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A chemical depilatory is a cosmetic preparation used to remove hair from the skin on the body. Currently, common active ingredients are calcium thioglycolate or potassium thioglycolate, which breaks down the disulfide bonds in keratin and weakens the hair so that it is easily scraped off where it emerges from the hair follicle.
This break down reaction is affected by the calcium hydroxide or the potassium hydroxide (both alkali). The resulting combinations of calcium hydroxide or potassium hydroxide and thioglycolic acid are calcium thioglycolate(CaTG) or potassium thioglycolate (KTG), respectively. The calcium hydroxide or potassium hydroxide are present in excess to enable the thioglycolic acid to react with the cystine present joining chains in hair protein. The reaction is:
- 2 HSCH2CO2H (thioglycolic acid) + R-S-S-R (cystine) → HO2CCH2-S-S-CH2CO2H (dithiodiglycolic acid) + 2 RSH (cysteine)
As the epidermis is also rich in keratin, the skin may become irritated and sensitive if the preparation is left on for too long. Chemical depilatories are used primarily for the arms and legs. They should not be used on the face unless specifically listed for that purpose on the product's label.