Cobalt and Copper Hexacyanoferrate Modified Carbon Fiber Microelectrode as an All-Solid Potentiometric Microsensor for Hydrazine

Abstract

This article reports on the novel mixed cobalt and copper hexacyanoferrate (CoCuHCF)-modified carbon fiber cylinder microelectrode (CFCME) and its application to potentiometric determination of highly toxic hydrazine. The substrate CFCMEs were fabricated in a standard manner using carbon fibers of 7 µm in diameter. The CoCuHCF film was deposited electrochemically by cycling the potential between 0 and +1.0 V (vs. Ag/AgCl) in a solution containing the precursor salts. It exhibited good chemical stability in the pH range from 1 to 9. The effects of the coverage/thickness of the CoCuHCF coating, of the pH of a measurement solution, and of the activation of the sensor, on the potentiometric response to hydrazine were examined in detail. The potentiometric behavior of CoCuHCF film in the presence of hydrazine, with the potential (emf) slope of –55 mV/decade, was compared with those of four single metal HCF films (CoHCF, CuHCF, FeHCF, and NiHCF) prepared in the same way, and the possible role of the CoCuHCF film components is explored and discussed. The emf versus log CHyd calibration plot was linear over 3 orders of magnitude, from 1.0×10–6 to 1.0×10–3 mol/L, with a correlation coefficient of 0.995. The limit of detection was found to be 5×10–7 mol/L. The response times of 10 to 30 s and the temperature coefficients of ca–2.7 mV/°C were obtained at various hydrazine concentrations. The repeatability of measurements was very good with the relative standard deviation ranging from 3 to 4% (n=10), depending on hydrazine concentration. The selectivity of the reported microsensor was found excellent, except in the case of a few negatively charged interferents for which it was greately improved by an additional Nafion coating. The useful lifetime of the all-solid microsensor was found to be more than 2 months when stored in air.