Fast determination of total fluoride by direct potentiometry in samples of aluminium fluoride and cryolite

Abstract

A direct potentiometric method for the determination of the total fluoride concentration in aluminium fluoride and cryolite is proposed. TISAB IV [disodium tartrate–tris(hydroxymethyl)methylamine–hydrochloric acid] buffer is used to complex the aluminium ions and NaOH is added to the standard solutions to keep constant both the ionic strength and the Na⁺ concentration as well as the pH. No hydroxide ion interference has been detected even though the pH of this solution is 8.57. A sensitivity loss has been recorded as the electrode ages; in spite of this, the potentiometric measurements are free of systematic errors. The response of the Radiometer and Orion fluoride electrodes with the presence of aluminium in the solution has also been studied, and it was noted that addition of aluminium ions to the calibration solutions was necessary when the Orion electrode was used in order to avoid high error levels. The main random error obtained in the whole process is the owing to the reading using the fluoride electrode. A comparison with the Willard–Winter distillation method was also performed.