Interference from chlorine and its elimination on the determination of total phosphorus in sea water by the peroxodisulfate digestion/molybdenum blue method

Abstract

Interference with chlorine on the formation of molybdenum blue was studied to determine the total phosphorus in sea water. When a sample containing chloride ion was digested with peroxodisulfate, chlorine was produced. The molybdenum blue formed in the absence of chlorine had absorption maxima at 880 and 710 nm, and the molar absorptivity at 710 nm was less than that at 880 nm. However, one of the absorption maxima of the molybdenum blue formed in the presence of chlorine shifted from 710 nm to 730 nm, and its molar absorptivity was larger than at 880 nm. The interference could be eliminated by boiling the digested sample and increasing the concentration of ascorbic acid in a mixed-reagent solution prepared by mixing acidic molybdate with ascorbic acid, but could not be eliminated by the addition of a sodium hydrogen sulfite solution.