INSOLUBLE INORGANIC SALTS AS CARBON-PASTE ELECTRODE MODIFIERS FOR PRECONCENTRATION AND VOLTAMMETRIC DETERMINATION OF OXALIC-ACID

A novel approach for chemical modification of carbon paste electrodes (CPEs) has been investigated. Selected water-insoluble inorganic salts as modifiers were found to exhibit preconcentration capability for ox alic acid under open-circuit conditions in a pH 4.3 buffered solution. After medium exchange to a pure electrolyte (Britton-Robinson buffer, pH 8), the accumulated oxalate was measured by square-wave anodic str ipping voltammetry with a peak current response at 1.2V (vs. Ag/AgCl). The water-insoluble inorganic salts as modifiers were active through the interfacial accumulation of insoluble oxalate by the exchange of a nions. The candidate inorganic salts should be, therefore, less insolu ble than the corresponding oxalates, whereas, the differences in solub ility between the modifiers and the corresponding oxalates are of mino r importance. Among the investigated inorganic salts, lead sulfate pro duced the highest response under the employed conditions and was there fore exploited as accumulating material for oxalic acid. With suitable preconcentration times, a linear calibration graph from 0.7 mg/L to 4 2.8 mg/L of oxalic acid in the analyte solution was obtained, and a de tection limit of 0.3 mg/L at a preconcentration time of 5 min was achi eved. The lead sulfate-modified CPE was used to determine oxalic acid in orange juice without pre-separation.