Carbonate-nitrate alkali sintering for fluoride determination in geological materials: mechanistic insights, interference control and validation with reference materials.

Reliable fluoride determination in geological materials is often limited by incomplete liberation of structurally bound fluoride and by solution-phase interferences from multivalent cations. In particular, Al and Fe can form fluoride complexes, whereas Ca and Mg may reduce free fluoride through precipitation or association reactions. This work illustrates a Na2CO3-KNO3 alkali sintering method coupled with fluoride ion-selective electrode measurement, in which thermochemical matrix decomposition is combined with citrate complexation, pH-controlled interference removal, and TISAB-based measurement. The method was optimized with respect to flux composition, flux-to-sample ratio, sintering temperature, and pH adjustment prior to TISAB addition. A flux ratio of 2:1, flux-to-sample ratio of 5:1, and sintering at 685 ± 5 [Formula: see text] for 30 min after attaining set temperature provided the most suitable conditions for fluoride recovery during initial optimization. Adjustment of the sample extract to pH ~8.5 after citrate addition improved fluoride recovery by reducing Al/Fe-related complexation, while citrate helped suppress Ca/Mg-induced fluoride loss. After TISAB addition, the final measurement solution was buffered to pH 5.5-6.0. The optimized method gave recoveries in the range of 94-98% for a suite of geological reference materials covering soils, stream sediments, lateritic material, mafic/intermediate rocks, quartz-rich rock, and phosphorites having fluoride concentration in the range of 133 to 40,400 mg kg-1. The method LOD and LOQ, estimated from processed blanks, were 15 and 50 mg kg-1, respectively. The study presents an integrated approach in which fluoride liberation and matrix-interference control are addressed together rather than as separate analytical steps. The present work advances conventional alkali sintering-ISE approaches by integrating Na2CO3-KNO3 assisted fluoride liberation with citrate and pH controlled solution-phase speciation management. This combined strategy enables effective decomposition of diverse geological matrices while minimizing matrix-induced suppression of free fluoride activity, thereby improving the accuracy of fluoride determination across compositionally varied reference materials.