Densityb

[g cm–3]

100% H2O2 150.2 –0.42 1.443

70% H2O2 125 –40 1.288

60% H2O2 119 –56 1.241

50% H2O2 114 –52 1.196

Water 100 0 0.997

a. Extrapolated values because decomposition will reduce boiling

point continuously.

b. 25 °C.

Fig. 7.116 Configuration of hydrogen peroxide in the solid phase.

The bond length between the two oxygen atoms of the H2O2 molecule is rather

long (Fig. 7.116). Compared to water, the energy content of H2O2 is much higher.

For water, the heat of formation (DH) [Eq. (107)] from the elements is as low as –

286 kJ mol–1, whereas for H2O2 [Eq. (108)] the corresponding value is only –

188 kJ mol–1 [7]. In consequence, H2O2 is less stable and can disproportionate into

water and oxygen:

H 2 _ 0_5 O 2 _ H 2 O D H _ _286 kJ mol _1 _107_

H 2 _ O 2 _ H 2 O 2 D H _ _188 kJ mol _1 _108_

Since the activation energy for the cleavage of the oxygen–oxygen bond is rather

low (DH = –71kJ mol–1) [7], traces of contaminants can start this reaction. Basically,

the decomposition is a redox process, with H2O2 either supplying electrons

and yielding oxygen, or accepting electrons and yielding water. Metal salts of different

states of oxidation can start the decomposition reaction. The first step can

be the reduction according to Eq. (109):

7.6 Hydrogen Peroxide Bleaching 851

2 Me 2_ _ H 2 O 2 _ 2 Me _ _ O 2 _ 2 H _ _109_

The alternative is the oxidation of a metal according to Eq. (110):

2 Me _ _ H 2 O 2 _ 2 H _ _ 2 Me 2_ _ 2 H 2 O _110_

The reaction certainly can also start with the reduced form of metal. The overall

reaction is identical, it being the formation of water and oxygen from H2O2 with

the redox system of the metal is acting as the catalyst [8].

The decomposition of H2O2 is, in addition, catalyzed by alkali, with the reaction

steps being as follows:

H 2 O 2 _ OH _ _ H 2 O _ HOO _ _111_

HOO _ _ H 2 O 2 _ H 2 O _ O 2 _ OH _ _112_

Since bleaching with H2O2 requires alkaline conditions, this decomposition

reaction is very important for its technical application.

Single electron transfer reactions of H2O2 with catalysts yield radicals, these

decomposition reactions taking place with either metals or with enzymes (e.g.,

catalase). Radical formation may also be the result of a thermal cleavage of the

oxygen–oxygen bond:

H 2 O 2 _ Me _ _ OH _ _ _ OH _ Me 2_ _113_

H 2 O 2 _ _ OH _ H 2 O _ _ OOH _114_

H 2 O _ _ OOH _ _ OO _ _ H 3 O _ _115_

The hydroxyl radical, the hydroperoxy radical, and the superoxide anion radical

are important intermediates. Each of these cause side reactions in bleaching processes,

with delignification as a positive and depolymerization of the cellulose as a

negative result. In general, radicals produce more negative effects than positive

results on delignification. Therefore, if present in higher amounts, transition metal

ions must be removed by acid washing or “neutralized” by chelation before and

during a peroxide treatment.

Tab. 7.46 Standard oxidation potential for hydrogen peroxide [7].

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