The effect of hydrolysed aluminium species in fluoride ion determinations

The limited ability of added ligands to mask the interference of Al(III) in fluoride determination by means of ion-selective electrode measurements has been re-examined, and an explanation based on competing equilibria developed. When the Al(III) level exceeds the fluoride content, the excess of A1(III) forms colloidal hydrous oxide particles in the pH range 4-9, and this solid strongly sorbs fluoride. Under these conditions, masking ligands have to promote both decomposition of AlF(x), soluble complexes and release of sorbed fluoride by substrate dissolution. The latter is a slow process, particularly with an "aged" gel, and long equilibration periods can be required for total fluoride release. Ion-chromatography studies have shown that the amount of fluoride lost through sorption on the hydrous oxide particles (isolated during the membrane filtration/degassing step) can be quite high. By analogy, preliminary phase separation of natural water samples must also separate sorbed fluoride and soluble fluoride complexes. In the presence of A1(III), accurate analysis for fluoride requires removal of the aluminium, or isolation of fluoride from the matrix, or very careful pretesting of masking efficiencies.