Abstract
In the analysis of fluorine in glass, the carbonate fusion is time-consuming when the melt is leached with water and filtered to remove the bulk of aluminum, and the result is erroneous when it is dissolved in water, neutralized to pH 6 and then the citrate buffer is added because of the coprecipitation of fluorine with aluminum hydroxide.
Four buffer solutions of citrate were examined in the ability of masking of aluminum. The more concentrated solution of citrate was found to be more satisfactory for the determination. It was unnecessary to add sodium chloride to the buffer solution. Since the TISAB (total ionic strength adjustment buffer) contains a very small amount of citrate and a large amount of sodium chloride, it is very unsatisfactory. The tolerance for aluminum increases markedly with a decrease in the concentration of fluoride ion in a buffer solution of 0.5 M citrate at pH 6. The reason is discussed theoretically. The following elements in the amounts(ppm) indicated can be tolerated; Ti, Cr, Pb, V(10), Ni, Mg(25), B(40), Cd, K, CO32-(50), Fe(150), Ca(200), Si(500), and P(10000).
According to the above result, the accuracy of the fluoride determination in the presence of an excessive amount of aluminum can be confirmed by repeated measurements after each 10- or 5-fold dilution until the last two data coincide while the concentration of citrate is kept 0.5 M, as the accuracy of the potential measurement is constant over a concentration level down to 10-5 M F- where Nernst's equation holds.
Fuse 0.15 g of glass with 1 g of sodium hydroxide, which has been dehydrated by heating with a small amount of ethanol, in a 100 mlnickel crucible at 500°C for 20 min. Cool and dissolve in 30 ml of hot water. Adjust the pH of the solution to 2 with HCl(1 : 1) in a polyethylene beaker. Add 10 ml of a buffer solution [294 g of sodium citrate dissolved in 1 l adjusted to pH 6 with HCl(1 : 1)], and dilute to 100 ml with water. Mix a 10 ml aliquot with 10 ml of the buffer and measure the potential.
In the case of phosphate rock, shake a 0.3 g sample with 5 ml HCl(1 : 1) and 150 ml of water in a 250 ml polyethylene volumetric flask for 1 hr, dilute to the mark, and treat a 50 ml aliquot of the solution by the procedure described above. For water sample, take an aliquot containing more than 0.5 mg F-, render alkaline with sodium hydroxide and evaporate to about 70 ml.
Results for five glasses, three phosphate rocks, and a well water agreed well with the certified values and those by other methods.
