The Chemistry Behind Hydrogen Peroxide Tooth Whitening

Objectives: Tooth whitening using hydrogen peroxide gel formulation is a complex process which involves both chemistry and physics. The objective of this work was to reveal the basic interactions between whitening agents and stain molecules in simple solutions, thus avoiding the complications from agent diffusion and light penetration in the tooth.

Methods: We formulated a whitening compound consisting of two components mixed at the point-of-use. The first component consists only of 37.2% hydrogen peroxide. The second component comprises of 0 to 5.91%wt/wt potassium hydroxide depending upon the pH required, and 0.004%wt/wt ferrous gluconate monohydrate catalyst.  A solution of tea was prepared by boiling 0.2g of loose leaf black tea in 10ml of water for 10min.  At the point of testing, four volumes of peroxide were added to one volume of activator and then to one volume of stain solution.

The absorbance of the solution at 450nm was measured using a model ELx800 plate reader (BioTek, USA) and reaction rates were deduced.

 In order to study the effect of visible light, we used the BriteSmile blue light illumination system (Discus Dental, USA) with illumination at a wavelength of 465nm, and power density 60mW/cm².  For infra-red irradiation we used an LED array at a wavelength of 850nm (Osram Opto Semiconductors, GmbH) at a power density of 50mW/cm² which caused heating to 35^oC.

Results:  The reaction rates (min^-1) deduced from these experiments are given in the table for a range of measured pH values, temperatures, and irradiation conditions.

pH	Temp (C)	Dark reaction rate	Blue reaction rate	IR reaction rate
8	35-37	0.133		0.153
8	22-24	0.120	0.229
7	22-24	0.069	0.138
6	22-24	0.048	0.110
5	22-24	0.034	0.101
4	22-24	0.031	0.078

Conclusions: It is clear that pH and illumination with blue light play a large and important role in the generation of radicals and the subsequent break down of stains. Conversely, Irradiation with IR light at 850nm serves mainly to heat the solution, giving a much lesser effect.