Sulphites

Sulphurous acid gives rise to normal sulphites of the type M₂SO₃ and acid salts of the type MHSO₃, where M represents a univalent metal atom. The normal salts are odourless and do not resemble the free acid or sulphur dioxide in their very harmful effect on living organisms. On the other hand, the acid sulphites readily yield sulphur dioxide; they have an acid reaction towards phenolphthalein, but are neutral towards methyl orange.

Other saline derivatives of sulphur dioxide are known corresponding with a hypothetical acid H₂S₂O₅; for example, alkali metabisulphites or pyrosulphites of the composition M₂S₂O₅ are known, which may be considered as structurally derived from two molecules of the hydrogen sulphites MHSO₃ by elimination of one molecule of water. They are well-defined compounds obtained by crystallisation from aqueous solutions of the sulphites of the alkali metals in the presence of excess of sulphur dioxide.

Sulphurous acid and the alkali sulphites show a marked tendency to react with sulphites of the heavier metals to form complexes in which the ordinary reactions of the heavy metal are often more or less obscured. The alkali sulphites and bisulphites are produced on the large scale by treating lime and magnesia or dolomitic limestone with sulphur dioxide in the presence of water, then adding the requisite amount of alkali sulphate to the filtrate and filtering off any insoluble sulphate; or, alkali carbonate or bicarbonate may be treated directly with sulphur dioxide.

The action of heat on sulphites varies both with the temperature and with the nature of the metal concerned. With sodium sulphite, Na₂SO₃, the chief reaction at 700° C. in vacuo is

4Na₂SO₃ ⇔ 3Na₂SO₄ + Na₂S,

and apparently lithium sulphite behaves similarly. In the case of calcium sulphite an analogous reaction takes place at about 650° C., but at 1100° C. the dissociation

CaSO₃ ⇔ CaO + SO₂

is practically complete. The residue contains calcium sulphate in addition to the oxide, probably because the equilibrium

3SO₂ ⇔ 2SO₃ + S

becomes effective at higher temperatures. Magnesium sulphite decomposes at 300° C. according to the equation:

4MgSO₃ = 2MgSO₄ + MgS₂O₃ + MgO.

At higher temperatures the following also occurs:

MgS₂O₃ = MgSO₃ + S,
MgSO₃ = MgO + SO₂.

The decomposition of the alkali pyrosulphites under the influence of heat follows a very complicated course. At 150° C. the potassium salt yields trithionate and sulphate, the ionic reaction being:

1. 2S₂O₅ → S₃O₆'' + SO₄'';
   
   above 200° C. sulphur dioxide is liberated, and if the partial pressure of the sulphur dioxide (from (iii)) is kept low, the reaction
2. S₂O₅'' ⇔ SO₃'' + SO₂
   
   proceeds from left to right. The main reaction at 250° C., however, produces thiosulphate, thus:
3. 3S₂O₅'' → S₂O₃'' + 2SO₂ + 2SO₂.
   
   A number of side reactions also occur, depending largely on concentration and temperature; the thiosulphate gradually disappears and the final state may be represented:
4. 2S₂O₅'' → ₂SO₄'' + SO₂ + S.
   
   The sodium salt behaves somewhat similarly, but the main reaction at 150° C. is that indicated in equation (ii).

The sulphites, both normal and acid, are easily oxidised, and in solution readily undergo atmospheric oxidation with the formation of sulphates. The oxidation proceeds more readily in neutral than in acid solution, and is accelerated by warming. The change in SO₂- content of a solution of potassium metabisulphite (0.1 per cent.) kept in a stoppered bottle, observed by titration at intervals of aliquot portions with standard iodine solution, has been observed to be as follows:

Time interval (hours)	0	24	48	72	96	144
Relative SO2-content	100	98.2	94.6	93.5	92.2	91.3

The oxidation can be considerably checked by the addition of certain organic compounds, such as sucrose, alcohols, quinol, and some organic acids. On the other hand it is accelerated by dextrose and certain other sugars, and also by the presence of Cu^•• or Fe^••• ions in solutions of P_H value 4 to 12.

Aqueous solutions of normal alkali sulphites are oxidised to dithionate by heating with lead dioxide, the latter being reduced to red lead. Manganese dioxide does not react in this way.

Analogous to the action of oxygen is that of sulphur, which slowly converts a sulphite in hot aqueous solution into the corresponding thio-sulphate, some trithionate being formed simultaneously. The reaction goes to completion with excess of sulphur, and is accelerated by the presence of sodium sulphide.

Crystalline hydrazine derivatives of certain sulphites, for example of zinc, cadmium, manganese, cobalt and nickel sulphites, have been prepared. When aqueous manganous sulphite containing excess of sulphurous acid is neutralised with hydrazine hydrate, a white crystalline compound, MnSO₃.N₂H₄.H₂SO₃, is formed. A solution of cobalt hydrogen sulphite similarly treated yields a red compound of composition 5CoSO₃.9N₂H₄.6H₂O, but if the hydrogen sulphite is added to an excess of concentrated hydrazine hydrate solution, a buff-coloured compound, CoSO₃.2N₂H₄.H₂O, results. A suspension of the latter salt treated with sulphur dioxide gives two red sulphites, CoSO₃.2N₂H₄.H₂SO₃.2H₂O and CoSO₃.N₂H₄.H₂SO₃.0.5H₂O. The other (red) compound similarly treated yields a brown substance, 2CoSO₃.N₂H₄.3H₂O. Similar products from sulphites of the other metals mentioned may be obtained.

By the action of alkali hydrogen sulphites on alkali nitrites, compounds are obtained the structure of which may be derived from that of ortho-nitrous acid, N(OH)₃, by substituting the sulphonic acid group, -SO₂.OH, for one or more hydroxyl groups.

The acid sulphites possess the property of forming crystalline additive compounds with aldehydes and ketones. Sodium hydrogen sulphite is therefore largely used for the purification of compounds of these classes; the sulphite adds on at the carbonyl group, >C:O, forming the grouping , and the original organic substance can be liberated

by suitable treatment with acid or alkali.

Calcium bisulphite is largely used in the manufacture of "sulphite pulp," from which paper and viscose silk may be produced. Wood chips or shavings are boiled with the bisulphite under pressure and so yield cellulose, which at the same time is bleached. The bisulphites of magnesium and the alkali metals are also sometimes used.

It is of interest that there is a frequent occurrence of isomorphism between the sulphites and the corresponding carbonates; this would appear, at first sight, to indicate quadrivalency of sulphur in the sulphites, but the evidence is untrustworthy and insufficient.