Mechanistic study and kinetic determination of vitamin C employing the sequential injection technique

For the first time, the robust sequential injection (SI) technique has been employed for full kinetic investigation of the oxidation reaction of vitamin C. Iron(III) in sulphuric 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 were found to be 1, 1, and -1 for vitamin C, iron(III) and hydrogen ions respectively. On the basis of these values a rate law was developed and a plausible mechanism was established. A kinetic method for the analysis 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 1.6x10(-3) mol dm(-3) ammonium ferric sulphate in 0.02 mol dm(-3) sulphuric acid with the aspiration volume of 944 mul 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 found to be accurate, precise and fast.