Analysis of Brass Plating Solution Using EDTA, Again

Complex salt of copper and zinc cyanide is decomposed into CuO and ZnO, by H2O2, and then dissolved in HNO3. Zinc can be titraled by EDTA in the presence of copper which is masked by adding a slight exess of thiosulfate at pH 5-6, using methanol and the PAN (α-pyridyl-β azonaphtol) indicator. Then copper is analyzed with EDTA, after breaking down the copper masked by thiosulfate, using hydrogen peroxide as fellows: 2Na2S2O3+H2O2→Na2S4O6+2NaOH The end point of above reaction is told by PAN indicator.