Physical and Chemical Properties and Definitions

Chlorine dioxide is a resonance-stabilized compound containing 19 valence electrons,

nine paired electrons, and one unpaired electron (Fig. 7.59). Chlorine dioxide

shows no noticeable tendency to dimerize, probably because the electron is

less localized on the central chlorine atom.

O Cl O O Cl O O Cl O O Cl O

Fig. 7.59 Resonance structures of chlorine dioxide.

The chlorine–oxygen bonds, which show predominantly double-bond character,

form an angle of about 117.5°, with a chlorine-oxygen bond length of 1.47 A. The

redox potential corresponding to

7.4 Chlorine Dioxide Bleaching 735

ClO 2_ aq _ _ _ e _ _ ClO _2 _56_

is 0.954 V. In aqueous solution, it depends linearly on the pH with a coefficient of

–0.062 V for increasing units of pH [6]. Another important half reaction involves:

ClO _

2_ _ 2 H 2 O _ 4 e _ _ Cl _ _ 4 OH _ _57_

with a redox potential E0 of 0.76 V. Chlorine dioxide has an oxidation state of +4,

and thus can accept five electrons per molecule to be reduced to chloride ions as

demonstrated by Eqs. (56) and (57). The molecular weight of chlorine dioxide is

67.457 g mol–1, and that of the equivalent weight 13.49 g mol–1 e– (67.457/5). The

charge of chlorine dioxide is mostly designated as “equivalent chlorine”, better

known as “active chlorine”, because this allows the combined addition of chlorine

and chlorine dioxide to be expressed as a single term.

To convert chlorine dioxide into active chlorine equivalents, the redox reaction

of elemental chlorine must be considered:

Cl 2 _ 2 e _ _ 2 Cl _ _58_

The equivalent weight of chlorine can be calculated as 35.46 g mol–1 e– (70.914/

2). The charge of chlorine dioxide can be expressed as active chlorine charge in

weight units (% on o.d. pulp or kg odt–1 pulp) by multiplying the weight of chlorine

dioxide by a factor of 2.629 (35.46/13.49), while considering an equivalent

electron transfer. The charge of chlorine dioxide, expressed as active chlorine, is

often related to the kappa number of the pulp before the bleaching treatment. As

an example, an unbleached softwood kraft pulp, kappa number 27, is treated in a

first chlorine dioxide stage, a so-called D0-stage. Assuming a kappa factor of 0.25

to ensure complete pre-delignification, would require the following amount of

active chlorine or chlorine dioxide:

27. 0.25 = 6.75% active chlorine (or equivalent chlorine) on o.d. pulp

= 67.5 kg active chlorine odt–1 pulp

= 67.5/2.629 = 25.68 kg ClO2 odt–1 pulp.

The freezing point of chlorine dioxide is –59 °C, and its boiling point 11 °C.

Chlorine dioxide is highly soluble in water. The vapor pressure, in kPa, is

expressed as:

log P _ 6_8676 _

1375_1

T _59_

where T is °K. Equation (59) reveals that chlorine dioxide boils at 9.65 °C. The partition

coefficient of chlorine dioxide between water and the gaseous state is 70 at

0 °C and 21.5 at 35 °C. The solubility of chlorine dioxide in chilled water (~5 °C) is

about 10 g L–1 while maintaining a reasonably low vapor pressure (~6.6 kPa). This

solution is rather stable, and can be stored in the dark for several months. Chlorine

dioxide is known to be quite light-sensitive and decomposes through the free

736 7Pulp Bleaching

radicals O and ClO to give ClO3/Cl2O6 in the gas phase [6]. Chlorine dioxide is

usually analyzed spectrophotometrically as it possesses a broad absorption band

with a maximum near 360 nm. Lenzi reports that the extinction coefficient of

1150 M–1 cm–1 exhibits only a slight variation with the acidity of the medium and

almost no cross-sensitivity with chloride ion, chlorate ion, and chlorine [7].

Дата добавления: 2015-10-21