Various colormetric methods for determining magnesium in natural waters have been studied, and the methods using 8-oxyquinolin, ammonium molybdate and titan yellow were studied most frequently(1)), following the studies on the interfering ions(2)). Brilliant yellow(3)), l-amino-2-naphthol-6-sulfonic acid(4)) and other new reagents were also used. E. D. T. A. was used, but the determination by using this reagent is not exact. Present reagent already reported by T. Ashizawa(5)), magneson II, is insoluble in water, soluble in alkali and hardly soluble in ethanol. In the existence of magnesium ion, the color of solution varies from pink-violet (in alkali) and orange (in ethanol) to blue-violet. This variation of color was evaluated photometrically by Shimadzu photoelectric spectrophotometer, and moreover the grades of interference by interfering ions were clarified.

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