Chemical Name：Diphenhydramine Hydrochloride
CAS：147-24-0
Synonyms:2-(Diphenylmethoxy)-N,N-dimethylethylamine hydrochloride; Benadryl (brand name)
Molecular Formula:C₁₇H₂₁NO·HCl
Molecular Weight:291.82 g/mol
Appearance:White crystalline powder
Melting Point:166–170 °C (331–338 °F)
Solubility - Highly soluble in water, ethanol, and chloroform
- Slightly soluble in acetone
- Insoluble in ether
pKa (Dissociation Constant):~8.3 (weak base)
Pharmacological Class:First-generation antihistamine (H₁ receptor antagonist)
Mechanism of Action:Blocks histamine H₁ receptors, exhibits anticholinergic and sedative effects
Common Uses
- Allergy relief (e.g., hay fever, hives)
- Motion sickness
- Sleep aid (short-term insomnia)
- Cold symptom relief
Stability:Stable under normal storage conditions (room temperature, dry place)
Safety Notes -
May cause drowsiness, dizziness, or dry mouth
- Avoid alcohol and CNS depressants
- Not recommended for long-term use without medical supervision
| Test Item | Specification | Test Method |
|---|
| Appearance | White or almost white crystalline powder | Visual inspection (EP/USP) |
| Identification | Complies with IR, HPLC, or TLC comparison | USP <621>, EP 2.2.24 (HPLC) |
| Assay (Purity) | 98.0%–101.0% (anhydrous basis) | USP <621> (HPLC/ Titration) |
| Water Content | ≤0.5% (Karl Fischer) | USP <921> (KF Titration) |
| Residue on Ignition | ≤0.1% | USP <281> (Sulfated ash) |
| Heavy Metals | ≤20 ppm (e.g., Pb, Cd, Hg) | USP <231> / ICP-MS |
| Related Substances | Individual impurity ≤0.5%, Total ≤1.0% | HPLC (USP <621>) |
| Chloride Content | 11.9%–12.5% (as HCl) | Argentometric titration |
| pH (1% Solution) | 4.0–6.0 | USP <791> (pH meter) |
| Melting Point | 166–170°C | USP <741> (Capillary method) |
| Microbial Limits | TAMC ≤1000 CFU/g, TYMC ≤100 CFU/g | USP <61>, EP 2.6.12 |
| Dissolution (Tablets) | ≥80% released in 30 min (if applicable) | USP <711> (Paddle method) |
Diphenhydramine Hydrochloride is a widely used first-generation antihistamine with additional anticholinergic and sedative effects. It is commonly marketed under the brand name Benadryl and is employed to treat allergies, insomnia, and motion sickness. Its ability to cross the blood-brain barrier leads to pronounced drowsiness, distinguishing it from non-sedating antihistamines.
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