DETERMINATION OF FOLIC-ACID IN SEA-WATER USING ADSORPTIVE CATHODIC STRIPPING VOLTAMMETRY

Folic acid is of biological origin and occurs ubiquitously in the natural system. Very little is known about its likely occurrence and distribution in natural waters. A voltammetric method is proposed to determine folic acid in sea and estuarine waters. The method is based on cathodic stripping voltammetry (CSV) preceded by adsorptive collection on a hanging mercury drop electrode. Using cyclic voltammetry at two pH values it is confirmed that folic acid undergoes reduction in three steps. The first step is electrochemically reversible producing a two-electron reduction peak at -0.75 V at pH 8.5. Comparative experiments indicate that the benzene ring is responsible for the adsorptive behaviour of folic acid on the mercury drop electrode. The adsorbed layer of folic acid is rearranged when the adsorption density is increased above similar to 5.10(-11) mol cm(-2) causing the appearance of a sharp capacitance peak. The best sensitivity for the determination of folic acid in sea water is obtained at pH 8.5 at a deposition potential of -0.25 V and using the square-wave modulation at frequencies between 25 and 100 Hz. The voltammetric response for 60 s adsorption was found to increase linearly with the folic acid concentration from the limit of detection (0.09 nM) to 500 nM. The method was applied successfully to the determination of dissolved folic acid in samples from the Mersey estuary. The folic acid concentration was found to lie between 0.1 and 3 nM increasing generally with decreasing salinity due to freshwater inputs.