# Does hydrogen peroxide - urea become more active when illuminated?

Dentists' tooth whitening systems and consumer kits use gels containing carbamide peroxide, also known as hydrogen peroxide - urea.

Dentists use a light activator: (image from Wikipedia).

Consumer kits typically include 35% carbamide peroxide gel. Some of them use an LED light too, while others don't.

Wikipedia seems to be in two minds about it, stating

Recent research has shown that the use of a light activator does not improve bleaching, has no measurable effect and most likely to increase the temperature of the associated tissues, resulting in damage

while also talking about which light is "best":

The ideal source of energy should be high energy to excite the peroxide molecules without overheating the pulp of the tooth

Is a light source relevant here, or is it merely a marketing gimmick?

• 'High energy' means using ultra-violet light and high intensity means lots of them. The absorption spectrum of hydrogen peroxide increases significantly below 300 nm so any light here will speed up its dissociation into radicals. However, UV of these wavelengths is dangerous which is why we add sunscreens when outside in high sunlight to prevent sunburn. – porphyrin Apr 29 '18 at 20:47
• @porphyrin Thanks, I wonder if the LED lights in consumer tooth whitening kits even reach the UV range. – MaxB Apr 30 '18 at 19:04
• in consumer LED's i'm sure you are correct that there is very little if any uv. In that case the effect of illuminating will be small as very few photons will be absorbed. – porphyrin May 1 '18 at 8:15

I hate to answer my own question, but after some digging, I believe that the use of a light source is in fact a gimmick, at any concentration of the reagent, contradicting the only other answer so far (But I'm happy to be corrected, if I'm wrong)

This meta-study https://www.ncbi.nlm.nih.gov/pubmed/22525016 concluded that at high concentrations of the reagent (typical of in-office use), the light is useless; and there was insufficient or no data for lower concentrations (as of 2012):

Light increases the risk of tooth sensitivity during in-office bleaching, and light may not improve the bleaching effect when high concentrations of HP (25-35%) are employed. Therefore, dentists should use the light-activated system with great caution or avoid its use altogether. Further rigorous studies are, however, needed to explore the advantages of this light-activated system when lower concentrations of HP (15-20%) are used.

A slightly more recent study came to the same conclusion https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4178330/ :

The in-office bleaching treatment of vital teeth did not show improvement with the use of light activator sources for the purpose of accelerating the process of the bleaching gel and achieving better results.

I'm inclined to think that if light were useful at all, they'd see this at higher concentrations of HP too, but I haven't seen any recent studies that address the lower concentrations specifically.

Before answering your question, we must review the mechanism of bleaching action. Hydrogen peroxide ($\ce{H2O2}$) is replacing other beaching agents as the preferred beaching agent in many applications. Although it was accepted that $\ce{H2O2}$ acts by destructively oxidizing colored organic molecules involved, its bleaching mechanism by such oxidation is still not well understood. However, recent works on the bleaching of textile by $\ce{H2O2}$ have identified that the hydroxyl radical ($\ce{HO^.}$) and perhydroxyl radical ($\ce{HO2^.}$) are the active oxygen species mostly responsible for such action [1]. The facts found by a separate study on the effects of the $\ce{HO^.}$ radical and $\ce{H2O2}$ on tooth bleaching suggest that $\ce{HO^.}$ has the main role in tooth bleaching with $\ce{H2O2}$ [2]

Suggested path for formation of $\ce{HO^.}$ and ($\ce{HO2^.}$) radicals from aqueous $\ce{H2O2}$ is as follows: $$\ce{H2O2 + H2O <=> HO2- + H3O+}$$ $$\ce{HO2- + H2O2 -> HO2^. + HO^. + HO-}$$

Now, we look at bleaching process in Dentists' office: Stable hydrogen peroxide-urea solid is commonly used in these situations to avoid unstable aqueous $30\%~\ce{H2O2}$. It is described in Wikipedia article on hydrogen peroxide - urea as follows:

Hydrogen peroxide-urea (also called Hyperol or artizone) is a solid composed of equal amounts of hydrogen peroxide and urea. This compound is a white crystalline solid, which dissolves in water to give free hydrogen peroxide.

In the Wikipedia article on tooth whitening mentioned in your question states that:

The bleaching agent is either carbamide peroxide, which breaks down in the mouth to form hydrogen peroxide, or hydrogen peroxide itself. The bleaching gel typically contains between 10% and 44% carbamide peroxide, which is roughly equivalent to a 3% to 16% hydrogen peroxide concentration. The legal percentage of hydrogen peroxide allowed to be given is 0.1-6%. Bleaching agents are only allowed to be given via dental practitioners, dental therapists and dental hygienists.

At a dentist's office, they may use the light source to accelerate (and light-influenced catalytic radical cleavage) above spontaneous reactions to generate $\ce{HO^.}$ and ($\ce{HO2^.}$)Radicals. To support my argument, I'd like to point out the following statement from the same source you noted above in the Wikipedia article on tooth whitening:

Use of light during bleaching increases the risk of tooth sensitivity and may not be any more effective than bleaching without light when high concentrations of hydrogen peroxide are used.

Thus, use of light source is not a gimmick. It is a necessity.

References:

1. The mechanism of hydrogen peroxide bleaching: J. Dannacher, W. Schlenker, Textile Chemist & Colorist, 1996, 28(11), 24-28.

2. Effects of the Hydroxyl Radical and Hydrogen Peroxide on Tooth Bleaching: K. Kawamoto, Y. Tsujimoto, Journal of Endodontics, 2004, 30(1), 45-50 (DOI: https://doi.org/10.1097/00004770-200401000-00010).

• I wonder if the second quote is accurate. 35% carbamide peroxide is available over the counter and online (in the U.S.) – MaxB Apr 29 '18 at 21:19
• I think you are right. $35\%$ carbamide peroxide approximately equals to $12\%$ $\ce{H2O2}$. One of my friends bought, I believe, $15\%$ $\ce{H2O2}$ solution online for his son's high school project. – Mathew Mahindaratne Apr 29 '18 at 21:52