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Development of an analysis method to determine peroxides in or released from tooth-whitening/dental-bleaching products. 2 Analysis of peroxides
2.1 Chemical analysis of peroxide in tooth-whitening productsTooth-whitening agents can contain the active substance hydrogen peroxide either in a free state or bound in one or more releasers. Several tooth-whitening products contain the releasers urea peroxide or calcium peroxide, which creates a chemical equilibrium with free hydrogen peroxide (the active substance). The methods for determination of hydrogen peroxide used so far are based primarily on chemical redox titrations or colorimetric methods. A draft proposal for the establishment of a Japanese Industrial Standard for tooth-whitening products (Annex 5) includes e.g. a potassium permanganate titration method for hydrogen peroxide determination, whereas urea peroxide is determined using sodium thiosulfate titration. Some products contain mixtures of different forms of peroxides, which, potentially, require their own separate chemical analysis. 2.2 Analysis of peroxide using catalase bioassayThe newly developed method to determine the peroxide content in tooth-whitening products involves enzymatic conversion of hydrogen peroxide (CAS no. 7722-84-1) to water and oxygen while continuously measuring the oxygen released. In an aqueous solution, hydrogen peroxide will spontaneously split into oxygen and water, but the process is very slow. If catalase is present, however, the rate of reaction is increased significantly. The reaction is as follows:
In cases where the catalase bioassay is to be used for the determination of the peroxide content in products that contain peroxide in a bound state, a chemical equilibrium will exist between the bound and the free hydrogen peroxide. In cases where, for instance, urea peroxide (CAS no. 124-43-6) is contained in the product, the equilibrium between urea peroxide and the free hydrogen peroxide is as follows:
The released hydrogen peroxide is cleaved by the catalase into water and oxygen according to formula (1) above, shifting the equilibrium towards the right. For the remaining releasers tested in this report, an outline of their chemical equilibrium as a result of the release of hydrogen peroxide is given below. The solubility of calcium peroxide (CAS no. 1305-79-9) is highly dependent on pH, as is the equilibrium created according to the reaction below:
Sodium perborate monohydrate (CAS no. 10332-33-9) is a dimer that is bonded together by genuine peroxygen bonds. In an aqueous solution at pH7, hydrogen peroxide and sodium borate are created and the following equilibrium is created:
Sodium percarbonate (CAS no. 15630-89-4) is highly water soluble and when dissolved in water releases carbonate and hydrogen peroxide according to the following reaction:
In products containing one or more releasers, the assay will promote the conversion of the bound fraction of peroxides to free hydrogen peroxide. In this way, determination of the total quantity of hydrogen peroxide in the product is achieved. Catalase (CAS no. 9001-05-2) can be purchased in a purified form, e.g. from Sigma-Aldrich. The enzyme is naturally present intracellularly in all aerobically respirating organisms, where its function is to break down the reactive oxygen radicals created as a by-product of the respiratory process. The content of peroxides in tooth-whitening products is usually stated as a percentage of weight. That is, in a tooth-whitening agent which contains, for instance, urea peroxide, the hydrogen peroxide represents only about 35% of the weight of the molecule. The bleaching effect of a product containing e.g. 18% urea peroxide will therefore approximately correspond to a product containing about 6% hydrogen peroxide. Similar differences in bleaching effect will apply for the remaining releasers.
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