Direct determination of caustic hydroxide by a micro-titration method with dual-wavelength photometric end-point detection

An automatic micro-titration method for the direct determination of caustic hydroxide in aluminate solution is proposed, which is based on acid-base titration with sulfosalicylic acid and phenolphthalein as the titrant and the indicator, respectively. Sulfosalicylic acid is also the complexing agent for aluminum to prevent the formation of aluminum hydroxide precipitate. Since only 30 μl of sample is injected into the titration cell, direct titration without dilution is possible; titrantʹs consumption is reduced to less than 7.5 ml even for the most concentrated samples. Furthermore, the end-point is detected by a light emitting diode (LED)-based dual-wavelength photometer, with one of the wavelengths as a reference for correcting the interference of barium carbonate precipitate. Samples containing 50–300 g l−1 (expressed as Na2O) of caustic hydroxide can be analyzed with a sampling frequency of 45 samples per hour. A relative standard deviation of 0.27% is obtained for 13 determinations of a typical sample containing 170.4 g 1−1 of sodium oxide.