Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

OXY-ACIDS OF SULPHUR. 127

For a further discussion of these formulas, see Nos. 155 and 156.

Regarding the constitution of sulphuric acid, it is to be noted:

1. Sulphuric acid contains two hydroxyl radicals: sulphuryl chloride
   (No. 149) and water react together with the formation of sulphuric acid,
   whereby the chlorine atoms are replaced by hydroxyl groups:
   . SO 2 C1 2 -» SO 2 (OH)C1 —> SO 2 (OH) 2.


2. The two hydroxyl groups of sulphuric acid are bound to the sulphur
   atom: Either one or both of these groups can be replaced by organic radi-
   cals, such as phenyl, whereby phenyl sulphonic acid, C 6 H 5 SO 2 OH, and diphe-
   nylsulphone, (C 6 H 6 ) 2 SO 2 , are formed. But the same substances can be pre-
   pared by the oxidation respectively of mercaptan, C 6 H 5 SH, and of diphenyl
   sulphide, (C 6 H 5 ) 2 S, in which the organic radical must be joined directly to
   the sulphur atom. From this it follows that both hydroxyl groups are
   held by the same sort of a bond:
   .OH
   os;


3. The way in which the two remaining oxygen atoms are bound is not
   known with certainty, but a mutual bonding of the two atoms to one another
   is improbable on account of the great stability of the SO 2 -grouping. Thus
   the structural formula is apparently
   O. /OH

PYROSTTLPHURIC ACID, HO.SO 2 —O—SO 2 .OH, may be regarded as derived

from two molecules of sulphuric acid by the loss of one molecule of

water; it is prepared by dissolving sulphur trioxide in sulphuric acid.^1 By

substituting the two hydroxyl groups of pyrosulphuric acid with chlorine

atoms, pyrosulphuric-acid-chloride is obtained which is stable and easy to

obtain pure. (See No. 150.)

CARO'S ACID, HO.SO 2 .OOH, is formed by the withdrawal.of a molecule

of water from between a molecule of sulphuric acid and a molecule of hydro-

gen peroxide.

PERSULPHURIC ACID, H 2 S 2 OS (see No. 94), is produced by the electrolysis of

H^Oj when two discharged monovalent acid-sulphate anions, HSO 4 , become

united; it may also be regarded as resulting from the condensation of one mole-

cule of sulphuric acid with one of Caro's acid.

THIOSULPHTJHIC ACID. Just as sulphuric acid may be formed by the

addition of an atom of oxygen, so thiosulphuric acid, H 2 S 2 O 3 , is produced by the

addition of an atom of sulphur to a molecule of sulphurous acid (cf. No. 90).

POLYTHIONIC ACIDS. Among the polythionic acids are included dithwnic

acid, H 2 S 2 O 6 , triihionic acid, H 2 S 3 O 6 , tetrathionic add, HjS^Og, and pentathionic

(^1) Concerning the various hydrates of SO
3 , see R. Knietsch, Ber. 34, 4100
(1901). Notice particularly the curves.