Improved simultaneous reaction-rate determination of phosphate and silicate by use of the continuous addition of reagent technique

The continuous addition of reagent (CAR) technique was used as an effective means of improving the simultaneous reaction-rate determination of phosphate and silicate based on the formation of the corresponding heteropolymolybdates. The special features of the CAR technique allow the flexible manipulation of the reactant concentrations and reaction kinetics for controlling the classical synergistic effect involved in this reaction. Hence, resolution of mixtures of the two anions with phosphate-to-silicate ratios from 10 : 1 to 1 : 20 at the micromolar level can be resolved with good accuracy (errors less than 10% in most cases) and precision (relative standard deviations about 2.5–4.5%). The method compares favourably with existing reaction-rate alternatives, especially in the resolution of mixtures with excess of phosphate over silicate, which is reported in this paper for the first time. The determination of phosphorus and silicon in sunflower was selected to validate the proposed method for the analysis of real samples.