Sulphur dioxide reduction

Prior to the announcement of the proposed legislation, internal debate between Presidential advisors was concerned over whether the bulk of the sulphur dioxide reduction requirements should be imposed on the 20 most heavily polluting plants, all situated in the Midwest, or spread around all US plants (New York Times. 10 June 1989). Original proposals, and earlier ones from Senators George Mitchell (Democrat, Maine) and Robert Byrd, would have forced the retrofit of FGD equipment or scrubbers. The eventual Bush proposals were directed at 107 plants in 18 states and allowed them to take several routes to compliance. 

The effect of temperature on the conductivity is shown in Figure 24.3, for an electrolyte composition of 74% sulphur dioxide. It may be seen that a decrease in temperature from 25 to —50°C decreases the conductivity by a factor of only about one-half, This small effect of the temperature on the conductivity explains the excellent high-rate capability of the lithium-sulphur dioxide system at low temperatures, coupled with the high exchange currents for the lithium oxidation and the sulphur dioxide reduction reactions. The small decrease of the conductivity with a lowering of the temperature can be attributed to the beneficial effects of the sulphur dioxide and solvent on the viscosity of the electrolyte. 

Two important redox potentials for reduction by sulphur dioxide in aqueous solution are ... 

Concentrated sulphuric acid is an oxidising agent, particularly when hot, but the oxidising power of sulphuric acid decreases rapidly with dilution. The hot concentrated acid will oxidise non-metals, for example carbon, sulphur and phosphorous to give, respectively, carbon dioxide, sulphur dioxide and phosphoric(V) acid. It also oxidises many metals to give their sulphates cast iron, however, is not affected. The mechanisms of these reactions are complex and the acid gives a number of reduction products. 

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 ... 

By the reduction of copper(II) chloride or a mixed solution of copper(II) sulphate and common salt by sulphur dioxide. 

Hydrobromic acid. Method 1 (from bromine and sulphur dioxide). A mixture of 600 g. (or 188-6 ml.) of bromine, 250 ml. of water and 760 g. of crushed ice is placed in a 1 6 litre round-bottomed flask and a rapid stream of sulphur dioxide (from a siphon of the liquefied gas) is passed into the flask, care being taken that the outlet of the gas-delivery tube is below the surface of the bromine layer. The rate of flow of the gas is adjusted so that it is completely absorbed. It is advisable to cool the flask in ice and also to shake the contents from time to time. The reduction is complete when the mixture assumes a uniform yellowish-brown or yellow colour, which is unaffected by further introduction of sulphur dioxide excess of the latter gas should be avoided as it will be... 

Determination of thallium as chromate Discussion. The thallium must be present in the thallium(I) state. If present as a thallium(III) salt, reduction must be effected (before precipitation) with sulphur dioxide the excess of sulphur dioxide is boiled off. 

Determination of chlorate as silver chloride Discussion. The chlorate is reduced to chloride, and the latter is determined as silver chloride, AgCl. The reduction may be performed with iron(II) sulphate solution, sulphur dioxide, or by zinc powder and acetic (ethanoic) acid. Alkali chlorates may be quantitatively converted into chlorides by three evaporations with concentrated hydrochloric acid, or by evaporation with three times the weight of ammonium chloride. 

Another group of Japanese workers91 found that the sulphoxonium salt, 7, was reducible to sulphoxides with either alkyllithiums or lithium dialkylcuprates, the exact reaction pathway being complicated by halide ions originating from the preparation of the metal alkyls. However, good yields of methyl phenyl sulphoxide were obtained by reduction of 7 with sulphur dioxide or a thiol in pyridine (equation 37). 

These topics are discussed in more detail in other chapters of this text. Formally, the pyrolytic elimination of sulphur dioxide from a sulphone, with the concomitant formation of a new carbon-carbon bond, constitutes a reduction at sulphur. These reductions have been valuable in the formation of new molecules, especially macrocycles and cyclophanes, and have been reviewed by Vogtle and Rossa205. Pyrolytic elimination of sulphur dioxide has been used by Julia and co workers in the formation of mixtures of isoprenoids206, and by Takayama and collaborators in the stereoselective synthesis of vitamin D, 19-alkanoic acids207. 

Sulphur dioxide is reduced by pentacyanocobaltate is several stages (c/. the reduction of p-benzoquinone), viz. 

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). 

One mole of iodine will consume 2 x 96 485 coulombs of electricity. The Karl Fischer titration is widely used for the determination of water in pharmaceuticals. Quantitation in this case is not based on the total amount of current which flows through the solution but the reduction of iodine is simply used to indicate the endpoint of the titration. The reagent consists of mixture of anhydrous methanol, anhydrous pyridine, iodine and sulphur dioxide. The equation for the reaction of water with the reagent looks complicated (see below)... 

In some cases, such as with the oxides of silver and copper, the sulphate can undergo further reduction by sulphur with formation of sulphide and sulphur dioxide the oxides of zinc, tin and iron are not greatly attacked by sulphur, whilst chromium trioxide reacts so violently as to cause inflammation of the sulphur.6... 

When sulphur dioxide alone is passed over carbon at a red heat the latter undergoes partial oxidation, the products being carbon monoxide, carbon oxysulphide and carbon disulphide no oxysulphide is obtained at a white heat.2 Both carbon monoxide and methane are oxidised by sulphur dioxide at high temperatures with formation of sulphur. It has been suggested that volcanic sulphur may, in part, be formed by the reduction of sulphur dioxide by methane, carbon monoxide and hydrogen, all of which are emitted by volcanoes.4... 

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