Modern inorganic chemistry

278 GROUP VI

Alternatively an ice-cold dilute solution of sodium peroxide is

passed through a column containing a cation-exchanger of the

synthetic type (p. 274) where the cation is hydrogen (i.e. H 3 O+), then

exchange occurs:

Na 2 O 2 + 2H 3 O+ -» H 2 O 2 + 2Na+ + 2H 2 O

(on exchanger) (on exchanger)

Hydrogen peroxide is obtained in aqueous solution at the bottom

of the column. This is a good method of preparation.

On a large scale, hydrogen peroxide is produced by the electrolysis

of ammonium hydrogensulphate, using a platinum anode and a lead

cathode separated by a diaphragm. The essential process occurring

is:

(NH 4 ) 2 S 2 O 8

s 2 or + 2E

>H?t

i.e.

and

2NH 4 HSO 4

2HSO4

2H + + 2e"

H 2 T

This is a process of anodic oxidation. The ammonium peroxo-

disulphate formed is then hydrolysed and the solution distilled in

vacua :

(NH 4 ) 2 S 2 O 8 + 2H 2 O -+ 2NH 4 HSO 4 -f H 2 O 2

The ammonium hydrogensulphate is returned to the electrolytic

cell. A process such as this yields an aqueous solution containing

about 30% hydrogen peroxide. The solution can be further con-

centrated, yielding ultimately pure hydrogen peroxide, by fractional

distillation; but the heating of concentrated hydrogen peroxide

solutions requires care (see below).

The above method has now been largely replaced by a newer

process, in which the substance 2-ethylanthraquinone is reduced by

hydrogen in presence of a catalyst to 2-ethylanthraquinol; when

this substance is oxidised by air, hydrogen peroxide is formed and

the original anthraquinone is recovered:

2-ethyl-anthraquinone +H 2 O;

OH

2-ethyl-anthraquinol