June, 1'9601 Thermodynamic Studies in Cobalt Complexes. V 841

Thermodynamic Studies on Cobalt Complexes. V. On the Solubility and

the Thermal Dissociation of Aquopentamminecobalt (III) Nitrate

and Bromide, and on the Thermodynamic Stability of

Monoacidopentamminecobalt(III) Complexes*

By Motoshichi MoRI and Ryokichi TsucHIYA

(Received December 2, 1959)

In the preceding papery the authors deter

mined the standard free energies of formation of

aquopentamminecobalt (III) chloride in the

solid state and in aqueous solution. In the

present study the standard free energies of

formation of aquopentamminecobalt(III) nitrate

.and bromide in the solid state were determind

by measuring their solubilities, and those of

nitratopentamminecobalt(III) nitrate and bromo-

pentamminecobalt(III) bromide in the solid

:state were determined by measuring their dis-

sociation pressures.

On the basis of these data the thermodynamic

.stability of aquopentamminecobalt(III) chloride,

nitrate and bromide, that of monoacidopent-

) amminecobalt(III) complexes and that of their

ions in aqueous solution were compared and

.discussed.

Experimental

Aquopentamminecobalt (III) nitrate was preci-

pitated with nitric acid from an aqueous solution

of its chloride. The bromide was prepared by the

.double decomposition of its chloride with ammoium

bromide in aqueous solution.

Solubilities of these aquopentamminecobalt(III)

.complexes were obtained by analyzing the cobalt

.content in their saturated solutions by iodometry in

the same manner as described in the second paper2)

of this series. The dissociation pressures of the

.complexes were measured with the same apparatus

,as described in the third paper3).

Results and Calculation

Solubility of Aquopentamminecobalt (III)

Nitrate.-The results obtained for aquopent-

amminecobalt(III) nitrate are listed in Table I.

As the activity coefficient of the compound

was not known, it was assumed that it was

approximately equal to that of hexammine-

cobalt(III) chloride4) in relatively lower con-

centrations. When aquopentamminecobalt(III)

nitrate is dissolved in water, the change is

expressed by

If the solubility of the salt in molality and

its mean activity coefficient are expressed by

m and rf respectively, the equilibrium constant

Fig. 1. The relationship between the equili-

brium constant of the solution and the

temperature.

−◎‑;aquopentamminecobalt(III) nitrate

‑○‑;aquopentamminecobalt(III) bromide

* Read at the Symposium on Coordination Compounds

of the Chemical Society of Japan, Fukuoka, Oct., 1959.

1) M. Mori, R. Tsuchiya and Y. Okano, This Bulletin,

32, 1029 (1959).

2) M. Mori, R. Tsuchiya and Y. Okano, ibid., 32, 462

(1959).

3) M. Mori and R. Tsuchiya, ibid., 32, 467 (1959).

4) M. Mori and R. Tsuchiya, J. Chem. Soc. Japan, Pure

.Chem. Sec. (Nippon Kagaku Zasshi), 79, 1164 (1958).