Instrumentation for microcoulometry by photocoupled amplifiers and the application to microdetermination of chloride ion

Abstract

A new type DC amplifier with a photo-coupler was constructed and used for the microcoulometric argentometry of chloride ion. The instrument was consisted of both the preamplifier for detection of the potential difference between indicator electrodes (Ag-AgCl electrode and SCE) in the cell and the power amplifier for supplying the electrolytic current to generate silver ion in the cell. Each amplifier was operated by different DC sources and the signal of the preamplifier was connected to the main amplifier by the photo-coupler. In order to control the current to be zero at the end point, the definite potential of inverse sign which had the same absolute value as that of the end point of the titration curve, was supplied between the indicator electrodes. When certain amount of chloride ion was added to the electrolyte containing potassium nitrate and ethanol, the potential difference appeared between the indicator electrodes. The current corresponding to the detected potential difference was used for the electrolytic generation of silver ion from the silver anode. The generation of silver ion was automatically controlled to be zero at the end point of the titration. In this method, the disturbance by the generating current to the accurate measurement of the potential difference between the indicator electrodes was well removed. Chloride ion down to 2μg in 50 ml electrolyte could be determined by this method within 10% error.