Separation of Fluorine by a Microdif fusion Method aided by Hexamethyldisiloxane

Abstract

A method for the separation and determination of fluorine in geochemical samples by diffusion with hexamethyldisiloxane (HMDS) has been established on the basis of the method described by Sara and Wanninen. A recommended procedure is as follows.
Pipet 5 ml of a sample solution into one side (B1) of an outer compartment of a polyacrylic diffusion dish placed at an inclined position (Fig.1). Then, put 2.5 ml of HMDS-saturated 2.7 moli/ perchloric acid and 2 ml of conc. phosphoric acid in another outer side (B2), and 5 ml of a trapping solution (0.1 mol/l sodium hydroxide solution) in an inner compartment (A). Put on the lid (C) sealing with vaseline, and mix the solutions in the outer compartment. Leave the dish overnight in a horizontal position at room temperature. Determine the fluoride concentration in the trapping solution with a fluoride ion selective electrode after neutralization and buffering.
The recovery of 5-500, ag fluoride ranges from 98 to 101% for 19 determinations by this procedure. Aluminium ion retard the diffusion of fluorine remarkably when only HMDS-saturated perchloric acid is used (Table 2), but can be masked by phosphoric acid (Table 3). Addition of 2 ml of phosphoric acid is sufficient for masking up to 20 mg of aluminium ion (Al/F molar ratio=300). The effect of diverse ions was examined (Table 4). Almost all the ions examined could be tolerated up to 100 times in molar of fluorine. Several types of natural water samples of high electrolyte contents (sea water affected by a submarine volcano, strong acid hotspring water and condensate of active fumarolic gases), an apatite sample and a standard rock sample JB-1 (basalt) were analysed for fluorine by the proposed method (Table 5). The results agreed well with those obtained by other methods.