A KINETIC SEQUENTIAL INJECTION ANALYSIS METHOD FOR SILICATE DETERMINATION IN WATER SAMPLES CONTAINING PHOSPHATES

Original Paper

Abstract

This manuscript describes the adaptation of Sequential Injection Analysis (SIA) for the determination of silica in boiler feed water containing phosphates as a routine method being adopted to a Petrochemical industry in Saudi Arabia (Ibn Zahr). The method is based on the reaction of ammonium molybdate with silica to form an intensely yellow complex (ammonium molybdosilicates) which is reduced to blue complex monitored spectrophotometrically at 813 nm. A comparison was made on the effectiveness of triammonium citrate and oxalic acid to eliminate phosphate by rendering it innocuous. Interference study of phosphate on this method was also studied. The sensitivity of the method was enhanced by sandwiching the reagents in the holding coil between two samples of 100 μL volume each and dispensing to the detector at a flow rate of 10 μL/s. The Reagent and samples were stacked into four distinct zones in a 190 cm long holding coil. Another 190 cm long reaction coil was used to ensure more uniform mixing of samples and reagent by radial dispersion. The sensitivity was further enhanced by heating the holding coil at 60 C in a thermostated water bath. To maximize zone penetration while minimizing dilution of the sample the reaction mixture was delayed at the holding coil for 3.53 mins (fixed time kinetic method). The effect of delay time, flow rate, temperature and volume of analyte and reagents on the sensitivity of this method was also studied. The linear working range is 0.5 ppm to 50 ppm. The detection limit is 0.5 ppm with a sample through put of 10 samples/hour. The method was validated by comparison with (APHA) batch spectrophotometric method. The results were found to be in agreement with these methods thus indicating accuracy. The method described has good reproducibility, rapid and suitable for online and in situ determination.