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.

Kinetic catalytic determination of trace Cu(II) in water samples with the thioglycolic/thiolactic acid-chromate reaction.

The use of two novel similar indicator reactions as applied to the kinetic determination of Cu(II) in water is investigated. The methods rely on the catalytic effect of the analyte on the oxidation of thioglycolic (TGA) and thiolactic (TLA) acids by chromate in acidic media. The extent of the reactions was followed spectrophotometrically at 345 nm. Pseudo-first-order rate coefficients, k(obsd), were determined as a function of catalyst concentration. Interference of Fe(III) and Pb(II) was suppressed by complexation with pyrophosphate. For the reaction of TGA, a linear regression for k(obsd) versus [Cu(II)] was obtained for the entire concentration range considered. Although the plot corresponding to TLA oxidation exhibits a sharp change of slope at approximately 1.8x10(-5) M Cu(II), it can still be described effectively by two linear regressions with different slopes. The reaction of TGA is more sensitive than that of TLA at low Cu(II) concentration. The opposite is true for higher catalyst contents. The detection limits were 65 microg L(-1) for TGA and of 80 microg L(-1) for TLA oxidation, respectively. The relative standard deviations, of 0.4% for TGA and 1.1% for TLA oxidation, respectively, were obtained for five replicate runs at 1000 microg L(-1). Samples of river and wastewater from the mining region of Baia-Mare, Northern Romania were analyzed using the more sensitive reaction of thioglycolic acid. Results were compared to those obtained by the officially standardized methods. Good agreement was obtained, even for an untreated sample. Measurements did not require prior separation of interfering species.

Catalytic determination of Pb(II) in the presence of Cu(II).

A kinetic method is presented to determine micro-molar amounts of Pb(II) from various river and wastewater samples, in the presence of trace copper. The procedure is based on the catalytic effect of both species on the oxidation of mercaptosuccinic acid by chromate in acidic media. The extent of the reaction is followed spectrophotometrically at 420 nm and pseudo-first-order rate coefficients of the rate-determining step are determined as a function of catalyst concentrations. The optimum operating conditions (ionic strength, temperature, and concentration of reagents) regarding sensitivity towards lead were established. Interference by several ionic species has been studied. The effect of Fe(III), the only severe interferent, is suppressed by complexation with 1,10-phenantroline. The bi-component calibration model employs an artificial neural network to compute the Pb(II) concentration from a k(obsd) value and the a priori-known Cu(II) concentration of the sample. Working concentration ranges are 20-2160 micro g L(-1) for Pb(II) and 80-650 micro g L(-1) for Cu(II), respectively. Detection limits are 20 micro g L(-1) Pb(II) and 80 micro g L(-1) Cu(II), respectively. The relative standard deviations (3 measurements) for four different testing points are lower than 2.5%. The method was applied to samples of river and wastewater of the mining region of Baia-Mare, Northern Romania. The results were compared to those obtained by an officially standardized AAS method. Good agreement was achieved. The method is inexpensive, fairly rapid, and sensitive. Its working range covers the exact range of concentrations usually encountered in the mentioned geographic area.