2002 Volume 51 Issue 12 Pages 1159-1164
The adsorption of vitamins at a mercury electrode was investigated by measuring differential capacity-time curves with a flow injection method, differential capacity-potential curves and cyclic voltammograms at a hanging mercury drop electrode. An experiment to measure the differential capacity-time curves was done as follows. A supporting electrolyte solution (0.05 M Na2SO4) was placed in a reservoir. Teflon tubing was used to connect the detector to a peristaltic pump and an injection valve. Solutions of vitamins were introduced via the injection valve. The adsorption of vitamins was almost irreversible. The adsorption of riboflavin and its reduced form (leucoflavin) was irreversible except at -0.25 V and -0.7 V. The less adsorption of leucoflavin i.d.fferential capacity-time curves is interpreted as follows. In the experiment involving the flow injection method, the molecules of riboflavin carried to the electrode are adsorbed and reduced to leucoflavin. Because the adsorption structures of riboflavin and leucoflavin are different, it takes time for the leucoflavin molecules to be adsorbed. During this time a part of the leucoflavin is carried away from the electrode. The adsorption of Vitamin B12 is irreversible between -0.1 V and -1.4 V. Vitamin B12 is considered to be one of the strongly adsorbed molecules at a mercury electrode. In the differential capacity-time curves between -0.4 V and -1.0 V, kinks are observed. The kinks are considered to be due to the different adsorption structures (flat and perpendicular) of vitamin B12. The reduced form of thioctic acid i.d.sorbed. In the differential capacity-time curves, two adsorption structures are found. One is reversible adsorption with (CH2)4COOH to the electrode and the other is irreversible adsorption with S atms to the electrode. It is considered that the irreversible adsorption of vitamins is favorable for adsorptive stripping voltammetry.
