The Accuracy of the Titrimetric Determination of Zinc with Potassium Ferrocyanide

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When zinc sulfate solution was titrated directly with potassium ferrocyanide, using diphenylamine as an indicator, equivalent point could not entirely be found owing to the gradual discoloration of the blue. The same results were observed when the titration was carried out either at room temperature or at 60℃. Addition of some ammonium chloride made the color change distinct. But deviation from calculated amount was great. About 7% of zinc was found to be present as simple zinc ferrocyanide. Presence of an excess of potassium ferrocyanide was favourable for the formation of zinc-potassium double ferrocyanide. Satisfactory results, therefore, were obtained when potassium ferrocyanide solution was titrated with zinc or small excess of the ferrocyanide was added in one stroke into zinc solution, and after the completion of the double salt, the excess of the ferrocyanide was titrated with suitable reagent such as zinc sulfate or potassium dichromate. On adding sulfates, chlorides, and nitrates of ammonia, potassium, and sodium, influences of these salts on the accuracy of the titration were studied. Among these salts, sodium sulfate was the only one that could be present without causing an error in the titration. In the presence of the others, consumption of potassium ferrocyanide varied with their amounts. Influence of ammonium chloride was greatest. On adding 10 g of it in 100 ml of solution, for instance, about -2 and -5% of errors were observed in direct and back titration, respectively. In the titration, therefore, ammonium and alkaline salts except sodium sulfate should be avoided. If they are present, correction must be made strictly under the same condition. The effects of amounts of indicators, potassium ferricyanide and those of acids and their concentration were also studied.
東北大学の論文

The Accuracy of the Titrimetric Determination of Zinc with Potassium Ferrocyanide

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