A sensitive immobilized enzyme electrode for l-lactate is described. Toluidine blue O (TBO) and lactate dehydrogenase (LDH) were co-immobilized onto the surface of a graphite electrode by means of a dialysis membrane. l-lactate is oxidized to pyruvate by LDH in the presence of nicotinamide adenine dinucleotide NAD+ which is then reduced to NADH. The NADH formed is electrocatalytically oxidized by TBO and the reduced mediator is detected at 0 V vs. a AgAgCl reference electrode. Different dialysis membranes were tested in order to improve stability. The most stable enzyme electrode was fabricated with a benzoylated membrane, showing a half-life of eight days. This amperometric electrode responds fast and linearly to l-lactate in a concentration range 10−6 − 6 × 10−5 M. The limit of detection is 4 × 10−7M. The enzyme electrode was applied to the determination of l-lactate in food samples. Results were compared to those obtained using a spectrophotometric method, showing a good agreement.

The adsorptive stripping voltammetric behaviour of folic acid and riboflavin was studied at a hanging mercury drop electrode by phase-selective a.c. voltammetry. In 0.1 M sodium acetate buffer (pH 5.0) a cathodic scan gave peaks at −0.29 and −0.55 V vs. Ag/AgCl for riboflavin and folic acid, respectively. The adsorptive stripping response was evaluated with respect to concentration dependence and preconcentration time. Both compounds can be simultaneous determined with a relative standard deviation of 1.4% at 5 × 10−8 M riboflavin and 0.74% at 10−8 M folic acid. The method compared favourably with liquid chromatography with UV detection and was succesfully applied to the simultaneous determination of both compounds in multivitamin preparations. The average contents of riboflavin and folic acid were found to be 15.19 mg ± 2.1% and 1.8 mg ± 2.6%, respectively.