Physical Properties of Ozone

Ozone (O3) is an allotropic form of oxygen. At ambient conditions, it is a pale blue

gas (= 2.1415 g L–1 at 0 °C and 101.3 kPa). It condenses into an indigo blue liquid

(–112 °C) and freezes to a deep blue-violet solid (–195.8 °C). Ozone has a bent

structure of C2v symmetry with an apex angle of 116°49′ and equal oxygen–oxygen

bond distances which are more closer to that of molecular oxygen as compared to

that of hydrogen peroxide. Hence, the bonds in ozone have considerable doublebond

character [6]. Data obtained from the microwave spectrum of the ozone molecule

have shown it not to be significantly paramagnetic [7]. Thus, the ozone molecule

can be pictured as a resonance hybrid consisting of four mesomeric structures,

as shown in Fig. 7.77.

778 7Pulp Bleaching

+O

O

O- O

+

O

O- -O

+

O

O -O

O

O+

1 2 3 4

Fig. 7.77 Resonance structures of ozone.

The contributions of forms 1 and 4, which have a positively charged terminal

oxygen with only six electrons, have been used to explain the electrophilic character

of ozone. As such, ozone falls into a moderately large class of 1,3-dipolar compounds

and will in certain reactions follow mechanisms typical of this class as a

whole [8]. In a simple molecular orbital representation of the ozone molecule,

each of the oxygen atoms is considered to be an sp2 hybrid and thus overlap of the

p-orbitals provides a molecular orbital containing four p electrons. The UV spectrum

of ozone shows an absorption maximum in 0.01M HClO4 at 260 nm with an

extinction coefficient of 2930 L.M–1.cm–1 [9]. In acid aqueous solution, the oxidizing

power is exceeded only by fluorine, atomic oxygen, OH radicals, and a few other

species [6]. The oxidation potential in aqueous solution is expressed by Eq. (83):

O 3 _ 2 H _ _ 2 e _ _ O 2 _ H 2 O _ E 0 _ 2_07 eV _83_

Ozone is thermodynamically unstable, and 1mol decomposes exothermically to

1.5 mol of molecular oxygen.

The solubility of ozone is an important criterion for ozone bleaching. The solubility

of ozone in equilibrium with its partial pressure is usually defined by

Henry’s law according to the following expression:

xO 3 _

PO 3

kH _84_

where xO3 is the dissolved ozone molar fraction (mol mol–1), PO3 is the ozone partial

pressure (kPa), and kHis Henry’s law constant (kPa mol fraction–1).

The ozone molar fraction can be transformed to a concentration of ozone, cO3

(in mol L–1 or mg L–1) by multiplying the molar fraction by 55.51or by 2.664. 106,

respectively. Ozone solubility is influenced by several factors, such as temperature,

pH, ionic strength and dissolved matter. Henry’s law constant, kH, depends on the

temperature, T, according to Eq. (85):

dLnkH

d _1_ T __ _

D H

R

kH _ k 0

H _ Exp _

D H

R

T _

T 0 __ _85_

where R is the gas constant and DH is the heat of solution of the gas. The parameters

k0

H and T0 refer to kH and T at standard conditions. Equations (84) and (85)

show that an increase in temperature is connected with a decrease in the dissolved

7.5 Ozone Delignification 779

780 7Pulp Bleaching

ozone concentration. The reduced ozone solubility at higher temperature can be

explained by a drop in the liquid phase driving force, and by a higher decomposition

rate. The pH is the predominant parameter which determines the stability of

ozone in aqueous solution (see Section 7.5.4), and hence also its solubility in

water. It is agreed that the dissolved ozone concentration increases with decreasing

pH. This is one of the important reasons why ozone bleaching is conducted

under acidic conditions, preferably in the pH range of 2–3. Quederni et al. have

determined the apparent Henry’s law constants for ozone solubility in water as a

function of temperature at pH 2 and pH 7 [10]. The relationship between kH and

the temperature for these two different pH levels is expressed in Eq. (86):

kH _ 101_3 _ Exp 20_7 _

T __ pH _ 7_

kH _ 101_3 _ Exp 18_1 _

T __ pH _ 2_

_86_

The solubility of ozone in water at 1atm, pO3 = 101.3 kPa and 0 °C calculates to

(101.3/1.966. 105). 2.664. 106 = 1. 37 g L–1 at pH 2, and to (101.3/

2.270. 105). 2.664. 106 = 1.18 g L–1 at pH 7. Figure 7.78 illustrates the course of

the equilibrium dissolved ozone concentration as a function of temperature in the

range of 15 to 50 °C for the two pH levels, considering typical conditions for medium

consistency ozone bleaching:

_ Generated ozone concentration in oxygen gas: 200 gm–3 = 9.3 Vol%

_ Total pressure in the mixer: 8 bar = 0.81MP a

_ Ozone partial pressure, pO3: 0.093. 810.4 = 75.4 kPa

10 20 30 40 50

p

O3

= 75.4 kPa

pH = 7 pH = 2

Dissolved O

conc. [mg/l]

Temperature [.C]

Fig. 7.78 Influence of temperature and pH on the dissolved

ozone concentration in water assuming a partial pressure,

pO3, of 75.4 kPa (according to results determined by Quederni

et al. [10]). The ionic strength is kept constant at 0.13mol L–1.

The ratio between the ozone solubility at pH 2 and pH 7 further increases with

rising temperature. The dependency of the dissolved ozone concentration on pH

was even more pronounced, according to results obtained by Sotelo et al. [11].

These authors found an increase by a factor of 1.6 (3.1) when comparing ozone

solubilities at pH 2.5 and pH 7.0 (pH 9.0) at 10 °C and an ionic strength of 0.15 M.

As already mentioned, gas absorption is also dependent on the ionic strength.

Generally, the dissolved gas concentration decreases as ionic strength increases.

The effect of ionic strength on the solubility of ozone is most pronounced in the

presence of phosphates, chloride or carbonate ions, whereas the addition of sulfate

ions exerts practically no change in solubility. According to Sotelo et al., the

dissolved ozone concentration is decreased by half when increasing the ionic

strength from 0.04 mol L–1 to 0.49 mol L–1 in the presence of sodium chloride (pH

5.94 and 1.1 kPa → 4.8 mg L–1 versus 2.4 mg L–1) [11].

In more general terms, both the influence of temperature and ionic strength is

described by Eq. (5) [12]:

kH _ Exp _

T _ 2_659 _ l _

688 _ l

T _ 16_808_ _87_

where kH, Henry’s constant is kPa.L.mol–1, T, temperature in Kelvin, and l is the

molar ionic strength. The temperature-dependence of the dissolved ozone concentration

is more pronounced using Eq. (86) than with Eq. (87), with an almost perfect

correspondence at temperature higher than 35 °C (Fig. 7.79).

0 10 20 30 40 50

p

O3

= 101.3 kPa

Oyama: ì = 0.13 M; Quederni et al.: ì = 0.13 M

ì = 1.00 M

Dissolved O

-conc [mg/l]

Temperature [.C]

Fig. 7.79 Comparison of calculated dissolved ozone concentration

as a function of temperature using Henry’s constants

from different literatures sources: Quederni et al. [10] versus

Oyama [13]. The influence of ionic strength was assessed

using Eq. (87).

7.5 Ozone Delignification 781

7.5.3

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