Bleaches

Chemistry: Foundations and Applications, (2004) by Henry A. Carter

Bleaches

When chlorine gas is bubbled through a cylinder of tomato juice, the chlorine/tomato juice mixture turns almost completely white within five minutes. This spectacular change is a result of the chemical action of chlorine, acting as an oxidizing bleaching agent, on the pigments in tomato juice. When old newspaper clippings, discolored through aging and exposure to light, are treated with 1 percent aqueous sodium borohydride solution, the paper is dramatically whitened within twenty minutes. In this instance, the paper has been restored to its original white color by the action of sodium borohydride acting as a reducing bleaching agent.

A bleaching agent is a substance that can whiten or decolorize other substances. Colored substances generally contain groups of atoms, called chromophores , that can absorb visible light having specific, characteristic wavelengths, and reflect or transmit the part of light that is not absorbed. For example, if a chromophore absorbs blue light, it will reflect light of the complementary color, and the chromophore-containing substance will appear yellow. Bleaching agents essentially destroy chromophores (thereby removing the color), via the oxidation or reduction of these absorbing groups. Thus, bleaches can be classified as either oxidizing agents or reducing agents .

Some of the use of bleaching agents are:

The bleaching of textiles and fabrics The bleaching of wood pulp The removal of stains Commercial and household laundering and cleaning As ingredients in scouring cleansers and dishwashing products The bleaching of hair

Oxidizing Bleaches

A large number of oxidizing bleaches were reviewed by Jules A. Szilard in Bleaching Agents and Techniques (1973). The oxidizing bleaches (and bleaching agents) in common use today are: chlorine, chlorine dioxide, alkaline hypochlorites, hydrogen peroxide, peroxygen compounds, and sunlight and artificial light.

Chlorine (Cl 2 ). The discovery of chlorine by the Swedish chemist Carl Wilhelm Scheele in 1774 marked the beginning of the modern era of bleaching. According to Sidney M. Edelstein in a 1948 journal article titled "The Role of Chemistry in the Development of Dyeing and Bleaching," French chemist Claude-Louis Berthollet was the first to use chlorine to bleach cotton and linen fabrics.

Chlorine has been used to bleach wood pulp. Many pulp mills employing the Kraft pulping process prepare sodium hydroxide (needed to digest wood chips) on-site via the electrolysis of brine , a concentrated aqueous solution of sodium chloride.

2NaCl + 2H 2 O → 2NaOH + H 2 + Cl 2          (1)

Chlorine is a side product. Subsequent chlorine bleaching of the brown pulp gives a product that can be used for the manufacture of writing and printing paper. Unfortunately, organic compounds in the pulp are both oxidized and chlorinated, yielding small quantities of organochlorine compounds, including

Table 1. Bleaching agents and their commercial uses.

BLEACHING AGENTS AND USES Bleaching Agent Commercial Use in Bleaching Chlorine Bleaching pulp and paper; making hypochlorites Chlorine dioxide Bleaching kraft paper and flour Sodium hypochlorite Household laundering and sanitizing Calcium hypochlorite Solid bleach used in sanitizing Sodium dichloroisocyanurate Sanitizing and dishwashing agents Hydrogen peroxide Bleaching textiles, fur, pulp and paper, and hair Sodium perborate Milder bleach for laundering; dry cleaning; denture cleaning; tooth powder; replacement for phosphates in detergents Light Bleaching paper artifacts Sulfur dioxide Preserving grapes, wine, and apples; removal of color during refinement of sugar Sodium sulfite; sodium bisulfite Anti-chlor (a reducing agent for removing oxidizing bleaches) Sodium dithionite Bleaching textiles, pulp and paper; removing rust stains Sodium borohydride Bleaching pulp and paper

dioxins. In fact, the most abundant dioxin produced by the pulp and bleaching process, 2,3,7,8-tetrachlorodibenzo- p -dioxin (2,3,7,8-TCDD), has been found to be both a carcinogen and a deadly toxin . Thus, chlorine as a bleaching agent is being replaced by the safer bleaching agents chlorine dioxide and hydrogen peroxide. In fact, the trend in the pulp and paper industries is toward totally chlorine free (TCF) bleaching. Chlorine is now used in the bleaching industry mainly to prepare hypochlorite solutions and dry bleaches such as calcium hypochlorite.

Chlorine Dioxide (ClO 2 ). Chlorine dioxide has been used as a bleaching agent both in its gaseous phase and in aqueous solution. Because of its explosive nature, chlorine dioxide in the gaseous phase is often diluted with nitrogen or carbon dioxide. If stored or shipped, chlorine dioxide is passed through cold water and kept under refrigeration.

Chlorine dioxide is prepared industrially via the reduction of sodium chlorate by sulfur dioxide in aqueous solution.