What exactly is FAS?
Direct Answer
Direct Answer Formamidinesulfinic acid (FAS) is a white crystalline powder or granular organic sulphur compound made by oxidising thiourea with hydrogen peroxide. It is a latent reducing agent that releases strong reducing power in hot alkaline solution, and it is used mainly for reductive bleaching in the paper industry.
Basic properties
| Item | Value |
|---|---|
| Molecular formula | CH₄N₂O₂S |
| Molecular weight | 108.12 |
| CAS number | 1758-73-2 |
| Appearance | White crystalline powder or granular solid, odourless; turns yellowish on prolonged storage |
| Melting point | 124 to 127 C, with decomposition |
| Density | Approx. 1.68 g/cm³ |
| Bulk weight | Approx. 750 g/L |
| pH value | Approx. 4.5 (1% or 10 g/L aqueous solution at 20 C); varies with solution concentration and temperature |
| Solubility | 26.7 g/L in water at 20 C; up to 100 g/L under strongly alkaline conditions, but the solution is comparatively unstable |
| Ionic charge | Anionic |
Note: solubility is 26.7 g/L in water at 20 C. The pH figure is measured on a 10 g/L aqueous solution at 20 C; the actual value varies with solution concentration and temperature.
Reduction mechanism
Under alkaline conditions FAS decomposes to form the strongly reducing sulfoxylate ion, SO₂²⁻:
(NH₂)₂CSO₂ + H₂O -> (NH₂)₂CO + H₂SO₂
The sulfoxylate ion, together with the nascent hydrogen it releases, performs the actual reduction. The end products are urea and sodium sulphate.
Three points that decide whether it works
- It must be used under alkaline conditions. In weakly acidic solution it is oxidised to formamidine sulfonic acid and loses its reducing power
- The order of addition must therefore be alkali first, then the product, with the alkali added only shortly before use
- Stock solutions should be prepared with cold water. Alkali-free solutions are almost stable for up to 6 hours at room temperature, whereas alkaline solutions are very unstable and should be prepared and used immediately
Bottom line
Formamidinesulfinic acid (FAS) is an alkali-activated reductive bleaching agent for pulp and paper. The essentials are alkaline conditions, alkali before the product, and prepare-and-use.
Sources
- PubChem CID 61274, thiourea dioxide (formamidinesulfinic acid) 分子式与分子量，https://pubchem.ncbi.nlm.nih.gov/compound/61274
- US5958184A, Process for producing thiourea dioxide（还原当量质量比、碱中水解式、木浆与脱墨浆等当量漂白实施例），https://patents.google.com/patent/US5958184A/en
- 我司产品 SDS 与 TDS（危害分类、运输分类、热分解温度、溶解度等物性数据）
- 我司产品安全数据表《Material Safety Data Sheet — THIOUREA DIOXIDE》（MSDS，签发日期 2023-03-03）
- 我司产品技术数据表《THIOUREA DIOXIDE Technical Data Sheet》（TDO-TDS，签发日期 2024-05-08）
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