Spectrophotometric study of the complex formation

Summary

• 1. The formation of nitrate complex compound of Pu⁴⁺ when the HNO₃ concentration is increased carries a stepwise character. The formation of mainly one or another complex ion takes place in a definite region of HNO₃ concentration. This does not exclude the possibility of coexistence in the given HNO₃ concentration range of some complex ions; a proof of this is given by the doubling and even tripling of the absorption band in the 480 m\gm region.

• 2. A high concentration of hydrogen ions or of cations having a significant hydration energy enhances the formation of complex ions with a high content of nitrate groups.

• 3. The formation of nitrate complex ions of tetravalent plutonium leads to a significant shift of the absorption band. At the same time there takes place a change of the values of the molar extinction coefficients which either increase or decrease, due to the perturbations of the 5f energy levels produced by the nitrate groups replacing the water molecules in the hydrate sphere of the ion.

• 4. The coordination number of plutonium in the formation of nitrate complex ions remains the same and equal to the number of water molecules forming the hydrate sphere of a hydrated Pu⁴⁺ ion. Since the literature data [5] give for tetravalent plutonium a coordination number equal to 8, the reaction of successive formations of nitrate complexes of plutonium may be written in the following form.

  $$[Pu(H_2 O)_{8 - n} (NO_3 )_n ]^{ + 4 - n} + NO_3^ - \rightleftharpoons [Pu(H_2 O)_{8 - (n + 1)} (NO_3 )]_{n + 1}^{ + 4 - (n + 1)} + H_2 O$$

  The composition of the complex ions which are formed for HNO₃ concentrations from 0.5 to 10–11 M is expressed by the formula [Pu(NO₃)_n(H₂O)_8−n]⁺⁴⁻ⁿ, where n=1,2...6.