Simultaneous Determination of Orthophosphate and Silicate Ions in River Water by Ion-Exclusion Chromatography with Postcolumn Spectrophotometric Detection Using Molybdate and Malachite Green

The simultaneous determination of orthophosphate and silicate ions in river water was examined using ion-exclusion chromatography with spectrophotometric detection at 680 nm after derivatization with molybdate and malachite green. In this study, optimization of the color-forming reactant composition to form the ion association complex of heteropolyacids with malachite green was examined. The optimum concentration of ethanol (30%) in the reactant accelerated the formation of the ion-association complex, resulting in high sensitivity for the detection of ions. Using the optimized reactant containing 100 mM H_2SO_4/10 mM Na_2MoO_4/0.05 mM malachite green/30% ethanol, the calibration curve of orthophosphate and silicate ions was linear in the range of 0～200 μg L^ as P and 0∼2000 μg L^ as Si with a good correlation coefficient of 0.998. The relative standard deviations of the peak areas and the retention time of orthophosphate and silicate ions were between 0.5 and 3.2% for five repeated measurements. The detection limits of orthophosphate and silicate ions were 0.3 and 2.5 μg L−1, respectively. The detection limit for the orthophosphate ion was 5 to 10-times improved when compared with those of previous studies using molybdenum-yellow or molybdenum-blue as a derivatization with molybdate. The recovery tests using river water for orthophosphate and silicate ions were 97.2% and 102.5%, respectively. The developed method was successfully applied to the simultaneous determination of orthophosphate and silicate ions in practical river-water samples