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What is Dichloromethane？
Dichloromethane (DCH, Methylene Chloride)
1. Overview & Chemical Identity
Dichloromethane is a volatile, colorless, chlorinated hydrocarbon with a faint, sweet, chloroform-like odor. It is a non-flammable liquid with high volatility and excellent solvent power for a wide range of organic materials. Its primary advantage is its ability to dissolve many compounds while having relatively low toxicity compared to other chlorinated solvents like chloroform or carbon tetrachloride, though it still requires careful handling.
Chemical Formula: CH₂Cl₂
CAS Number: 75-09-2
IUPAC Name: Dichloromethane
Common Names: Methylene chloride, Methylene dichloride
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2. Key Technical Parameters & Specifications
The following specifications are typically based on industrial or reagent grade purity (e.g., ≥99.5% – 99.9%).
| Parameter / Property | Typical Specification / Value | Standard / Test Method |
|---|---|---|
| Physical Properties | ||
| Appearance | Clear, colorless liquid, free of suspended matter | Visual (ASTM D2090) |
| Odor | Characteristic, sweetish, chloroform-like | - |
| Boiling Point | 39.6 °C (103.3 °F) at 760 mmHg | ASTM D1078 |
| Freezing Point | -96.7 °C (-142.1 °F) | - |
| Density at 20°C | 1.325 – 1.335 g/cm³ | ASTM D4052 |
| Refractive Index (n²⁰D) | 1.423 – 1.425 | ASTM D1218 |
| Vapor Pressure at 20°C | 47 kPa (350 mmHg) | - |
| Evaporation Rate (BuAc=1) | ~27 (Highly volatile) | - |
| Purity & Composition | ||
| Assay (DCM) | ≥ 99.5% (by GC) | ASTM D5329 / GC-FID |
| Water Content | ≤ 0.02% (200 ppm) | Karl Fischer Titration (ASTM E203) |
| Acidity (as HCl) | ≤ 0.001% (10 ppm) | ASTM D1613 |
| Non-volatile Residue | ≤ 0.001% (10 mg/100mL) | ASTM D1353 |
| Key Impurities (Max Limits) | ||
| Chloroform (CHCl₃) | ≤ 0.05% | |
| 1,2-Dichloroethane | ≤ 0.05% | Gas Chromatography |
| Tetrachloroethene | ≤ 0.05% | |
| Methanol / Ethanol (Stabilizers) | 50 – 500 ppm (commonly added) | |
| Performance & Safety | ||
| Flash Point | None (Non-flammable by standard tests) | |
| Auto-ignition Temperature | 556 °C | |
| Explosive Limits in Air | None (Non-flammable) | |
| Solubility in Water | Slightly soluble (~1.3% w/w at 20°C) | |
| Water Solubility in DCM | 0.2% w/w at 20°C | |
| Key Solvent Properties | ||
| Kauri-Butanol (KB) Value | ~136 (Indicates strong solvency for resins) | ASTM D1133 |
| Health & Exposure | ||
| OSHA PEL (8-hr TWA) | 25 ppm | |
| ACGIH TLV (8-hr TWA) | 50 ppm |
3. Stabilizers
Pure DCM is susceptible to decomposition in the presence of light, air, and moisture, which can produce small amounts of hydrochloric acid (HCl) and phosgene (COCl₂, highly toxic). Therefore, commercial grades are almost always stabilized.
Common Stabilizers: Small amounts of ethanol, methanol, or cyclohexene (typically 50-500 ppm) are added to neutralize any acids formed and prevent degradation.
Implication for Use: Users must be aware of the stabilizer type, as it can affect certain chemical reactions or analytical results. For sensitive applications (e.g., spectroscopy, synthesis), specially purified or "unstabilized" grades are available.
4. Primary Applications
Paint Stripping & Removal: Major industrial use due to its powerful penetration and ability to dissolve many paints and coatings without damaging underlying substrates.
Chemical Synthesis & Pharmaceutical Manufacturing: Widely used as a reaction solvent, particularly for temperature-sensitive reactions due to its low boiling point, and for extraction/purification steps.
Metal Cleaning & Degreasing: An effective vapor degreasing solvent.
Polyurethane Foam Production: Used as a blowing agent to create foam cells.
Food & Flavor Industry: Historically used as an extraction solvent for caffeine (decaffeinated coffee), hops, and flavorings. Strict residual limits apply.
Adhesives & Aerosols: Component in some adhesive formulations and aerosol products.
5. Health, Safety & Handling (Crucial)
Inhalation Hazard: DCM's high vapor pressure and density (heavier than air) create a significant inhalation risk. It is a central nervous system depressant and can cause dizziness, nausea, and at high concentrations, unconsciousness or death.
Metabolite Toxicity: The body metabolizes DCM to carbon monoxide (CO), which can lead to carboxyhemoglobin formation, reducing blood oxygen-carrying capacity. This is a unique and serious risk, especially for individuals with heart conditions.
Carcinogenicity: Classified as a probable human carcinogen (IARC Group 2A, OSHA, NTP) based on animal studies. It requires strict exposure controls.
Skin Contact: Can cause irritation and defatting of the skin; it is readily absorbed through the skin.
Required PPE: Chemical-resistant gloves (e.g., Viton®, Silver Shield®), chemical splash goggles, and adequate ventilation. Air monitoring and respiratory protection (e.g., supplied-air respirator) are mandatory for operations where exposure limits could be exceeded.
Storage: Store in a cool, well-ventilated area away from heat and incompatible materials (strong bases, active metals like aluminum powder). Keep containers tightly closed.
