MgSO₄,7H₂O

Relative molecular mass. 246.5

Chemical name. Magnesium sulfate (1:1) heptahydrate; CAS Reg. No. 10034-99-8.

Other name. Epsom salt.

Description. Brilliant, colourless crystals or a white, crystalline powder; odourless.

Solubility. Freely soluble in water; practically insoluble in ethanol (~750 g/l) TS.

Category. Cathartic drug.

Storage. Magnesium sulfate heptahydrate should be kept in a well-closed container.

Additional information. Magnesium sulfate heptahydrate effloresces in warm, dry air.

Requirements

Magnesium sulfate heptahydrate contains not less than 99.0% and not more than the equivalent of 100.5% of MgSO₄, calculated with reference to the dried substance.

Identity tests

A. Dissolve 10 mg in 2 mL of water and add 1 mL of ammonia (~100g/l) TS; a white precipitate is produced which redissolves after adding 1 mL of ammonium chloride (100 g/l) TS. Then add 1 mL of disodium hydrogen phosphate (40 g/l) TS; a white, fine crystalline precipitate is formed.

B. A 20 mg/mL solution yields reaction A described under 2.1 General identification tests as characteristic of sulfates.

Heavy metals. Use 1.0 g for the preparation of the test solution as described under 2.2.3 Limit test for heavy metals, Procedure 1; determine the heavy metals content according to Method A; not more than 10 μg/g.

Arsenic. Use a solution of 5 g in 35 mL of water and proceed as described under 2.2.5 Limit test for arsenic; the arsenic content is not more than 2 μg/g.

Chlorides. Dissolve 0.85 g in a mixture of 2 mL of nitric acid (~130 g/l) TS and 20 mL of water, and proceed as described under 2.2.1 Limit test for chlorides; the chloride content is not more than 300 μg/g.

Iron. Use 2.0 g; the solution complies with the 2.2.4 Limit test for iron; not more than 20 μg/g.

Clarity and colour of solution. A solution of 1 g in 10 mL of water is clear and colourless.

Loss on drying. Dry 0.5 g at 110 - 120 °C for 1 hour and then at 400 °C to constant mass; it loses not less than 0.48 g/g and not more than 0.52 g/g.

Acidity or alkalinity. Dissolve 1.0 g in 10 mL of water and add 0.05 mL of phenol red/ethanol TS; not more than 0.2 mL of hydrochloric acid (0.01 mol/l) VS or sodium hydroxide (0.01 mol/l) VS is required to obtain the midpoint of the indicator (pink).

Assay. Dissolve about 0.25 g, accurately weighed, in 100 mL of water, and proceed with the titration as described under 2.5 Complexometric titrations for magnesium.

Each mL of disodium edetate (0.05 mol/l) VS is equivalent to 6.018 mg of MgSO₄.

Additional requirements for Magnesium sulfate heptahydrate for parenteral use

Complies with the monograph for "Parenteral preparations".

Bacterial endotoxins. Carry out the test as described under 3.4 Test for bacterial endotoxins; contains not more than 0.09 IU of endotoxin RS per mg.