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

Abstract

• Redox behaviour of bare CaCl 2 melts -explored using cyclic voltammetry at 900 °C at inert tungsten electrode. • Effect of addition of CaCl 2 .2H 2 O and Ca(OH) 2 in CaCl 2 melt studied using cyclic voltammetry. • Peak current densities observed in bare melts enhanced with addition of CaCl 2 .2H 2 O or Ca(OH) 2 . • Complex reduction behaviour attributed to formation of CaOHCl in melt at high temperatures. • Peak analysis of cyclic voltammograms and semi-integral curves carried out; cathodic reduction mechanism proposed. Redox behaviour of CaCl 2 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. CaCl 2 prepared from vacuum drying of CaCl 2 .2H 2 O 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, CaCl 2 .2H 2 O and Ca(OH) 2 were added in pre-electrolysed CaCl 2 melts and those were again analysed by cyclic voltammetry. Enhancement of peak current density of redox peaks, which were observed in pre-electrolysed CaCl 2 melt, was seen. This was attributed to the increased concentration of CaOHCl in the melt in presence of CaCl 2 .2H 2 O 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 CaCl 2 .2H 2 O and Ca(OH) 2 in melt.