What is sodium thiosulfate pentahydrate?
Sodium thiosulfate pentahydrate (Na₂S₂O₃·5H₂O) is a colorless transparent monoclinic crystal or white granular solid with a regular blocky or prismatic crystal appearance and a smooth surface with a glassy luster. The crystals are susceptible to weathering in dry environments and may be slightly deliqued when exposed to humid air.
1. In terms of solubility, it is easily soluble in water, with a solubility of about 70g/100ml at 50 ℃ and slightly soluble in ethanol. The aqueous solution is weakly alkaline, and its solubility increases significantly with increasing temperature. When heated to about 48 ℃, it will lose its crystalline water and gradually decompose into anhydrous sodium thiosulfate, while releasing sulfur and sulfur dioxide gases.
2. In terms of reducibility, it is converted into tetrasulfonate (S ₂ O ₂) or sulfate (SO ₂ ²) by strong oxidants (I ₂, Cl ₂), which can react with elemental iodine to form sodium tetrasulfate (used for iodometric titration). When it encounters acid, it quickly decomposes to produce sulfur precipitation and releases irritating sulfur dioxide gas.
3. In terms of stability, it is stable under neutral conditions and releases SO ₂ and S precipitates during acid decomposition (equation: S ₂ O 3 8315+2H++2s → S ↓+SO ↑+H ₂ O). Its stable chelating ability makes it play an important role in fields such as photographic fixation, heavy metal wastewater treatment, and pharmaceutical detoxification. At the same time, due to its low toxicity and easy storage characteristics, it has become a widely used multifunctional reagent in laboratories and industry.
Finally, its core characteristics are complexation (formation of soluble complexes with heavy metal ions), detoxification (neutralization of chlorides, halogens, and other toxic substances), and fixation (dissolution of unexposed silver halides).
| Category | Parameter | Value/Description |
|---|---|---|
| Basic Information | Chemical Formula | Na₂S₂O₃·5H₂O |
| Molecular Weight | 248.18 g/mol | |
| CAS Number | 10102-17-7 | |
| Common Names | Hypo, Sodium Hyposulfite | |
| Physical Properties | Appearance | Colorless transparent monoclinic crystals or white granules |
| Density | 1.69 g/cm³ | |
| Melting Point | 48°C (decomposes, losing crystal water) | |
| Decomposition Temperature | >100°C (fully decomposes into Na₂S, S, and SO₂) | |
| Solubility (20°C) | 70.1 g/100 mL water | |
| Solubility | Highly soluble in water, slightly soluble in ethanol, insoluble in ether | |
| Aqueous pH (10% solution) | 6.5–9.0 (weakly alkaline) | |
| Chemical Properties | Redox Behavior | Reacts with I₂ to form Na₂S₄O₆; reacts with Cl₂ to form SO₄²⁻ |
| Acid Reactivity | Decomposes in strong acids (e.g., HCl) to release S and SO₂ | |
| Complexation Ability | Forms stable complexes with Ag⁺, Hg²⁺ (e.g., [Ag(S₂O₃)₂]³⁻) | |
| Application Parameters | Fixing Solution Concentration (Photography) | 20%–40% aqueous solution |
| Antidote Dosage (Cyanide Poisoning) | Intravenous: 12.5–25 g (adult dose, combined with sodium nitrite) | |
| Chlorine Neutralization Ratio (Water Treatment) | Molar ratio Cl₂:S₂O₃²⁻ = 4:1 | |
| Safety & Storage | Toxicity | Low toxicity (LD₅₀ oral, rat: >5 g/kg) |
| GHS Labeling | Hazard Statements: H315 (Causes skin irritation) | |
| Storage Conditions | Sealed, dry, cool, and dark place (humidity <60%, temperature <25°C) | |
| UN Number | Not classified as hazardous | |
| Industrial Standards | Purity (Industrial Grade) | ≥99.0% |
| Heavy Metals (as Pb) | ≤10 ppm | |
| Arsenic (As) Content | ≤3 ppm | |
| Notes | Stability | Efflorescent in dry air; slightly deliquescent in humid environments |
| Incompatibility | Avoid direct contact with strong oxidizers (e.g., KMnO₄, HNO₃) and strong acids |
What applications can they be used in?
Applications in photography and influence industries
From the perspective of photography and affecting the industry, sodium thiosulfate pentahydrate can be used as a fixer, and its high efficiency and low residue characteristics are widely favored by traditional photographic processes. It is an irreplaceable chemical. In the process of film and photographic paper development, sodium thiosulfate pentahydrate can be used to dissolve unexposed silver halide and form a soluble silver thiosulfate complex to prevent continued sensitization. Thus fixing the influence.
Applications in the pharmaceutical and public health industries
In terms of medical and public health applications, sodium thiosulfate pentahydrate can be used in a wide range of scenarios. It can be used in first aid for cyanide poisoning, radioactive iodine protection, or dermatological treatment. In the first aid of cyanide poisoning, sodium thiosulfate pentahydrate can be used as a cyanide antidote to convert highly toxic hydrogen ions into low-toxicity thiocyanate by combining with nitrite. It is often used in industrial accidents or in emergency situations where cyanide is accidentally ingested. In radioactive iodine protection, it can be used in nuclear radiation emergencies to reduce the accumulation of radioactive iodine in the thyroid gland and reduce radiation risk. Utilizing its mild sulfur release properties in skin treatment can dilute the solution for the treatment of scabies or fungal infections
Applications in environmental protection and water treatment
In terms of environmental protection and water treatment, it is mainly used in the field of residual chlorine removal, heavy metal wastewater treatment, industrial waste gas treatment and other business functions. In the field of residual chlorine removal, it is mainly used in waterworks or swimming pools, which can effectively neutralize residual free chlorine and prevent corrosion of pipelines or human bodies. In heavy metal treatment, it can form stable complexes with heavy metal ions such as lead and gonium, reduce their toxicity and promote precipitation separation. In industrial waste gas treatment, it can absorb sulfur dioxide from sulfur-containing waste gas and reduce atmospheric pollution.
Industry-Specific Specifications
| Industry | Purity | Key Metrics | Application Example |
|---|---|---|---|
| Photography Grade | ≥99.5% | Low halide residue (Cl⁻ <0.01%) | Film/paper fixing solutions |
| Pharmaceutical Grade | ≥99.9% | Sterile, pyrogen-free, heavy metals <5ppm | Cyanide antidote formulations |
| Industrial Water Treatment Grade | ≥98.0% | Water-insoluble matter <0.1%, pH 6.5–8.5 | Chlorine neutralization, heavy metal removal |
| Laboratory Reagent Grade | ≥99.0% | Complies with GB/T 637-2006 | Iodometric titration standard |
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