Modern inorganic chemistry

GROUP VI 303

is very low because the large HaSO^ and HSO^ ions can move only

slowly through the viscous acid in an electric field. Only recently

has it been possible to find an acid-base indicator sufficiently resis-

tant to the oxidising and sulphonating action of the concentrated

acid to be used in it; this indicator shows the acid to be quite strong.

THE SULPHATES AND HYDROGENSULPHATES

The hydrogensulphates (or bisulphates) containing the ion

are only known in the solid state for the alkali metals and ammonium.

Sodium hydrogensulphate is formed when sodium chloride is

treated with cold concentrated sulphuric acid:

NaCl + H 2 SO 4 -> NaHSO 4 + HClt

It may also be obtained by crystallising sodium sulphate from a

dilute sulphuric acid solution:

Na 2 SO 4 + H 2 SO 4 -> 2NaHSO 4

The hydrogensulphate ion dissociates into hydrogen and sulphate

ions in solution; hence hydrogensulphates behave as acids.

When solid sodium hydrogensulphate is heated, sodium^4 pyro-

sulphate' is formed; further heating gives sodium sulphate and

sulphur trioxide:

2NaHSO 4 T> Na 2 S 2 O 7 + H 2 Ot
Na 2 S 2 O 7 -» Na 2 SO 4 + SO 3 T
Electrolysis of the hydrogensulphate of potassium or ammonium
can yield a peroxodisulphate and thence hydrogen peroxide.
The sulphates of many metals are soluble in water, but those of
barium, lead, mercury(I), calcium and strontium are insoluble or
only sparingly soluble. Soluble sulphates often crystallise out as hy-
drates, for example the vitriols such as, FeSO 4 .7H 2 O; NiSO 4 .7H 2 O;
CuSO 4 .5H 2 O and double salts, for example FeSO 4 .(NH 4 ) 2 SO 4.
6H 2 O, and the alums, for example KA1(SO 4 ) 2 .12H 2 O. In these salts,
most of the water molecules are attached to the cation; the remaining
water molecules are connected by hydrogen bonds partly to the
sulphate ions and partly to the cationic water molecules (for example
CuSO 4 .5H 2 O, seep. 412).
The sulphates of the alkali and alkaline earth metals and man-
ganese(II) are stable to heat; those of heavier metals decompose on
heating, evolving sulphur trioxide and leaving the oxide or the
metal: