Simultaneous Kinetic Spectrophotometric Determination of Ascorbic Acid and L-Cysteine by H-Point Standard Addition Method

Abstract.

The H-point standard addition method (HPSAM) is applied to kinetic data for the simultaneous determination of ascorbic acid (AA) and L-cysteine (Cys). If present in excess amounts, iron(III) is quantitatively reduced to iron(II) by AA or Cys which in turn, in the presence of phenanthroline (phen), produces a colored iron(II)-phen complex showing maximum absorbance at 510 nm. HPSAM is based on the difference of the reaction rates between AA and Cys with Fe(III). The linear dynamic ranges for the two analytes AA and Cys are 0.50–4.00 ppm and 1.00–8.00 ppm, respectively. The RSD% for 6 replications of AA and Cys at 3.00 ppm is 2.30 and 2.70, respectively. The predictability of the method is checked by determining the two analytes in real samples. The various effects of several similar reducing agents are investigated.