THE EFFECT OF NOS-ELECTROLYTES O S T H E COAGULATION O F COLLOIDS. PART I11 Copper Ferrocyanide Sol BY SUBODH GOBIXDA CHAUDHURY AXD TIRMALA P A D A CHATTERJEE
In Parts I and 11’ it has been shown that the effect of a non-electrolyte on the coagulation of colloids by electrolyt,es depends mainly on a change in two physical factors: A change in the dielectric constant of the medium, and (I) h change in the solid-liquid interfacial tension. The effect of the (2) diminution of the dielectric constant is in general to sensitise the sol, whereas the effect of the diminution of surface tension is to stabilise it. On the basis of these considerations we have explained the apparent paradox of higher adsorbability and lower coagulation power. The considerations hold only for those cases, Ghere there i s no chemical interaction when the non-electrolyte i s added and also where the presence of the nonelectrolyte does not inJluence the specific adsorbabilities o j the ions. The present work was undertaken with a view to investigate more fully the case of copper ferrocyanide hydrosol*. The effect of adding the following substances, methyl alcohol, ethyl alcohol, urea, cane sugar, gelatin, acetic acid on the coagulating concentrat,ions of the electrolytes, hydrochloric acid, sulphuric acid, citric acid, potassium chloride, barium chloride, and aluminium chloride, have been studied. Experimental Preparation of the Sol. The sol was prepared by precipitating copper ferrocyanide by mixing equivalent amounts of CuSOi and K4Fe(CS)G solutions and washing the precipitate till it peptises. The sol, so prepared, was then dialysed till the dialysate gave neither any test for sulphate nor for ferrocyanide. K i t h this sol, equicoagulating concentrations of different electrolytes in presence of non-electrolytes were determined by noting the time, when the light from a filament lamp fed with a constant current just disappears on account of increasing turbidity of the sol. 5 C.C. of the sol were always mixed with 5 C . C . of the electrolyte. The required amount of electrolyte was taken from a burette and the volume was made up to j C.C.before mixing it with colloid. In the following tables the amount of electrolyte just necessary to produce a rate of coagulation such that the turbidity becomes great enough to render the filament invisible to the unaided eye in 3 to 4 minutes, has been expressed in millimoles per litre of the mixture of colloid and electrolyte (Le. the end concentrations have been given). The limit,ing concentrations can be drtermined with a drop, i.e. .05 cc. of electrolyte can be taken:J. Phys. Chem., 32, 1231, 1872 (1928). 2 C f . Sen: J. Phys. Chem., 29, 587 (1928).
EFFECT O F SOS-ELECTROLYTES ON COAGULATION OF COLLOIDS
24.5
TABLE I* S o r . of ccs. of methyl alcohol in IO0 cc. of the final vol.
HC1( - )
,03872 ,03765 .03j81
0
2 , j 3
IO
,02806
25
.OIjOO
Citric acid (-,
H?SO,( - )
,08642 ,07901 .07037 ,05679 ,02457
2.1619 2.0309 1.8998 1.7033 1.1792
KC1 ( - )
BaC12(-)
AlClai - )
,07009 ,06338 .0j965
.001j3
.00072
.0014?
.0007
,00139
.04jj2
.0012;
.0253j
.00103
.oo070 ,00069 .00060
I
*Th,esign ( +1 means Stabilisation. ( - ) means Sensitisation. I,
,J
(0)
means No effect.
TABLE 11* Copper Ferrocyanide sol-Ethyl
alcohol AlC13 ( - )
IOO C.C. of the final volume
,03872 .03j81 ,03194
0 2
8
5 IO
,02710
25
,01839
.08889 .08520
,07901 ,07160 ,04938
2.1946 1.9981 1.8343 1.637: 1.1792
,06710 ,05806 ,04687 ,033jj ,01342
,00144
,000708
.oo137 ,00117
,00069 ,00068 ,00066
.00092
,0006 I
.OOIZ?
TABLE III* Copper Ferrocyanide Sol-Crea Conc. of urea by wt. per IOO C.C. of the final solution 0
. 1 5 gms. .3
”
6
’’
1.j
”
,
\Yt. of sugar
added per IOO c.c per the final solution 0 .25
.5 I 2.5
gms.
’’ 11
”
HC1 ( - )
,03872 ,03562 ,03194 ,02613 ,01819
HC1
(0)
,04068 ,04068 ,04068 ,04068 ,04068
H2S04( - )
,08397 ,07284 ,06271 ,04938 ,03458
Citric acid ( - )
2.1618 1.9653 1.7688 1.6378 1.3102
TABLE IT’* Citric H$04 ( - ) acid ( 0 )
,08889 .09135 ,09259 ,09382 ,09629
KC1 (+)
,06710 ,06710 .oj1j8 ,07381 ,07426
IiCl (+)
2.2274
,06710 ,06979 ,07157 ,07381
2.2274
,077oj
2.2274 2.2274
2.2274
BaCh ( - )
.oo151 ,00149 ,00149
,00146 ,00141
BaC12 ( 0 )
,00144
.00144 ,00144 ,00144 ,00144
,00064 ,00064 ,00066 ,00066 ,00068
AlClr io,
,00069 ,00069 ,00069 ,00069 ,00069
246
SUBODH GOBINDA CHAUDHURY ASD KIRYALA PADA CHATTERJEE
TABLE V* Streneth of aceticacid added in the HC1 ( -1 final solution in terms oi normality 0
,04068
,0029N ,0058 N ,0116X ,0348K
,0405
,0402 ,03972 ,03972
HgSOa
Citric acid ( - j
,08642 ,08642 ,08642 ,08642 ,08642
2.2274 2.2274 2.1944 2.1619 2.0636
KCl ( - j
,06934 ,06934 ,06822 ,06710 ,06598
RaC12 ( - )
,00146 ,00129 ,00127
,00124 .00117
AlCY8 ( - )
,00069 .00040
,00037 .00035 ,00034
TABLE VI* Copper Ferrocyanide Sol-Gelatin W t . of gelatin added per IOO C.C. of the final solution
HC1 ( - )
,00025
gms.
.0005
”
,001
”
,04068 ,03875 ,03681 ,03390
,0025
”
.02228
0
H2SO4( - )
Citric acid ( - )
,8642 2.2274 .083yj I ,9653 ,08147 1.9653 ,07654 1.8998 ,04938 1,7033
KC 1 (-1
,06934 ,0822 ,06598 ,06039 ,04921
BaC12 ( - j
,00146 ,00146 .OOI~I
.oo161 .oo17j
.11C13 ( - j
.000708
.000708 .000724 .000724 ,000756
Discussion Mukherjee, Chowdhury and Mukherjee investigated the effect of adding the following substances: methyl and ethyl alcohols, formic, acetic and oxalic acids on the coagulation of arsenious sulphide sol by the electrolytes hydrochloric acid, sulphuric acid, potassium chloride, lithium chloride, barium chloride, and aluminum chloride. They observed that methyl and ethyl alcohols do not act in the same way for a number of electrolytes. Thus ethyl alcohol stabilises the arsenious sulphide sol against coagulation by potassium chloride, lithium chloride and barium chloride whereas methyl alcohol sensitises it against these electrolytes. Besides, a sensitising effect was observed of both alcohols against hydrochloric acid and aluminium chloride. Sulphuric acid has neither a sensitising nor a stabilising effect. We find on the other hand for copper ferrocyanide sol that both methyl and ethyl alcohols have got uniformly sensitising effect against the electrolytes x e have used as has also been found in the case of ferric hydroxide sol (Part 11). Obviously the effect varies from sol to sol and the valency of the coagulating ions gives no clue to its behaviour. The behaviour of copper ferrocyanide sol in presence of methyl and ethyl alcohols is simple, as they show uniform sensitising effect. For other substances the specific nature of the reaction becomes apparent. Vrea sensitises copper ferrocyanide sol against the three acids and against barium chloride to a slight extent. I t shows stabilisation against potassium
EFFECT O F KON-ELECTROLYTES ON COAGULATIOS OF COLLOIDS
247
chloride. Aluminium chloride has very little stabilising effect. The data with cane sugar again show the specificity of these effects. It stabilises the sol against potassium chloride and sulphuric acid and does not change the coagulating concentration against other electrolytes. The coagulating concentrations of hydrochloric acid and sulphuric acid do not change on addition of acetic acid, a result identical with that observed for arsenious sulphide sol on the addition of acetic and propionic acid. A sensitising effect is observed in all three cases against potassium chloride and alluminium chloride. This regularity breaks down again, if we compare the results when barium chloride is the coagulating agent. A stabilising effect is observed when arsenious sulphide sol is used but with copper ferrocyanide sol, acetic acid sensitises it against coagulation by barium chloride. Of the above substances all excepting urea and gelatin’ decrease the dielectric constant. Thus cane sugar also reduces the dielectric constant whereas urea increases it.* It is obvious that several factors are a t work and that it is a priori difficult t o predict what is going to happen in particular case. The specificity of stabilising or sensitising effect is to be attributed to the fact that what we observe is the net result of changes in a large number of variables which affect the coagulating concentrations. The experiments with urea are instructive for we get a sensitising effect against three electrolytes though the dielectric constant is increased by its addition. On comparing the effect of urea with that of ethyl alcohol we find that the sensitising effect of urea against the three acids is equally marked in both cases. On the basis of the views of Weiser3 one would have expected a stabilising effect of urea, for, in his opinion “the extent of the sensitisation 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.” Mukherjee, Chowdhury and Mukherjee pointed out the desirability of simultaneously measuring the charge of the colloidal particles under these conditions. Discussions on the amounts of adsorption of the coagulating ion that is necessary at the coagulating concentration are rather wide of the mark, for, the tacit assumption that’ is always made, namely, that coagulation takes place at a particular critical potential, has no basis on facts (vide Mukherjee and Chowdhurf, Our knowledge about the mechanism of coagulation of these colloids by simpler inorganic electrolytes is as yet too insufficient to enable us to enter into a reasonable speculation regarding the nature of influences at work in the phenomena dealt with in this paper. Thus it is very difficult to account for the peculiar effects of cane sugar where we find that in four cases there is no change in the coagulating concentration, unless we assume a variation in the coagulation potential. Cane sugar decreases the dielectric constant as does methyl and ethyl alcohols. It also affects the adsorption of coagulating ions5 About gelatin we have no definite data. 2Harrington: Phys. Rev. (21,8, 581 (1926:; Lattey: Phil. Mag. (6), 41, 829 (1921) J. Phys. Chem., 2 8 , 1253 (1924). See also Mukherjee, Roy Chowdhury and Chowdhury‘s paper communicated. Sen: Kolloid-Z., 38, 310 (1926).
248
SUBODH GOBISDA CHAUDHURY AND SIRhZAL.4 PADA CHATTERJEE
and also slightly diminishes the surface tension of the medium a t the concentrations used. Cane sugar changes the activity' of cations, hydrogen and barium; but it has no effect on the coagulating concentration. What is the probability that a complicated change in all these variables, which together with some others determine the coagulating concentrations, will leave the net effect equal to zero in all these cases? Urea and cane sugar stabilise the sol against the same electrolyte, potassium chloride. It is a t present not a t all definite that we have even got a clear idea of the relevant physical and chemical properties which determine the coagulating concentration. -4 change in the surface tension or rather the change in the energy associated with conglomeration of two particles which depends on capillary forces is always kept out of consideration in discussion on coagulations obviously because of the difficulties of experimentally determining them. Khile measurements of charge carried out in this laboratory are gradually forcing us to the conclusion that the cohesive forces play a very significant part in determining the coagulating concentrations we also feel that, in effect', this statement signifies a t the present state of our knowledge that all the difficulties in t,he way of explaining the facts are being referred to one group of undetermined properties of the system. We could not carry out' a sufficient number of charge measurements within the time at our disposal excepting the data given in the tables below. In the following experiments 50 C.C.of the so1 were always mixed with 50 C . C . of the non-electrolyte. The required amount of nonelectrolyte was taken from a burette and the volume was made up to so C.C.
TABLE TI1 Cataphoretic experiment' Copper Ferrocyanide Sol--Urea. Temp. 3 j" Kt. of urea added 100 C.C. of the
pel
final solution 0
. 6 gms. 1.5
*V in em. per see. per vol. per ern.
4 0 . 1 X 10-j 4 8 . 6 X IO-5 5 3 . 4 X IO-^
* Corrected for viscosity, taking the value for water a t this temp. to be unity.
It would appear from the above that not only does urea increase the dielectric constant but it also increases the rate of migration. Assuming that the rate is proportional to the density of the charge we find that the sol ought to be stabilised. The only factor resulting from an increase in the dielectric constant as such, which tends to sensitise the sol is the increased intensity of adsorption of the cation. Sugar on the other hand lowers the dielectric constant and also diminishes the rate of migration though slightly at higher concentration. From the single measurement given in Table KO.TIII, we find that the net effect of sugar a t the concentrations given is to reduce the Anderson (Trans. Faraday Soc., 19, 635 (1923-24)) concluded that as sugar increases the activity of cations this fact explains the lower coagulating concentration of these cations in the presence of sugar (Sen: loc. cit.!.
EFFECT O F SOX-ELECTROLYTES O S COAGULATIOS O F COLLOIDS
249
charge considerably but we find instead of a sensitisation a stabilisation in two cases out of six (Table IV) and this is to be attributed to a lowering of the critical potential, consequent on a decrease in the colloid-liquid interfacial tension. With other electrolytes, the sol is neither sensitised nor stabilised, which means the effect of greater adsorbability is just counteracted by lowering in colloid-liquid interfacial tension. Unless we know exactly the dielectric constants and the surface tensions of the mixtures of electrolyte and non-electrolyte, we cannot speak anything more definite. TABLE
TI11
Cataphoretic experiment. Copper Ferrocyanide Sol-Cane Sugar. Kt. of sugar added
*V in cm. per sec. per volt per cm
per IOO C.C. of the final solution 0
. 5 gms. I
)’
gm (in presence of 2 . 5 gms. I
.004
K’KC1)
40.1 X IO-^ 4 1 . 6 X IO& 4 0 . 5 X IO-^ 3 4 . 7 x 10-6 3 7 . 5 x 10-6
* Corrected for viscosity taking the value for water a t this temperature to be unity.
These measurements amply bear out our contention that it is unjustifiable t o discuss the observations on coagulation with mixtures of electrolytes and in the presence of non-electrolytes on the basis of amounts of adsorption of the coagulating ion necessary for coagulation as affected by simple considerations in changes in adsorbability or variation in the original charge. It is not denied that t’hese are factors t o be considered but that there are other equally influential factors such as the electrical forces opposing collisions, the electrical adsorbability, the changes in the energy associated with the agglomeration of the particles, and most of all the change in the critical potential of the sol on the addition of a non-electrolyte. Our best thanks are due to Prof. J. K. Mukherjee D.Sc. for his advice and for facilities for carrying out this work. Physical Chemistry Laboratory, Uniuersity College of Science,
99 U p p e r Circular R o a d ,
Cnlcutla.
