Simultaneous and automatic determination of hydroxide and carbonate in aluminate solutions by a micro-titration method Original Research Article

An automatic micro-titration method for the simultaneous determination of hydroxide and carbonate in sodium aluminate solution is proposed. Thirty μl of the sample is injected and propelled into a titration cell by a reagent solution containing sulfosalicylic acid, barium chloride, and phenolphthalein. Sulfosalicylic acid is both the titrant and the complexing agent for aluminum to prevent the formation of an aluminum hydroxide precipitate. Firstly, hydroxide is titrated by sulfosalicylic acid to the end point indicated by phenolphthalein while carbonate is present as barium carbonate. More reagent solution is gradually added until all the barium carbonate is dissolved. A light-emitting diode and phototransistor-based photometer is employed to record the absorbance change during the titration process, from which the end points for both hydroxide and carbonate could be obtained. The proposed method can be employed to analyze industrial sodium aluminate solutions containing 50–300 g l−1 hydroxide and 0–50 g l−1 carbonate (expressed as Na2O) with a sampling frequency of 45 samples per hour. The relative standard deviations are 0.28% for 170.4 g l−1 hydroxide and 0.37% for 35.6 g l−1 carbonate (both as Na2O, n=13).