Errors in the direct potentiometric electrode method of fluoride determination: adsorption, illumination and temperature effects

Abstract

Pre-soaking of clean plastic bottles for 1 day in dilute hydrochloric acid reduces adsorption of fluoride from dilute fluoride solutions, stabilising storage. Changes in ambient illumination cause an electrode potential drift, implying an increase in the fluoride concentration on commencement of irradiation. The potential does not necessarily return to the original value on cessation of irradiation. The extent of temperature dependence of the electrode is directly proportional to the state of deterioration (“age”) of the electrode membrane. Older electrodes show greater potential drift per degree of temperature change than newer electrodes; this is often large in the 15–30 °C range. Although the extent of drift is variable, the direction of drift is uniform, a temperature rise producing an apparent fall in the fluoride concentration. A temperature hysteresis exists on sample heating and cooling.