Dithionite detection

The second region is the mixed kinetic transport-controlled region, and the most negative part of it can also be used for kinetic and mechanistic studies of the electron-transfer reaction after the experimental currents have been compensated for transport limitations. Finally, a second wave is observed at potentials higher than 0.5 V vs. AglAgCl, which can be attributed to the oxidation of sulphite to sulphate. However, this wave is not further considered because the oxidation mechanism of sulphite showed poor reproducibility (see section 6.3), and sulphite detection in dyeing processes is not of great importance compared with dithionite detection. 

Note Phosphoric acid [8] and hydrochloric acid [6, 9] have both been suggested in the literature as substitutes for phthalic acid. The addition of sodium dithionite [9] is also occasionally mentioned and sometimes no additives are employed [10]. The alternative reagents offer no advantages over the phthalic acid containing reagent since they usually cause more background coloration. The limits of detection are about 0.1 —0.5 pg per chromatogram zone [5]. 

The parent acid has no independent existence and has not been detected in aqueous solution either. Sodium dithionite is widely used as an industrial reducing agent and can be prepared by reduction of sulfite using Zn dust, Na/Hg or electrolytically, e.g. ... 

The first step of the reaction involves nitration of the aromatic skeleton of the substance to be detected. Then the aromatic nitro derivatives so produced are reduced with sodium dithionite, in acid medium, to the corresponding amines these are then diazotized and coupled with N-(l-naphthyl)-ethylenediamine to yield an azo dye (cf. Fig. 21). 

FPR studies at low temperature detect the presence of one iron-sulfur center and molybdenum. At low temperature a sample of nitrate reductase reduced by dithionite shows a rhombic signal (gm,x = 2.04, gmed = 1.94, and gnm = 1.90). This signal accounts for 0.84 spins/... 

D = -0.7 cm", E/D = 0.26 for the S = 3/2 center in dithionite-reduced AvV (B. J. Hales, unpublished results). MCD magnetization data for dithionite-reduced Avl and Avl (8) indicate that the EPR-detectable S = 3/2 paramagnets are also responsible for the temperature-dependent MCD transitions. Therefore, by analogy with the MoFe protein it seems probable that the S = 3/2 paramagnetic chromophore in dithionite-reduced Avl is a V-Fe-S cluster. However, the EPR and MCD studies indicate that this cluster has magnetic and electronic properties distinct from that of the Mo-Fe-S clusters in conventional Mo nitrogenases. 

A considerable amount of information regarding flavin semiquinone reactivity as well as the environment of the bound flavin coenzyme has accumulated over the years from studies of flavoenzyme systems which produce semiquinones either on photochemical reduction or upon reduction by one electron equivalent of dithionite, but which do not form a detectable semiquinone intermediate during catalytic turnover. For example, the correlation of anionic semiquinone formation and the ability to bind sulfite at the N(5) position in a number of flavoenzyme... 

A nitrochromone has three easily reducible functions and, for its reduction, conditions and reagents which have little or no effect on the carbonyl or the 2,3-double bond should be chosen. Nitrochromones are reduced by tin-hydrochloric acid, zinc-ammonium chloride, iron-acetic acid, iron-hydrochloric acid or sodium dithionite. It may be easier to control the severity of the conditions in catalytic hydrogenation. Scheme 29 shows that with proper choice of conditions (temperature, pressure, solvent, catalyst), it is often possible to optimize the yield of the desired product (527). Extending the reaction time from about 30 min to 2.5 h increased the yield of the chromanone (528) and none of the hydroxylamine (529) was then detected (70JCS(C)2230). 

Simultaneous detection of indigo and sodium dithionite for control of dyeing processes... 

The aim of an indigo sensor is to keep the leuco-indigo concentration in the solution at a constant value. In the past, different methods were developed for detection of the indigo and sodium dithionite concentration, but up to now with limited success. The sodium dithionite concentration can be determined by volumetric titration with iodine2 22 or with K3[Fe(CN)6]23. The endpoint detection of these titrations can be done visually22,24"25 or... 

---