Magnesium Sulphate, MgSO₄

Magnesium Sulphate, MgSO₄, is known anhydrous, and with 12, 8, 7, 6, 5, 4, 2, 5/4, and 1 molecules of water.

Rhombic prisms of the heptahydrate separate from solutions containing magnesium sulphate at ordinary temperatures. It is the principal magnesium salt of commerce, and is known as Bittersalz, from its bitter taste, or as Englisches Salz or Epsom salts, from its occurrence in the mineral spring at Epsom. It frequently forms a fibrous efflorescence in dry mines, occurs in crystals in the Kentucky caves, and is being deposited in the Gulf of Kara-Boghaz, in the Eastern Caspian. The naturally occurring solid heptahydrate is known as epsomite, or, in the Stassfurt deposits, as reichardtite.

It was formerly prepared by crystallisation from natural waters, as Grew obtained it, in 1695, from the Epsom mineral spring. Dolomite was also frequently decomposed with sulphuric acid: the soluble magnesium sulphate being separated from the sparingly soluble calcium sulphate. Crude kieserite, MgSO₄.H₂O, was also converted into epsomite. It is now commonly prepared from the mother-liquor remaining after the preparation of magnesium chloride. Magnesium hydroxide is precipitated by adding milk of lime and dissolved in sulphuric acid. The salt can be purified by crystallisation.

The density of the crystals is 1.676, and they have an appreciable vapour pressure. Pickering says the pure crystals are stable in air, though they seem to lose water in dry air or over calcium chloride.

MgSO₄.7H₂O is soluble in dilute alcohol and insoluble in absolute. Its solubility in water has been expressed by 0.47816t°+25.76. By 20.5+0.2276t° from 0°-123° C., and by 48.5 -0.4403t° from 123°-190° C.: in parts MgSO₄ per 100 parts water.

A labile variety of the heptahydrate sometimes crystallises from cold supersaturated solutions in monoclinic tablets that are more soluble than the ordinary salt. This β-salt is isomorphous with FeSO₄.7H₂O, and sometimes forms when crystals of the hexahydrate are in contact with their mother-liquor at 0°-10° C.

At 100° C. MgSO₄.7H₂O becomes the dihydrate, at 130°-140° C. the monohydrate, and anhydrous at 280° C. When strongly ignited it obstinately retains traces of sulphuric acid. Its heat of solution is -3.863 Cals.

The dodecahydrate separates in large crystals from concentrated solutions of the sulphate when cooled below 0° C.: it is stable only between its crvohydric point -3.9° C., and 1.8° C. when it decomposes into the heptahydrate.

The octahydrate separates at 25° C. from solutions of magnesium sulphate containing some sodium sulphate. At 30° C., if the proportion of sodium sulphate is increased to about 32 molecules per cent., the octahydrate transforms into the heptahydrate. Air drying results in the same change.

The hexahydrate occurs dimorphically as tetragonal crystals that change readily into monoclinic prisms. The former, or α-variety, was obtained by inoculating a concentrated solution of the sulphate with a small tetragonal crystal of copper and zinc sulphates. The latter, or β-variety, crystallises from warm solutions of magnesium sulphate. It is more soluble than the heptahydrate. It has been obtained by crystallisation above 40° C., and even at temperatures as low as 25° C., though the transition temperature from hepta- to hexa-hydrate is 48.2° C. The presence of sodium chloride lowers this temperature. Its density is 1.751.

The heptahydrate when kept over sulphuric acid in air passes into the pentahydrate, which has a density of 1.869. The pentahydrate, however, is apparently unstable, like the tetrahydrate. The heptahydrate passes into the former at 77.5° C. and into the latter at a similar temperature. These two salts begin to transform into the monohydrate at 67°-68° C.

The dihydrate has a density of 2.373 when prepared by boiling the heptahydrate with absolute alcohol. It also results from heating the heptahydrate to 100° C. It separates in bundles of slender needles when magnesium sulphate solution containing magnesium chloride is concentrated at 65° C. These soon alter to transparent crystalline masses of MgSO₄.5/4H₂O.

From solutions containing different proportions of magnesium sulphate and chloride at 25° C. the series of hydrates from the hepta to the tetra separate.

The heptahydrate decomposes into the monohydrate at 130°-140° C. Its density is 2.478. Bischof assigned a density of 2.517 to the naturally occurring kieserite. Kieserite occurs, as an essential constituent of Abraumsalz, in the upper layers of the Stassfurt deposits, most frequently in white granular masses and less frequently in monoclinic crystals. It is only slightly soluble in water, which, however, slowly converts it into the heptahydrate. The partially hydrated substance is hard like stone - " kieseritsteine." Boiling water more rapidly completes the hydration, and the heptahydrate can then be crystallised from the solution. Kieserite is formed when a concentrated solution of magnesium sulphate is poured into a hot, saturated solution of magnesium chloride. The monohydrate will only separate from solutions of magnesium sulphate above 67° C., but in presence of other substances it may separate at 25° C.

Kieserite is separated from the more soluble portion of the Abraumsalz by water, and sent into commerce as block kieserite that has been partially converted into the heptahydrate.

The heat of solution of MgSO₄.H₂O has been given as 13.300 Cal. and as 12.131 Cal.

Anhydrous magnesium sulphate prepared by heating the heptahydrate to 200° C. has a density of 2.709, though a value as low as 2.628 has been found. It melts at 1124° C. but decomposes considerably below that temperature, and retains traces of sulphuric acid after strong ignition. Its heat of solution is 20.425 Cal., its heat of formation is 302.300 Cal., and it is nearly insoluble in concentrated hydrochloric acid.

It cakes in water, but dissolves rapidly when pounded up in it. The residue contains MgS and MgO when carbon reduces magnesium sulphate at lower temperatures; at higher temperatures and with less carbon the products are oxide, sulphur dioxide, and sulphur. Above 700° C. hydrogen sulphide reduces it to oxide, and carbon monoxide to oxide, sulphur dioxide, and sulphur.