# Thioglycolic acid

Thioglycolic acid (TGA) is the organic compound HSCH$_2$CO$_2$H. TGA is often called mercaptoacetic acid (MAA). It contains both a thiol (mercaptan) and carboxylic acid functional groups. It is a colorless liquid with a strongly unpleasant odor. TGA is miscible with polar organic solvents.

## Uses

TGA is used as a chemical depilatory and is still used as such, especially in salt forms, including calcium thioglycolate and sodium thioglycolate. TGA is the precursor to ammonium thioglycolate, which is used for permanents. TGA and its derivatives break the disulfide bonds in the cortex of hair. One reforms these broken bonds in giving hair a "perm". Alternatively and more commonly, the process leads to depilation, as is done commonly in leather processing. It is also used as an acidity indicator, manufacturing of thioglycolates, and in bacteriology for preparation of thioglycolate media. Thioglycolysis reactions are used on condensed tannins to study their structure.
TGA has also been used to soften nails, either to reshape pincer nails into the correct position or to help the topical antifungal agent terbinafine penetrate the nail.
Organotin derivatives of thioglycolic acid isooctyl esters are widely used as stabilizers for PVC. These species have the formula R$_2$Sn(SCH$_2$CO$_2$C$_8$H$_{17}$)$_2$.
Sodium thioglycolate is a component of thioglycolate broth, a special bacterial growth media. It is also used in so-called "fallout remover" or "wheel cleaner" to remove iron oxide residue from wheels. Ferrous iron combines with thioglycolate to form red-violet ferric thioglycolate.

## Production

Thioglycolic acid is prepared by reaction of sodium or potassium chloroacetate with alkali metal hydrosulfide in aqueous medium. It can be also prepared via the Bunte salt obtained by reaction of sodium thiosulfate with chloroacetic acid:
    ClCH$_2$CO$_2$H + Na$_2$S$_2$O$_3$ → Na[O$_3$S$_2$CH$_2$CO$_2$H] + NaCl
    Na[O$_3$S$_2$CH$_2$CO$_2$H] + H$_2$O → HSCH$_2$CO$_2$H + NaHSO$_4$

## Reactions

Thioglycolic acid with a pK$_a$ of 3.83 is an acid about 8.5 times stronger than acetic acid (pK$_a$ 4.76):
    HSCH$_2$CO$_2$H → HSCH$_2$CO$_2$$^−$ + H$^+$
The second ionization has a pK$_a$ of 9.3:
    HSCH$_2$CO$_2$$^−$ → $^−$SCH$_2$CO$_2$$^−$ + H$^+$
Thioglycolic acid is a reducing agent, especially at higher pH. It oxidizes to the corresponding disulfide (2-[(carboxymethyl)disulfanyl]acetic acid or dithiodiglycolic acid):
    2 HSCH$_2$CO$_2$H + "O" → [SCH$_2$CO$_2$H]$_2$ + H$_2$O

### With metal ions

Thioglycolic acid, usually as its dianion, forms complexes with metal ions. Such complexes have been used for the detection of iron, molybdenum, silver, and tin. Thioglycolic acid reacts with diethyl acetylmalonate to form acetylmercaptoacetic acid and diethyl malonate, the reducing agent in the conversion of Fe(III) to Fe(II).

## History

Scientist David R. Goddard, in the early 1930s, identified TGA as a useful reagent for reducing the disulfide bonds in proteins, including keratin (hair protein), while studying why protease enzymes could not easily digest hair, nails, feathers, and such. He realized that while the disulfide bonds, which stabilize proteins by cross-linking, were broken, the structures containing these proteins could be reshaped easily, and that they would retain this shape after the disulfide bonds were allowed to re-form. TGA was developed in the 1940s for use as a chemical depilatory.

## Safety and detection

The LD$_{50}$ (oral, rat) is 261 mg/kg, LC$_{50}$ inhalation for rat is 21 mg/m$^3$ for 4 h, and LD$_{50}$ dermal for rabbit is 848 mg/kg. Mercaptoacetic acid in hair waving and depilatory products containing other mercapto acids can be identified by using thin-layer chromatography and gas chromatography. MAA also has been identified by using potentiometric titration with silver nitrate solution.