Sodium Thioglycolate - Production CAS 367-51-1
Sodium Thioglycolate: Production and Analytical Control
1. Introduction
Sodium thioglycolate (sodium mercaptoacetate, CAS 367-51-1) is an organosulfur compound with the formula NaSCH₂COO and a molecular weight of 114.10 g/mol. It is a white to off-white, hygroscopic crystalline powder with a characteristic mercaptan odor, highly soluble in water (>600 g/L at 20°C) and melting above 300°C. As a potent reducing agent containing both thiol (-SH) and carboxylate (-COO⁻) functional groups, sodium thioglycolate finds applications in mineral flotation, cosmetics (depilatories and hair waving), corrosion inhibition, and bacteriological media.
2. Synthesis Methods
Two principal industrial routes are employed for the production of sodium thioglycolate.
Neutralization Route: The most direct method involves the neutralization of thioglycolic acid (TGA) with sodium hydroxide: HSCH₂COOH + NaOH → NaSCH₂COO + H₂O. This exothermic reaction (ΔH ≈ -50 kJ/mol) requires careful temperature control. In practice, thioglycolic acid is cooled to 5°C, and a 40% sodium hydroxide solution (also at 5°C) is added slowly with stirring. When the temperature reaches 35°C, hydrogen sulfide gas begins to evolve; addition is stopped when the solution turns pink and stable crystals precipitate. The product is then cooled to room temperature, filtered, washed with ethanol, and dried below 80°C.
Direct Exchange Route: An alternative method involves the reaction of sodium chloroacetate with sodium hydrosulfide: ClCH₂COONa + NaHS → NaSCH₂COONa + NaCl. This route can produce sodium thioglycolate with high yields (>95%) at pH 11 and 40°C. However, a side reaction can produce the disulfide by-product NaOOCCH₂-S-CH₂COONa.
3. Purification and Quality Control
For industrial and analytical applications, sodium thioglycolate is available in both technical and reagent grades.
| Parameter | Technical Grade | Reagent Grade |
|---|---|---|
| NaTGA Assay (iodometric) | ≥90% | ≥98–99% |
| Disulfides | ≤3% | ≤0.5% |
| Iron (Fe) | ≤50 ppm | ≤5–10 ppm |
| Chlorides | Not specified | ≤0.005% |
| Loss on Drying | ≤2% | ≤0.5% |
Purification typically involves decolorization with activated carbon to reduce iron content to <5 ppm, followed by vacuum drying at 60°C to achieve moisture ≤2%. Analytical methods for quality control include iodometric titration for assay determination (≥96.5% iodometric for high-purity grades), ion chromatography for chloride and impurity analysis, and ICP-OES for trace metal analysis (iron, heavy metals).
4. Stability and Storage
Sodium thioglycolate is hygroscopic and sensitive to oxidation. Exposure to air or contact with iron can cause discoloration (yellow to brown/black), indicating degradation. Oxidation produces purple-red disulfides, which are catalyzed by copper, iron, and manganese ions. Recommended storage conditions: sealed containers, under inert gas (nitrogen), at 2–8°C or –20°C for long-term storage (up to 3 years at –20°C). The product should be protected from moisture, air, and light. At ambient temperature, 70% aqueous solutions may form 1–2% thioglycolides per month, which hydrolyze back to the original compound under acidic or alkaline conditions. Thioglycolate salts may lose purity on storage even when air is excluded.
5. Applications
Mineral Flotation: Sodium thioglycolate is used as a depressant and inhibitor in the flotation separation of copper-molybdenum sulfide ores. It serves as a more environmentally friendly alternative to sodium cyanide and sodium sulfide, acting as a powerful depressant for copper sulfide and galena. It enhances the recovery rate of precious metals during flotation.
Cosmetics: Its primary cosmetic use is in depilatories (hair removal creams). It also functions as a hair waving and straightening agent, antioxidant, and reducing agent.
Analytical Reagent: Used as an acidity indicator and in the preparation of thioglycolate media for bacteriology. Thioglycolate broth is used for the cultivation of anaerobic and microaerophilic bacteria.
Corrosion Inhibition: Acts as a chelating agent for metal ions; ferrous iron combines with thioglycolate to form red-violet ferric thioglycolate. Used in "fallout remover" formulations to remove iron oxide residues from steel surfaces.
6. Safety and Toxicology
Sodium thioglycolate is classified as hazardous: harmful if swallowed (H302), causes skin irritation (H315) and serious eye irritation (H319). The acute oral LD₅₀ in rats is approximately 500 mg/kg. Dermal toxicity studies in rats and mice have shown skin irritation at the site of application at doses ≥45 mg/kg, with epidermal hyperplasia and liver effects at higher doses. The compound is toxic to aquatic life with long-lasting effects (Aquatic Chronic 3).
7. Conclusion
Sodium thioglycolate is a versatile reducing and complexing agent produced by two primary routes: neutralization of thioglycolic acid with sodium hydroxide or direct exchange from sodium chloroacetate and sodium hydrosulfide. Rigorous quality control, including iodometric assay, ion chromatography, and ICP-OES analysis, ensures product purity for both technical and reagent-grade applications. Proper storage under inert atmosphere at low temperature is essential to prevent oxidation and maintain stability. Its applications span mineral flotation, cosmetics, analytical chemistry, and corrosion inhibition, making it a valuable industrial chemical with expanding uses in sustainable technologies.