Photoelectrochemical determination of ascorbic acid using methylene blue immobilized in alpha-zirconium phosphate

Photoelectrochemical analysis of ascorbic acid is reported in aqueous solut ion at a platinum channel electrode using the dissolved phenothiazine dyes methylene blue and methylene green. This is achieved by measuring the curre nt produced by immediate electrooxidation of the reduced form of the dyes p roduced during the 1:1 photoreaction between the dyes and ascorbic acid ind uced by visible light. This behavior is similar to that reported previously for toluidine blue, and rate constants for the photochemical reaction of 3 60 +/- 20 mol(-1) dm(3) s(-1) for methylene blue and 150 +/- 20 mol(-1) dm( 3) s(-1) for methylene green are determined using simulation of the observe d photocurrent as a function of flow rate. The light used for this determin ation was at 620-625 nm and came from a monochromated 1000 W xenon are lamp . The technique is extended to the immobilization of methylene blue in a zi rconium phosphate lattice, which has a layer structure. Methylene blue was chosen since it intercalates to the greatest extent as evidenced by the siz e of new peaks apparent in its powder X-ray pattern. The dye/phosphate comp ound is mixed with graphite and mineral oil such that there is a 10:1 ratio of dye/phosphate to graphite to produce a viable photo-electrode. It is fo und that the photoreaction proceeds as in solution and that a light pen (la ser pointer) producing 4 mW of light of wavelength 620-670 nm is sufficient to produce significant photocurrents with ascorbic acid which permit the q uantitative determination of the latter at concentrations up to 1.0 mM. Fin ally a prototype sensor design is described, incorporating an all-in-one un it comprising the photo-electrode and light pen together. This is used to m easure the amount of ascorbic acid in the commercial headache remedy Resolv e. A value of 31.0 +/- 1.0 mg per sachet is found, compared with 30 mg/sach et determined independently.