465. トリウム塩素錯体ならびにトリウム硝酸錯体の安定度定数

抄録

Stability constants of ThCl³⁺ and ThNO³⁺₃, K_c1 and K_c2 were determined electrometrically by means of the pCl-stat in solutions of KNO₃, NaClO₄ and NaNO₃-NaClO₄. The results are presented as a function of concentration, ranging from 0.05 to 1.6mole/l at temperatures of 15°, 25° and 35°C.
The value of K_c1 increased markedly with decreasing concentration, especially in the range of concentration below 0.5mole/l. For example, whereas K_c1 was 6 and 2.8 (mole/l)^-1 respectively at 0.5 and 1mole/l, extrapolation to zero concentration gave a thermodynamic stability constant K_a as high as 37(mole/l)^-1.
From the value of K_a and its temperature dependence, the free energy and the heat of complex formation were estimated to be -2.14 and nearly zero kcal/mole, respectively. Hence, a value of 7.2 e.u. was derived for the entropy of complex formation.
The apparent stability constant of ThCl³⁺ in a solution of KNO₃ was smaller than that in a solution of NaClO₄ by an amount of 15 to 50%, depending on the concentration of solution. This discrepancy was explained by assuming the formation of ThNO³⁺₃. The estimated stability constant of ThNO³⁺₃ varied from 4 to 0.75(mole/l)^-1 with an increase of concentration from 0 to 1 mole/l.