Comparison of spectrophotometric and potentiometric detection for the determination of water using Karl Fischer method under flow injection analysis conditions

Abstract

Flow injection procedures for the determination of water in methanol using Karl Fischer method are presented. Both spectrophotometric and potentiometric detection systems were investigated and critically compared with respect to their performance characteristics. A novel tubular differential potentiometric flow through cell has been developed. Using spectrophotometric detection the determination of water at a sampling frequency of about 120 h −1 is possible in the concentration range 0.01–1%. Precision is better than 3% R.S.D. over the entire concentration range. The detectability of the method is severely affected by the presence of water traces in the carrier solvent but with appropriate preventive measures, concentrations as low as 0.002% are accessible. The specifications of the potentiometric variant compare well with the spectrophotometric detection method, yet the former offers slightly better precision but suffers from reduced linear range. Automation of the two methods has been realised and the applicability for on-line monitoring purposes are outlined. Particular emphasis is paid to the calibration problematic. Modified reverse FIA has also been applied and shown to be a simple and reliable way of detecting deviations from nominal values as required in several process control situations.