Magnesium Carbonate (molecular weight: 84.31) appears as a white granular powder. It decomposes at 350°C and loses carbon dioxide at 900°C. It is slightly soluble in cold water and dissolves slowly in acids. Its trihydrate exists as colorless needle-like crystals with a melting point of 165°C and a relative density of 1.850. Its pentahydrate exists as white monoclinic crystals with a relative density of 1.73, which decompose upon heating in air. It can be synthesized by introducing carbon dioxide into a magnesium salt aqueous solution while adding sodium carbonate to obtain the hydrate; drying this at 50°C or below yields the anhydrous form. Magnesium carbonate is used in the manufacture of magnesium salts, magnesium oxide, fireproof coatings, inks, glass, toothpaste, and rubber fillers.
Physicochemical Properties
Magnesium carbonate exists either as basic hydrated magnesium carbonate or normal hydrated magnesium carbonate. Depending on the crystallization conditions, the product is categorized as “light” or “heavy,” with the light variety being more common. Light forms include MgCO₃·H₂O, while heavy forms include 5MgCO₃·Mg(OH)₂·3H₂O, 5MgCO₃·2Mg(OH)₂·7H₂O, 4MgCO₃·Mg(OH)₂, and 3Mg(OH)₂·4H₂O. At room temperature, it typically exists as a trihydrate.
Light magnesium carbonate is a white, loose powder or friable blocks. It is odorless with a relative density of 2.2 and a melting point of 350°C. It is stable in air and, when heated to 700°C, releases carbon dioxide to form magnesium oxide. It is nearly insoluble in water, though it causes a slight alkaline reaction in water. It is insoluble in ethanol and dissolves in dilute acids with effervescence. Light magnesium carbonate is primarily used as a filler and reinforcing agent for transparent or light-colored rubber products, as it does not change the refractive index of the rubber while enhancing its wear resistance, flex resistance, and tensile strength. It is also used as an additive in paints, inks, and coatings, and is widely applied in the toothpaste, pharmaceutical, and cosmetic industries.
Identification Methods
To 0.2g of magnesium carbonate, gradually add dilute hydrochloric acid (1+3) mL; it will dissolve and effervesce. Add ammonia solution (40mL ammonia mixed with water to 100mL) to make the solution alkaline; this solution will show a magnesium salt reaction.
Magnesium Salt Reaction: To the solution above, add ammonium chloride solution (1+10) and ammonium carbonate solution (20g dissolved in 20mL ammonia, diluted to 100mL with water); no precipitate should form. Subsequently, add disodium hydrogen phosphate solution (1+8) to produce a white crystalline precipitate. This precipitate is insoluble when treated with ammonia solution.
Fields of Application
As an additive, magnesium carbonate is frequently used as a pharmaceutical intermediate, antacid, desiccant, color fixative, carrier, and anti-caking agent. In food, it serves as an additive and magnesium supplement. In fine chemicals, it is used to produce chemical reagents. In the rubber industry, it acts as a reinforcing agent and filler. It is also used in heat-insulating and high-temperature-resistant fireproof materials, as a crucial chemical raw material in wire and cable manufacturing, for high-quality glassware, and to provide surface luster in enamel and ceramics. It is further utilized in the production of magnesium salts, pigments, paints, cosmetics, shipbuilding, and boiler manufacturing. In the coating industry, it serves as an accessory pigment and filler for paints and inks to increase whiteness and opacity. It is also used by athletes as a drying agent for hands during competitions.
Food Additive Standards
- Chinese Name: Magnesium Carbonate (including light and heavy magnesium carbonate)
- Function: Food industry processing aid
- Usage Regulations: Generally permitted as a flour treatment agent, with a maximum dosage of 5.0g/kg (general food) and 1.5g/kg (wheat flour).
- ADI: No special restrictions specified (FAO/WHO, 1994).
Chemical Properties
White, friable blocks or white loose powder; odorless. Relative density of 2.2, melting point 350°C. Soluble in dilute acid solutions, nearly insoluble in water (forming a weakly alkaline solution), and insoluble in ethanol. Stable in air; decomposes to magnesium oxide when heated to 700°C.
Production Method
Dissolve equal amounts of crystalline magnesium sulfate and sodium carbonate in 10 times their volume of water. Heat to 60–80°C and mix thoroughly; precipitation occurs simultaneously with the evolution of CO₂. Filter the precipitate, wash thoroughly with hot water at 70–80°C, and finally dry at 50–60°C to obtain the product.
Reaction:
MgSO4+Na2CO3→Na2SO4+MgCO3↓