Redox behaviour of CaCl2 melts in presence of moisture as impurity. Part I: Cyclic voltammetry

Abstract Redox behaviour of CaCl2 melt at 900 °C is well known; what is not completely established so far is how the voltammetric behaviour of melt would be in presence of moisture. CaCl2 prepared from vacuum drying of CaCl2.2H2O retained some residual moisture that eventually led to formation of CaOHCl at 900 °C as a result of which two redox couples were observed in the cyclic voltammograms at tungsten electrode. In order to decipher whether the redox peaks were a reflection of formation of single electro-active species, CaCl2.2H2O and Ca(OH)2 were added in pre-electrolysed CaCl2 melts and those were again analysed by cyclic voltammetry. Enhancement of peak current density of redox peaks, which were observed in pre-electrolysed CaCl2 melt, was seen. This was attributed to the increased concentration of CaOHCl in the melt in presence of CaCl2.2H2O and Ca(OH)2. Subsequent to voltammetric peak analysis and logarithmic analysis of semi-integral curves for all melts, a mechanism involving formation of ( CaOH ) 2 2 + adsorbed undergoing a one-step two-electron transfer process in the first redox step was proposed. Second redox step resulted due to adsorption of O 2 - on tungsten, and its peak current density also got enhanced due to addition of CaCl2.2H2O and Ca(OH)2 in melt.