Modern inorganic chemistry

THE ELEMENTS OF GROUP I 149

H

O

HO x B; + 2H 2 O

V X

O

H

H

O

HO x BxX X OH

O

H

In the presence of glycerol or mannitol (polyhydroxo compounds)

boric acid behaves as a much stronger acid; the reaction can be

represented as:

(

—C-OH\

2H 2 O

The acid can then be titrated with sodium hydroxide using phenol-

phthalein as the indicator. Boric acid was known as "boracic acid'

and was used extensively as a mild antiseptic. Borates are rarely

simple salts although a few salts of formula MBO 3 (where M is a

trivalent metal) are known. More commonly, the 'borate' anion is

built up of BO 3 units into chains, rings or sheets, just as silicates

are built up from units of the group SiO 4. Sodium heptaoxo-

tetraborate (borax) Na 2 B 4 O 7 .10H 2 O is alkaline in solution since

it is hydrolysed. It can be titrated against hydrochloric acid using

methyl red as the indicator :

2H 5HO -» 4HBO

Borax is used in the production of pyrex glass, ceramics, as a flux

in soldering and welding, and in laundering to impart a glaze to

linen.

Sodium "perborate' NaBO 2 .H 2 O 2 .3H 2 O, or more correctly sodium
dioxoborate peroxohydrate, is an important additive to washing
powders, behaving in water like a mixture of sodium borate and
hydrogen peroxide (a mild bleach). It is manufactured by treating
a solution of borax with sodium peroxide followed by hydrogen
peroxide or by the electrolysis of a solution containing borax and
sodium borate with platinum electrodes.