The use of total ionic strength adjusting buffers in determining fluoride concentration in soil solutions by ion selective electrometry

Abstract

Total ionic strength buffers based on citrate at pH 7.5 and cyclohexylenediaminetetra‐acetic acid (CDTA) at pH 5.2 were evaluated for use when determining fluoride (F) concentration in soil solutions. Both buffers achieved acceptable recoveries of F added to ultra‐filtered (0.025 μm) solutions. Recovery of F added to unflltered and 0.22 μm filtered solution was low when the CDTA buffer was used. This effect was attributed to the adsorption of F by micro‐particulate materials present in these samples. The use of this buffer would need to be restricted to samples in which micro‐particulates were not present. The performance of the citrate buffer was not affected by the presence of microparticulates; however, electrode response times, and departure from Nernstian response at low F concentrations, were greater when this buffer was employed. Fluoride was found to be present at low concentrations in the soil solution of the 60 virgin highly weathered soils tested. The F concentration in 75% of the surface samples was <3 μM, while all subsoil samples had soil solution F concentrations <1 μM.