Sulfite Sensors

Sulfite is used in large amounts in paper production and as a reduc-tant in various chemical reactions. Fonong (1986a) immobilized sulfite oxidase (EC 1.8.3.1) covalently on a collagen membrane which was attached to a hydrogen peroxide-indicating Pt electrode. H2O2 liberated in the enzymatic reaction  

The application of immobilized sulfite oxidase in the Multipurpose Bioanalyzer (Provesta, USA) has been described by Smith (1987). 1.8 U of the enzyme was entrapped in a gel at the active tip of an oxygen probe. The sensor is suitable for sulfite determination in food samples down to 100 ppm and has a lifetime of 84 days. 

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A similar concept as for carbonate ionophores has also been used for sulfite sensors. In this case, the well-known addition of sulfite to benzaldehyde derivatives by reversible formation of a covalent bond has been taken advantage of (Figure 29). However, while used for ion-selective optodes, this approach has not been tested for ISEs. Optodes based on the addition of H2O or alcohols to phenyl ketones have also been reported for the detection of humidity and alcohols in beverages, respectively, but because of the nonionic nature of these analytes, ISEs based on these ionophores are not feasible." ... 

That work was extended to the zincfll) and free-base porphyrin derivatives, and the electrocatalytic activity of polymeric films demonstrated for reduction of nitrite [30] and sulfite in water/ethanol solution [31]. The modified electrodes prepared by successively cycling the potential in the -1.20 to 1.50 V range (50 times), exhibited high electrocatalytic activity for reduction of SO2 at pH 1.0, which was significantly decreased at pH 3.5. The modified electrodes were explored as sulfite sensors in wine samples where the detection and quantification limits, respectively, of 1.40 and 4.68 mg/L were achieved for the polymeric Ni-(5-N02)TRP derivative. High selectivity and electrocatalytic activity for oxidation of gallic acid was also reported. 

T. Garcia, E. Casero, E. Lorenzo, and F. Pariente, Electrochemical sensor for sulfite determination based on iron hexacyanoferrate film modified electrodes. Sens. Actuators B chem. 106, 803 (2005). 

The oxidation of sulfite and thiosulfate becomes facile in the presence of iodide and novel disposable microband sensor electrodes have been developed by Williams and coworkers [187] to allow fast amperometric determination. A similar approach was proposed for the determination of sulfite in wine [188]. In this method, a coulometric titration is carried out in which S(IV) is indirectly oxidized to S(VI). Speciation of SO2 and sulfite was achieved down to micromolar levels. Sulfide and hydrogen sulfide can be determined elec-trochemically in the presence of an iodide mediator [189]. This process may be further enhanced at elevated temperatures. 

Sulfor oxidizing bacteria oxidize sulfide and sulfite to sulfate. The decrease of oxygen [94,95] as well as the alteration of pH [96] can be used as indicators of these reactions. With a Thiobacillus thioparus-containing sensor, a detected limit of 4 pmol/1 sulfite is reached [94]. The detection limit of a sensor with Thiobacillus thiooxydans for sulfide is 0.02 mmol/1 only [95]. 

Suzuki M, Lee S, Fujil K, Arikawa Y, Kubo 1, Kanagawa T, Mikami E, Karube 1 (1992) Determination of sulfite by using microbial sensor. Anal Lett 15 973- 982... 

We can create optical sensors for specific analytes by placing a chemically sensitive layer at the end of the fiber. An optical fiber sensor is called an optode (or opt rode), derived from the words "optical and "electrode. Optodes have been designed to respond to diverse analytes such as sulfites in food, nitric oxide in cells, and explosives in groundwater.12... 

Two disparate translation methods are investigated for the measurement of sulfur dioxide. Both involve interaction with an aqueous solution. In the first, collected S(IV) is translated by the enzyme sulfite oxidase to which is then measured by an enzymatic fluorometric method. The method is susceptible to interference from i CWg) efforts to minimize this interference is discussed. The second method involves the translation of SO2 into elemental Hg by reaction with aqueous mercurous nitrate at an air/liquid interface held in the pores of hydrophobic membrane tubes. The liberated mercury is measured by a conductometric gold film sensor. Both methods exhibit detection limits of 100 pptv with response times under two minutes. Ambient air measurements for air parcels containing sub-ppbv levels of SO2 show good correlation between the two methods. 

To date various polymeric electron transfer systems have been incorporated to enzyme sensors for glucose, lactate, aldose, pyruvate, ascorbate, choline, acetylcholine, cholesterol, formaldehyde, phenoUcs, nitrate, sulfite and histamine. 

More recently, in the determination of cyanide with a metallic silver-wire electrode Frenzel ei al.[38] have shown that relatively non-selective potentiometric sensors may be used to make selective determinations by enhancing the selectivity using gas-diffusion separations. Potential interferences from sulfide, sulfite and nitrite which may form interfering gaseous species were removed by a pre-oxidation with permanganate and dichromate. 

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