COAGULATION O F COLLOIDAL SOLUTIONS BY ELECTROLYTES : INFLUENCE O F CONCENTRATION OF SOL m
BY
E. F. BURTON
A N D MISS
E. BISHOP
I. Introduction The phenomenon of the coagulation of colloidal solutions by the addition of various electrolytes has occasioned more work probably than any other phase of colloidal study. As a result of early experiments by Schulze,’ Linder and Picton,2 and others and later work by Freundlich3 and his co-workers, accepted qualitative laws have been laid down. ( I ) The coagulation seems t o depend only upon the ion in the electrolytic solution which bears a charge opposite in sign to that borne by the colloidal particles. (2) The activity of any such ion in any electrolytic solution depends very greatly on the valency of the ion. The efficiency of any electrolytic ion is expressed by a quantity called the coagulative power which is defined as follows: To a given volume of colloidal solution is added a quantity of electrolyte just sufficient to produce coagulation of the particles ; if the molecular concentration of the electrolyte in the total volume of the mixture is c (now usually expressed in millimols per litre), then I / G is called the coagulative power of the given electrolyte on the given sample of colloid. It has been known for many years that this coagulation is an electrical phenomenon and it is well established that in coagulation the particles of the sol are wholly or partially discharged. One would naturally expect that the coagulating power of an ion would depend upon its valency, i. e., upon its electrical charge and at first sight one might conclude that the coagulating powers of uni-, di-, and trivalent ions might be in the ratio of I : z : 3 . The experimental results referred to above pointed, however, t o a cpmplex relation between the coagulative powers of these ions. Averages of powers
702
E . F. Burton aiid fiIiss E . Bishop
of uni-, di- and trivalent ions gave ratios of the order
I
: 30 :
1000.
Attempts have been made to see if such experimental numbers are given by any reasonable theories. Such theories have been suggested by Whetham4and Robertson3 which lead to the expression I : x : x2 as the above ratios, where x is a constant ; an entirely different theory suggested by Freundlich3 arrives a t the ratios 1’’ : 2” : 3”, where 1%is ‘a constant. For certain values of x and n these two formulae may be made to approximate to the experimental ratios I : 30 : 1000. In spite of the support given to such formulae by experimental results, this law of the dependence of coagulating power on valency, which has been called the Schulze-LinderPicton law, has been very seriously questioned. (See Bancroft,6 etc.) A very impartial summary of the present experimental evidence has been recently given by Wo. Ostwald.’ In the following table taken from the latter paper are collected the coagulating powers with regard to arsenious sulphide sol of various electrolytes with uni-, di- and trivalent cations, the active coagulating ions since the particles of arsenious sulphide are negatively charged. As is quite apparent from this table, the coagulating power of univalent (or divalent or trivalent) ions cannot be said to be the same. However, leaving out the single inorganic salt thallium sulphate and the complex organic salts quoted from Frkundlich, there is a distinct break between the univalent and the divalent powers. The same can hardly be said for the transition between divalent ions and trivalent ions. There is this to be said, however, comparing the results for these two classes of ions given by Linder and Picton, the values of C for the trivalent ions are all quite below the value for the divalent ions. OstwaldYand others are inclinded to throw over the Schulze-Linder-Picton law entirely. In making this sweeping condemnation of the former view these authors seem to have lost sight of one other variable, namely, the concentration of the colloidal solution on which coagulation experiments have been done.
Coagulatioiz of Colloidal Solutiom by Electrolytes
703
TABLE I Coagulation of Arsenious Sulphide C = millimols per litre necessary to cause coagulation Univalent ions ~~
Electrolyte
Schulze' C
Linder and Pictonz
c
-
'/3
'/2
&So4
K2 oxalate KNOs '12 Na2SO4 KI
c
-
Acetic acid Oxalic acid '/2 HzS03 K3 citrate K acetate '/2 Li2SOA LiN03 LiCl '/4 K4Fe(CN), Na acetate
Freundlich
124.4 109.0
-
123. I
-
l/2
104.7 137.4 102.2
117.0
-
110.8 (97 ' 9) -
49.5
-
- 73.9 73.9 73.9 62.9 IO1 . o
'./2
H2SO4
"03
HCl HI HBr Guanidinnitrate 2! ' T1,SOI Strychninnitrate Anilinchloride p-Chloranilinchloride Morphinchloride Neufuchsine
109.0 103.5 95.8 92.4 57.5 58.7 57.5 56.0
-
I
.60
-
-
42.3
5 1 .o
-
30. I
-
30.8
-
16.4
-
8.0 2 .j 2
I .08
0.425
0.114
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' E . F. Burton and Miss E. Bishop
TABLE I1 Divalent ions Electrolyte
Schulzel
c
-
Linder and Picton2 C
Freundlich C 0.810
2.10
-
2.02 2.02 I .96 I .68 '
1.65 .60 1.52 I .42 I,37 1.34 I .31 I
ci.685 0.649
-
1.31 I .31 I .29 I .29 1.29 I .23 I . 23 1.18 I . 14 I . 14 I .OI
-
0.954 0.924 0.911 0.599 0.322 0.225
,
. -
-
0,635 0.691 0.717 0.687
-
0.642
-
Coagulation of Colloidal Solutions by Eloctrolytes
705
TABLE I11 Trivalent ions Electrolyte
Schulze' C
0.316 0.123
-
0. I 1 2
-
0.090
-
0.141 0.077 0.063
-
Linder and Picton2 C
I
C
Freundlich
0.216 0.154
-
0.136 0.080
0.074 0,074 0.074
0.062
-
0. I 0 2
0.092 0.040
-
0.040
Results given below suggest that the definition given above for the coagulating power of an electrolyte with regard t o a chosen colloidal solution, say arsenious sulphide, has no meaning as stated, because the power varies widely with the concentration of the disperse phase of the sol itself. 11. Experimental Work
The extremely small concentration of the trivalent ions is so remarkable t h a t the problem of finding how the coagulative power of a given electrolyte depends on the concentration of the colloid suggested itself. The results given by these experiments show quite remarkable differences in the action of uni-, di- and trivalent ions as the concentration of the colloidal solution is altered. Three examples of aqueous colloidal solutions were used, Schulze does not give these numbers but in the table C has been calculated from Schulze's results. Linder and Picton merely give relative results taking aluminium chloride as unity. Ostwald recalculates Linder and Picton's results by making the values of C lor potassium chloride the same in the first two columns.
706
E. F . Burto% and M i s s E . Bishop
viz., gum mastic sol, Bredig copper sol and arsenious sulphide sol. ( I ) The gum mastic sol was prepared by adding 2 5 cc of a strong alcoholic solution of gum mastic to one litre of distilled water. The conductivity of this stock mastic solution was 3 X IO-? a t 20' C. (2) Bredig copper sol: this was made in the ordinary way by arcing between pure copper terminals held below the surface of distilled water. The conductivity of this solution was about 8 X IO-^ at 20' C and the concentration of the stock solution varied from 0 . 3 2 to 0.36 gram of copper per litre. (3) Arsenious sulphide sol: prepared in the ordinary way by bubbling hydrogen sulphide gas through a solution of white arsenic. The sol was freed from excess of hydrogen sulphide by bubbling air through it. The conductivity was about I X IO-? at 20' C and the concentration of the stock solution varied from 2 . 1 3 to 2.8 grams per litre for different samples. The method of procedure was the same in each case. Preliminary experiments on a sample of given concentration fixed the amount of a given electrolyte necessary to produce coagulation. Then into each of five test-tubes were placed 30 cc of the given sample of sol and different numbers of drops of the electrolytic solution of known strength near the proper coagulating amount, were added to the different tubes, additional drops of purest distilled water being added in each case so as to bring the final volume up to the same for each testtube. This was done for each concentration of the colloidal solution and after repeated trials the smallest amount of electrolyte which would cause the colloid to settle even after a considerable interval of time, and leave the liquid clear above, was determined. All of the solutions were treated in the same way as far as possible; each was shaken slightly after the electrolyte was added and the water used for dilution for a set of concentrations was of the same conductivity. The weak solutions as a rule took longer to settle so all of the
Coagulation of Colloidal Solutions by Electrolytes
707
solutions were allowed to stand some days before final examination. TABLE IV Gum Mastic Colloidal Solution Coagulating concentration of electrolyte in millimols per litre
-e
%6
Ei
;i;
0 +
R
c, a,
Percentage concentration in terms of original sol sample
Conductivity of original sol.
Gi
2O0C
IOO
5 0 33.3
25
16.6
IO
6.6
j
3.3
12.5
---_________--
.oIj.OIZ.009.006 - I 2.88 X 10-5.043.030.024.021 Alum. __--------inium I1 3.79 X I O + .036.024 - . oj~.012 .009 - .006.006.003 sulphate .043 -.OS0 - . o I ~ - . o o ~ - G i 3 . 1 0 X 10-~.061 _ _ _ _ . _ _ _ _ _ _ _ _ - -
TABLE V Gum Mastic C,olloidal Solution Coagulating concentration of electrolyte in millimols per litre
Electrolyte IOO
Percentage concentration in terms of original: sol sample
.I
jo
.;_________----
Potassium chloride
205.2
1
25
I
I
16.6
I
IO
243.0 297.7 ' 464.7 631.6
6.6
-
1
j
-
-.-______----
Calcium chloride
(33%)
11.53 .
~
___
- 14.43 ~___ -_ -
13.48 , 14.43
Aluminium sulphate 1 0.0611 0.0431 0.0301 - I 0.0121 - lo.00~ Electrical conductivity of original sol = 3.10 X IO-^ a t 20'
C.
I n the cases of gum mastic and arsenious sulphide, we are dealing with negatively charged particles and consequently the metal ions in an electrolyte are the powerful coagulating ions. Series of experiments were carried out with solutions of the salts, potassium chloride, calcium chloride,
708
E . F . Burton and Miss E . Bishop
and aluminium sulphate as examples of uni-, di- and trivalent metallic ions. In Table IV results using aluminium sulphate with three different samples of mastic solution are recorded; in Table V are given the results for the three above-named salts on the same stock solution of mastic. All of these results are illustrated graphically in Figs. I and 2 .
25%
503
Fig.
I
Fig.
2.
100%
Coagulation of Colloidal Soldions by Electrolytes
709
These results are, of course, not quoted from single experiments but are typical of numbers which can easily be duplicated. In Fig. I and Table IV we have an example of the nature of the agreement found in various experiments with the same colloidal solution but different samples. In each curve, Figs. I to 4, the concentration of the disperse phase of the colloidal solution is plotted along the abscissa in percentages of the original concentration,which corresponds to the point a t the extreme right; the ordinates give the concentration of the coagulating salt calculated in millimols per litre of the sample actually under observation. In the first place, a glance a t Table V shows the great variation in the concentration necessary to produce coagulation as one goes from univalent to divalent and trivalent ions; we get here numbers such as might be suggested by the Schulze-Linder-Pictonlaw with proper values of x inserted. However, when one compares how the concentration of the coagulating salt varies with the concentration of the particles in the colloidal solution, one is struck with the diverse nature of this variation. In every case, when dealing with a trivalent coagulating ion, the concentration of the coagulating ion varies almost directly as the concentration of the colloidal particles. Absolutely contrary to the variation with trivalent ions is that given by univalent ions; as the concentration of the colloidal particles is decreased, the number of electrolytic ions necessary per cc to produce coagulation actually increases-an increase which becomes very rapid as the concentration of the colloid is decreased further and further. Intermediate between the univalent and trivalent ions are the divalent ions. In this case the necessary calcium ion concentration a t first slightly increases as the concentration of the colloidal sol is decreased and then becomes constant for further decrease in colloidal concentration. Whereas the variation in electrolytic concentration in the case of univalent ions is from 205.2 to 631.6and in the case of trivalent from 0.061to 0.009, while the corresponding variation of the
'
E . F . Burton aizd Miss E . Bishop
710
divalent concentration is from 11.5 to 14.4,we may probably be justified in treating the concentration of the divalent ion as sensibly constant. The striking concIusion from these results is that manifestly if we are to select any numbers by which to test shch a law as the Schulze-Linder-Picton law, it is of the utmost importance to take the concentration into account. From Table V we see a t once that the ratios given between the coagulating powers (CI, Cz, C,) of the various ions at maximum and one-tenth maximum concentration of the colloid are, respectively, given by 1
E
c, .. - = 1
1
205.2
: r r . 5 : 0.06 and 631.6 : 14,4 : 0.012
c 3
from which we deduce that C1 : Cz : C3 = r : 17.4 : 3333 and
I
: 41 :
52000.
TABLE VI Arsenious Sulphide Colloidal Solution Coagulating concentration of electrolyte in millimols per litre
! Electrolyte
.___.
Percentage Concentration in terms of original sol : 2.8 grms. per litre IO0
Potassium chloride Zinc sulphate
59.3 0.59'
50
25
67.8
76.3
16.6
6.6
215
----80.5
89.0
106.0
----0.738 0.738 0.738 0.738
0.886
-----
0.012 0.012 Aluminium sulphate 0.01; 0.014 0.012 Electrical conductivity of original sol = 8.45 x IO-^ a t 20' C.
0.012
In Table VI, similar results are recorded for the colloidal solution arsenious sulphide; the table gives comparable results for potassium chloride, zinc sulphate and alilminium sulphate on one and the same sample of arsenious sulphide. Fig. 3 illustrates the results of this table and shows exactly the same characteristics as the mastic curves.
Coagulation of Colloidal Solutiolis by Electrolytes
7 1I
In the case of colloidal copper since the particles are positively charged, the powerful ions are the acid radicle ions;
Fig. 3
as sources of typical mi-, di- and trivalent ions solutions of potassium chloride, potassium sulphate and potassium phosphate were used. The results are given in Table V I I ; the TABLEVI1 Copper Colloidal Solution Coagulating concentration of electrolyte in millimols per litre
Electrolyte
Percentage concentration in terms of original sol : 0.32 grms. per litre 16.6 ___-
Potassium chloride Potassium sulphate
I .83 2.14 2.14 -- - _ _ _ 0.041 0.041 0.041
_______
5
IO
___-
2.29 0.041
2.75
3.21
-0.041 0.044' _____
Potassium phosphate lo. 044 0 . 0 2 0 0.017 0.011 0.006 lo. 005 Electrical conductivity of or-ginal sol = 7.52 X IO-^ a t 2 0 ' C.
table gives comparable effects of the three named salts on one and the same sample of copper colloidal solution. Fig. 4 illustrates the results of this table, and here again we
712
E . F . Burton and M i s s E . Bishop
have results for the action of uni-, di- and trivalent negative ' ions similar t o results of the positive ions, a circumstance
Fig. 4
which points to some far-reaching, underlying reason for this curious relation between coagulating power of ions and the concentration of the colloidal solution. 111. Conclusions
As a conclusion to these experiments we may say that the coagulative power of any given ion varies with the concentration of the disperse phase of the colloidal solution according to the following laws: I. For univalent ions the concentration of ion necessary t o produce coagulation increases with decreasing concentration of the colloid-this increase being very rapid with low concentrations of the colloid. 11. For divalent ions the concentration of ion necessary t o produce coagulation is almost constant and independent of the concentration of the colloid. 111. For trivalent ions the concentration of ion necessary to produce coagulation varies almost directly with the concentration of the colloid. As has been shown formerly in the work of Neisser and
Coagulation of Colloidal Solutions by Electrolytes
7 13
Friedemann,8 Bechhold,Y Buxton and his co-workers,I o and Burton,ll on such solutions as mastic, platinum, gold and silver, there exist what have been called coagulation zones. That is, if one adds to various samples of the colloidal solution gradually increasing amounts of an electrolyte containing a coagulating trivalent ion, coagulation is produced when a certain concentration of this ion is reached and then for larger concentrations coagulation does not take place SO readily. It is found, in this latter region, that the charge on the particle has been changed, as though the particle had adsorbed an overdose of the coagulating ion and had thereby been stabilized. Larger and larger concentrations of the electrolyte again causes rapid coagulation. However, in the above experiments we can hardly be encountering this phenomenon because in all cases the coagulation observed was that which first appears, i. e., coagulation by a minimum quantity of electrolyte. By contrasting the action of univalent and trivalent ions as the colloidal solution is diluted one is driven to the conclusion that we are dealing with two widely differing phenomena. That is, the essential mechanism by which univalent ions cause coagulation must be quite different from that by which trivalent ions produce this result. Or, from another point of view, there are possibly several elements entering into the process of coagulation. One set of these elements is particularly accentuated when one deals with univalent ions, another set is predominant in the use of trivalent ions. For example, on account of the very large amount of electrolyte added for the univalent ion compared with the amount added for the trivalent ion, the electrical conductivity in the former case is very considerably higher than in the latter case. Now if electrical conductivity is for any reason a potent factor in causing coagulation-a circumstance that may very well be-the influence of this factor might easily dominate the action with univalent ions but be of quite secondary importance in the case of trivalent ions. Combined with such an effect, there might be an influence of valency quite in the
714
E. F . Burto1.z and M i s s E. Bishop
way that Whetham pictures it (or of adsorption as Freundlich pictures it) which might be the dominant feature with the trivalent ion but of secondary importance with the univalent ion. That some such combination of effects is working is suggested by the relative action of the divalent ions as the colloidal solution is diluted. The averages of the ordinates of two of the curves in Figs. 2, 3 and 4, respectively, would give curves not unlike the third curve in each of these figures. I n any of these’cases by altering the potency which we assign to the trivalent ion effect in relation to the univalent ion effect, we could actually reproduce the divalent curves. Such considerations would justify one in enunciating the following principles of coagulation : I. There are, at least, two properties of the system made up of the colloidal solution plus electrolyte which have influence in determining the coagulating power of any ion. 11.These two influences are such that they tend to counteract one another to a certain extent. 111. These influences are siich that one of them dominates the action of univalent ions, while the other dominates the action of trivalent ions. In the action of divalent ions, the two influences seem to be somewhat equalized. There are many possibilities to suggest for these influences. I n addition to electrical conductivity and valency cited above, we must remember that in all this work the influence of the ion bearing the same charge as the colloidal particle has almost always been ignored. This has often been commented on (see BancroftJ6 Ostwald’). It does seem unscientific to ignore completely this ion charged similarly t o the colloidal particle; one would suspect that it would oppose the action of the other ion. Now in dealing with the coagulating power of a univalent ion one always has present in the solution in equimolar concentration another ion of equal or greater valency, while in testing a trivalent ion, there is present in equimolar concentration another ion which is always of less valency than the ion tested. A proper appreciation of the
Coagulation of Colloidal Solations by Electrolytes
715
concomitant action of these other ions charged similarly to the colloidal particle may throw a flood of light on ’the whole process of coagulation by electrolytes. In particular, this other ion may have a peptizing effect, or its presence in solution may exert a definite effect on the adsorption of ions by the colloidal particle. An exhaustive series of experiments with a carefully selected list of electrolytes, introducing as many combinations of valency as possible, would undoubtedly be of use provided a colloid of constant concentration were used throughout. Hans Schulze: Jour. prakt. Chem., 25, 431 (1882);27, 320 (1883); 32,390 (1884). Linder and Picton: Jour. Chem. SOC.,67,63 (1895). 3 H. Freundlich: Kapillarchemie, 1909; Zeit. phys. Chem., 73,385 (1910); 79,168(1912);80,564 (1912);85,641 (1913);86,458(1914);later papers in Kolloidchem. Beihefte, 1915 and 1916. Ishizaka: Zeit. phys. Chem, 83,97 (1913). Whetham: Phil. Mag., (5) 48, 474 (1899);“Theory of Solution,” 396 ( I 902). Robertson: “Physical Chemistry of Proteins,” I 14(1918). Bancroft: Second Report on Colloid Chemistry, Brit. Ass. Adv. Sci., 2 (1919); Jour. Phys. Chem., 19,e t seq.. Wo. Ostwald: Zeit. Kolloidchemie, 26,28, 69 (1920). 8 Neisser and Friedemann: Munch. med. Wochenschr., 1 1 (1903). Bechhold: Zeit. phys. Chem., 48,385 (1904). l o Buxton and co-workers: Zeit. phys. Chem., 57, 47, 64, 76 (1907); 60,469,489 (1908);Jour. Med. Research (Boston), 20, 1 1 3 , 311 (1909). l 1 Burton: Phil. Mag., (6)12, 476 (1906);“Physical Properties of Colloidal Solutions,” 151 (1916). Department of Physics L‘niversity of Toronto J d y 80,1920
