A direct potentiometric method for the determination of the total fluoride concentration in aluminium fluoride and cryolite is proposed. TISAB IV [disodium tartrate-tris(hydroxy-methy1)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.
The interferences in the determination of Ni by flame atomic absorption spectrometry due to the presence of copper and fluoboric acid have been studied according to a previously proposed calibration method ' that allows the investigation and quantification of the matrix effect. The ability of traditional calibration methods, like standard addition (SAM) or direct calibration approach, to overcome matrix effects has been studied. Such investigations have shown that direct calibration could provide as far as a -42% error in nickel determination, whereas SAM error could be about +17%. However this large error could be decreased to f 2% when the mathematical model here proposed is applied.Once the method is verified to be able to solve matrix effects, it was applied to the determination of nickel in a real sample of copper fluoborate. The
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