Setsuo Furihata1, Kazuo Akashi2, Jen-chi Huang3 and Shun-ichi Kurosawa4
1Faculty of Engineering, The University of Tokyo, Tokyo. Present address: Department of Industrial Chemistry. Faculty of Engineering, Tokai University, Hiratsuka
A modified method for purification of the fused LiCl-KCl eutectic mixture and the unfavorable effect of various impurity ions such as Ag+, Sb3+, Pb2+, Fe2+, Cr3+, Zn2+, Mg2+ and OH- contained in the fused salt were investigated. A very small amount of chloride or hydroxide was added to the LiCl-KCl melt purified, and then the reduction wave in the polarography of the impurity ions was recorded by using a Pt micro-electrode and a Pt/Pt2+ reference electrode at 673∼723 K.
The bubbling of dry HCl and Ar and filtration through a pyrex or silica glass filter were very effective for the purification of the fused LiCl-KCl mixture. After the filtration, the residual current decreased to a value less than 5 μA at a -2.0 V potential indicated by a Pt micro-electrode (0.8 mmφ) vs reference electrode Pt/Pt2+ 9 mol/m3. This value shows that the LiCl-KCl eutectic mixture is purified sufficiently for successive experiments.
The half wave potential of Ag+ was -0.8∼-0.9 V and the impurities having a low boiling point such as SbCl3, which increased the residual current level, could be easily removed by only Ar bubbling. Other cations causing the increase of residual current over the range of -1.0∼-2.0 V were Pb2+, Fe2+, Zn2+ and Cr3+. The remarkable increase of residual current at more negative potentials than -2.0 V which may be due to the reduction wave of OH- and Mg2+ could be observed.
The adsorption of moisture into the fused salt from air increased the OH- content and the residual current level. Accordingly it is very important to perform careful dehydration of the fused salt used as a solvent melt for correct electrochemical measurements.
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