CHEMISTRY OF BLEACHING Tooth whitening continues to be an evolving science. Nathoo has developed three classifications of extrinsic dental stain: 1. N1 type dental stain or direct dental stain: Colored material (chromogen) binds to the tooth’s surface and causes discoloration. The color of the chromogen is similar to that of dental stain. 2. N2 type dental stain or direct dental stain: Colored material changes color after binding to the tooth. 3. N3 type dental stain or indirect dental stain: Colorless material or a pre-chromogen binds to the tooth and undergoes a chemical reaction to cause a stain. The whitening action of bleaching is believed to be linked to the degradation of high molecular weight and complex organic molecules that reflect a specific wavelength of light that is responsible for the color of the stain. The resulting degradation products are of lower molecular weight and composed of less complex molecules that reflect less light, resulting in a reduction or elimination of discoloration. Darkly pigmented organic material responsible for enamel discoloration is composed of carbon ring structures with unsaturated, double-carbon bonds. With further oxidation these products are modified to hydrophilic, nonpigmented carbon structures with saturated carbon bonds (i.e., the saturation point). Ideally, this is the point at which whitening should be terminated. If the degradation process continues, however, there is further decomposition of organic matrix, which can lead to complete oxidation with generation of carbon dioxide and water, resulting in a total loss of enamel matrix protein. Peroxides and oxygen free radicals are reactive oxygen agents that are formed as natural products in all living systems that use oxygen. Reactive oxygen is usually only at very low concentrations, because natural antioxidant protective systems prevent their accumulation. Reactive oxygen in the presence of free metal ions, such as Fe or Cu, combines to cause oxidative damage to the tissue. The safe use peroxides in dentistry requires adherence to practices that minimize oxidative damage to oral cavity tissue. Hydrogen peroxide is more stable; thus it has a longer shelf life in acidic solutions. However, it is more effective as a bleaching agent at ph values closer to the dissociation constant. Also, solubility factors suggest that alkaline solutions would be less likely to penetrate the pulp than acidic solutions. Alkaline solutions cause less demineralization of tooth surfaces than acidic agents. The active ingredients in the bleaching agents must be quickly reactive with stains, non-reactive with dental or oral tissues, pleasant to use, and stable over long periods of shelf storage Hydrogen peroxide readily breaks
down to oxygen and water and is often accelerated by enzymes, such as peroxidase. Carbamide peroxide and hydrogen peroxide are most common and have been used in dentistry for decades. Other names for carbamide peroxide are urea peroxide, carbamyl peroxide, and perhydrol urea. The bleaching process is based on the oxidation of the bleaching agent. Oxidation is the chemical process by which organic materials are converted into carbon dioxide and water. The oxidationreduction reaction that takes place in the bleaching process is called the REDOX REACTION. Bleaching slowly transforms the organic substance in the stained tooth into chemical intermediaries that are lighter in color than the original tooth shade. In a Redox Reaction the peroxide (oxidizing agent) has free radicals with unpaired electrons, which it gives up, becoming reduced. The stained tooth structure accepts these electrons and becoming oxidized, thereby reducing the organic colorants. The free radicals produced by the peroxides are perhydroxyl and nascent oxygen. Of these, the perhydroxyl is a move potent free radical, which is responsible for a better bleaching action. In order to promote the formation of perhydroxyl radicals, the peroxide is buffered to a pH range of 9.5 to 10.8. The buffering provides a greater amount of perhydroxyl free radicals, which result in a better bleaching effect. A HIGHER PERCENTAGE OF STRONGER FREE RADICAL IS SEEN WITH THE BUFFERING OF HYDROGEN PEROXIDE The most common bleaching materials used are hydrogen and carbamide peroxide. Carbamide peroxide first breaks down into hydrogen peroxide, which then further liberates the perhydroxyl and nascent oxygen free radicals. Unlike the hydrogen peroxide, the carbamide peroxide-bleaching agent must remain in contact with the teeth for a longer period of time to obtain complete efficiency of the reaction. Carbamide peroxide is less irritating to the gingival tissues thus better tolerated by the patients when used as a home bleaching agent.