On the Volumetric Estimation of Zinc

Abstract

The potassium ferrocyanide method for titrating zinc using ferrous sulphate or diphenylamine as an internal indicator requires much experience before an operator becomes skilled in its use. The author is frequently bothered to judge the end point in the titration with potassium ferrocyanide. Studing the titration of zinc, the author proposes a simple and convenient method for the titration of zinc.
The method consists in precipitating zinc as sulphide with H₂S using monochloracetic acid and sodium monochloracetate as a buffer solution at about pH 2.4. Zinc sulphide, thus obtained is sandy precipitate and can easily be filtered. After washing several times with water zinc sulphide is transferred to a flask with a filter paper,and it is dissolved in a definite amount of about 0.35 N HCl, followed by boiling for about 5 minutes to expel H₂S. The acid solution is then cooled and diluted to about 200 cc, and the excess acid is successfully titrated with standard NaOH (about 0.35 N) using methyl orange as an indicator.
The NaOH solution is standardized against pure ZnO dissolved in HCl as previously explained. Phenolphthalein cannot be used, for Zn(OH)₂ begins to precipitate at about pH 6, as is shown in Fig. 1. The method is proved to be useful in analysing alloys containing zinc, such as brass and zinc duralumin.