THE USE OF TOTAL IONIC-STRENGTH ADJUSTING BUFFERS IN DETERMINING FLUORIDE CONCENTRATION IN SOIL SOLUTIONS BY ION-SELECTIVE ELECTROMETRY

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 mum) solutions. Recovery of F added to unfiltered and 0.22 mum 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 micro-particulates; 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 muM, while all subsoil samples had soil solution F concentrations <1 muM.