Thioglycolic acid

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.

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 R2Sn(SCH2CO2C8H17)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:

:ClCH2CO2H + Na2S2O3 → Na[O3S2CH2CO2H] + NaCl :Na[O3S2CH2CO2H] + H2O → HSCH2CO2H + NaHSO4

Reactions

Thioglycolic acid with a pKa of 3.83 is an acid about 8.5 times stronger than acetic acid (pKa 4.76): :HSCH2CO2H → HSCH2CO2− + H+ The second ionization has a pKa of 9.3: : HSCH2CO2− → −SCH2CO2− + 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 HSCH2CO2H + "O" → [SCH2CO2H]2 + H2O

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 LD50 (oral, rat) is 261 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.

References

References

1. ''[[Merck Index]]'', 11th Edition, '''9265'''
2. [https://www.cdc.gov/niosh/npg/npgd0610.html NIOSH Pocket Guide to Chemical Hazards]. CDC.
3. {{PGCH. 0610
4. (22 January 2013). "What exactly is Mercaptan?".
5. (11 September 2014). "Thioglycolic acid :Understanding the risk of specific chemicals of interest - PREVOR".
6. ''The Merck index'', 14th ed.; O’Neil, Maryadele J., Ed.; Merck & Co., Inc.: Whitehouse Station, NJ, 2006; p. 9342.
7. Robert Rippel "Mercaptoacetic Acid and Derivatives" in ''Ullmann's Encyclopedia of Industrial Chemistry'', 2012, Wiley-VCH, Weinheim. {{doi. 10.1002/14356007.a16_265.
8. (2011-09-01). "Characterization of oxidized tannins: comparison of depolymerization methods, asymmetric flow field-flow fractionation and small-angle X-ray scattering". Analytical and Bioanalytical Chemistry.
9. (2021-03-17). "Detection and Identification of Oxidation Markers of the Reaction of Grape Tannins with Volatile Thiols Commonly Found in Wine". Journal of Agricultural and Food Chemistry.
10. (2016-08-01). "Douglas bark dry fractionation for polyphenols isolation: From forestry waste to added value products". Industrial Crops and Products.
11. (2017-05-01). "Phenolic compounds of Moroccan red press wines: Influence of fining agents and micro-oxygenation treatments". LWT.
12. (2012). "Novel treatment using thioglycolic acid for pincer nails". Wiley.
13. (2017). "New Polyurethane Nail Lacquers for the Delivery of Terbinafine: Formulation and Antifungal Activity Evaluation". Elsevier BV.
14. (2016-03-17). "What is a fallout remover".
15. "SONAX Xtreme Wheel Cleaner" Safety data sheet
16. McKetta, John J. Jr.. (1981-01-01). "Encyclopedia of Chemical Processing and Design". CRC Press.
17. Lyons, Edward. (1927-08-01). "Thioglycolic Acid as a Color Test for Iron". Journal of the American Chemical Society.
18. "Limit Test of Iron".
19. Cosmetic, Toiletry, and Fragrance Association (CTFA). Thioglycolic Acid. 1987;(1987a). Submission of unpublished data by CTFA Code No. 3-25-2
20. Lee CW, Phil M. The detection of iron traces on hands by ferrozine sparys: a report on the sensitivity and interference of the method and recommended procedure in forensic science investigation. ''J Forensic Sci.'' '''1986,''' 31:920-930.
21. [http://www.nap.edu/readingroom.php?book=biomems&page=dgoddard.html National Academies Press:Biographical Memoirs:David Rockwell Goddard:by Ralph O. Erickson]
22. Goetz N, Gataud P, Bore P. Determination of mercaptoacetic acid in hair waving and depilatory products. ''Analyst.'' '''1979,'''104:1062-1069
23. 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.
24. 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.