Potential application of the reaction between hydroquinone and chromate with respect to the kinetic determination of iron

The analytical potential of the reaction between hydroquinone and chromate in acidic media is explored with respect to the kinetic determination of iron in water samples. The extent of the reaction is followed spectrophotometrically at 350 nm. The reaction occurs more quickly in the presence of the metal ion, but the values of absorbance at reaction initiation and completion are not altered. No other transitional metal ion affects the course of the reaction, regardless of its concentration. This fact represents the most eye-catching and analytically exploitable aspect of this indicator reaction. Three procedures used to obtain calibration graphs from the same kinetic data are discussed: slope, fixed and variable time techniques. The reaction follows a sequence of two consecutive steps, both of first-order with respect to the colored species. First-order kinetics is preserved in the presence of iron. Curve fitting is used to determine the corresponding rate coefficients. The slope method requires much data and uses plots of rate constants against analyte concentration for calibration purposes. In this case, the best detection limit (0.5 mg l^–1) is given by the faster stage. On the other hand, the rate-determining step enables more precise results. The fixed and variable time methods rely on similar principles: they register either the value of absorbance achieved at a predetermined reaction time (here, 50 s) or the time interval required for the absorbance to drop to a predetermined value (here, 0.15 absorbance at 350 nm). In both cases, ratios between the average value from the blind runs and all individual values are plotted against the analyte concentration. The best results (detection limit of 0.3 mg l^–1) are derived from the variable time procedure. Advantageously, neither of the techniques require the entire kinetic curve, and so sophisticated equipment is not needed.