Free sulfite

An aqueous solution of sodium sulfite, which was prepared at ambient temperature (or bisulfite, but free of thiosulfite) at a concentration of about 20%, is added to the reaction vessel. Usually 1.05-1.1 mol of the sulfite is required. The temperature is kept at 70-90°C with continuous stirring. The reaction can be finished when analytical data, e.g., content of free sulfite, are sufficient to fit the specification. Time required for the second step is around 30 min to 1 h. Finally the pH is adjusted to the desired level. 

M. Galloway and S.A. Soper, Contact conductivity detection of polymerase chain reaction products analyzed by reverse-phase ion pair microcapillary electrochromatography, Electrophoresis, 23 (2002) 3760-3768. M. Masar, M. Dankova, E. Olvecka, A. Stachurova, D. Kaniansky and B. Stanislawski, Determination of free sulfite in wine by zone electrophoresis with isotachophoresis sample pretreatment on a column-coupling chip, J. Chromatogr. A, 1026 (2004) 31-39. 

Sulfate. As for the production of hydrogen sulfide from sulfur dioxide/sulfite at least three possible pathways for the light-dependent synthesis of hydrogen sulfide in response to sulfate can be assumed, i.e. first the light-dependent reduction of sulfate to carrier-bound sulfide followed by a release of the sulfide moiety from its carrier second the light-dependent reduction of sulfate to carrier-bound sulfide followed by an incorporation of the sulfide moiety into cysteine and subsequent degradation of cysteine third the release of sulfite from carrier-bound sulfite followed by reduction of free sulfite to sulfide (see Figure 1). 

The pH dependenee of both the oxidative and reductive half-reactions of sulfite oxidase has reeently been examined (Brody and Hille, 1999). From a comparison of the pH dependence of kcat with the limiting rate constants for the two half-reactions, kred and kox, respectively, it is evident that k d i s principally rate-limiting above pH 7, but at lower pH kox becomes increasingly important, kred is essentially pH-independent, consistent with a reaction mechanism in which nucleophilic attack by the substrate lone pair on a Mo=0 group initiates the eatalytie sequence. The pH dependence of kred/ d " " indieates an aetive site group having a p a of 9.3 must be depro-tonated for reaetion of oxidized enzyme with free sulfite, possibly Tyr 322 whieh from the erystal strueture of the enzyme constitutes part of the... 

This work shows that oxygen-free sulfite in lime/limestone slurries, exposed to sulfur dioxide, slowly decomposes under process conditions. In fact, auto-redox reactions of sulfur oxyacids can occur in all coal desulfurization systems, including coal-gasification systems and impurities present in commercial flue gas systems are capable of catalyzing the reaction under process conditions. Our experiments indicate that any large-scale coal utilization will depend on appropriate control of the autoredox reactions of sulfur species. 

The sulfur moiety of cysteine is derived ultimately by the reductive assimilation of inorganic sulfate. Sulfate can be reduced in plants by two pathways. One pathway involves free sulfite as an intermediate which is reduced by sulfite reductase to form free sulfide. The other involves carrier-bound sulfite (carrier-S-SOj) which is reduced by thiosulfonate reductase to yield carrier-bound sulfide (carrier-S-S ). Although the relative physiological importance of the two pathways has not been firmly established, the indispensability of thiosulfonate reductase (even in the presence of sulfite reductase) for sulfate reduction in Chlorella mutants indicates the physiological importance of bound sulfite for this organism (Schmidt et al., 1974). Further details of the reduction of sulfate are presented in Chapter 5. 

A i °c. based on total free sulfite Ki c- based on total free sulfite pH value calculated as sulfurous acid probably in bisulfite formf... 

The a-hydroxysulfonates are not oxidized by iodine in neutral or acid solutions, but this does occur in alkaline solutions where it is preceded by dissociation into the free sulfite. They are also not readily oxidizable by oxygen. 

Part of the dissociated sulfite (largely in the form of bisulfite or HSO3 ) can bind reversibly or irreversibly to certain components of the food. This fraction is known as combined sulfite the fraction not bound to the food is called free sulfite and remains dissociated in dynamic equilibrium. The sum of the two fractions is called total sulfite. Thus, it is often important to measure both free and bound forms of sulfite that are present in foods. 

The amount or presence of sulfite in each of these states (freely, reversibly or irreversibly bound) depends on a number of factors such as the level of addition, the type and composition of the food or beverage, and the processing and storage conditions. For example, reversibly bound sulfite may dissociate into free sulfite when a food is treated with acidic solutions and heated to boiling (Fazio and Warner, 1990 Liick and Jager, 1997). On the other hand, the free sulfite fraction is rapidly converted to molecular sulfur dioxide when the sulfited food is acidified (Wedzicha, 1992). Analytical determination of sulfite does not therefore reflect the preservatives that were initially added. 

Bendtsen, A. B. and S. S. Jorgensen. 1994. Determination of total and free sulfite in unstabilized beer by flow injection analysis. J. AOAC Int. 77 948-951. 

This has been shown by 0 NMR to occur without Co-0 bond breaking. The intramolecular redox reaction follows first-order kinetics, k = 1.4 X 10 s" at 25°C. The associated value of AV is large and positive, -1-34 cm moP at 25°C, consistent with rate-limiting formation of SOJ. The radical then attacks a second complex, and is converted to 804 by oxygen atom abstraction. The Pt(IV) complex [Pt(NH3)50S02] reacts with free sulfite to yield an intermediate identified as the S-bonded bis(sulfito) complex m-[Pt(NH3)4(S03)2] 2H20. In acidic solution it is converted (k = 5.6 x 10 s at 25°C) to a planar Pt(II) complex and SO3, which is hydrolyzed to sulfate ... 

Fig. 4. Intercellular location of the enzymes of the bound and free pathways for the assimilation of inorganic sulfur in maize, a C4 plant. Sulfate is delivered to the leaf by the vascular bundle (V.B.) and assimilated via the bound pathway in the bundle sheath cells. Assimilation of free sulfite (including SO2) and free sulfide (including H2S) occurs via the free pathway in mesophyll cells. C, Carrier of the bound pathway. (Adapted from Schmutz and Brunold, 1985.)...

Siegel, L. M., H. Kamin, D. C. Rueger, R. P. Presswood, and O. H. Gibson An Iron-Free Sulfite Reductase Flavoprotein from Mutants of Salmonella tryphimu-rium. In Flavins and Flavoproteins (H. Kamin, ed.), p. 523. Baltimore University Park Press. 1971. 

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