Overcoming interference from hydrolysable cations during the determinationof sulphuric acid by titration

During the neutralisation of mineral acids, hydrolysable cations precipitat e and consume the titrant. Depending on the concentration and type of catio n, this consumption leads to erroneously high titration results. The follow ing cations were tested: iron(III), iron(II), nickel, cobalt, copper and ma nganese. Various methods have been suggested to overcome this interference, but all suffer from certain limitations - especially at high cation to fre e acid ratios. A simple and practical method is suggested to overcome the e ffect at high cation concentrations. It is based on the addition of alkalin e salt (80% m/v) in methanol to the sample. This addition enhances the acti vity of the protons, which coincides with a shift in the inflection of the titration curve. This results in a shift of the titration end point to a lo wer pH, and a better resolution of the free acid and coinciding cation curv e. This effect, combined with performing the titration in a methanolic envi ronment, decreases the hydrolysation of the cations and improves the inflec tion of the titration curve. Previous method could tolerate a ferric ion to sulphuric acid ratio of 2:1. The suggested method improves on this ratio s o that a more than 10-fold excess of ferric cation concentration (the most serious interference) could safely be tolerated during the free acid titrat ion. The method was statistically evaluated. It proved to be accurate and p recise and a percentage R.S.D. of between 2.6 and 0.7 was obtained for vari ous cation-containing, sulphuric acid solutions. (C) 2001 Elsevier Science B.V. All rights reserved.