Abstract: A new type of chemically modified electrode (CME) was fabricated by electrodeposition of palladium(II)-substituted Dawson type heptadecatungstodiphosphate, K-8[P2W17O61Pd(H2O)] (abbreviated as P2W17Pd in the following), onto a carbon fiber microelectrode (CFME). A pair of waves was observed on the P2W17Pd CFME, which is ascribed to the redox process of the palladium center in the heteropolytungstate. After continuous potential scanning for 30 min in a pH 4.0 buffer, 92% of the original electrode response remained for the P2W17Pd CFME. The P2W17Pd CFME had high electrocatalytic activity for nitrite reduction and exhibited good reproducibility and stability. The catalytic peak current was found to be linear with the nitrite concentration in the range of 1.0 x 10^-7 similar to 1.2 x 10^-3 mol L-1 (at 25°C) with a correlation coefficient of 0.9886 The detection limit (signal/noise = 3) was found to be 2.0 x 10^-8 mol L-1. The response time of the microsensor for nitrite measurement was less than 15 s. For 10 parallel measurements of 1.0 x 10^-5 mol L-1 nitrite, the relative standard deviation (RSD) was found to be 4.5%. The sensitivity of the microsensor was 0.57 nA (µmol l-1)-1. The P2W17Pd CME was applied successfully as an electrochemical detector (ECD) to determine the nitrite level in rat brain by flow injection analysis (FIA) coupled with microdialysis sampling. The linear range was over three orders of magnitude and the detection limit was 3.0 pmol for nitrite determination. The mechanism of the catalytic reaction was also addressed.

Abstract: For the 1st time, the robust sequential injection (SI) technique was employed for full kinetic study of the oxidation reaction of vitamin C. Iron(III) in sulfuric acid media was used as an oxidant and 1,10-phenanthroline as an indicator and the resulting solution of tris 1,10-phenanthroline-iron(II) complex was monitored spectrophotometrically at 510 nm. The reaction orders with respect to each reagent were determined by the SI-technique and are 1, 1, and - 1 for vitamin C, iron(III) and hydrogen ions respectively. From these values a rate law was developed and a plausible mechanism was established. A kinetic method for the anal. of vitamin C in drug formulations based on the results obtained above was thus validated. The drug in the range 20-300 ppm was determined by the kinetic method using 1.6 x 10^-3 mol L-1 ammonium ferric sulfate in 0.02 mol L-1 sulfuric acid with the aspiration volume of 944 µL and the fixed-time of 180 s. The results thus obtained by the SI-kinetic method were statistically compared with those obtained by the British Pharmacoebia standard method and are accurate, precise and fast.

Abstract: A rapid and convenient method for the determination of furfural is presented that is based upon sequential perturbation of the Mn(II)-catalyzed B-Z oscillating system with different amounts of furfural using a continuous-flow stirred tank reactor (CSTR). When the sample was injected, the change in the amplitude and/or period was linearly proportional to the logarithm of the concentration of furfural over the range 3 x 10^-8~1 x 10^-5 mol L-;1. This method gave a detection limit of 3 x 10^-9 mol L-;1 under optimum conditions. Finally, the possible mechanism of furfural perturbation in the oscillating reaction is discussed. When the furfural was injected into the Mn(II)-catalyzed B-Z oscillating system, the change in the amplitude and/or period was linearly proportional to the logarithm of the concentration of furfural over the range 3 x 10^-8~1 x 10^-5 mol L-;1, with a detection limit of 3 x 10^-9 mol L-;1 under optimum conditions.