expressed in milliequjvalents per gram of sulfur is in inverse order to the precipitating power. However, if one compares the fraction of the precipitation value adsorbed, the adsorption is in the same order as the coagulating power.
COAGULATION OF SOLS BY ELECTROLYTES
1175
3. The order of adsorption above the precipitation value is the same as that of the precipitating power: T h > Nd > A1 > Ba > Sr > Ca. 4. The displacement of hydrogen ions during the stepwise addition of electrolytes to both Raffo and Selmi sols has been followed potentiometrically, using the glass electrode. 5. The hydrogen-ion displacement by the several cations is in the same order as the coagulating power. 6. Equivalent amounts of hydrogen ion are not displaced at the precipitation value of the several cations, as claimed by Bolam and Muir. The displacement approaches equivalence only when the precipitation values are of the same order of magnitude. 7. With cations of the same valence, the order of hydrogen-ion displacement as well as coagulating power is related to the size of the ions. At the same concentration, the largest ion produces the greatest displacement. Thus we find the order to be as follows: K > N a > Li, and Ba > Sr > Ca. 8. A mechanism has been proposed to account for the above-mentioned phenomena of adsorption and displacement of hydrogen ion which accompany the processes of potential reduction and coagulation. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
BASSETT AND DURRANT: J. Chem. SOC.1931, 2919. BOLAM AND BOWDEN: J. Chem. SOC.1932, 2684. AND BOWDEN: J. Chem. SOC.1932, 2684. Cf. BOLAM BOLAM AND MUIR:J. Chem. SOC.1933, 1022. Cf. BONVARLET: Rev. g6n. colloides 8, 300 (1930). MACINNES AND DOLE:Ind. Eng. Chem., Anal. Ed. 1, 57 (1929). NEIDLE:J. Am. Chem. SOC. 38, 1270 (1916). RAFFO:Kolloid-8. 2, 358 (1908). SOBRERO AND SELMI:Ann. chirn. phys. [31 28, 210 (1850); J. prakt. Chem. 49, 417 (1850). VOETAND BALKEMA: Rec. trav. chim. 62, 371 (1933). WEISER:J. Phys. Chem. 36, 1, 1368 (1931). WEISERAND CUNNINQHAM: J. Phys. Chem. 33, 301 (1929); Colloid Symposium Monograph 6, 319 (1928). WEISER AND GRAY:J. Phys. Chem. 36, 2178, 2796 (1932).
