Determination of fluoride in soil water extract through ion chromatography

Abstract

Estimation of water-soluble fluoride content is essential in polluted soils so that necessary corrective measures can be taken to prevent or minimize its entry into the food chain. Although fluoride can be estimated through titrimetry, colorimetry or through ion-selective electrode, its estimation in soil extract through ion-chromatography is generally not done. This is because considerable amount of acetate is found in the extract, which elute closely with fluoride during separation and a number of polyvalent anions accumulate in the column causing high background noise. An ion chromatographic method has been standardized for determination of fluoride in soil extracts. For developing the method, IonPac AG4A-SC guard column, IonPac AS4A-SC analytical column, ASRS-I conductivity suppressor and conductivity detector were used. Sample volume was 20 μL. Solution of Na2B4O7 (0.005 M) was used as eluent with 2.0 mL min−1 flow rate and 2.6 min after injection 0.05 M Na2B4O7 was passed to wash the column. Washing was continued for 1.5 min and thereafter, columns were equilibrated with 0.005 M Na2B4O7 eluent. The whole operation was completed within 13 min. Washing the column with concentrated eluent (i.e., 0.05 M Na2B4O7) was found necessary after each analysis because background conductivity as well as noise increased probably due to contamination of column by polyvalent anions. Water extracts of 118 different soils were estimated for fluoride content. Fluoride content in these soils decreased with increasing acidity. A considerable number of deep tube well sources of drinking water in the fluorosis-affected villages of Mandla district were found to contain fluoride above the safe limit.