Control of pH
Control of pH is vital for effective and cost-efficient bleaching. Bleach chemicals release their bleaching power as they decompose. A slow rate of decomposition is desirable because it allows the active chemicals time to diffuse through the plant material to the pigments which are to be destroyed.
The rate of bleach decomposition is governed by the pH of the bleach bath. Incorrect pH will either release the active bleaching compound too slowly, resulting in poor bleaching, or too rapidly, which wastes expensive chemical. Individual bleaching agents work best over narrow pH ranges (Figure 1).
When the bleach bath is not in use, bleach decomposition can be slowed by adjusting the pH to a value at which the bleach can be considered stable (Figure 1).
To reactivate a bleach, add the appropriate alkali or acid to bring the pH within the working range for the particular bleach being used (black region in Figure 1).
If used below pH 8.5, hypochlorite bleaches produce hypochlorous ions which attack the colourless cellulose fibres rather than the lignin. This wastes bleach power and produces a brittle product.
A pH meter or pH paper can be used to monitor the pH of the bleaching solution. Strong mineral acids such as hydrochloric acid can be used with extreme caution to decrease pH. Weaker acids such as acetic or formic acid are much safer.
Adding an alkali will increase pH. Sodium carbonate is a weaker but safer alkali than sodium hydroxide. Sodium silicate is the preferred alkali for use with peroxide bleach as it has useful metal chelating properties. It is essential to use personal protection equipment (PPE) to protect your skin and eyes from splash injuries when using strong acids or alkalis.
Additives
Bleach decomposition is catalysed by trace metal impurities in the water supply. Add a chelating agent, such as sodium EDTA (at 0.1 to 1% depending on water quality), to immobilise these impurities and avoid wasting a lot of bleach. Do not use bore water rich in iron.
Wetting agents, such as soaps or polyglycol ethers, are usually added at about 1% concentration to increase contact between the plant material and bleach liquor. Household detergents or industrial chemicals such as Tween® or Triton™ X-100* are satisfactory.
Bleach baths
Use about 20 parts of liquor to one part of material. Bleach concentration can only be determined from experience and depends on many factors: the normal range would be from 1 to 4% of bleaching agent. Do not expose the bleach solution to strong light, because it causes rapid decomposition of the bleach.
If chlorite is to be used, bleaching tanks should be made of type 316 or 317 stainless steel to resist corrosion. For peroxide, type 304 stainless steel is adequate. Alternatively, tanks can be lined with PVC or butyl rubber sheeting. Polyethylene can be used temporarily.
Some form of agitation of the bleach liquor is advisable. A heater is recommended for peroxide and chlorite baths.