Sulphite

Determination of sulphite by oxidation to sulphate and precipitation as barium sulphate Discussion. Sulphites may be readily converted into sulphates by boiling with excess of bromine water, sodium hypochlorite, sodium hypobromite, or ammoniacal hydrogen peroxide (equal volumes of 20-volume hydrogen peroxide and 1 1 ammonia solution). The excess of the reagent is decomposed by boiling, the solution acidified with hydrochloric acid, precipitated with barium chloride solution, and the barium sulphate collected and weighed in the usual manner (Section 11.72). 

Determination as copper (I) thiocyanate, CuSCN. The solution (100 mL) should be neutral or slightly acid (hydrochloric or sulphuric acid), and contain not more than 0.1 g SCN . It is saturated with sulphur dioxide in the cold (or 50 mL of freshly prepared saturated sulphurous acid solution added), and then treated dropwise and with constant stirring with about 60 mL of 0.1M copper sulphate solution. The mixture is again saturated with sulphur dioxide (or 10 mL of 

Conversion of thiosulphate to sulphate and determination as barium sulphate Discussion. Thiosulphates are oxidised to sulphates by methods similar to those described for sulphites (Section 11.74), e.g. by heating on a water bath with an ammoniacal solution of hydrogen peroxide, followed by boiling to expel the excess of the reagent. The sulphate is then determined as barium sulphate, BaS04. 

One tried-and-tested technique is the volumetric method (iodometric titration) described in Section 3.2.6.1, involving the use of a known quantity of iodine and back titration of the iodine not consumed. 

Where the sulphite concentration is higher (over 50 mg/1) and where 

For low concentrations - and in the presence of sulphide and thiosulphate -the polarographic method (Section 3.2.6.3) can be used. 

In the iodometric determination method, iodine is used to oxidize the sulphite ions to sulphate ions  

The quantity of elemental iodine consumed in this reaction is equivalent to 

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Anthraquinone can be brominated, chlorinated directly to the tetrachloro (I, 4, 5, 8-) stage, nitrated easily in the 1-position, but gives the 1,5-and 1,8-dinitro-derivalives on prolonged nitration the nitro groups in these compounds are easily displaced by neutral solutions of alkali sulphites yielding the corresponding sulphonic acids. Sulphonation with 20-30 % oleum gives the 2- 2,6- and 2,7-derivatives in the presence of Hg the 1- 1,5- and 1,8- derivatives are formed. 

It is readily oxidized by air to benzoic acid. With aqueous KOH gives benzyl alcohol and benzoic acid. Gives addition products with hydrogen cyanide and sodium hydrogen sulphite. 

CCls CHO. A colourless oily liquid with a pungent odour b.p. 98°C. Manut actured by the action of chlorine on ethanol it is also made by the chlorination of ethanal. When allowed to stand, it changes slowly to a white solid. Addition compounds are formed with water see chloral hydrate), ammonia, sodium hydrogen sulphite, alcohols, and some amines and amides. Oxidized by nitric acid to tri-chloroethanoic acid. Decomposed by alkalis to chloroform and a methanoate a convenient method of obtaining pure CHCI3. It is used for the manufacture of DDT. It is also used as a hypnotic. 

CH OfiSj, H2C(S03H)2- a colourless, crystalline solid which readily absorbs water vapour decomposes on distillation. The potassium salt is prepared by heating methylene chloride with an aqueous solution of potassium sulphite under pressure at 150-I60" C. The free acid is obtained by decomposing the sparingly soluble barium salt with sulphuric acid. The aryl esters are very stable, but the alkyl esters decompose on heating to give ethers. Resembles malonic acid in some of its reactions. 

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. ... 

C HgNjOjS. Colourless needles, with iH20. Prepared by reducing diazotized sulphanilic acid with an excess of sodium sulphite. It is a typical hydrazine in its reactions with ketones, and with acetoacetic ester. The latter reaction gives rise to the tartrazine dyestuffs, and is much used commercially. 

SiOs] [0aSS(0)2] ions. See sulphur oxyacids. Formed from sulphites, [SOj] , and SO2. 

Sulphur dioxide, SO2, m.p. — 72-7°C, b.p. — I0"C. Colourless gas with characteristic smell. Formed by burning S, metal sulphides, H2S in air or acid on a sulphite or hydrogen sulphite. Powerful reducing agent, particularly in water. Dissolves in water to give a gas hydrate the solution behaves as an acid - see sulphurous acid. Used in the production of SO3 for sulphuric acid. 

Arsenates(V) are more powerful oxidising agents than phos-phates(V) and will oxidise sulphite to sulphate, hydrogen sulphide (slowly) to sulphur and, depending on the conditions, iodide to iodine. 

Oxygen does not read with alkalis. Sulphur dissolves slowly in strong alkalis to give a mixture of sulphite [sulphate(IV)] and sulphide initially ... 

However, the sulphide ion can attach to itself further atoms of sulphur to give polysulphide ions, for example Sj , Sj , and so these are found in solution also. Further, the sulphite ion can add on a sulphur atom to give the thiosulphate ion, S203 which is also found in the reaction mixture. 

Sulphur dioxide is also produced by the action of an acid (usually concentrated sulphuric since it is involatile) on a sulphite or hydrogensulphiie. for example... 

Liquid sulphur dioxide is a solvent for a number of substances, for example iodine, sulphur, some sulphites, potassium iodide and sulphur dichloride oxide, SOClj (see below). The liquid can be assumed to ionise slightly, thus ... 

Hence, for example, sulphur dichloride oxide behaves as an acid and a sulphite as a base thus ... 

Although sulphur dioxide, as a gas, is a reducing agent in the sense that it unites with oxygen, free or combined (for example in dioxides or peroxides) most of its reducing reactions in aqueous solution are better regarded as reactions of sulphurous acid (in acid solution), or the sulphite ion (in alkaline solution). 

The sulphite ion, SO3, has a pyramidal structure and the short S—O bond length suggests the presence of double bonding, i.e. 

Sulphites react with molecular oxygen (or air) to give sulphates, a reaction catalysed by certain ions (for example Fe, Cu, arsenate(III) ion, AsO ) and inhibited by, for example, phenol, glycerol and tin(II) ions, Sn ... 

Sulphites react with oxidising agents, for example mangan-ate(VII) and dichromate(VI) ... 

This reaction is a useful test for a sulphite or for moist sulphur dioxide, which turns dichromate paper (filter paper soaked in potassium dichromate) from yellow to green. 

Sulphites are oxidised by chlorine water and solutions con-tainingchloric(I) (hypochlorous)acid or the chlorate(I) (hypochlorite) ion... 

In the presence of strong reducing agents the sulphite ion acts as an oxidising agent some examples are ... 

In strongly acid solution, substances which are normally reducing agents reduce sulphur dioxide solution or sulphites, for example iron(II) and zinc ... 

If a solid sulphite is heated with zinc dust (or carbon) the sulphite is reduced to sulphide ... 

The reducing action of sulphurous acid and sulphites in solution leads to their use as mild bleaching agents (for example magenta and some natural dyes, such as indigo, and the yellow dye in wool and straw are bleached). They are also used as a preservative for fruit and other foodstuffs for this reason. Other uses are to remove chlorine from fabrics after bleaching and in photography. 

Evaporation and crystallisation of the sodium sulphite solution gives crystals of the heptahydrate NajSOj.THjO. However, on evaporation of the hydrogensulphite solution, the solid obtained is chiefly sodium pentaoxodisulphate(IV) ( metabisulphite ) Na2S20j, and contains little if any of the hydrogensulphite. However, the hydrogen sulphite ion is obtained when the solid redissolves in water ... 

Alternatively these salts can be prepared by first saturating a known volume of alkali with sulphur dioxide, giving a solution of the hydrogensulphite, from which sulphite can be prepared by the addition of a second equal volume of alkali. 

The redox properties have already been considered. A number of reactions of soluble (alkali metal) sulphites are noteworthy ... 

On boiling a solution of a sulphite with sulphur a thio-sulphate(VI) is formed, and sulphur dissolves ... 

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