弱酸性溶液中でのNpの電極反応

Electrode reaction of Np in a weakly acidic solution

北辻 章浩  ; 大内 和希  ; 音部 治幹  

Kitatsuji, Yoshihiro; Ouchi, Kazuki; Otobe, Haruyoshi

中性から酸性溶液中でのネプツニウムイオンの金電極上での電解酸化還元反応を調べた。pH2より酸性度の高い溶液中では、Np(V)の還元電流は水素発生電流に隠れ電位窓内に観測できなかった。より酸性度の低い溶液中ではNp(V)の還元に起因する電流ピークがボルタモグラム上に観測された。この還元反応の詳細を調べたところ、還元生成物が電極上に析出することが分かった。また、析出により電極が不活性化すること、析出化学種の溶出によると考えられる酸化電流ピークが観測されること、還元による析出量に限度があることなどが分かった。これまでに明らかにしたウラン(V)イオンの還元-析出電極反応と比較し、両者の違いについて考察した。

Electrolytic reactions of Neptunium ions in a weakly acid solution were investigated by using gold electrode. It was reported previously that electrolytic reduction of Neptunium (V) ion based on mediator reaction with Np(III)/Np(IV) couple was observed in the acidic solution when bulk electrolysis was carried out at the potential around -0.2 V versus Ag/AgCl reference electrode. The reduction of Np(V), however, was not observed at the pH higher than 2. This is attributable to low reaction rate of electron exchange reaction between Np(V) and Np(III). Electrode reaction of Np(V) at more negative potential was investigated by cyclic voltammetry. The current peak due to reduction of Np(V) was observed at ca. -0.75 V, and oxidation current was appeared at +0.5 V. The reactions was determined to be reduction of Np(V) and oxidation of deposited species of reduction product of Np on the electrode. It was found that stripping current due to oxidation of deposit was saturated in spite of longer preelectrolysis time. This phenomenon is different from uranium.