Perhydroxyl ion

The perhydroxyl ion, OOH , derived from the dissociation of hydrogen peroxide in alkaline solution, is generally considered the active agent in the bleaching of wood pulp using hydrogen peroxide. This ionization may be expressed by the following reversible reaction ... 

Figure 3. Scheme of ozone decomposition mechanism in water. P = promoter (e.g. ozone, methanol), S = scavenger or inhibitor (e.g. /-butanol, carbonate ion), I = initiator (e.g. hydroxyl ion, perhydroxyl ion) (adapted by Beltran [35]). 

The perhydroxyl ion, HO2, is formed by hydrogen peroxide dissociation in base ... 

Though hydrogen peroxide is stable in acidic medium, but bleaching occurs by the addition of alkali or by increased temperature. Hydrogen peroxide liberates perhydroxyl ion (HO ) in aqueous medium and chemically behaves like a weak dibasic acid. The perhydroxyl is highly unstable and in the presence of oxidisable substance (coloured impurities in cotton), it is decomposed and thus bleaching action takes place. Sodium hydroxide activates hydrogen peroxide because ion is neutralised by alkali which is favourable for liberation of HO. ... 

The process of regulation or control of perhydroxyl ion to prevent rapid decomposition of bleach and to minimise fibre degradation is described as stabilisation. Stabilisers for peroxide normally function by controlling the formation of free radicals. These are complex blends of a selection of materials serving a number of functions. They could include any of the following ... 

In practice cotton bleaching with hydrogen peroxide is carried out at 90-100°C, but the temperature may be increased to 120°C in the case of pressurised equipment with a corresponding reduction in process time. The rate of bleaching increases with the increase in temperature, but at the same time solution becomes unstable and degradation of cotton increases. Below 80 C, the evolution of perhydroxyl ion is very slow so also the rate of bleaching. 

This explanation, however, is no longer tenable. There is no absolute certainty about the nature of the bleaching action but it is believed that the perhydroxyl ion is the active species. These ions are formed when hydrogen peroxide dissociates in the following manner ... 

It is a well known fact that bleaching is more rapid in alkaline than in acid solutions. This may well be because the hydroxyl groups present in the alkaline liquor neutralize the hydrogen ions thereby promoting the liberation of perhydroxyl ions. 

Cotton is usually bleached in a 1-volume liquor at the boil. The most important factor in bleaching is to achieve the right degree of stability in the bleach liquor. If the pH be too low no perhydroxyl ions are set free and bleaching does not take place when the liquor is too unstable the whole of the oxygen is liberated and escapes into the atmosphere before it has had time to act upon the cotton. Reference has already been made to the catalytic activity of copper and iron, and neither of these metals should enter into the construction of bleaching vats at any point. Stainless steel (F.M.B. or F.D.R. qualities) is entirely satisfactory, but stoneware, concrete-lined, or plastic-lined vessels are also used. Iron or steel vessels such as kiers can be used if a silicate cement lining is applied. 

In an aqueous solution, NaClO does not exist as a complete rmit, but as sodium ions, Na+, and hypochlorite ions, CIO". The ingredient responsible for the bleaching action in this type of bleach is the hypochlorite ion. Many other solid and liquid bleaches contain hydrogen peroxide, H2O2, instead of sodium hypochlorite. In these bleaches, the active substance in solution is the perhydroxyl ion, HOO. What do the CIO and HOO" ions have in common ... 

Because the compormds HCl and H2O are more stable than the hypochlorite and perhydroxyl ions, these reactions occur. The reactions result in a bleaching action because the released oxygen... 

The bleach performance profile of aerial bleach catalysts is qnite different from that of peroxide bleaching. Whereas a wide range of oxidizable stains is attacked by the activated catalyst in the presence of perhydroxyl ions, in combination with atmospheric oxygen, only oily food stains, such as tomato oil, mango or annatto-derived stains [53], are targeted. It is assumed that under aerial bleach conditions the catalyst indnces the formation of hydroperoxides in the food stain. In a second... 

For an efficient cell all processes must occur rapidly. Reactants must be able to reach the electrodes easily so that porous electrodes with large internal surface areas, saturated with electrolyte, are used. The pore sizes are often graded from large on the gas side of an electrode to small on the electrolyte side. Good catalyst materials ensure rapid electrochemical reactions and, in combination with a higher temperature, help to suppress the cathodic formation of perhydroxyl ions by the reaction... 

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