Determination of fluorine and chlorine in geological materials by induction furnace pyrohydrolysis and standard-addition ion-selective electrode measurement

Abstract

Fluorine and chlorine in geological materials are volatilized by pyrohydrolysis at about 1150° in a stream of oxygen (1000 ml/min) plus steam in an induction furnace. The catalyst is a 7:2:1 mixture of silica gel, tungstic oxide and potassium dihydrogen phosphate. The sample/catalyst mixture is pyrohydrolysed in a re-usable alumina crucible (already containing four drops of 1 + 3 phosphoric acid) inserted in a silica-enclosed graphite crucible. The absorption solution is buffered at pH 6.5 and spiked with 1.6 μg of fluoride and 16 μg of chloride per g of solution, to ensure rapid and linear electrode response during subsequent standard-addition measurement. The simple plastic absorption vessel has ⪢99.5% efficiency. The 3 s limits of detection are 5–10 μg/g and 40–100 μg/g for fluorine and chlorine respectively. The procedure is unsuitable for determining chlorine in coal.