A precise coulometer

Abstract

This paper is a contribution to the application of coulometry to accurate analysis, and describes an apparatus which can be used for the accurate assay of “pure” sodium carbonate. An instrument has been built which measures coulombs with a probable error of ±25 p.p.m. as the product of a constant current and its time of flow. The current is maintained constant by an electric servo-system, and it is adjusted so that the voltage drop across a precise resistor is equal to the e.m.f. of a standard cell. Time is measured by a quartz-crystal clock. The resistor and cell are checked against local standards, which in turn have been calibrated against international standards by the National Physical Laboratory. The clock is checked against a broadcast frequency and the General Post Office Speaking Clock. Thus the quantity of electricity for a titration is referred ultimately to the fundamental standards of mass, length and time; the titre is independent of knowledge of the purity of any chemical substance.