Sodium sulphate, Na₂SO₄

The sulphate is often called "Glauber's salt," on account of its application in the seventeenth century as a medicine by the physician Glauber, the specific being known as "sal mirabile Glauberi." Its purgative action seems to be a phenomenon dependent on osmosis.

The anhydrous sulphate is a constituent of oceanic salt deposits, and is called thenardite. An isomorphous mixture with potassium sulphate is known as glaserite; a double salt with magnesium sulphate as astrakanite, and with calcium sulphate as glauberite.

Sodium sulphate is an intermediate product in the manufacture of sodium carbonate by the Le Blanc process. It is also a byproduct in the manufacture of nitric acid by the interaction of sodium nitrate and sulphuric acid:

2NaNO₃+H₂SO₄=Na₂SO₄+2HNO₃.

In the Stassfurt deposits sodium chloride and magnesium sulphate monohydrate or kieserite are present, and on cooling the solution to -3° C. sodium sulphate crystallizes out. A mixture of sodium chloride, magnesium sulphate, and sand also reacts at dull red heat to form sodium sulphate and magnesium silicate, with evolution of chlorine. In Hargreaves's process sodium chloride reacts with a mixture of sulphur dioxide, steam, and air, producing sodium sulphate and hydrochloric acid. Sodium sulphate is also produced by treating the calcium sulphide of the alkali-waste with sodium hydrogen sulphate formed in the manufacture of nitric acid:

2NaHSO₄+CaS=Na₂SO₄+CaSO₄+H₂S.

Sodium sulphate is a white, tetramorphous substance, crystallizing below 200° C. in the rhombic or monoclinic system, from 200° to 500° C. in another rhombic form, and above 500° C. in the hexagonal system. It forms a heptahydrate and decahydrate. For the melting-point of the anhydrous salt Ruff and Plato give 880° C.; Wolters, 881° C.; McCrae, 881.5° and 885.2° C.; Arndt and Naeken, 883° C.; Heycock and Neville, 883.2° C.; Ramsay and Eumorfopoulos, van Klooster, and Dana and Foote, 884° C.; Bocke, 888° C.; and Huttner and Tammann, 897° C. Nacken observed neither decomposition nor volatilization at the melting-point. The transition-point of the monoclinic into the second rhombic form is given by Nacken as 234° C., by Huttner and Tammann as 235° C., and by Boeke as 239° C. The density of the anhydrous solid is given by Retgers as 2.673 at 15° C., by Krickmeyer as 2.671 at 20° C.; for that of the fused salt from the melting-point to 1000° C., Brunner gives the interpolation-formula

d_t =2.065-0.00045(t - 900°);

and for the decahydrate Clarke gives 1.465, Andreae 1.4665, and Rosicky 1.49.

For the temperature range 28° to 57° C. Schuller found for the specific heat 0.2293, and for 17° to 98° C. Regnault found 0.2312. For the heat of formation from the elements Thomsen gives 328.6 Cal., and Berthelot 328.1 Cal. Thomsen's value for the heat of neutralization is 31.38 Cal., and for the heat of solution of the anhydrous sulphate he found 0.46 Cal. For the heat of solution of the decahydrate Thomsen gives -18.8 Cal., and Berthelot -18.l Cal. The heat of hydration of the anhydrous salt to the decahydrate is given by Thomsen as 19.22 Cal., and by Berthelot as 18.64 Cal.

The solubility of the anhydrous salt diminishes with rise of temperature from 33° to 120° C., as indicated in the table of solubility.

Sodium sulphate forms double salts with potassium sulphate and magnesium sulphate.

Solid Phase, Na₂SO₄,10H₂O

Temperature, °C.	0	10	20	30	32.75
Grams Na2SO4 in 100 grams water	5.0	9.0	19.4	40.8	50.65

Solid Phase, Na₂SO₄,7H₂O

Temperature, °C	0	5	10	15	20	25
Grams Na2SO4 in 100 grams water	19.5	24	30	37	44	53

Solid Phase, Na₂SO₄

Temperature, °C	33	35	40	50	60	80	100	120	140	160	230
Grams Na2SO4 in 100 grams water	50.6	50.2	48.8	46.7	45.3	43.7	42.5	41.95	42	44.25	46.4

Solubility-curve of sodium sulphate

Lowel gives the transition-point of the heptahydrate into the anhydrous salt (E) as 24.4° C., and that of the decahydrate into the anhydrous salt (F) is given by Richards and Wells as 32.383° C., and by Dickinson and Mueller as 32.384° C. The solubility relations are graphically represented in fig.

A saturated solution of sodium sulphate containing 42.2 grams of sulphate per 100 grams of water boils at 101.9° C. at 751 mm. Pressure, or 108.668° C. at 760 mm. pressure.

Sodium sulphate readily forms a supersaturated solution in water. When a solution is cooled to about 5° C., the heptahydrate crystallizes out. The crystals of the decahydrate weather in air, owing to loss of water of crystallization.

Reduction of sodium sulphate with charcoal at red heat produces sodium sulphide and carbon monoxide, along with sodium polysulphides and carbon dioxide. Addition of alcohol to a solution of sodium sulphate in aqueous hydrogen peroxide precipitates a complex derivative of the formula Na₂SO₄,9H₂O,H₂O₂.