NOTES
1764
VOl. 64
poorer, but, in general, does not differ by more than lOql,.
Preparation of Cadmium Chloride.-The anhydrous cadmium Chloride used waa prepared from Baker Chemical Company analytical reagent cadmium chloride containing 2.5 molecules of water. The anhydrous salt was prepared TABLE I1 by recrystallizing CdC12.2.5H20from an aqueous solution, SMOOTHED HEATCONTENTS, ENTROPIES A N D FREEENERGYslightly acidified with hydrochloric acid. These crystals were dehydrated in a tube oven, heated to 150°, through FUNCTIONS FOR SOLIDSODIUJ~ OXIDE which was passed a stream of dry hydrogen chloride. (NazO, mol. wt. = 61.98) The excess hydrogen chloride was removed by means of a current of dry nitrogen. The resulting salt then was Hi,- H n e s . ~ , ST - s ~ s . 1 6 , (FT -THnga"') T ,OK. csl./mole cal./deg./mole cal./deg./mole evacuated until successive samples, taken a t hour intervals, gave a constant pH reading. 400 1 ,789 5.27 18.79 Measurement of pH.-The pH was measured with a 500 3 ,683 9.49 20.11 Beckman Laboratory Model G pH meter using a Beckman 600 5,676 13.12 21.65 general purpose shielded glaas electrode and a fiber type sealed calomel Beckman electrode. The hydrolysis cell 700 23.23 7,768 16.31 contained openings for two electrodes, a thermometer, a 800 9 ,959 19.27 24.81 conductivity water inlet and a nitrogen inlet and. outlet. 12,247 900 26.34 21.96 The cell containing the wei hed cadmium chlon.de was 14,636 1000 24.48 27.83 flushed with nitrogen gas, before and durlng the time the 17,122 1100 29.27 purified water was added. A magnetic stirring bar agitated 26.85 the solution for 15 minutes before the pH readings were Acknowledgments.-The authors wish to ac- taken. When cadmium chloride is placed in water, these dissociknowledge the support of this research by the United States Navy through the Callery Chemical ation and association reactions may occur
Company under a contract with the Bureau of Aeronautics.
HYDROLYSIS OF C14DMIU&1CHLORIDE AT 25' 1 3 KARL ~ H. GAYERA N D RUDYM. HAAS Department o f Chemtstrv, Wayne Stale University, Delrozt, Mzchigan Received Y a y 0, 1060
The study of the hydrolysis reactions and the evaluation of equilibrium constants for cadmium chloride has been undertaken by other investigators. If a true hydrolysis constant can be obtained for any species of cadmium chloride over a definite concentration range, it can be related t o an over-all reaction. The hydrolysis constants (Kh) and the reactions predicted by others are: at 100" for Kullgren' reported a Kh of 3.3 X the reaction CdClf €320 CdClOH H+; Chaberek, Courtney and Martel12 calculated R Kh of 2.5 X lo-'* at 30" in a 0.1 M potassium chloride medium for the reaction Cd++ H20 Cd(0H) + H+. According t o this last reaction it is assumed that the chloride ion from the potassium chloride has no effect on the hydrolysis reaction of the Cd++ ion. Doubt is cast on this assumption because it is further assumed that the concentration of the Cd++ ion is equal to the total concentration of cadmium chloride used. Our results seem to indicate that the major hydrolysis product at 25" is CtlC1OI.Z.
+
+
+
+
Experimental Purification of Water.-Water was prepared in a manner similar to that described by Gayer and Woontner.3 Ordinary distilled water was distilled in a Pyrex distillation apparatus from an oxidizing solution of sodium hydroxide and potassium permanganate. The resulting water was then redistilled by adding one drop of orthophosphoric acid per liter of water. Last traces of carbon dioxide were removed by boiling the h a 1 distilled water for about 15 minutes. Purification of Nitrogen.-Purified tank nitrogen was passed through several gas washing bottles of dilute sodium hydroxide, sulfuric acid and water. (1) C. Kullgren, Z. physik. Chem. (Leiprig), 86, 466 (1913). (2) 9. Chaberek, Jr.. R. C. Courtney and A. E. Martell, J . Am. Chem. Soc., 74, SO57 (1952). (3) K . H. Gayer and L. Woontner, J . Chum. Ed., 93,296 (1956).
+ +
CdC12 CdCl+ C1C d C l + s Cd++ C1CdClz C1CdCLCdCli 2C1- If CdCla"
+
(1) (2) (3)
+
(4)
In order to obtain the activities of these species, it is necessary to know the dissociation constants of the reactions involved. These are not available as such. As an approximation, however, it was assumed by Harned and Fitzgerald that the first reaction goes to completion, the third and fourth do not occur, and the second is the only one in which an equilibrium is involved.* By means of electromotive force measurements, they calculated the dissociation constant ( K ) for this reaction to be 1.1 X lo-*. Other Kt values given in the literature, assuming only the presence of Cd++, CdC1+ and C1-ions are 0.0106 and 0.0101.6
- aCd"
2--
aCl-
aCdCIC
- CCd* CCl-
.fCdHfCI-
CCdCl+
fCdCIC
Here a, C and f refer to the activity, concentration and activity coefficient, respectively. The various concentrations of CdCl+, C d + + and C1- ions can be evaluated by setting X . = Cd, X M = C1- and M - X = CdCl+, where M IS the molar concentration of the cadmium chloride used. The activity coefficients can be obtained by solving the regular form of the Debye-Huckel equation
+
where Zi is the charge on the ion, a, is its effective diameter and u is the ionic strength of the solution. The constants A and B were taken as 0.509 and 0.330, respectively, and.the effective diameters7 of the Cd++, C1-, CdC1+ and H + ions were taken as 5 X 10-8, 3 X 10-8, 4 x 10-8 and 9 X 10-8 cm., respectively. By the method of successive approximations, the following ionization equation can be solved by successively approximating X, the concentration of the Cd++ion. The activity coefficient of the Cd++ion, fx is given in the regular form of the Debye-Huckel equation
logf, =
-2 . 0 4 d 4 r M
1
+ 1.65 X 10-8-
(4) H. 9. Harned and M. Fitrgerald, J . A m . Chem. SOC.,68, 264 (1936). ( 5 ) H. L. Riley and V. Gallafant, J . Chem. SOC.,514 (1932). 2 6 , 592 (6) E. C. Righellato and C. W. Dsvies, Trana. Faradoar SOC., (1930). (7) J. Kiellsnd, J . Am. Ckem. Soc., 59, 1 6 i 5 (1937).
NOTES
Nov., 1960
The hydrolysis constant for cadmium chloride is then calculated from a value of the concentrations of CdCl+, Cd++, C1- and H + (from pH measurements) ions and the activity coefficients of these ions. All the following hydrolysis reactions will probably occur, to some extent; i t can however he assumed that one of them predominates over the rest. C d + + H20 _J CJ(OH)+ H + (1) C d + + 2H20 I_Cd(OH)2 2H+ (11) CdCl+ H2O )r CdClOH H + (111) The K h equations for the above are given in consecutive order. CCd(OH)+CH+ fCd(OH)+ fH+ = Khi CCdu fedtt -CUT+- Ho+)CH+ fCd(OH)+ fE+
+ +
+ + +
+
cia*- (H+- a091f;cd*-
(E+- Ho+)l
1765
due t o the hydrolysis of the cadmium chloride. Again assuming that only one of the above reactions occurs, the quantity of CdClOH or Cd(OH)+ formed is equal to the increase in the hydrogen ion concentration, and the quantity of Cd(0H)Z formed is equal to one-half the increase of the hydrogen ion concentration. The concentrations Ccd++ and CCdCl+ and the activity coefficients fed" and fCdCI+ are the values of these species after hydrolysis has occurred whereas Ca* and CGCI+ and fCd* and faE,+are the values of these ions, respectively, before h y d r o e hrts orcurred. The terms [Cd++ - l/z(H+ - Ho+)],CdC1+ ( H + - Ho+), etc., that is, those containing the bar symbols, simply illustrate how the measured values were corrected to give the equilibrium values used in the calculations. Approximations .C(H'- Hof)
