Generic Names
- BP: Heavy magnesium oxide and Light magnesium oxide
- JP: Magnesium oxide
- PhEur: Magnesii oxidum ponderosum and Magnesii oxidum leve
- USP: Magnesium oxide
Chemical Name and CAS Registry Number
Magnesium oxide [1309-48-4]
Molecular Formula & Molecular Weight
MgO | 40.30
Manufacturing Process
Magnesium oxide occurs naturally as the mineral periclase. It can be produced by the calcination of the mineral magnesite (magnesium carbonate) or magnesium hydroxide. Magnesium hydroxide is obtained by treating seawater or brine with lime water. Purification methods include crushing and screening, heavy media separation, and froth flotation. Magnesium oxide can also be produced through the thermal decomposition of magnesium chloride, magnesium sulfate, magnesium sulfite, hydromagnesite, and basic carbonate (5MgO⋅4CO2⋅5H2O), followed by purification via filtration or sedimentation.
Category
Tablet and capsule diluent.
Applications in Formulations
Magnesium oxide is used as an alkaline diluent in solid dosage forms. It is also used as a food additive and an antacid, either alone or in combination with aluminum hydroxide. Additionally, magnesium oxide is employed as an osmotic laxative and as a magnesium supplement for the treatment of magnesium deficiency.
Physical Properties
Magnesium oxide exists in two forms: Light magnesium oxide (large bulk volume) and Heavy magnesium oxide (small bulk volume). USP 25 defines both forms under a single monograph, whereas other national pharmacopeias define them under separate monographs. USP 25 specifies that 15g of Heavy magnesium oxide occupies a volume of approximately 30mL, while 20g of Light magnesium oxide occupies a volume of approximately 150mL.
Pharmacopeial Standards
See Table I.
Table I: Pharmacopeial Standards for Magnesium Oxide
| Test Item | JP 2001 | PhEur 2002 | USP 25 |
| Identification | + | + | + |
| Loss on Ignition | ≤ 10.0% | ≤ 8.0% | ≤ 10.0% |
| Color of Solution | — | + | — |
| Free Alkali and Soluble Salts | + | — | ≤ 2.0% |
| Soluble Substances | — | ≤ 2.0% | — |
| Acid-insoluble Substances | ≤ 0.1% | ≤ 0.1% | ≤ 0.1% |
| Arsenic | ≤ 10 ppm | ≤ 4 ppm | — |
| Calcium | — | ≤ 1.5% | ≤ 1.1% |
| Calcium Oxide | + | — | — |
| Carbonates | + | — | — |
| Heavy Metals | ≤ 40 ppm | ≤ 30 ppm | ≤ 20 μg/g |
| Iron | ≤ 500 ppm | ≤ 0.07% | ≤ 0.05% |
| Chlorides | — | + | — |
| Heavy Magnesium Oxide (Bulk density) | — | ≤ 0.1% | — |
| Light Magnesium Oxide (Bulk density) | — | ≤ 0.15% | — |
| Fluorides | ≤ 0.08% | — | — |
| Sulfates | — | ≤ 1.0% | — |
| Assay (Content) | ≥ 96.0% | 98.0–100.5% | 96.0–100.5% |
General Properties
- Acidity/Alkalinity: pH = 10.3 (saturated aqueous solution)
- Boiling Point: 3,600°C
- Melting Point: 2,800°C
- Particle Size Distribution: 99.98% of particles are smaller than 45 μm (for light magnesium oxide).
- Refractive Index: 1.732
- Solubility: Soluble in dilute acids and ammonium salt solutions; practically insoluble in pure water (solubility increases in the presence of carbon dioxide); insoluble in ethanol (95%).
- Specific Gravity: 3.581 g/cm³ at 25°C.
Stability and Storage Conditions
Magnesium oxide is stable at room temperature and standard pressure. However, it forms magnesium hydroxide in the presence of water. Magnesium oxide is hygroscopic; when exposed to air, the light form absorbs water and carbon dioxide more rapidly than the heavy form.
Bulk materials should be stored in airtight containers in a cool, dry place.
Incompatibilities
Magnesium oxide is a basic oxide and can react with acidic compounds in the solid state to form salts (e.g., magnesium ibuprofen) or cause the degradation of base-labile drugs. The literature reports the adsorption of various drugs onto magnesium oxide, including antihistamines, antibiotics (especially tetracyclines), salicylates, atropine sulfate, scopolamine hydrobromide, and anthranilic acid derivatives. Magnesium oxide can complex with polymers such as Eudragit RS to prolong drug release and can react with sodium phenobarbital in the solid state. It has an adverse effect on the stability of diazepam and can influence the bioavailability of trichlormethiazide and certain anti-arrhythmic drugs.
Safety
Magnesium oxide is widely used as an excipient and therapeutic agent in solid preparations. Therapeutically, it is taken orally in doses of 250–500 mg as an antacid and 2–5 g as an osmotic laxative. When used as an excipient, high oral doses may still produce laxative effects, but it is generally regarded as a non-toxic substance.
Handling Precautions
Observe routine precautions associated with the material environment and quantity handled. Magnesium oxide may be harmful if inhaled, ingested, or absorbed through the skin in large quantities and can irritate the eyes and respiratory system. The use of gloves, safety goggles, and a dust mask or respirator is recommended. In the US and UK, the occupational exposure limit (8-hour TWA) for magnesium oxide (calculated as Mg) is 10 mg/m³ for total dust and 4 mg/m³ for respirable dust. The short-term (15 min) exposure limit for respirable dust is 10 mg/m³.
Regulatory Status
Listed as GRAS (Generally Recognized as Safe). Accepted for use as a food additive in Europe. Included in the FDA Inactive Ingredients Guide (for oral capsules and tablets). Licensed for use in non-parenteral preparations in the UK.
Notes
The EINECS number for magnesium oxide is 215-171-9.