It is prepared on the industrial scale for the manufacture of
sulphuric acid, for the preparation of sodium sulphate by the
Hargreaves process, and for use as a bleaching-disinfecting agent
and as a preservative. When compressed it is also used largely
as a refrigerating agent, and in virtue of its property of neither
burning nor supporting combustion it is also used as a fire extinct or.
The solution of the gas in water is used under the name of sulphurous
acid. The free acid has not been isolated, since on evaporation
the solution gradually loses sulphur dioxide. This solution possesses reducing
properties, and gradually oxidizes to sulphuric acid on exposure.
When heated in a sealed tube to 180° C. it is transformed into sulphuric
acid, with liberation of sulphur. Numerous salts, termed
sulphates, are known. Since the free acid would be dibasic, two
series of salts exist, namely, the neutral and acid salts. The neutral
alkaline salts are soluble in water and show an alkaline reaction,
the other neutral salts being either insoluble or difficultly soluble
in water. The acid salts have a neutral or slightly acid reaction.
The sulphates are prepared by the action of sulphur dioxide on the
oxides, hydroxides or carbonates of the metals, or by processes of
precipitation. Sulphur-ous acid may have either of the constitutions
/OH 0 OH
0:5 or , or be an equilibrium mixture of these
oH o H
two substances. Although the correct formula for the acid is not known, sulphates are known of both types. Sodium sulphite is almost certainly of the second and unsymmetrical type. Two ethyl sulphates are known, the first or symmetrical form being derived from sulphuryl chloride and alcohol, and the second and unsymmetrical from sodium sulphite and ethyl iodide; the junction of one ethyl group with a sulphur atom in the second salt follows because it yields ethyl sulphonic acid, also obtainable from ethyl mercaptan, C2H5SH. Two isomeric sodium potassium sulphates are known, and may be obtained by neutralizing acid sodium sulphite with potassium carbonate, and acid potassium sulphite with sodium carbonate; their formulae are: 02SK(0Na) and 02SNa(0K). There are various haloid derivatives of sulphurous acid. Thionyl fluoride, SOFZ, has been obtained as a fuming gas by decomposing arsenic fluoride with thionyl chloride (Moissan and Lebeau, Compt. fend., 1900, 130, p. 1436). It is decomposed by water into hydrofluoric and sulphurous acids. Thionyl chloride, S0Cl2, may be obtained by the action of phosphorus pentachloride on sodium sulphite; by the action of sulphur trioxide on sulphur dichloride at 7? -80° C. (Joum. Chem. Soc., 1903, p. 420); and by the action o chlorine monoxide on sulphur at low temperature. It is a colourless, highly refracting liquid, boiling at 78°; it fumes on exposure to moist air. Water decomposes it into hydrochloric and sulphurous acids. 0n treatment with potassium bromide it yields thionyl bromide, S0Br2, an orange-yellow liquid which boils at 68° C. (40 mm.) (Hartoz and Sims, Chem. News, 1893, 67, p. 82).
Sulphur trioxide, 503, mentioned by Basil Valentine in the 15th century, was obtained by N. Lemery in 1675 by distilling green vitriol. It may be prepared by distilling fuming sulphuric acid, or concentrated sulphuric acid over phosphorus pent oxide, or by the direct union of sulphur dioxide with oxygen in the presence of a catalyst, such as platinized asbestos (see SULPHURIC Ac11>). This oxide exists in two forms. The a- form is readily fusible and melts at 14-8° C. It corresponds to the simple molecular complex S03. The B- variety is in fusible, but on heating to 50° C. is transformed into the a- form. It corresponds to the molecular complex (S0;)2. /Vhen perfectly dry this oxide has no caustic properties; it combines rapidly, however, with water to form sulphuric acid, with the development of much heat. It combines directly with concentrated sulphuric acid to form pyrosulphuric acid, H2520-, It reacts most energetically with many organic compounds, removing the elements of water in many cases and leaving a carbonized mass. It combines directly with many elements and compounds and frequently acts as energetic oxidizing agent. It finds considerable application in the colour industry.
Sulphuryl fiuoride, SOZFQ, formed by the action of fiuorine on sulphur dioxide (H. Moissan, Compt. fend. 132, p. 374), is an exceedingly stable colourless gas at ordinary temperatures, becoming solid at about -120° C. Sulphuryl chloride, SOQCI2, first obtained in 1838 by Regnault (Ann. chim. phys., 1838, (2), 69, 170), b the action of chlorine on a mixture of ethylene and sulpfiur dioxide, may also be obtained by the direct union of sulphur dioxide and chlorine (€SD€C1&lly IQ the presence of a little camphor); and by heating chlorsulphonic ac1d 1n the presence of a catalyst, such as mercuric sulphate (liawlewski, Bef., I?Q7, 30 p 765); QSQZCLQH =5(), Cl, + HQSO4. it is a colourless fummg liquid which boils at 699 C. and which is readily decomposed by water into sulphuric and hydrochloric acids. Fluorsulphonic acid, SOQF-OH, is a mobile liquid obtained by the HCUOH of an excess of hydrofluoric acid on well-cooled sulphur trioxide. It boils at 162-6° and is decomposed violently bv water. Chlorsulphonic acid, S0¢Cl~0H, first prepared by A. Williamson (1f'0€~ R022 S06-, 1856, 7, p. II) by the direct union of sulphur trioxide with hydrochloric acid gas, may also be obtained by distilling concentrated sulphuric acid with phosphorus oxychloride: 2H2$04-ljPQCla=2S02Cl-0H-l-HCl-1-HP03. It is a colourless fummg liquid which boils at 152-153° C. When heated under pressure it decomposes, forming sulphuric acid, sulphuryl chloride, &c. (Ruff, Bef., 1901, 34, p. 3509). It is decomposed by water with explosive violence. Disulphuryl chloride, S205Cl2, corresponding to pyrosulphuric acid, is obtained by the action of sulphur trioxide on sulphur dichloride, phosphorus oxychloride, sulphuryl chloride or dry sodium chloride:6S03 -l-2P0Cl3 = P205 -I-3S205Cl2; SZCI2 + 5503 = S205Cl2 'l' 5502; S03 'ff' S02Cl2 = S205Cl2; + = S20;, Cl2+Na2S0.i. It may also be obtained by distilling chlorsulphonic acid with phosphorus pentachloride: 2S02Cl-OH -l-PCl5 = S205Cl2 +POCl;i --2HCl. It is a colourless, oily, fuming liquid which is decomposed by water into sulphuric and hydrochloric acids. An oxychloride of composition S203Cl4 has been described. Sulphur sesquioxide, S203, is formed by adding well-dried flowers of sulphur to melted sulphurtrioxide at about 12-15° C. The sulphur dissolves in the form of blue drops which sink in the liquid and finally solidify in blue-green crystalline crusts. It is unstable at ordinary temperatures and rapidly decomposes into its generators on warming. It is readily decomposed by water with formation of sulphurous, sulphuric and thiosulphuric acids, with simultaneous liberation of sulphur. Hyposulphurous acid, H2S204, was first really obtained by Berthollet in 1789 when he showed that iron left in contact with an aqueous solution of sulphur dioxide dissolved without any evolution of gas, whilst C. F. Schonbein subsequently showed the solution possessed reducing properties. P. Schutzenberger (Compt. fend., 1869, 69, p. 169) obtained the sodium salt by the action of zinc on a concentrated solution of sodium bisulphite: Zn -|- 4NaHS03 = Na2S204 + ZnS03 + Na2S03 + 2H20, the salt -being separated from the sulphates formed by fractional precipitation. A solution of the free acid may be prepared by adding oxalic acid to the solution of the sodium salt. This solution is yellow in colour, and is very unstable decomposing at ordinary temperature into sulphur and sulphur dioxide. A pure zinc salt has been prepared by Nabl (Monatr, 1899, 20, p. 679) by acting with zinc on a solution of sulphur dioxide in absolute alcohol, whilst H. Moissan (Compt. fend., 1902, 135, p. 647) has also obtained salts by the action of dry sulphur dioxide on various metallic hydrides. Considerable controversy arose as to the constitution of the salts of this acid, the formula of sodium salt, for example, being written as NaHS02 and Na2S204; but the investigations of C. Bernthsen (Ann., 1881, 208, p. 142; 1882, 211, p. 285; Bef., 1900, 33, p. 126) seem to decide definitely in favour of the latter (see also T S. Price, Joum. Chem. Soc.; also Buchercr and Schwalbe, Zeit. angew. Chem 1904, 17, p. 1447). Although this acid appears to be derived from an oxide S203, it is not certain that the known sesquioxide is its an hydride.
Persulphuric an hydride, S201, is a thick viscous liquid obtained by the action of the silent discharge upon a mixture of sulphur trioxide and oxygen. It solidifies at about 0° C. to a mass of long needles, and is very volatile. It is decomposed readily into sulphur trioxide and oxygen when heated. Water decomposes it with formation of sulphuric acid and oxygen: 25201 + 4H20 = 4H2S04 + 02. Persulphuric acid, HS04, the acid corresponding to 5207, has not been obtained in the free state, but its salts were first prepared in 1891 by H. Marshall (]oum. Chem. Soc., 1891, p. 771) by electrolysing solutions of the alkaline bi sulphates. The potassium salt, after recrystallization from warm water, separates in large tabular crystals. Its aqueous solution gradually decomposes with evolution of oxygen, behaves as a strong oxidant, and liberates iodine from potassium iodide. Solutions of per sulphates in the cold give no precipitate with barium chloride, but when warmed barium sulphate is precipitated with simultaneous liberation of chlorine 1 KQSQOS + BaCl¢ = BaS04 + K¢S04 + Cl2. The conductivity measurements of G. Bredig point to the salt possessing the double formula. Thiosulphuric acid, formerly called hypo sulphurous acid, HQSQO3, cannot be preserved in the free state, since it gradually decomposes with evolution of sulphur dioxide and liberation of sulphur: HQSZO3 = S+S02-l-H20. The salts of the acid, however, are stable, the sodium salt in particular being largely used for photographic purposes under the name of “ hypo." This salt may be prepare by digesting flowers of sulphur with sodium sulphite solution or by boiling sulphur with milk of lime. In this latter reaction the deep yellow solution obtained is exposed to air when the calcium poly sulphide formed is gradually converted into thiosulphate by oxidation, and the calcium salt thus formed is converted into the sodium salt by sodium carbonate or sulphate. The thiosulphates are readily decomposed b mineral acids with liberation of sulphur dioxide and precipitation of, sulphur: Na2S203 -1- 2HCl = 2NaCl S -l' 502 H2Q- ThQY form many double salts and give a dark violet coloration with ferric Chloride solution, this colour, however, gradually disappearing on standing, sulphur being precipitated. The acid is considered t0 possess the structure 02S(SH) (OH). SiHC€ S0dl{m 'fh105Phat€ f€aCtS with ethyl bromide to give sodium ethyl th1osulphate, wh1ch on treatment with barium chloride gives presumably barium €¥hYl thiosulphate. This salt, on standing, decomposes into barium dithionate, BaS205, and diethyl disulphide, (C2Hf»)252» Wh1Ch Points to the presence of the SH group in the molecule. I The zhionic acids are a group of sulphuf-C0f1t21f1§ U§ acids 0f §§ 9e"? formula H2S, .O¢, where »=2, 3. 4. 5 and Possibly 64, DI* 19“'§ acid, I-125206, prepared by ]. Gay-Lussac in 1819, is usua y obtaine
