One of the most commonly used ingredients in over-the-counter acne treatments in cream, gel, and wash form is benzoyl peroxide. It is an anti-bacterial agent that kills the bacterium (Propionibacterium acne) involved in the formation of acne. The formulation of these products is extremely difficult owing to the instability of benzoyl peroxide. Benzoyl peroxide is soluble in polyethylene glycol (PEG) and isopropyl myristate, but it is not stable in these solvents. Also, it is practically insoluble in water and only slightly soluble in ethanol but is very stable in the presence of these solvents. The stability of benzoyl peroxide in PEG or isopropyl myristate is increased upon addition of water or ethanol. Students take on the role of the process chemist and use peroxide strips to investigate the stability of benzoyl peroxide in an over-the-counter acne medication in solutions of water, ethanol, PEG, 50:50 ethanol:PEG, 50:50 water:PEG, isopropyl myristate, and 50:50 ethanol:isopropyl myristate.
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