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.