Reduction with Sulphite

Since sulphite and sulphate solutions are colourless and the complex ion solution is strongly coloured and die colour of the reduced form is less intense but pink, the kinetics can be followed by measuring the absorbance at the wavelength of maximum absorption of the reactant. 

Prepare a 0.01 M solution of the complex in water and run its absorption spectrum in a 10 mm cell over the range 360-800 nm. Locate the wavelengths of maximum absorption. 

Repeat at at least 3 other higher temperatures. For the run at the highest temperature, measure an aliquot of the final solution, add to it gradually, with stirring, a solution of barium chloride in 2.5 M HCl. Allow the precipitate to settle and determine its [sulphate] as in Sec.7.3.4. 

Repeat the series of experiments above replacing sulphite by a solution of hydrazinium chloride or sulphate. 

The photoreduction of the complex compound has been studied in solution and in the solid state. In either case, the photoreaction can be represented by  

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Nitrophenylhydrazine may be similarly prepared from p-nitroplienyl-diazonium chloride by reduction with sodium or ammonium sulphite ... 

The carbanions take up 02 and these take up protons to give hydroperoxides in good yields. But because they are explosive in nature, they are not usually isolated and on reduction with sodium sulphite on trialkyl phosphite give alcohols. Alcohols can also be prepared via hydroperoxy molybdenum complexes and alkyl boranes. These reactions are summarized as follows ... 

Chlorine also converts the selenium and sulphur of dried chamber mud into the volatile chlorides, from which the selenium can be liberated by decomposition with water followed by reduction with an alkali sulphite.4... 

Isobutyraldehyde is another reagent that has been applied to condensation with amino acids [261 ]. The reaction of methyl ester hydrochlorides of amino acids with isobutyraldehyde and sodium hydrogen sulphite proceeds in a solution of sodium carbonate according to Scheme 5.23. The azomethines produced were chromatographed either as such or as N.-alkyl esters after reduction with zinc powder in methanolic hydrogen chloride. N-Isobutylidene methyl esters of most of amino acids, except His, Arg, Trp,... 

Addition of chlor-platinous acid or a solution of platinous chloride to a metallic carbonate or chloride.1 For this purpose the chlor-platinous acid is best made by dissolving platinous chloride in hydrochloric acid. This ensures the freedom of the final product from the metallic chlor-platinate. A less troublesome method is to reduce chlor-platinie acid with sulphur dioxide if a pure product is not required, but this is not easy to carry out satisfactorily otherwise. If, for example, reduction of the chlor-platinic add is not complete, the metallic platinite wifi be contaminated with platinate. If, on the other hand, excess of sulphur dioxide is present, the metallic chlor-platinite becomes contaminated with sulphite derivatives. 

The reagent is sometimes employed by adding sodium sulphite Na2S03 to the acidified solution to be reduced. Some reductions with SO2- are as follows ... 

The solution is intensely violet, but the salt is readily precipitated by addition of alcohol. Reduction with alkaline hydroxylamine chloride solution yields potassium aquo penta-ferrocyanide, K3Fe(CN)5.H20, whilst with alkaline potassium sulphite the sulphito penta-ferrocyanide, K5Fe(CN)5.S03, is obtained. 

Metal ions forming chloride complexes which give the same reactions as Sb(V) with RB interfere in the determination. These are Au(III), Tl(III). Ga(III), and Fe(III). Gold can be separated after reduction to the element with sulphite. Gallium and iron can be separated by extraction as chloride complexes before the oxidation of antimony to Sb(V). Small amounts of Fe(III) are masked with phosphoric acid. 

Berzelius and Marcet, by the action of moist chlorine on carbon disulphide, obtained a crystalline compound for which Kolbe found the empirical formula CCI2SO2 (C=6, 0 = 8, S=i6, Cl = 35 5). He called it sulphite of perchloride of carbon. By the action of potash on it he obtained compound (I), which by reduction with nascent hydrogen yielded (II)-(IV) ... 

Bromine.— The reaction mechanisms of a relatively new species, the perbromate ion, are currently under investigation. The redox reactions with a number of reductants have been studied, and in reactions with sulphite and arsenite it... 

The method was also used for the determination of cysteine and glutathi but its application presents, mainly in biological materials, the doubts expressed in the previous paragraphs. For the determination of cystine a method is employed based on the reduction of cystine in an acidic medium with sodium amalgam or on the reaction with sulphite at pH 8-12 according to equation ... 

It is prepared commercially by treating benzene diazonium chloride with sodium sulphite and then reducing the mixture with zinc dust and ethanoic acid. It can also be prepared by reduction of benzene diazonium chloride with SnCl. ... 

The dichromate ion oxidises iron(II) to iron(III), sulphite to sulphate ion, iodide ion to iodine and arsenic(III) to arsenic(V) (arsenate). Reduction of dichromate by sulphite can be used to prepare chrome alum, since, if sulphur dioxide is passed into potassium dichromate acidified with sulphuric acid, potassium and chromium(III) ions formed are in the correct ratio to form the alum, which appears on crystallisation ... 

Benzenediazonium chloride reacts in solution with sodium sulphite to give benzenediazonium sodium sulphonate, which when treated with sulphurous acid undergoes reduction to phenylhydrazine sodium sulphonate. The latter readily hydrolyses in the presence of concentrated hydrochloric acid to give... 

It may also be prepared by the reduction of phenyldiazonium chloride with the calculated amount of a solution of stannous chloride in hydrochloric acid, but the yield is not so high as that obtained by the above sulphite method ... 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

Reduction to S(IV) is the basis of two spot tests of Feigl, although these apply to other S(VI) classes, e.g. sulphonamides. Thus Feigl and Lenzer fused the sample with alkali to yield sulphite, then treating with hydrochloric acid and warming to expel sulphur dioxide they detected the latter with nickel(II) hydroxide on test paper, which yielded ultimately the black Ni(IV) oxyhydrate (see also Section C). In the other test FeigF fused the sample with sodium formate/alkali, cooled and acidified with sulphuric acid to liberate sulphur dioxide in this case also. This was detected by a ferric chloride/potassium ferricyanide reagent which yielded a blue colour (Prussian, Turnbull s). 

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