A novel method for the determination of vitamin C in natural orange juices by electrochemical monitoring of dediazoniation of 3-methylbenzenediazonium tetrafluoroborate Original Research Article

A novel procedure for the determination of L-ascorbic acid (vitamin C, AA) in natural orange juice samples is described. First, we examine the kinetics of reactions of m-methylbenzenediazonium (mMBD) tetrafluoroborate in aqueous solutions. The method involves two electrochemical procedures: by directly monitoring diazonium salt loss (dediazoniation reaction) and by monitoring the derivatization product from coupling unreacted diazonium ions with 2-naphthol-6-sulphonic acid to give an azo dye (coupling reaction). The diazonium ions, dediazoniation products and derivatization products like azo dyes are electroactive compounds, showing well-defined peaks at −50 and −430 mV for diazonium ions, at +1.236 V for m-cresol (major dediazoniation product), both in 0.1 M HCl, and −659 mV for the azo dye in 0.01 M NaOH (all peak potentials vs. Ag/AgCl reference electrode). Secondly, a differential pulse polarographic method based on the accelerating effect of AA on the dediazoniation reaction of mMBD is described. This reaction is strongly accelerated in the presence of AA. The observed rate constant is linearly correlated with the total AA concentration and the calibration graph is linear over at least three orders of magnitude. mMBD kinetics are first-order (with respect to mMBD) in absence of and in presence of AA.