ADSORPTION BY PRECIPITATES VI1 BY HARRY B. WEISER
The Influence of Non-Electrolytes on the Precipitation of Sols by Electrolytes Thirty years ago Bodlander’ found that small amounts of electrolytes caused rapid clarification of kaolin suspensions, whereas soluble non-electrolytes appeared to have no effect even when added in large quantities. The latter observation has been disproven by the recent investigations of Rona and Gyorgy2 using a large variety of non-electrolytes such as diphenylthiocarbamid, chloroform, tributyrin, camphor, thymol, a number of alcohols, and urethanes. The presence of every compound except those that are virtually insoluble in water, was found to increase the velocity of kaolin sedimentation. Since the effect of the non-electrolytes could not be due to the slight change in the viscosity of the medium, Rona and Gyorgy concluded that the compounds added, produced a sensitization of the suspended particles toward the electrolytes always present and thus increased the velocity of sedimentation. Such a sensitization of colloids by non-electrolytes has been observed by a number of investigators. Thus Billitzer3 found that a negatively charged platinum sol could be sensitized, discharged, or even changed into a negative sol by the addition of suitable amount>sof alcohol; Wo. Ostwald4 precipitated a silver sol with propyl alcohol; and Klein5 showed that negatively charged sols of arsenious sulphide, gold, silica, and ferric oxide were agglomerated in part by alcohols; whereas positively charged ferric oxide and silica were not. Freundlich and Rona6 observed that the addition of camphor, thymol, and urethanes to ferric oxide sol reduced the precipitation concentration of electrolytes. For example, the precipitation value of sodium chloride in the absence of non-electrolytes was 35 millimols per liter. This value was decreased to 2 7 in the presence of I O millimols per liter of camphor; and to 2 0 in the presence of 5 millimols per liter of thymol. The sensitization of sols by non-electrolytes has been attributed by Wo. Ostwald’ and Cassuto8 to a change in the dielectric constant of the medium. Freundlichg suggests that the sensitization results from the lowering of the charge on the particles by adsorption on their surface of the organic nonJahrbuch Min., 2, 147 (1893). Biochem. Z., 105, 133 (1920);cf. Quincke: Drude’s Ann., 7 , 5 7 (1902). SZ. physik. Chem., 45, 312 (1903). 4 “Grundriss der Kolloidchemie”, 471 (1909). Kolloid-Z., 29, 247 (1921). Biochem. Z., 81, 87 (1917). 7 LOC. cit. “Der kolloide Zustand der Materie”, 152 (1923). 9 “Kapillarchemie”, 637 (1922).
I254
HARRY B. WEISER
conductor which has a dielectric constant appreciably lower than that of water. It has been shown that the charge, e, on a single colloidal particle, e=
E Dr (r+d)
d where E is the potential difference of the double layer a t the surface of a spherical particle of radius T , D is the dielectric constant and d the thickness of the double layer. From this it follows that the lower the dielectric constant the lower the charge on a particle. This would mean that the amount of a precipitating ion that must be adsorbed to effect neutralization will be less and so the precipitation value of an electrolyte will be decreased as Freundlich observed with ferric oxide sol. Furthermore, as the velocity of migration, u, of a colloidal particle in an electric field is given by the expression :
u=-
EHD 4 lr rl,
a decrease in D will cause a falling off in IC. That such is the case was demonstrated by Freundlich and Rona who found for a pure ferric oxide sol, u = 0.4 X I O - ~cm/sec for a potential of one volt/cm, while for a sol containing 5 millimols of camphor per liter, u = o . ~ X I Ocm/sec -~ and for one containing 2 5 millimols thymol per liter u=0.3X10-4 cm/sec. From the slope of the adsorption isotherms, Freundlich also deduced that the decrease in precipitation value will be appreciable only for electrolytes containing univalent precipitating ions that are weakly adsorbed; and will be very slight or negligible for electrolytes containing strongly adsorbed precipitating ions. From conclusions drawn by Freundlich as a result of his investigations on ferric oxide sol, it would appear to follow that the addition to colloids of non-electrolytes having a lower dielectric constant than water, will always decrease the precipitation value of electrolytes: and that this effect will be greater the lower the dielectric constant of the non-conductor added. These deductions appear to be disproven by the investigations of Kruyt and van Duinl on arsenious sulphide sol. They found that the effect of non-electrolytes such as phenol and isoamyl alcohol on the precipitation value of electrolytes was determined by the nature of the precipitating ion; for univalent and trivalent ions the precipitation value was lowered; while with bivalent and tetravalent ions it was raised. Moreover, for any given electrolyte the change in the precipitation value seemed to be independent of the dielectric constant of the non-electrolytes employed. Kruyt and van Duin2 studied the adsorption of non-electrolytes by charcoal and found for a given electrolyte, that there was a parallelism between the effect of different nonelectrolytes on the precipitation value for colloidal arsenious sulphide and the adsorption of the non-electrolytes by carbon. However, they were unable to account for the fact that the same non-electrolyte appears t o stabilize a 1
Kolloidchem. Beihefte, 5, 1270 (1914). Cf. van Duin: Kolloid-Z., 17, 123 (1915).
ADSORPTION BY PRECIPITATES
1255
sol toward certain electrolytes and t o sensitize it toward others. The experiments to be recorded in the next section throw some light on the mechanism of the process which accounts for these apparent anomalies. Experiments with Colloidal Arsenious Sulphide
For a, satisfactory study of adsorption during the precipitation of sols, the latter must not be too dilute, otherwise the change in concentration of electrolyte due t o adsorption, will be insufficient to measure accurately. As Kruyt and van Duin did not give the concentration of the sol they employed, it was necessary, at the outset, to determine the effect of concentration of sol on the precipitation value of an electrolyte in the presence of a constant amount of non-electrolyte. The initial experiments were made with colloidal arsenious sulphide prepared by dropping a solution of arsenious oxide into water through which was bubbled a stream of carefully washed hydrogen sulphide. The sol was washed free from hydrogen sulphide by hydrogen and kept in an atmosphere of the latter gas. Precipitation experiments. The concentration of the original sol used in these experiments was 12.5 gms. per liter. From this, more dilute sols were prepared and the precipitation value of electrolytes was determined for each, both in the presence and absence of phenol. The procedure was as follows: After determining the approximate precipitation value, I O cc. of colloid was diluted in a test tube with 5 cc. of water or 5 cc. of phenol solution and allowed to stand 2 0 minutes. This was then placed in the outer compartment of a small mixing apparatus similar to that employed in earlier experiments.' In the inner compartment was placed a definite amount of electrolyte diluted to 5 cc. The electrolyte was measured with a 2 cc. pipette graduated in 0.05 cc. After mixing, the contents were poured into the test tube that held the colloid originally and the mixture allowed to stand 2.5 hours, shaking at the end of the one- and two-hour intervals. To determine whether or not precipitation was complete, the contents of the test tube were centrifuged
TABLEI Precipitation of Colloidal Arsenious Sulphide Concentration of colloid
Precipitation Values Milliequivalents per liter without phenol with phenol* KCI BaClt KC1 BaC12
I
100% (12.55
g. Per
50% 25%
12.5%
1)
75.6 81.8 83.9 86.9
1.713 1.325
1.440 I .400
44.4 50.6 53 * 1 54.4
2.288 2 , IO0
2.025 2.000
1256
HARRY B. WEISER
and a portion of the supernatant liquid examined for the presence or absence of the slight greenish opalescence which serves to detect very minute amounts of the colloidal sulphide. The results with KCl and BaC12 are given in Table I and a,re shown graphically in Fig. I . The concentration of the sols is expressed in percent, taking the most concentrated as 1oo0/0. The curves given in Table I were obtained by plotting concentration against ratio of each precipitation value for an electrolyte to that of the strongest colloid. It will be noted that the precipitation value of KC1 increases and that of RaCl2 decreases with dilution of the pure sol, a circumstance that has been considered in detail in an earlier paper’.
FIG. I
The presence of phenol lowers the precipitation value of KC1 and increases that of BaC12 for sols of all concentrations between 12.5 gms. and 1.5 gms. As2& per liter. I n both cases the “phenol” curves are slightly above those for the pure sol, showing that the percentage lowering of the precipitation value of KC1 in the presence of phenol is relatively greater than the percentage raising of the precipitation value of BaC12. Since the effect of phenol on the precipitation value is of the same general character with sols of widely varying concentrations, there appeared to be no objection to working with a sol that was sufficiently concentrated to make satisfactory adsorption studies.
If the addition of a non-electrolyte to a sol sensitizes it, one should not expect the precipitation value of BaCh to be raised as the observation discloses. I n view of this apparent abnormality, and the relative ease with which barium ion may be determined, the following experiments were carried out: It was necessary, first of all, to determine whether an accurate estimation of barium could be made by precipitation a,s sulphate in the presence of phenol and of isoamyl alcohol. That this can be done is clear from the results of the experiments recorded in Table 11. 1
Weiser and Nicholas: J. Phys. Chem., 25, 742 (1921).
1257
ADSORPTION BY PRECIPITATES
TABLEI1 Standardization of Barium Chloride BaCL approx. N/5o cc
I
ontents of solution analyzed Non-electrol yte cc
Phenol
Isoamyl alcohol 17.0 g/l
66.0 g/1
.... 50
.... .... ....
....
50
....
Water cc
Weight of Bas04
IO0
0.1162
IO0
50
0.1163 0.1158
50
0.I 162
The arsenious sulphide sol used in the following experiment was prepared as previously described; it contained 31.3 gms. AszSaper liter. The precipitation value of BaClz was determined both in the absence and in the presence of phenol and of isoamyl alcohol. For the puroose of the experiments to be described later on, the precipitation value with mixtures of BaCL and AlC13 in the presence of phenol, were also determined. In accord with earlier observations, the precipitating action of such mixtures j s approximately additive'. All values are given in cubic centimeters of solution employed. The solutions of non-electrolytes used, were approximately saturated at ordinary temperatures. The effects are therefore the maximum that could be obtained by the procedure employed. TABLE 111 Precipitation of Colloidal Arsenious Sulphide
No.
Solutions mixed that effect precipitation Total volume 2 0 cc. Colloid Phenol BaClt AlCIa 31.3 g per I 66.0 g per I N/SJ N/IOO
I
IO
0
I1
IO
5 5 5 5 5
IO
I11
IO
IV V
IO TO
HzO
3.14 4.23
...
... 3.14 2.00 I .oo
1.51 0.36 0.83 1.18
3.94 3.14
...
I .06
0.25
1.61
...
6.86 0.77 3.49 1.50 2.17 2.82
Isoamyl alcohol 17.0 g. per I
VI VI1
IO
IO
5 5
Adsorption Experiments. Adsorption experiments were carried out in the following manner: To IOO cc. of colloid was added 50 cc. of water or nonelectrolyte and the mixture placed in the outer compartment of a mixing 1
Weiser: J. Phys. Chem., 25, 665 (1921);28, 232 (1924).
1258
HARRY B. WEISER
apparatus of suitable dimenions. In the inner compartment was placed the required amount of electrolyte diluted to 50 cc. After standing 2 0 minutes the contents of the apparatus were shaken thoroughly and then transferred to a 2 5 0 cc. bottle. After standing 2 % hours the precipitate was matted in the bottom of the vessel by the aid of a centrifuge and a definite volume pipetted off for analysis. Since the solution usually contained a minute trace of sulphide, it was allowed to stand 24 hours and was then filtered through a small filter paper. The subsequent analysis was carried out as nearly as possible under the same conditions as were used in standardizing the solution. The results of a series of seven experiments (corresponding in number to those in Table 111) are recorded in Table IV and shown graphically in Figure 2. TABLEIV Adsorption of Barium Ion by Arsenious Sulphide with
Solutions mixed IOO cc. colloid contg. 31.5 g.
No.
I I1 111 IV
V
BaSOl rqmaining in I80 cc.
AS^&
N/5o BaClz cc
Phenol cc
31.40 31.40
0
42.35 42-35
50
31.40 31.40 20.00
HrO cc
Average
Barium adsorbed Grams per mole AsZS3
68.60 68.60
0.0503
0.0502
0.805
7.65 7.65
0.0749 0.0753
0.0751
0.708
50 50
15.00'
0.0531 0.0530
0.0531
0.657
50 50
21.70~
20.00
702
0.0312 0.0315
0.0314
0.538
I O . 00
50
I O . 00
50
28. 2oa 28. 2 0 3
0.0132 0.0136
0.0134
0.381
0.0680 0.0679
0.0680
0.753
0.0520
0.713
0
SO
15.001
21.
0.0501
Isoa my 1 alcohol
VI VI1
64 64
39.36 39.36
50 50
IO.
31.40 31.40
50
16. 104 1 6 . 104
50
3.60 N/IOO dc13. +++ 11.80 8.30 cc. N/IOOAlCls cc. N/IOOfilch. ' + 2.50 N/IOOAlCl,. CC.
J
CC.
IO.
0.0519 0.0521
ADSORPTION BY PRECIPITATES
I259
The results recorded in Table IV are so significant that but little comment is necessary. Comparing the results of experiment I with those of experiment 11, it will be observed that thk adsorption is less in the presence of phenol although the amount of electrolyte that must be added to effect precipitation is greater. This means that the sol is sensitized, in the sense that less barium ion must be adsorbed in order to lower the charge on the particles below the critical value necessary for agglomeration and precipitation. The fact that a higher concentration of barium must be present to cause precipitation of the sensitized sol is due to the marked influence of the adsorption of phenol on the adsorption of barium ion. This is demonstrated clearly in experiment
FIG. I
I11 in which the adsorption of barium in the presence of phenol is determined a t the concentration of electrolyte that will cause precipitation in the absence of phenol. It will be noted that the adsorption is cut down almost 20%. This cutting down of the adsorption is not due to the very low concentration of aluminum ion (0.0018N) since there is very little if any antagonistic action under these conditions. Similar results were obtained with isoamyl alcohol as shown by experiments VI and VII. Comparing experiment I11 with VII, it will be noted that the adsorption of barium a t the precipitation value of BaClz in the presence of .phenol is less than that in the presence of isoamyl alcohol. This indicates that phenol has sensitized the sol more than the alcohol, a result that might be expected from a comparison of the dielectric constants of the two compounds. It should be pointed out however, that the concentration of phenol is higher than that of the alcohol. Moreover we have no measure of the relative adsorbability of the two non-electrolytes by the colloidal particles. One
1260
HARRY B. WEISER
compound that is strongly adsorbed may have a greater effect than the same concentration of another compound having a higher dielectric constant, that is less strongly adsorbed. Attention should be called to Figure 2 which shows that the adsorption of barium below the precipitation concentration follows the well-known adsorption isotherm. Such a result was predicted by Freundlich and was indicated by some earlier observations made in this laboratory1. The truth of this important assumption is now definitely established by direct experimental observation. As already recorded the precipitation value of KC1 is lowered quite appreciably by the presence of phenol. As Freundlich pointed out2 this is probably due to the relatively flat character of the adsorption isotherm for univalent precipitating ions. With such ions the change in adsorption is usually very slight for relatively large changes in concentration ; conversely a slight decrease in the adsorption necessary for lowering the charge on the particles below the critical value will be marked by an appreciable lowering of the precipitation value. Thus a lowering of 10% in the amount that must be adsorbed, such as observed in the preceding experiment, may cause a lowering in the precipitation value of 50% or more, if no other factor enters in. As a matter of fact, the presence of phenol doubtless cuts down the adsorption of potassium ion3,a circumstance that would tend to increase the precipitation value. This effect is of course much less marked than in the case of BaClz since the concentration of potassium ion is approximately I O O times greater than that of barium ion. Moreover, the high precipitation value of KC1 results, in part, from the fact that the adsorption of chloride ion a t certa.in concentrations, is comparable to that of potassium ion4. Since the presence of phenol doubtless cuts down the adsorption of chloride ion as much or more than that of potassium ion, this amounts t o a sensitization of the sol and a consequent descrease in the precipitation value. With BaClz as precipitant, the concentration of the relatively slightly adsorbed chloride ion a.t the precipitation value is too low to have any appreciable influence. We may now consider what will be the influence of a like amount of phenol or isoamyl a.lcoho1on the precipitation value of a salt with a trivalent precipitating ion such as A1C13. On account of the steep slope of the adsorption isotherm for aluminum ion as compared with that of a univalent ion, one should expect the small sensitization of the sol to lower the precipitation value but slightly5. Opposing this, is the cutting down of the adsorption of Weiser: J. Phys. Chem., 28, 232 (1924). ‘LKapillarchemie”,638 (1922). 9 According to Lachs and Michaelis (Kolloid-Z., 9 , 275 ( I ~ I I the ) , adsorption of chloride ion from solution by charcoal is not changed appreciably by the presence of henol or isoamyl alcohol. On account of the high precipitation v a h e of potassium c d r i d e and the relatively small change in concentration of potassium ion after precipitation of arsenious sulphide sol, it was impossible to determine accurately the adsorption of potassium ion in the presence and the absence of non-electrolytes. Cf. Weiser: J. Phys. Chem., 25, 680 (1921);28, 241 (1924). 6 Cf. Freundlich: LOC.cit.
ADSORPTION BY PRECIPITATES
1261
aluminum ion by adsorbed non-electrolyte. This latter effect should also be small on account of the very strong adsorption of aluminum ion even in low concentration. The sum of these two opposing influences would be expected to give a net change in the precipitation value that is very slight or negligible. Kruyt and van Duin claimed, however, that the precipitation value of KA1(S04)zis lowered quite appreciably by the presence of phenol or isoamyl alcohol. It is impossible to say how this result was obtained; but we have been unable to confirm it by repeated observations on several different sols and on several concentrations of the same sol. A few of these observations are given in Table V. For the more concentrated sols the difference in precipitation value is too small to be detected. With very dilute sols the
TABLEV Precipitation of Colloidal Arsenious Sulphide with Aluminum Salts ~
Electrolyte
AICl3 AIC13 AIC 13 AlC13 KAI(SOJ2 KAl(S04)t KAl(S04)2
Concentration of colloid g Per 1
Concentration of phenol; milliequivalents per 1
Precipitation values milliequivalents per I without with phenol phenol
31.50
170
0.075
6.17 34.70 0.385 13.43 34.70 I.I O *
170
0.025
0.075 0.025
158
0.063
0.063
170
0.012
0.013
170
0.048
158
0.081 0.013
0.048 0.081 0.014
170
* Sol. prepared by the method of Freundlich and Nathanson: Kolloid-Z., 28,258 (1921). precipitation value appears to be slightly greater rather than less in the presence of phenol. Since the concentration of aluminum ion necessary for precipitation is so low and the adsorption so great, the amount that remains in solution after precipitation is, unfortunately, too small to detect the very slight difference in adsorption that must exist in the presence and the absence of non-electrolytes. Experiments with Colloidal Hydrous Oxides
Precipitation Experiments. Colloidal hydrous ferric oxide and hydrous chromic oxide were prepared by Neidle’s methodl and dialyzed in the hot for several days. The precipitation concentrations of various electrolytes with and without the presence of phenol and isoamyl alcohol were determined as previously described for arsenious sulphide, except that the solutions after mixing were allowed to stand quietly for 2 9 5 hours after which they were centrifuged to determine whether precipitation was complete. The results which are recorded in Table VI, indicate that the hydrous oxide sols 1
J. Am. Chem. SOC.,39, 7 1 (1907).
I 262
HARRY E. WEISER
are sensitized but slightly by the presence of phenol or isoamyl alcohol. The precipitation concentration of KC1 is lowered a small amount by the presence of a non-electrolyte while that of salts with multivalent precipitating ions is changed but little if at all. These results are in agreement with those of Freundlich on hydrous ferric oxide to which reference has already been made. From these observations we should expect phenol and isoamyl alcohol to have little influence on the adsorption of multivalent ions during the precipitation of the colloidal oxides.
TABLE VI Precipitation of Colloidal Hydrous Oxides Colloid
Electrolyte
Fed&
KC1 KzCz04 &.So4 KzCz04 &SO4
Crz03
Precipitation value milliequivalents per liter. without with with non-dec trolyte phenol isoamyl alcohol
Solutions mixed with IOO cc colloid contg. 0 . 2 g Cr2O3 3/50 KzS04 hon-electrolyte cc cc 25
25
0 0
25
isoamyl alcohol 50 50 phenol 50
25
50
25
25
37.1 0,405
40.5 0.405 0.433 0,630 0.637
36.2
....
*...
0.424
....
0.610
....
0.632
Sulphate adsorbed gms. per mole Crz03
RaSOcremaining in 170 cc. gms.
H20 cc
average
75
0.0361
75
0.0359
0.0360
5.00
25
0.0363 0.0361
0.0362
4.95
25
I
4.96
ADSORPTION BY PRECIPITATES
I 263
TABLEVI11 Adsorption of Sulphate Ion by Hydrous Ferric Oxide
with Njgo KzS04 cc 20
20
20 20
20 20
BltSOl remaining in 17.5 cc ems. average
Solutions mixed 100 cc colloid contg. 3.51 g Fe& non-electrolyte HzO cc cc 0 80
0.0296
0
80
0.0292
isoamyl a.lcoho1 50 50 phenol
30 30
0.0293
50 50
30 30
0.0294
Sulphate adsorbed gmp. per mole Fez08
0.0294
2.50
0.0293
2.49
0.0291
2.45
0.0289 0.0293
As was to be expected from the precipitation experiments, the adsorption of sulphate by the hydrous oxidee is approximately the same in the presence as in the absence of phenol and isoamyl a.lcoho1. With both sols there appears to be a slight decrease in adsorption in the presence of the non-electrolytes. While this is as it should be, the results are scarcely accurate enough definitely to establish such a tendency. Summary and Conclusions I. A study has been made of the adsorption during the precipitation by electrolytes, of negative arsenious sulphide sol and of positive hydrous ferric oxide and hydrous chromic oxide sols both in the presence and the absence of phenol and of isoamyl alcohol. The adsorption of a non-electrolyte by the particles of a sol decreases 2. the stability of the latter, in the sense that less of a precipitating ion must be adsorbed in order to decrease the charge below the critical value necessary for agglomeration and precipitation. The extent of the sensitization depends on the concentration and adsorbability of the non-electrolyte and its dielectric constant. This effect tends to lower the precipitation value of an electrolyte. The amount of the lowering is greatest for electrolytes with wea.kly adsorbed precipitating ions that precipitate o ~ l yin relatively high concentration.
3 . The adsorption of a non-electrolyte by the particles of a so1 cuts down the adsorption of the precipitating ion of the electrolyte added to produce coagulation. This effect tends t o raise the precipitation value of a n electrolyte. '
I 264
HARRY B. WEISER
4. Since the factors given in ( 2 ) and (3) have opposite effects on the precipitation value of electrolytes, it follows that, depending on the conditions, the precipitation value may be increased, decreased or remain unchanged in the presence of a non-electrolyte.
5. The adsorption of barium ion by colloidal arsenious sulphide has been determined at a number of concentrations below the precipitation value. On plotting the results of these observations, a typical adsorption isotherm was obtained.
I am indebted to Miss Charlotte Schaler, an Honors Student in Chemistry, for preparing the colloidal solutions used in this investigation and for making a number of the precipitation experiments. Department of Chemistry, The Rice Institute, Hotinton, Texas.
