Effects of acidity and molybdate concentration on the kinetics of the formation of the phosphoantimonylmolybdenum blue complex

The effects of variation of acidity and molybdate concentration on the spectrophotometric determination of phosphate by the method of Murphy and Riley were studied. The [H+]:[Mo] ratio was found to be a crucial parameter which not only influences the form of the final reduced complex, but also plays a key role in controlling the reaction kinetics. Normal colour formation of the reduced phosphoantimonylmolybdenum blue complex was only observed with a ratio between 60 and 80; the reaction was faster at low acidities. Silicate is a possible source of interference at high temperatures, but the formation of the silicoantimonylmolybdenum blue complex was found to be much slower than that of the corresponding phosphorus species. © 1990.