Consecutive chelatometric titration of sum of calcium and magnesium, manganese, and zinc in acid mine drainage

Abstract

A new consecutive titration procedure with ethylene-diaminetetraacetic acid (EDTA) is proposed for the rapid determination of Ca2+, Mg2+, Mn2+ and zn2+ in acid mine waters. The procedure is based on the fact that manganese is liberated by a selective decomposition of its cyano-complex when a suitable reducing agent is added to the solution containing cyano-complexes of these metal ions. The reaction is expressed as: [Mn(CN)6]3-+e+EDTA4-→MnEDTA2-+6CN- The sum of Ca and Mg is first obtained by direct titration at pH 10, by masking Zn2+ and Mn2+ with KCN and with thymol phthalein complexone (T. P. C.) as indicator. Manganese is then demasked by heating the solution to 60 to 70°C with sodium L-ascorbate and a known excess of EDTA, so that EDTA equivalent to manganese may be consumed. The remaining EDTA is titrated with standard Mg solution. Mn is calculated by difference. Zn can be determined by demasking the Zn cyano-complex with formaldehyde and titrating the liberated Zn with EDTA. Extension to other metal combinations is suggested. © 1967, Sage Publications. All rights reserved.