ELECTROKINETIC POTENTIAL OF SILVER IODIDE GEORGE N. GOROCHOVSKY State Optical Institute, Leningrad, U.S.S.R.
Received June 16, 1934 I. INTRODUCTION
It might be assumed that the electrokinetic potential of halogen salts of silver against pure water would be zero. However, Labes (8),Mukherjee and Kundu (12), Kruyt and Van der Willigen (7), Lange and Crane (9), and Basinski (3) have shown that the electrokinetic potential, t, of these salts (in particular, silver iodide) a t the point of equivalence, i.e., a t equal concentrations of silver and of halogen ions in solution, has a rather considerable negative value. Zero value of the electrokinetic potential was attained only by having a rather considerable excess of silver ions in solution. At higher concentrations of the soluble silver salt the silver halide is charged positively, but on subsequent dilution of the solution with water the potential falls to lower values and finally becomes negative. This phenomenon is not confined to silver halides. Lottermoser and Riedel (10) consider the negative charge in pure water to be proper to all substances and confirmed this with many examples. Labes, Mukherjee and Kundu, as well as Lange and Crane, measured the potential by the rate of the endosmosis, that is, on the boundary of the coarsely dispersed silver halide, while only the last authors measured systematically the whole curve of the dependence of r-potential of silver iodide on the concentration of the excess ion constituting the lattice of the salt (silver ion or iodide ion). In other researches electrophoresis of sols was used, but the data obtained were either only approximate or concerned with the influence of the dilution with water on the r-potential of the positively charged sol, i.e., the simultaneous change and concentration of the electrolyte and the concentration of the dispersed substance. My aim has been to measure the curve r-potential-concentration of electrolyte, similar to that one obtained by Lange and Crane, but with sols of highly dispersed silver iodide. 11. METHOD OF l-POTENTIAL DETERMINATION
r-potentials were determined by rate of the electrophoresis of sol observed through an ultramicroscope. The method was that of P. Tuorila (14). Observations were made with the help of the slit ultramicroscope of 465
466
GEORGE N. GOROCHOVSKY
C. Zeiss. The cell used was pasted together by means of picein; it contained two platinum electrodes and a tightly fitting glass cover. The distance between the electrodes was 35 mm., and the depth of the cell 5 mm. On the upper side of the cell a slight scratch was made with a diamond. The cell was set on the vertically displaceable stage of the microscope, and the latter was focused first on the scratch and then on the illuminated layer of sol. If the displacement of the microscope tube, D, is known as well as the true thickness of the glass plate, d, then the thickness of the layer of liquid between the inner surface of the upper glass and the plane under observation will be as follows: x=:%D-- d n
2
m
where nl and n2 are the refractive indices at the interfaces glass-exterior medium and glass-sol. Since the objective was used with water immersion, we may put nl - n2 = nH20
x=D-- d nHZ0
where
d
- = the optical thickness of the plate, is established.
nH20
The observed velocity of electrophoresis of the dispersed phase a t different distances from the walls of the cell is the algebraic sum of the true velocity and of that of endosmose of the solution relative to the walls of the cell. According t o Smoluchovsky (13) the velocity of endosmose in the closed cell a t the distance x from one of the two parallel sides distant 1 apart is:
U , = Uo [l
- 6 (T -
$1
where U ois the velocity of endosmose close to a side. The rate of motion of the particles observed by the ultramicroscope is equal to velocity of electrophoresis, v, at the depth where U , = 0, Le., when
XI = 0.2111 and 2 2 = 0.7891. The parabolic curve of Smoluchovsky’s function near this “neutral layer” is nearly a straight line. Hence, by measuring the velocity of motion of particles in the electric field, once a t a depth somewhat smaller and then at one greater than 0.211E, it is possible to find the true velocity of electrophoresis, v , by interpolation.
or
ELECTROKINETIC POTENTIAL OF SILVER IODIDE
467
A difference of potentials of about 12 volts was applied to the electrodes, about 3.5 volts per centimeter. By means of a stop watch the time of passing of the observed particle between two selected divisions of the ocular-micrometer was measured, first in the field, where the positive pole was on the left, second, in the case of short-circuited electrodes (for in a wide cell there are always curious, partly convectional currents of liquid, more or less constant in velocity), and lastly in the reversed field. Measurements of this kind were repeated a t both depths four to seven times. The potential was calculated with the usual equation for spherical particles, due to Debye and Hiickel:
where 7 is the viscosity, taken as the viscosity of pure water, H is the potential gradient, and D is the dielectric constant, considered equal to 81. Sols were prepared in the following way. To 37.5 cc. of the solution of one of the electrolytes (potassium iodide or silver nitrate) during exactly 3 minutes and with constant stirring, 12.5 cc. of the second electrolyte was added. The mixture was made in yellow light. The sol was violently shaken and left standing for three to four hours in order to attain a state of comparative equilibrium. After that the {-potential was measured. Two series of experiments were undertaken with three different concentrations of silver iodide in each, viz., M/400, M/4000, and M/20,000. These two series were made a t different times and differed from each other by the following conditions: (1) In the &st series common distilled water was used, while the water used for the second series was carefully distilled three times through a silver condenser. (2) In the first series, sols were made in such a way that the first solution (37.5 cc.) contained that electrolyte of which there was an excess, while in the second series that electrolyte was used one of the ions of which communicated the charge to sol, which fact was discovered from the results of the first series. Experiments showed that the potential did not change with time. M/4000 AgI, 10-6 M K I . . ................ After 2 hours After 48 hours
= -50 mv. = -56 mv.
M/400 AgI, 10-4 M AgN03.. . . . . . . . . . . . . . After 3 hours After 48 hours
= +38 mv. = $37 mv.
M/ZO,OOO AgI,
i
M AgNOs.. ........... After 3 hours After 24 hours
M/400 Agl, 10-6 M K1.. . . . . . . . . . . . . . . . . . After of an hour After 5+ hours
= -25 mv. = -26 mv. = -45 mv. = -48mv.
468
GEORGE N . GOROCHOVSKY
111. POTENTIALS OF SOLS WITH DIFFERENT CONCENTRATIONS O F SILVER
IODIDE
The results of measurements made during the Jirst series of experiments are shown in table 1. Several values in one column correspond to the potentials of sols made up a t different times; these data give an indication of the reproducibility of the experiments. It should be noted that sols with the contents of M/10 potassium iodide are already of great electrical conductivity, and consequently the data obtained are approximate. TABLE 1 Potentials o.f sols w i t h different concentrations os silver iodide POTENTIAL ~~
CONCENTRATION OF EXCESB ELECTROLYTQ
M
AgNOa
XI
gram-molea per liter
ram-moles per liter
m
M
M
4000
20,000
mv.
mv.
mu.
-
10-2 10 -a 10-4 10-4.2 10 -6
+58
$49, $44 +21, +36 +I5 +I -16, -13, -35 - 53
+55 +38
0
-
10-8
10-8
~
Concentration of AgI in sol
10-8 10-6 10-6 10-4.2 10-4 10-8 10-2 10-1.2 10-1
-
-48
-44 -50 -44 -55
-60,
-50
-62
-51 - 53 73 approx. -100
-
-
-
approx. -170
+32 $12, -2 +25
-
-31,
-36
-43
-53
-56 -50,
-50
-50
-53 -65, -49
-
approx. -105
f-potentials of the second series of experiments are given in table 2. Basinski (3) for the more concentrated sol (N/50silver iodide) obtained the following values of (-potential: r-potential
At M AgN03 At 10-3 M AgN08 At 10-2 M KI
= +46.9 mv. =
+47.3 mv.
= -70.1 mv.
i.e., values coinciding practically with those which refer to our M/400 sol. The results are shown graphically in figures 1 and 2. The more the sol is diluted, the greater is the difficulty in charging it
469
ELECTROKINETIC POTENTIAL OF SILVER IODIDE
TABLE 2 Potentials of sols with digerent concentrations of silver iodide {-POTENTIAL CONCHlNTRATION OF EXClllSS ELECTROLYTE
Concentration of AgI in sol
M AgNOa
KI
oram-moles per liter gram-moles per liter
10-2 10-3 10-4 10-6 10-4 10 - 8
457
mv.
mn.
mu.
$58 $37 +38
$30 10 - 10 -24 -40
-
10-4
10-3 10-2 10-1
M 20,000
f50 43 $45, $41 0 (?I
+
10-8 10-6 10-5
M 4000
-23 -48, -45, -4 -36 -37 -69 approx. -135
FIQ. 1
$20, 0
-29
-
-43 -41
-47 -75 approx. -115
+
-
-33, -37 -32 -39 49 71
-
-
FIG.3 470
ELECTROKINETIC POTENTIAL OF SILVER IODIDE
471
positively and the less is its {-potential in the region of positive charge. Accordingly the isoelectric point (l'= 0) is shifted; if in the case of the sol of M/400 silver iodide, the concentration of silver ion exceeds at this point the concentration of iodide ion by lo6, then in the case of the sol with content of silver iodide fifty times less, the concentration of silver ion exceeds the concentration of iodide ion by log. Outwardly this change is manifested by a quite obvious shift of the domain of minimum stability of the sols. The curves obtained differ to a great extent from the endosmotic curve of Lange and Crane (see figure 3, where the average curves of both series of tests are shown). While it is possible to explain fully this last divergence by the difference of the methods of measurement and by the fact that the conditions of adsorption of ions on silver iodide in statu nascendi can differ considerably from the adsorption on the precipitate previously made, the divergence between the potential curves of sols of dissimilar concentrations requires a special explanation. IV. INFLUENCE O F DILUTION UPON {-POTENTIAL
It may be supposed that {-potential depends on the conditions of preparation of the sol. The more the .initial solutions are diluted, the larger would be the particles and the less the relative quantity of adsorbed silver ions, which according to Kruyt's view (6) are adsorbed chiefly on the edges of the crystals. Endosmotic experiments of Kruyt showed that fused silver iodide differed from precipitated since it was not possible to charge it positively even with high concentrations of silver nitrate. The possibility indicated is not peculiar to heteropolar crystals, but also occurs with noncrystalline spherical particles. The dependence of the velocity of transference (and consequently of {-potential) on the radius was established by means of experiment. Illig and Schonfeld ( 5 ) have observed this by the endosmose of water through diaphragms with pores of different size; Mooney (11)with emulsions of oil in water; Alty (1)with bubbles of gas in water. In later researches this dependence is expressed mathematically (2, 4). In connection with such hypotheses it is interesting to compare the results obtained with phenomena found in the usual dilution of sols. Basinskil showed that there is no marked influence of dilution by ultrafiltrate on the {-potential of the positively charged sol, but the same dilution of the negatively charged sol leads to a decrease of the potential. The dilution with water of the positively charged silver iodide sol gives a sharper curve for {-potential than that in our work. But it is necessary 1
Reference 3, pp. 332-9.
THE JOURNAL
The curve in our figure 4 is drawn from table 44.
OF PHYSICAL CHEMISTRY, VOL. 39, NO, 4
472
GEORGE N. GOROCHOVSKY
to take into consideration that the reduction of concentration of excess electrolyte and that of the dispersed phase take place simultaneously on dilution with water; thus, the positive potentials (concentration of silver iodide from M/1250 to M/5000) belong to sols which approach the most concentrated of my sols, and negative potentials (concentration of silver iodide from M/10,000 to M/100,000) belong to sols whicli are for the most part more diluted than the least concentrated of my sols. Under such circumstances I consider that the data given by this author confirm the presence of a concentration dependence of t-potentials of sols. In order to verify these results, I also made experiments on the dilution of the sol of M/400 silver iodide by means of isoelectrolytic solutions, that is, by solutions containing the same quantity of excess electrolyte as TABLE 3 Potentials obtained after dilution of the MI400 silver iodide sol I-POTENTIAL
CONCENTRATION OF EXCESS ELECTROLYTE
KI
AgNOt
gram-moles per liter gram-moles per lifer
10-2 10 -a 10-4 10-5 10-8
mo.
10-4
I
Ad
++4958 .
10 - 8 10 -6 10-3 10-2 10-1
The same but diluted to M/ZO,OOO
M / 4 0 0 sol AgI
,
+41 $23 -53 (?) -55 -51 -51 -82 approx. -145
mv
mv.
+30 +I3 $8 -23, -28 -28, -32
+27 +I6 10 -27 -22 -49 -39 -51 70
-40
-52 -58
I
Diluted sol with M/250KNOs
-71 approx. -120
+ -
the initial sol, and also by means of solutions which contained excess electrolyte with M/250 potassium nitrate (see table 3). It is difficult t o suppose that this electrolyte influences the l-potential, but as it was being formed in dissimilar quantities in sols of different concentrations of silver iodide, it was necessary to take this fact into consideration. I t follows from these data that simple dilution of the sol gives exactly the same general results as have already been observed: the diluted sol has a smaller positive charge than the concentrated sol, and the isoelectric point is shifted from the point of equivalence. The presence of potassium nitrate has practically no effect on the value of the c-potential. Therefore the conditions of preparing a sol do not cause a dependence of {-POtential on the concentration of the dispersed phase. It seemed possible to explain the results otherwise. The fact that the
473
ELECTROKINETIC POTENTIAL O F SILVER IODIDE
flocculation value depends on the concentration of sol is generally explained in the following way: the higher the concentration of dispersed phase, the greater will be the proportion of the dissolved electrolyte which partakes in adsorption and the less the equilibrium concentration of electrolyte as compared with the initial concentration. But in our case this reasoning leads to conclusions contrary to experiment. V.
pH
O F SOLS O F SILVER IODIDE
Let us examine the problem of the structure of the double electric layer of silver iodide. The rule by Paneth-Fajans states that those ions are adsorbed on the surface of the heteropolar crystal which give, together with the oppositely charged ions of the crystal lattice, the least soluble
TABLE 4 Results of p H measurements CONCENTRATION OF DISPERSED EXCESS CONCENTRATION OF SILVER ION
M /400 The made up a01
10-2 M 0
~120,000
Centrifuging
The made up sol
1
0
Dilution of sol M/400
6.05
10-8 M
5.17
M
5.83
5.80
6.31
6.68 6.27
10-8
AgI
5.32
1’
5.70 5.27 5.73
5.83 5.16
6.31
6.40
0 10-4 M
M
6.25
0
,
5.41
compounds. We suppose that the “composition” of the second layer of the dou.ble layer is also determined by this rule, i.e., that the latter layer is formed for the most part from ions which give together with ions of the first layer the least soluble compounds. In the case of negatively charged silver iodide sol, iodide ions constitute the first layer; it is evident that potassium and hydrogen ions can form the second diffuse layer. In positively charged sols silver ions constitute the first layer and either nitrate or hydroxyl ions constitute the second layer. According to what precedes, in the case of the positively charged sol hydroxyl ions form chiefly the second layer, as silver hydroxide is but slightly soluble. In this case the equilibrium H+ OH- a HzO is displaced and the intermicellar liquid is slightly acid. The higher the concentration of dispersed phase, the more the liquid will be acidified. In compliance with
+
474
GEORGE N. GOROCHOVSKY
the decrease of concentration of “free” hydroxyl ion, the relative amount of hydroxyl ion in the diffuse layer will be less in the concentrated sol than in the diluted one. The layer will be more diffused and the value of the c-potential will increase. In order to verify such an hypothesis, the pH values of sols were measured by means of a glass electrode. The determinations with positively charged sol were rather difficult, since the more positive the sol, the more intensely it was precipitated on the negatively charged glass and on the film of the electrode as well. In consequence, the potential proved to be unstable and badly reproducible (3 to 5 millivolts, and sometimes more). Table 4 gives the results of the pH measurements made a t different times. They give generally a negative answer to our problem. However, the negative results do not exclude the possibility of such an explanation of the phenomenon. Ions constituting the d8use layer of the double electric layer can be active in regard t’othe glass electrode, and then the value of pH will tell us nothing of the structure of the double layer. VI. SUMMARY
1. Electrokinetic potentials of sols of silver iodide with M/400, M/4000 and M/20,000 silver iodide made in different excesses of ions (silver or iodide) were electrophoretically measured. 2. The curves (-Celectrolyte of silver iodide sols do not coincide with the curve in the case of endosmose through the precipitate (Lange and Crane). 3. The less the concentration of the dispersed phase in sols, the greater are the concentrations of silver ion necessary to charge it positively and the further the isoelectric point is removed from the point of equivalence in the direction of the exqess of silver ions. 4. The dilution of sol M/400 silver iodide by means of solutions containing the‘same quantities of excess electrolyte as the initial sol leads also to decrease of positive charge of the sol. 5. The aheration of the concentration of dispersed phase does not cause any change of pH of the sols.
I wish to express my deepest gratitude to Mr. K. S. Ljalikov, to whose initiative this work was due and who invariably gave it his closest attention. REFERENCES ALTY,T.: Proc. Roy. SOC.London 1%A, 315-40 (1924). AUDUBERT, R . : J. chim. phys. 30, 89-101 (1933). BASINSKI, A. : Kolloidchem. Beihefte 36, 257-349 (1932). J. J. : Z. Elektrochem. 39, 526-31 (1933). BIKERMAN, N . : Wiss. Veroffentlich. Siemens-Konzern 7, 1, ILLIG,K., AND SCH~NFELDT, 294-300 (1928). (6) KRUYT,H. R.: Physik. Z. Sow. Union 4, 295-303 (1933).
(1) (2) (3) (4) (5)
ELECTROKINETIC POTENTIAL OF SILVER IODIDE
(7) KRUYT,H. R., (8) (9) (10) (11) (12) (13) (14)
475
AND VAN DER WILLIGEN, P. C.: Z. physik. Chem. 139A, 53-63 (1928). LABES,R.: Z. physik. Chem. 116, 1-64 (1925). LANGE,E., AND CRANE,P. W.: Z. physik. Chem. 141A, 225-48 (1929). LOTTERMOSER, A., AND RIEDEL,W.: Kolloid-Z. 61,30-9 (1930). MOONEY, M.: Phys. Rev. [2] 23, 396-411 (1924). J., AND KUNDU,P.: Quart. J. Indian Chem. SOC.3, 335-41 (1926). MUHHERJEE, SMOLUCHOVSKY, M.: Handbuch d. Elektrizitat v. Grata 2, 382-3(1921). TUORILA, P.: Kolloid-2. 44, 11-22 (1928).
