|Molar mass||92.12 g·mol−1|
|Melting point||−16 °C (3 °F; 257 K)|
|Boiling point||96 °C (205 °F; 369 K) at 5 mmHg|
|US health exposure limits (NIOSH):|
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
|what is: / ?)(|
Thioglycolic acid (TGA) is the organic compound HSCH2CO2H. TGA is often called mercaptoacetic acid (MAA). It contains both a thiol (mercaptan) and carboxylic acid functional groups. It is a colourless liquid with a strongly unpleasant odor. TGA is miscible with polar organic solvents.
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 that 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. In fact thioglycolysis reactions used on condensed tannins to study their structure.
Applying TGA can soften nails and then fix pincer nails in the correct position.
Thioglycolic acid is prepared by reaction of sodium or potassium chloracetate with alkali metal hydrosulfide in aqueous medium. It can be also prepared via the Bunte salt obtained by reaction of sodium thiosulphate with chloroacetic acid:
- ClCH2CO2H + Na2S2O3 → Na[O3S2CH2CO2H] + NaCl
- Na[O3S2CH2CO2H] + H2O → HSCH2CO2H + NaHSO4
It is about 100x stronger acid than acetic acid with pKa of 3.83:
- HSCH2CO2H → HSCH2CO2− + H+
The second ionization has a pKa of 9.3:
- HCH2CO2− → −SCH2CO2− + H+
It is a reducing agent, especially at higher pH's. It oxidized to the corresponding disulfide:
- 2 HSCH2CO2H + "O" → [SCH2CO2H]2 + H2O
With metal ions
TGA, usually as its dianion, forms complexes with metal ions. Such complexes have been used for the detection of iron, molybdenum, silver, and tin. TGA reacts with diethyl acetylmalonate to form acetylmercatoacetic acid and diethyl malonate, the reducing agent in conversion of Fe(III) to Fe(II).
The 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 LD50 (oral, rat) is 261 mg/kg, LC50 inhalation for rat is 21 mg/m3 for 4 h, and LD50 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.
- Merck Index, 11th Edition, 9265
- CDC - NIOSH Pocket Guide to Chemical Hazards
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- Robert Rippel "Mercaptoacetic Acid and Derivatives" in Ullmann's Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH, Weinheim. doi:10.1002/14356007.a16_265
- Cosmetic, Toiletry, and Fragrance Association (CTFA). Thioglycolic Acid. 1987;(1987a). Submission of unpublished data by CTFA Code No. 3-25-2
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- National Academies Press:Biographical Memoirs:David Rockwell Goddard:by Ralph O. Erickson
- Sigma-Aldrich MSDS. http://www.sigmaaldrich.com/safety-center.html?cm_sp=Search-_-MSDS-_-MSDS1 (accessed Nov 10, 2013). Product Number – T3758
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- Goetz N, Gataud P, Bore P. Gas-chromatographic determination of mercaptoacetic acid in hair-waving and diplatory products. Cosmet Sci Technol Ser. 1985, 4:65-79.
- Vandeputte M, Dryon L, Van Den Winkel P, Mertens J, Massart DL. Determination of thioglycolic acid using a silver sulfide single crystal electrode. Analysis. 1975,3:500-504.
- Okada K, Okada E. Novel treatment using thioglycolic acid for pincer nails. J. Dermatol. 2012, volume 39, pp. 996-999.