Stability constants of ThCl
3+ and ThNO
3+3, K
c1 and K
c2 were determined electrometrically by means of the pCl-stat in solutions of KNO
3, NaClO
4 and NaNO
3-NaClO
4. 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 ThCl
3+ in a solution of KNO
3 was smaller than that in a solution of NaClO
4 by an amount of 15 to 50%, depending on the concentration of solution. This discrepancy was explained by assuming the formation of ThNO
3+3. The estimated stability constant of ThNO
3+3 varied from 4 to 0.75(mole/l)
-1 with an increase of concentration from 0 to 1 mole/l.
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