T H E h1E:CHANISAI O F T H E MUTUAL COAGULATION PROCESS. 11. B Y HARRY B . W E I S E R A S D THOMAS S. CHAPMAS
In a recent communication' dealing with the mutual coagulation process it was demonstrated that, when a given series of positive sols is arranged in order of the optimum concentration for mutual coagulation on mixing with negative sols, the order of the positive sols may vary widely with different negative sols. I t was shown further, that the zone of complete mutual coagulation of two sols of opposite sign may be quite narrow or very broad. The reason for the observed behavior is that the precipitating power of positive sols for negative sols is not determined exclusively by the charge on the colloidal particles. Other factors which influence the process are: ( I ) the interaction between stabilizing ions, (2) the presence in the sols of precipitating ions which were not removed in the process of preparation and purification, and (3) mutual adsorption of colloidal particles, that is independent of their charge. With reference to the first of these factors it was shown that complete mutual coagulation is not due in general t,o interaction with the consequent removal of the stabilizing electrolytes of the oppositely charged sols; but that this may be important in certain cases. The effect of unadsorbed electrolytes in the sols as a factor in the process will be considered in this paper. The third factor, mutual adsorption of particles that is deTermined by their specific nature rather than by their surface charge, must await consideration until some way is found for the quantitative evaluation of the mutual adsorption process. I n the preparation of hydrophobic sols which owe their charge to the preferential adsorption of ions, the peptization is accomplished by the presence of an electrolyte containing a relatively strongly adsorbed anion or cation. In general, the intermicellar solution will contain more or less of the peptizing electrolyte, the amount depending on the method of preparation and the extent of purification. The effect of such electrolyte impurities on the mutual coagulation process has been pretty generally overlooked although it may be a very important factor especially if one of the ions is multivalent. For example, if the excess alkali ferrocyanide used in the preparation of a negatively charged copper ferrocyanide sol is not removed completely and this sol is used to coagulate positive sols, the ferrocyanide ion in the intermicellar solution will exert a precipitating action on the positive sols that is independent of but supplements the mutual coagulation of the oppositely charged particles. In the earlier experiments the attempt was made to minimize this effect as far as possible by working with well purified sols. J. Phys. Chem., 35, j43
(1931).
HARRY B . WEISER A N D THOMAS S. CHAPMAN
714
I n the following experiments, sols of varying purity were employed and the effect on the width of the mutual coagulation zone and the composition of the coagulum was determined.
Experimental Preparatzon of Sols. Positive hydrous ferric oxide sol and negative sols of arsenic trisulfide, hydrous stannic oxide and copper ferrocyanide were prepared by the same procedure outlined in the paper to which reference has been made.' Hydrous alumina was formed by adding ammonia to a solution of aluminum chloride short of precipitation and dialyzing in the hot.? The purity of the sols was regulated by the time of dialysis. In the case of hydrous alumina and ferric oxide, eight or ten liters of sols were prepared and divided into a number of equal portions which were dialyzed simultaneously and continuously in the hot. From time to time samples were removed for analysis and the dialysis was stopped when the desired Cl/oxide ratio was attained. Method of Procedure. The procedure followed in locating the zone of complete mutual coagulation was essentially the same as that described in the first paper. The sols were mixed always in such amounts that the total volume was I O cc. The limits of the zone of mutual coagulation were first determined within I cc, after which a series of tests were made which located the limits of the zone to within 0.1 cc of the respective colloids. The mixing of the two portions of oppositely charged sols was made as rapidly as possible and the mixture was allowed t o stand 30 minutes, after which it was centrifuged for I minute and the supernatant solution examined for complete coagulation. This was readily detected visually with colored sols but with colorless ones a portion of the sol was pipetted off, treated with an electrolyte having a multivalent precipitating ion and examined after a few minutes for the presence or absence of a floc. A typical example of the data obtained in a given case is shown in Table I.
TABLE I Mutual Coagulation of Fez08 Sol and Cu2Fe(CN)6 Sol Fe201 2 . 7 7 g/l
Cu2Fe(CN)e 4.18g/l
CC
CC
3 2
6 8 6 7 5 0 4.9
3 3 5 0 5 1
Observations
Excess Cu2Fe(CN)Bsol Complete mutual coagulation Complete mutual coagulation Excess Fez08 sol
Range of complete mutual 70FelOscoagulation by weight
24
6 to 39 9
Ferric Oxzde Sol and Negatzve Sols. Some preliminary experiments were carried out which indicate the effect of electrolyte impurities in the oppositely charged sols on the width of the mutual coagulation zone. Two samples of ferric oxide, stannic oxide, and 1 J. Phys. Chem., 35, 543 (1931). *Cf. Weiser: J. Phys. Chem., 35, I370 (1931).
T H E MECHANISM O F T H E MUTUAL COAGULATION PROCESS
715
copper ferrocyanide sols were prepared. One set of samples was relatively impure and is designated “impure”; the other set of samples, having been subjected to prolonged dialysis, was relatively pure and is referred to as “pure.” The zone of mutual coagulation for all the possible combinations was determined and the results summarized in Table I1 and Fig. I . Since the terms “pure” and “impure” as applied to the sols are relative, the results ’P“..’
‘I-,.,..
0
X Fe,O,
by Weight FIG.I
Influence of Purity of Sols on the Range of Mutual Coagulation.
TABLEI1 Mutual Coagulation of Positive and Negative Sols of Varying Degrees of Purity Positive sols
Negative sols
Range of corn lete mutual coaguPation % FelOI by weight
Fez03“pure” I . 5 2 g/l FezO3“pure” Fez03 “impure” I . 30 g/l Fez03“impure”
c u ~ F e ( c N“impure” )~ 2 . 8 5 g/l CUzFe(CN)6 “pure” 2 . 5 0 g/l CuzFe(CN)G“impure” c u ~ F e ( c N“pure” )~
30. I to 6 6 . o 40.6 to 5 8 . 7 30.1 t 0 4 3 . 0 2 8 . 6 to 4 2 . 4
Fez03“pure” Fez03‘‘pure” Fe203“impure” Fez03“impure”
Sn02 “impure” 3.23 g/l SnOz “pure” 2 . 11 g/1 SnOz “impure” SnOz ‘(pure”
33.4 to 5 5 . 2 65.0 to 6 5 . 5 14.2to30.0 2.6 to 3 . 1
are qualitative; but they show the marked effect of purity of sol on the position and width of the zone of mutual coagulation. I t will be noted that in general the zone is broadened by the presence in the intermicellar liquid of an electrolyte which contains an ion of high precipitating power. Observations were next made with ferric oxide sols of varying degrees of purity as indicated by the C1/Fez03 ratio, and the negative sols of copper ferrocyanide, stannic oxide, and arsenic trisulfide. I n every case the negative
HARRY B . WEISER AND THOMAS S. CIIAPMAN
716
sols were prepared in such a way that they would be expected to be quite pure. Actually, however, in the nature of things the stannic oxide sol was the only one in which the intermicellar liquid was practically free from electrolyte. I n the preparation of this sol only a small amount of NHaOH was required for peptization of the hydrous oxide. The excess was removed by boiling and further purification was accomplished by dialysis. The As& sol was washed with hydrogen to remove the excess HnS. Unfortunately, the extent of purification of the sol is limited by the fact that As& hydrolyzes. Hence if the HzS content is reduced too low the intermicellar solution is contaminated with H3AsO8. The copper ferrocyanide sol was purified by prolonged dialysis in the cold but it still contained some of the peptizing electrolyte as evidenced by the fact that the sol gave the Prussian blue test promptly on adding a trace of ferric salt. The results of a series of experiments carried out as indicated in Table I are summarized in Table I11 and reproduced graphically in Figs. 2 , 3, and 4.
TABLE I11 Coagulation of Positive Fe2O3 Sols of Varying Purity and Xegative Sols Positive F e p 0 3sol Purity g Fe203/1 C1/Fe2O3
0.0615 0.0480 0.0402 0.0169 0.0071 0.0027
2.77
Cu2Fe(CN)B4 . 1 8 g/l
,, ,,
3 .OI 2 .83 3.04 3.03 2 ' 94
1,
0.0615
2.77
A s ~ 3S, ~ 8 0 g/l
0,0480
11
0.0071
3 .OI 2.83 3.04 3.03
0.0027
2.94
0.0402
0.0169
0.0615 0,0480 0.0402 0.0169 0.0071 0.0027
2.77
3 .OI 2.83 3.04 3.03 2.94
Range of complete mutual coagulation 70 FelOl by weight
Kegative sol
,, ,, ff 1,
,
24.6 26.2 24.2 30.8
to to to to
39.9 42.0 39.5 45.2
5 1 . 5 to 5 8 . 5 5 2 . 3 to 5 6 . 1 5 1 . 7 to 5 6 . 0
5 0 . 5 to 5 7 . 4 5 0 . 9 to 6 1 . 0 j 4 , 9 to 93 s
,,
1 5 . 6 to 2 8 . 2 1 4 . 3 to 2 9 . 6 1 6 . 8 to 3 3 . 2
,, ,, ,,
3 4 . 8 to 5 1 . 4 . 4 to 6 2 . j 7 6 . 0 to 77.0
SnOz 3 . 5 1 g/l
,,
These data demonstrate conclusively that as the electrolyte content of one sol is diminished the precipitating action of the other becomes more pronounced. Thus with the several negative sols the width of the zone of complete mutual coagulation remains almost constant with decreasing purity of the ferric oxide sol until the C1/Fe203 ratio is approximately 0 . 0 2 a t which
T H E MECHANISM O F T H E MUTUAL COAGULATION PROCESS
% Fe,O,
b y Weight FIG.2
Zone of Mutual Coagulation with Fe20s sols of Varying Purity and As2& Sol.
7. F e l C 3 by Weight FIG.3 Zone of Mutual Coagulation with Fez03Sols of Varying Purity and .4s2Sl Sol.
7. Fe,C, by Weight FIG.4 Zone of Mutual Coagulation with Fe201Sols of Varying Purity and S n 0 2 Sol.
7'7
HARRY B. WEISER AND THOMAS S. CHAPMAN
7 18
point the ultrafiltrate from the sol is free from chloride ion (HC1). The width of the zone then increases or decreases with further decrease in the Cl/FezOJ ratio depending on the purity of the negative sol. With the copper ferrocyanide sol containing a small excess of KaFe(CN)Band the As2S3sol containing a small amount of H3As03 or HzS, the zone widens out in the direction of greatly increasing the relative amount of Fez03in the precipitate. This is probably due chiefly to the precipitating action of the electrolyte in the intermicellar solution. With the highly pure stannic oxide sol, on the other hand, the zone of complete mutual coagulation is narrowed down as the purity of the ferric oxide sol increases. This means that if one mixes two oppositely charged sols the ultrafiltrates from which contains little or no electrolyte, the zone of mutual coagulation will be narrow. I n other words, if the mutual electrical neutralization of oppositely charged particles is the prime factor determining mutual coagulation, the zone will be relatively narrow.
Hydrous Alumina Sol and Negative Sols. Observations similar to those with hydrous ferric oxide were made with alumina sols of varying purity. The data which are summarized in Table IV and shown graphically in Figs. 5 , 6, and 7 confirm those obtained in the preceding section. It should be noted that the alumina sols were weaker, less pure, and had a higher precipitating power for the negative sols than
TABLEIV Coagulation of A1203 Sols of Varying Purity and Negative Sols Positive AlrOa sol Purity g ALOs/l 0.152
0.134 0.0865 0.0740 0.0647 0.0490 0.I52
0.134 0.0865 0.0749 0.0647 0.0490 0.152
0.134 0.0865 0.0740 0.0647 0.0490
0.391 0.381 0.446 0,454
Negative sols
Cu2Fe(CN)B4.18g/l JJ
7.2
0.408
0 .5 0 5
0.408 0.391 0.381 0.446 0,454 0405
0.408
4.4to 6.8 4.7 to 7 ' 5
to
12.0
8.2 to 14.5 8.8to 28.8 10.6to 2 5 . 7
0.505
0.391 0.381 0.446 0.454
Range of corn lete mutual c o a g d t i o n % AliOa by weight
3.8 g/l I,
2 . 9 to
4.9 to 7.4to 8.3 to 9.8to 11.6to
6.4 6.9 14.0
17.5 19.8 14.9
6 3 to 9.7 6.9to 18.3 1 1 . 7 to 16.6 14.6to 19.5 1 5 . 0 to 22.4 18.4to 25.9
T H E MECHANISM O F T H E METVIAL COAGULATION PROCESS
r6 AI,O,
by Weight FIG.5
Zone of Mutual Coagulation with A1,03 Sols of Varying Purity and Cu?Fe(CN)6Sol.
% Al,03 by Weight FIG.6 Zone of Mutual Coagulation with Al,Os Sols of Varying Purity and As& Sol.
719
HARRY B . WEISER A N D THOMAS S. CHAPMAK
FIG.7 Zone of Mutual Coagulation with AI2O3Sols of Varying Purity and SnOl Sol.
the ferric oxide sols. The purest alumina sol had a Cl/oxide ratio of 0.0490 while the purest ferric oxide sol had a Clloxide ratio of 0.00~7.Moreover, the ultrafiltrates from all the alumina sols contained chloride. As in the case of the experiments with ferric oxide sols the zone of complete mutual coagulation widened in the direction of a higher percentage of alumina in the precipitate as the chloride content of the sol diminished. The zone did not narrow down sharply with stannic oxide and alumina as it did with the highly purified ferric oxide but with purest alumina the width of the zone was distinctly less with stannic oxide than with either arsenic trisulfide or copper ferrocyanide.
Summary and Conclusions The following is a brief summary of the results of this investigation: I . A study has been made of the effect on the mutual coagulation process of the presence of unadsorbed electrolytes in the sols. 2 . The zone of complete mutual coagulation was determined for positive hydrous alumina and ferric oxide sols of varying purity and well purified negative sols of arsenic trisulfide, stannic oxide, and copper ferrocyanide. 3. The presence of free electrolytes in the oppositely charged sols has a marked effect on the width of the zone of mutual coagulation and the composition of the coagulum. 4. The zone of mutual coagulation is quite narrow if the intermicellar solution in both sols contains little or no free electrolyte or if the free electrolyte which is present contains ions of low precipitating power. In such
T H E MECHASISM O F T H E MUTUAL COAGVLATIOS PROCESS
721
cases the electrical neutralization by mutual adsorption of oppositely charged colloidal particles is the prime factor in determining the mutual coagulation. j. The zone of mutual coagulation is broad if one of the sols, say the positively charged sol, is quite pure and the negatively charged sol contains an electrolyte with an anion of high precipitating power. In this case the precipitating anion in the intermicellar solution exerts a coagulating action on the positive sol that is independent of but supplements the coagulation by mutual adsorption of oppositely charged particles. 6. When a given series of positive sols is arranged in the order of optimum concentration for mutual coagulation on mixing with negative sols, the order of the positive sols will be the same for different negative sols only in case the mutual coagulation process results primarily from mutual adsorption of oppositely charged colloidal particles. 7 . Since the nature and amount of electrolyte impurities in oppositely charged sols has a marked effect on the width of the zone of mutual coagulation and the composition of the coagulum, it is not surprising that the order of precipitating power of a series of positive sols for different negative sols usually exhibits considerable variation. The Rice Institute, Houston, Texas.
