Exploitation of reaction mechanisms for sensitivity enhancement in the determination of phosphorus by sequential...

The molybdenum blue method using antimony and ascorbic acid was studied for the determination of phosphorus (as orthophosphate) by means of sequential injection analysis (SIA). In order to avoid the interference of the Schlieren effect in the photometric measurements a stopped-flow kinetic approach was adopted monitoring the absorbance of the reaction bolus inside the flow cell. Aiming at enhancing the sensitivity of the method, the effect of the order of addition of the reactants was studied. It was found that the best sensitivity was attained by adding separate reagents and acidifying only after the phosphate, molybdate and antimony solutions were already mixed; the reductant (ascorbic acid) was then added. In this way a sensitivity enhancement in excess of 10 times was obtained when compared to the addition of the phosphate solution to the acidified mixture of molybdate and antimony. It is proposed that the difference in sensitivities could be explained by the existence of different mechanisms for the formation of the intermediate phosphoantimonyl molybdic acid (PMA). Thus the selected sequence in the order of addition, where sulphuric acid is added to the mixture of the other reactants would lead to higher production of PMA in turn conducting to a faster reduction reaction. The resulting SIA method was validated finding limits of detection (3s/m) and quantification (10s/m) of 0.0077 and 0.026 mg-P L^-1 respectively. Linearity was confirmed in the range up to 2 mg-P L^-1. Precisions (s_r, n = 10) were in the range 1.8%-4.0%. 32 water samples of different types and origins were analysed by the proposed method and by ion chromatography, obtaining a regression curve y = 0.990× - 0.0019, with a determination coefficient R² = 0.973.