Ammonium thioglycolate

Ammonium thioglycolate, also known as perm salt, is the chemical compound with the formula HSCH₂CO₂NH₄. Being the salt of a weak acid and weak base, ammonium thioglycolic acid exists in solution as an equilibrium mixture of the salt itself as well as the free carboxylic acid thioglycolic acid (HSCH₂CO₂H) and ammonia : HSCH₂COO^- + NH₄⁺ ? HSCH₂COOH + NH₃.

Clear colorless solution with characteristic odor

acts as a reducing agent in permanent-waving formulations for hair treatment and in shrink-resistant treatment of wool.

Solutions of various strengths are used for hair waving and for hair removal.

Ammonium thioglycolate is contained in permanent waves solutions and causes contact dermatitis in hairdressers.

ChEBI: Ammonium thioglycolate is an organic ammonium salt having thioglycolate(1-) as the counterion. Also known as perm salt, it has use in perming hair. It has a role as a reducing agent and an allergen. It contains a thioglycolate(1-).

Colorless to faint pink liquid with a repulsive, skunk-like odor.

Water soluble.

Ammonium thioglycolate may be sensitive to heat. Ammonium thioglycolate is incompatible with acids.

Flash point data for Ammonium thioglycolate are not available. Ammonium thioglycolate is combustible.

Not classified

This substance is contained in “basic” permanent waves solutions and causes contact dermatitis in hairdressers.

Poison by intravenous and intraperitoneal routes. An allergen; can cause contact dermatitis. Emits hydrogen sulfide. See also SULFIDES. When heated to decomposition it emits very toxic NOx, SOx, and NH3.

When discussing the chemistry of perms, one should consider two chemical facts. First is the thiol-disulfide equilibrium:
RSH + R'SSR' ? R'SH + RSSR'
where R and R' are organic substituents such as methyl (-CH3), ethyl (-C₂H₅), or -CH₂COO^-.
The thiol-disulfide exchange reaction is accelerated by bases such as ammonia, because the base generates some thiolate anion (RS^- ), which attacks the disulfide. Thus the ammonia plays multiple roles (and more, see below) in this application.
The second chemical fact is that polar molecules are less volatile than nonpolar ones. So the glycolate substituent makes the thiol nonvolatile and hence non-odorous. An added advantage is that the glycolate confers some solubility in water. One could almost certainly use HSCH₃ and ammonia to give a perm, but there would be serious olfactory consequences.

A solution containing ammonium thioglycolate contains a lot of free ammonia, which swells hair, rendering it permeable. The thioglycolic acid in the perm solution reduces the disulfide cystine bonds in the cortex of the hair. In a sense, the thioglycolate removes crosslinks. After washing, the hair is treated with a mild solution of hydrogen peroxide, which oxidizes the cysteines back to cystine. These new chemical bonds impart the structural rigidity necessary for a successful perm. The rigidification process is akin to the vulcanization of rubber, where commonly polysulfide linkages are used to crosslink the polymer chains. However, not as many disulfide bonds are reformed as there were before the permanent. As a result, the hair is weaker than before the permanent was applied and repeated applications over the same spot may eventually cause strand breakage.