Mechanism of salt error on the color development of phosphomolybdenum blue

Abstract

The molybdenum blue method, a widely used method for the determination of phosphate, is based on the formation and reduction of 12-phosphomolybdic acid, in which ascorbic acid with antimony(III) and tin(II) chloride function as a reductant. In the method, chloride ion in the sample affects the color intensity of the produced molybdenum blue. This phenomenon is called the salt error. The levels of the salt error are known to be less when ascorbic acid with antimony(III) is used compared to when tin(II) chloride is used. The present study examined the difference between ascorbic acid with antimony(III) and tin(II) chloride at levels of the salt error observed in molybdenum blue formation. Molybdenum blue reduced by ascorbic acid with antimony(III) or tin(II) chloride was extracted into MIBK, and the amounts of antimony or tin in the MIBK phase were determined by AAS. The atomic ratios of antimony to phosphorus and tin to phosphorus were determined. When 12-phosphomolybdic acid was formed from 30-50 μg of phosphate-phosphorus and reduced by ascorbic acid with antimony(III), the antimony to phosphorus ratio was found to be ca.2 in the presence and absence of chloride ion in a sample. Under the same condition, when tin(II) chloride was used, the amounts of tin in the MIBK phase decreased as the chloride ion concentrations of the sample increased, with tin to phosphorus ratios being ca.2 at 0% of chloride ion, and ca.0.7 at 5% chloride ion. The results indicate that chloride ion in a sample reacts with tin(II) forming a complex, thereby, chloride ion interferes with tin(II) associating with molybdenum blue. Meanwhile, because the chloride ion does not form a complex with antimony(III), antimony(III) can associate with molybdenum blue regardless of the presence of the chloride ion.