Overcoming interference from hydrolysable cations during the determination of sulphuric acid by titration Original Research Article

During the neutralisation of mineral acids, hydrolysable cations precipitate and consume the titrant. Depending on the concentration and type of cation, this consumption leads to erroneously high titration results. The following cations were tested: iron(III), iron(II), nickel, cobalt, copper and manganese. Various methods have been suggested to overcome this interference, but all suffer from certain limitations — especially at high cation to free acid ratios. A simple and practical method is suggested to overcome the effect at high cation concentrations. It is based on the addition of alkaline salt (80% m/v) in methanol to the sample. This addition enhances the activity 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 lower pH, and a better resolution of the free acid and coinciding cation curve. This effect, combined with performing the titration in a methanolic environment, decreases the hydrolysation of the cations and improves the inflection 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 so that a more than 10-fold excess of ferric cation concentration (the most serious interference) could safely be tolerated during the free acid titration. The method was statistically evaluated. It proved to be accurate and precise and a percentage R.S.D. of between 2.6 and 0.7 was obtained for various cation-containing, sulphuric acid solutions.