2012 Volume 128 Issue 10_11 Pages 584-589
The time-dependence of molecular weight of gelatin in Zn electrowinning was investigated by gel permeation chromatography to make clear the effect of various factors on the degradation of gelatin. The molecular weight of gelatin decreased significantly with increasing the time even in the case without electrolysis. Although gelatin acted as a polarizer for Zn deposition, shifting the cathode potential to less noble direction, the polarization effect gradually decreased with time. The decrease in polarization effect of gelatin with time was apparently caused by the decrease in molecular weight due to the degradation. The degradation of gelatin was accelerated with electrolysis, and the degradation was faster at anode than at cathode. However, the difference in degradation rate between anode and cathode was small, showing that the degradation of gelatin was accelerated even at cathode. The observed degradation constant k’ of gelatin for hydrolysis reaction in solution without electrolysis was linearly increased with concentration of sulfuric acid. The degradation constant k’ of gelatin increased significantly with solution temperature, but no k’ depended on the existence of Zn2+, and Cu2+, ions in solution. The linear relationship was observed between lin k’ and 1/T , showing the formation of Arrhenius’ equation. The activation energy for hydrolysis of gelatin in electrowinning solution containing 1.5 mol/dm3 of H2SO4 calculated by the inclination of Arrhenius plot was to be 60.1 kJ·mol-1, and the following equation was derived. k’ =7.4×103 exp(-60.1×103/RT) min-1
