Determination of the concentration of hydrofluoric acid in the solutions used for chemical polishing of crystal

Acid-base and potentiometric titrations are performed in order to determine fluoride concentration in solutions used for the chemical polishing of crystal. The main results presented in this paper show that: (i) the potential values measured with the specific LaF 3 electrode are not solely linked to the presence of fluoride ions F - but also to the presence of hexafluorosilicic acid, H 2 SiF 6 ; (ii) the proposed technique is feasible because the determination of the H 2 SiF 6 concentration is quantifiable even if H 2 SO 4 is the main acidic species in the liquid medium. The classical curve obtained by the acid/base titration (pH versus volume of added base solution) show the occurrence of two peaks that characterise, respectively, the neutralisation of H 2 SiF 6 by soda at pH<4·3 and the decomposition of SiF 2- 6 ions by soda leading to the probable formation of Si(OH) 4 .