Comparison between amperometric and true potentiometric end-point detection in the determination of water by the Karl Fischer method

Abstract

A rapid and sensitive method using true potentiometric end-point detection has been developed and compared with the conventional amperometric method for Karl Fischer determination of water. The effect of the sulphur dioxide concentration on the shape of the titration curve is shown. By using kinetic data it was possible to calculate the course of titrations and make comparisons with those found experimentally. The results prove that the main reaction is the slow step, both in the amperometric and the potentiometric method. Results obtained in the standardization of the Karl Fischer reagent showed that the potentiometric method, including titration to a preselected potential, gave a standard deviation of 0·001 1 mg of water per ml, the amperometric method using extrapolation 0·002 4 mg of water per ml and the amperometric titration to a pre-selected diffusion current 0·004 7 mg of water per ml. Theories and results dealing with dilution effects are presented. The time of analysis was 1–1·5 min for the potentiometric and 4–5 min for the amperometric method using extrapolation.