Sulphite sensor

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

The determination of formaldehyde, a substance of great interest on account of its toxicity and high significance to the resin, fertilizer and explosive manufacturing industries, among others, entails the use of rapid, convenient, safe methods. One sensor developed for this purpose uses the manifold shown in Fig. 4.6 to handle the reagents involved, viz. / -rosaniline (PRA) and sulphite. The instability of sulphite ion calls for on-line mixing with PRA dissolved in 0.4 M HCl prior to injection of the sample. After the... 

From what is described in this section, it can be concluded that the kinetics of the oxidation reaction of sulphite and dithionite at a platinum electrode in alkaline solution are strongly affected by the nature of the platinum surface. This is important when a platinum electrode is used for a quantitative investigation of the kinetics of the oxidation of sodium dithionite and/or sulphite or as electrode material in the development of a sensor for the measurement and/or control of dithionite and/or sulphite concentrations. However, for sodium dithionite, it has no serious consequences because the limiting-current at 0.45 V vs. SCE does not change as a function of scan number. However, this oxidation is still irreversible (no return peak observed) which means that, in the onset of the voltammetric wave, the current is controlled by charge-transfer kinetics. Therefore, it is possible to investigate and obtain the mechanism of the oxidation of sodium dithionite, which is explained in the next section. 

Numerous catalytic layers made by different immobiUzing techniques have been used to enhance the selectivity of amperometric sensors. One example chosen from a large variety was a platinum microelectrode coated with polyvinylpyridine carrying a layer of anodicaUy deposited paUadium/iridium oxides (Shi et aL 2001). Polyvinylpyridine is an electricaUy conducting polymer. The sensor has been used to determine sulphur dioxide in gaseous phase as weU as sulphite in solution. 

Figure 7.24. Sensor for sulphite or SO2 with catalyst layer of PVP/Pd/Ir02...

Corbo, D. and M. Bertotti. 2002. Use of a copper electrode in alkaline medium as an amperometric sensor for sulphite in a flow-through configuration. Anal. Bioanal. Chem. 374 416-420. 

Sulfadimethoxine (SMD) is a dmg frequently used to prevent the spreading of diseases in freshwater fish aquaculture, but its control is important to avoid environmental contamination. A potentiometric sensor selective to SMD was built inside a micropipette tip [211] by immersing a PVC membrane loaded with 1 % of meTO-tetra(phenyl)porphyrinate manganese(lll) chloride in a reference solution. This electrode gave near Nemstian response (54.1 mV/decade) and excellent detection limit (2.4 x 10 mol L ) was attained. For the quantification of SO2, a selective electrode was prepared by incorporation of Cio-(tetraphenyl)porphyr-inate zinc(ll) in a PVC membrane plasticized with 2-nitrophenyl-phenylether [212]. The new electrode presented a selective response to sulphite, with good linearity in an interval larger than four decades of concentration and slope of -59.5 mV per decade, and detection limit of 3.7 x 10 mol L . This sensor presented high... 

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