Consecutive spectrophotometric determination of phosphate and silicate in a sequential injection lab-at-valve flow system.

A new highly sensitive and selective sequential injection lab-at-valve spectrophotometric method for the consecutive determination of silicate and phosphate is described. The proposed method is based on the formation of specific ion-association complexes (IAs) of 12-heteropolymolybdates of phosphorus and silicon (12-MSC) with Astra Phloxine. The addition of an external reaction chamber (RC) to the SIA manifold made it possible to significantly improve the conditions for the formation of the analytical form used. The formation of the IA took place in the RC; the solution is mixed by passing an air flow through it. The interfering effect on the determination of phosphate from silicate was completely eliminated by choosing an acidity at which the rate of 12-MSC formation is very low. The use of secondary acidification in the determination of silicate led to the complete exclusion of the influence of phosphate. The tolerable ratio of phosphate to silicate and vice versa is about 100-times, which allows the analysis of most real samples without the use of masking agents or complex separation steps. The determination ranges are 3.0-60 μg L for phosphate as P(V) and 2.8-56 μg L for silicate as Si(IV) at a throughput of 5 samples h. The detection limits are 5.0 and 3.8 μg L for phosphate and silicate, respectively. Silicate and phosphate were determined in the tap water, river water, mineral water, main water of the Krivoy Rog (Ukraine) region, and the certified reference material of carbon steel.