The Hydrolysis of Protactinium(V) Studied by Capillary Diffusion

Abstract

The mean diffusion coefficient of ²³³Pa has been measured simultaneously with those of ²²Na and ¹⁵²Eu in 0.5 M (Na, H)ClO₄ solutions with the pH ranging from 0.3 to 13, by the “open-end capillary method” optimized in order to obtain reproducible and reliable D values at T = 25°C. In the case of Eu(III), the results tend to give higher β₁₃ and β₁₄ hydrolysis constants than the values generally acccepted, but these data are probably affected by the formation of polynuclear or colloidal species as soon as the hydrolysis process is involved. For Pa(V), results are in agreement with the existence of the following two equilibria (I = 0.5 M, T = 25°C):

$$\begin{gathered} PaO\left( {OH} \right)^{2 + } + H_2 O \Leftrightarrow PaO\left( {OH} \right)_2^ + \left( {orPaO_2^ + + H_2 O} \right) + H^ + \hfill \\ with\log K_{h2} = - 11.62 \pm 0.02 \hfill \\ PaO\left( {OH} \right)_2^ + \left( {orPaO_2^ + + H_2 O} \right) + 2H_2 O \Leftrightarrow Pa\left( {OH} \right)_5 + H^ + \hfill \\ with\log K_{h3} = - 6.96 \pm 0.02 \hfill \\ \end{gathered} $$

However, unusual behavior is observed at a pH value around 1.3. A third equilibrium in basic media leads to the formation of a negatively charged species (log K _h4 = −9.03 ± 0.1 at I = 0.5 M). Finally, the presence of chloride in solution (0.1 M; pH = 1 and 4) and carbonate-bicarbonate ions (0.1 M; pH = 9.4 and 11.0), which cannot be neglected in most of the natural waters, decreases the measured values for the diffusion coefficient of Pa(V) appreciably compared to noncomplexing media.