Determination of chloride and fluoride in boronated water

Abstract

The performance of chloride-and fluoride-selective electrodes in boronated waters, typical of a pressurised water reactor (PWR) primary coolant, has been investigated. The precision and sensitivity of the mercury(I) chloride-mercury(II) sulphide membrane electrode were unaffected by the boric acid medium and the electrode was adequate for measurements below the specified limit of 150–200 µg l^–1 of chloride. Measurements of fluoride concentration in the range 25–250 µg l^–1 were straightforward, with a relative standard deviation of about 2%. The lanthanum fluoride membrane electrode can, therefore, be used to check that PWR primary coolant is within the specified limit of 150 µg l^–1. The boric acid in the sample causes a bias in direct potentiometry for fluoride, but this can be corrected by a simple mathematical equation. Alternatively, the bias can be avoided by the use of known addition potentiometry. Fluoride electrodes deviate from the theoretical sensitivity below a concentration of about 25 µg l^–1, but still have ample sensitivity down to 1 µg l^–1. In this concentration range, however, the response is very slow, taking 40–120 min to reach equilibrium and there is considerable variation between individual electrodes.