Chemistry Reference
In-Depth Information
to tears and holes during after-treatments (for example dyeing, merceris-
ing, thermofixation).
Apart from the basic products, hydrogen peroxide and sodium hydrox-
ide, an industrial bleaching bath contains many additives, the most impor-
tant of which are discussed below. From an analytical point of view, it is
evident that, for the development of a sensor for hydrogen peroxide, each
of the additives used must be investigated to determine whether and to
what extent they can interfere.
4.2.2
Stabilisation of hydrogen peroxide
Already at a pH of 11, the balance of Equation 4.1 is situated too far to the
right because too much of the radical dioxygen anion is formed
12
. Besides
activation (with sodium hydroxide), hydrogen peroxide should also be sta-
bilised by shifting the balance of Equation 4.3 more to the left. This means
that HO
2
-
should be made more stable, which can be achieved by spread-
ing the strongly localised charge present at the terminal oxygen atom. This
can be realised by inserting a cation in solution with a high charge density.
This ion will tend to associate with HO
2
-
since it is relatively unstable itself.
An ion that qualifies for this task is Mg(II), which is brought into the bleach-
ing solution in the form of magnesium sulphate
16
. Even though it is correct
that such ions are stabilised by solvation (with water), formation of com-
plexes with HO
2
-
creates even more stable configurations
12
.
In a strongly alkaline solution, magnesium ions will immediately pre-
cipitate. Therefore, they are first dissolved in an aqueous solution of water-
glass. Waterglass is a sodium silicate, having the six-chain structure as its
main component. The magnesium ions are bound to the waterglass on the
terminal -O
-
- groups. In fact, an exchange reaction between the sodium
ions and the magnesium ions occurs, resulting in improved stabilisation of
sodium ions in solution by hydration, so that Mg(II) is able to replace Na(I)
in the waterglass structure. The magnesium ions are now bound to the
waterglass in such a way that, when they are dissolved in the alkaline
bleaching solution, they will not precipitate; however, they will still be able
to stabilise HO
2
-
(Fig. 4.1).
Waterglass also has a second advantageous property - the stabilisation
of hydrogen peroxide
16
. It masks metal ions and their oxides and hydrox-
ides (such as those from iron, copper and manganese). The latter com-
pounds can strongly catalyse the decomposition of hydrogen peroxide to
water and oxygen. During this decomposition, the radical dioxygen anion
is formed as an intermediate, and hence it can occur in a highly increased
concentration in the presence of metal ions, metal oxides and metal hydrox-
ides. Cotton that is to be bleached always contains small amounts of metal
ions, the quantity being dependent on the place of origin and the climato-
