A STUDY OF T H E ACTIVITY OF CADMIUM IODIDE I N AQUEOUS SOLUTION BY FREDERICK H. GETMAN
I n a previous paper' we presented the results of an experimental study of the activities of the chloride, bromide and sulphate of cadmium in aqueous solution. The present paper embodies the results of similar studies which have since been made of solutions of cadmium iodide. The experimental procedure has been identical with that followed in the preceding investigation. I n the present series of experiments the data has been obtained from measurements of two entirely independent galvanic systems represented by the following schemes:
(I) and
(2)
Cd Cd
- CdL(m) - AgI - Ag, - Cd12(m) - Pb12 - Pb, Hg.
The latter cell has been shown by Gerke and others2 to be reversible to the lead ion. Materials (I) C a d m i u m Iodide. Chemically pure cadmium iodide was recrystallized either from conductivity water, or alcohol, before use in preparation of the solutions. Silver Iodide. This salt was prepared from a solution of chemically (2) pure silver nitrate by treatment with pure potassium iodide, care being taken t o conduct all operations in a dark room in order to prevent possible reduction of the silver halide by light. (3) Lead Iodide. Lead iodide was prepared by precipitating a solution of chemically pure potassium iodide with a slight excess of chemically pure lead nitrate. The resulting precipitate was washed with distilled water until the wash water was free from nitrate, after which it was allowed to dry at room temperature. Hydriodic A c i d . A ,507~ solution of hydriodic acid, obtained from (4) the Eastman Kodak Company, was redistilled and a solution of the approximate concentration desired was made u p by diluting the fraction boiling a t 126' to 127O, under 760 mm. pressure, with the estimated amount of conductivity water. The exact concentration was subsequently determined by precipitating a known weight of the solution with silver nitrate and weighing the resulting precipitate of silver iodide. ( 5 ) Lead. Stick lead obtained from two different, reliable sources, and guaranteed to be free from silver and other metals, was used in preparing all of the amalgams. Getman: J. Phys. Chem., 32, 91 (1928). 2Gerke: J. Am. Chem. SOC., 44, 1701 (1922);Taylor: 38, 2295 (1916);Taylor and Perrott: 43, 489 (1921).
ACTIVITY O F CADMIUM IODIDE IN AQUEOUS SOLUTION
941
(6) Silver. The silver employed in the preparation of the silver electrodes was obtained by electrolysing a solution of pure silver nitrate between an anode of pure silver and a cathode of sheet platinum. The resulting crystals of electrolytic silver were first washed with distilled water, then with a solution of ammonium hydroxide and finally with conductivity water, after which they were rapidly dried in a current of dry, warm air. (7) Mercury. All of the mercury used was exceptionally pure, having been furnished through the courtesy of the Eppley Laboratories. (8) Solutions. All solutions were made up by direct weighing of both solute and solvent, except in the case of solutions of hydriodic acid, where, as pointed out above, the concentration was determined gravimetrically. All concentrations are expressed in mols of solute per 1000grams of solvent. Conductivity water having an average specific conductivity of 2 X IO-^ mols a t 25' was used in the preparation of the solutions,
Experimental Data I n Table I is given the measured values of the electromotive force of cells set up according to the scheme Cd - CdIn(m) - AgI - Ag.
TABLE I Conc. (m) (Xlols per 1000 gm.
Hz0)
1.0274
0.4883 0.202I
0.0969
E.M.F.
(25')
volts
0.37402 0,38688 0.39926 0,41024
Conc. (m) (Mols per I W O gm.
E.M.F. (25")
0.0389 0.0197 0.0098 0.0039
0.42500 0.43250 0.442SO 0.46475
H2O)
volts
The data of the foregoing table are represented graphically in Fig. I. The tendency of the electromotive force to fall off more and more rapidly with time as the concentration of the electrolyte is diminished was found to be even more marked in the case of the iodide than with the chloride and bromide of cadmium. I n fact i t was almost impossible to secure satisfactory results with cells in which the concentration of the cadmium salt was much below 0.005 molal. This instability is probably due to several causes, among which increasing solubility of the depolarizer with dilution of the electrolyte is considered to be significant. I n order to compute the activity coefficients of solutions of cadmium iodide from the foregoing data it is necessary to know the potential of the silver-silver iodide electrode in terms of the hydrogen electrode. This has been determined by Noyes and Free': as well as by Gerke2 and Pearce and Fortsch3. From measurements of the electromotive force of the cell
Pt - H ~ ( atm.) I - HI(m) - AgI - Ag, Noyes and Free: J. Am. Chem. SOC.,42, 476 (1920). 'Gerke: J. Am.Chem. Soc., 44, 1684(1922). BPearce and Fortach: J. Am. Chem. Soo., 45, 2852 (1923).
942
FREDERICX E. GETMAN
the following values of the electrode potential were obtained: 0.1522 volt (Noyes and Free), 0.1501 volt (Gerke), and 0.1513 volt (Pearce and Fortsch). The mean of these three values, 0.1512 volt, has been taken as the basis of our calculations. On the assumption that cadmium iodide in aqueous solution behaves as a bi-univalent electrolyte its activity coefficients can be computed by means of the familiar formula,
Putting E" = 0.3976 volt, the value of the electrode potential as determined by Horschl, and passing to common logarithms, we have
E
=
0.3976
- 0.08873 log (1.588my).
In Table I1 will be found the values of y, computed by substituting in the foregoing equation the successive values of the electromotive force, E , derived from the smoothed values of the electromotive force, E', as read from the curve in Fig. I and referred to the hydrogen electrode. 1
Horsch: J. Am.Chem. SOC.,41, I787 (1919).
943
ACTIVITY O F CADMIUM IODIDE I N AQUEOUS SOLUTION
TABLE I1 Activity Coefficients of Solutions of Cadmium Iodide m
E'
I .o
0.3765 0.3865 0.3995 0.4095 0.4192 0.4325 0.4425 0.4580
0.5
0.2 0. I
0.05 0.02
0.01
0.005
E 0.5277
0,5377 0.5507 0.5607 0.5704 0,5837 0.5937 0.6092
Y
m
0.022
__
0.033 0,059
0.16 0.19 0.23 0.30 0.43 0.53 0.63
0.091 0.14
0.25 0.39 0.52
In the fifth column of the table, the corresponding values of the conductivity ratio, CY, a t 2 s 0 , are given for comparison with y. These ratios were read from a curve plotted from the conductivity data of Jones'. The results of a similar series of measurements of the electromotive force of cells set up according to the scheme Cd are given in Table 111.
- CdIZ(m) - PbIz - Pb, Hg, TABLE 111
Conc. (m) (Mob per 1000 gm.HsO)
E.M.F. (E')
I . 2967
0.16777 0.16738 0.18069 0.19227 0.20165 0.21419
1.0274 0.4897 0.2030 0.0968 0.0389
volta
Conc. (m) (Mol8 per 1000 gm. H a )
0.0196 0.0099
E.M.F. (E') volts
0.22321 0.23316 0.24632 0.24869 0.25423
0.0050
0,0040 0.0030
The potential of the Pb(Hg), Pb12, I- electrode was computed from the electromotive force of the cell Pt-HZ(1 atm.) - HI(m) - PbIz - Pb, Hg, as recorded in Table IV. TABLE IV Electromotive Force of the Cell Pt-HS(1 atm.) - HI(o.0891.M) - PbIz Pb, Hg
-
E.M.F. (obs.)
0.22530 0.22458 0.22435 0.22512
Barom.
E
E.M.F. (corn;)
0.0003
0.2256 0,2248 0.2246 0.2253 0.2258 0.2252
0.0002 0.0002
0.0002
0.22555
749.1 749.8
0.22497
750.0
o.ooc2
0.0002
Mean H. C. Jones: Carnegie Institution Publication, No. 170, p. 49.
0.~252
944
FREDERICK H. GETMAN
The observed values of the electromotive force were corrected for deviations from normal barometric pressure by means of the formula,
E
=
0.0296log 760/p,
where p is the observed barometric pressure. By interpolation in the table of activity coefficients of hydrochloric acid a t 25", as compiled by Lewis and Randall', the value of y for 0.0891molal acid is found to be 0.823. On the justifiable assumption that the activity coefficients of hydrochloric and hydriodic acids are equal a t corresponding dilutions we let y = 0.823. Substituting the mean value of the electromotive force, as given in Table IV, in the familiar equation for normal electrode potential,
+ = E" +
E = E" we have
0.2252
E" =
or
0.1183 log
(my),
0.1183log (0.0891 X 0.823),
0.3594volt.
The estimated value of this electrode potential given by Gerke2is 0.3579volt. Accepting the value 0.3594,as here determined, we calculate the values of the activity coefficients given in Table V.
TABLE V Activity Coefficients of Solutions of Cadmium Iodide m
E'
E
1.3 I .o
0.1677
0.5271
0.017
0.1715
0.5309
0.020
0.5394
0.031
0.2
0.1800 0 .I923
0.058
0.1
0.2010
0.5517 0.5604
0.05
0'
5699 0.5824
0.14
0.02
0.2105 0.2230
0.01
0,2335
0.40
0,005
0.2470
0.5929 0.6064
0.5
V
0.092
0.26
0.56
It will then be seen that the values of y calculated from the measured electromotive forces of the two different cells are in close agreement. The mean values of y given in Tables I1 and V are plotted against the logarithms of the corresponding concentrations in Fig. 2 . Similar curves for the chloride and bromide of cadmium, as given in our preceding paper, together with the curve of conductance ratios for cadmium iodide, are also plotted in Fig. 2. b Discussion of Results The manner in which the activity coefficients of solutions of cadmium iodide vary with concentration resembles that of the chloride and bromide. As was pointed out in our previous paper, the values of the activity coeffil
Lewis and Randall: "Thermodynamics," p. 362. Gerke: loc. cit., and Chemical Reviews, 1, 384 (1925).
ACTIVITY O F CADMIUM IODIDE IN AQUEOUS SOLUTION
945
cients of the individual cadmium halides in aqueous solution differ from each other much more widely than do the corresponding values of y for the halides of the alkali metals. When the conductivity ratios of solutions of cadmium iodide are plotted against the logarithms of their respective concentrations, we find a much greater divergence from the corresponding curve of activity coefficients than with solutions of either the chloride or bromide.
Fro. 2
KO trustworthy freezing point data for cadmium iodide being available it has not been possible t o compare the values of y derived from cryoscopic data with those computed from electromotive force measurements. It is hoped, however, that experiments now in progress in this laboratory may soon supply the data necessary for making this interesting comparison. summary (I)
The following cells have been studied:
(I) (2)
(3)
- CdI*(m) - AgI - hg, Cd - CdI:(m) - PbI: - Pb, Hg, Pt-H:(I atm.) - HI(m) - PbI: - Pb, Hg. Cd
946
FREDERICK 1%. GETMAN
From the measured values of the electromotive force of these cells, (2) a t 2 5 O , the values of the activity coefficients, y , of solutions of cadmium iodide down to 0.005 molal have been calculated. (3) The values of y derived from a series of measurements of each of the two cells designated as ( I ) and ( 2 ) were found to be in close agreement. (4) The activity coefficients of the cadmium halides in aqueous solution differ from each other, forming a progressive series in which the values of y for the bromide occupy a position approximately midway between those of the chloride and iodide. Hillside Laboratory, Stamford,
Conn.
