Simultaneous determination of tyrosine and Dopa using rotating carbon paste disk electrode

The anodic oxidation of tyrosine and Dopa in 1.08N H2SO4 was investigated at the carbon paste electrode by the method of chronopotentiometry, cyclic voltammetry, and voltammetry using rotating carbon paste disk electrode. Chronopotentiometric transition time constants, i0τ1/2/C, where C represents the bulk concentration of tyrosine and Dopa {β-(3, 4-dihydroxyphenyl) -l-alanine}, i0 is the current density and τ the transition time, are 630 and 425 μA·sec1, 2/ mM·cm2, respectively. In case of the voltammetry using rotating carbon paste electrode, the limiting current of tyrosine and Dopa in the same acid concentration is proportional to the square root of the speed of rotation of the electrode. Therefore, both electrode reactions are diffusion controlled processes. Anodic oxidation waves of both substances depend upon the concentration of H2SO4. The chemical reaction following the electrode reaction takes place especially in case of Dopa in the concentration more than 5N H2SO4. The limiting current of tyrosine in concentrated H2SO4 solution decreases with the increase of concentration of H2SO4 because of the increase of viscosity. Therefore, the concentration of H2SO4 has to be kept in the range of 0.91.2 N. The reproducibility of the limiting current for the anodic oxidation of tyrosine in 1.08 N H2SO4 is excellent in case of the successive use of the same electrode and even in the use of 8 times the decrease of the limiting current is only 2.5%. The limiting current of Dopa in 1.08N H2SO4 reproduces very well and the limiting current of 1.97 mM Dopa is 139.0±0.97 μA in the 95% confidence level. Recommended procedure is as follows : add 3 ml 36N H2SO4 to the sample solution containing 0.11 mM tyrosine and Dopa. Dilute to 100 ml with distilled water. Rotate the carbon paste disk electrode at the constant rotation speed of 900 rpm. Scan to the anodic side in the range from +0.4 to +1.3 volts (vs. SCE). Measure the limiting current for the oxidation of Dopa in the conventional way. Oxidation wave of tyrosine shows a current-potential curve of peak type. Limiting current at the peak potential, however, is proportional to the concentration of tyrosine. Fe (III), Br and I interfere. Aromatic primary amines interfere with the determination of Dopa because of the formation of the reaction product between oxidation product of Dopa and aromatic primary amine.