At the present time most of the acid is made in Bohemia. The operation is commenced by roasting pyrites, by which ferrous sulphate and basic ferric sulphate, Fe,S,O,, are formed. On lixiviating the mass a solution of the sulphates is obtained, which, on evaporation and ignition, gives a residue of the basic sulphate alone. The presence of the ferrous salt is avoided as far as possible on account of the formation of sulphur dioxide as above given. On submitting the basic salt to distillation the following reaction takes place :

The residue is the caput mortuum or colcothar used in polishing. The acid is also prepared by dissolving sulphur trioxide in concentrated sulphuric acid :

This would require about 45 per cent. of the trioxide, but the commercial acid often contains less than 20 per cent. of it.

Properties.-Fuming sulphuric acid is a thick, oily liquid, colorless when pure, but often of a brownish color from small quantities of organic matter. As ordinarily found, its specific gravity is rarely above 1.865. Since the introduction of the trioxide into commerce the liquid may be obtained in sealed tubes or bulbs. It is of semi-solid consistence, and of the specific gravity 1.900. When the acid is cooled, large crystals separate, which are of a white color and melt at 35°.

Disulphuric acid forms both acid and normal salts; for example, with sodium we have NaHS,O, and Na,S,O,.

The Nordhausen acid has not any use in pharmacy, but is employed largely in the arts for dissolving indigo and in the preparation of artificial alizarin.

THIOSULPHURIC ACID, H2SO

This acid is not known in the free state, but only in combination in certain salts, of which sodium thiosulphate, Na,S,O,. 5H,O, is the most important. The salt is improperly called hyposulphite of sodium, and consequently the acid is also miscalled hyposulphurous acid, a name which belongs to the compound H2SO,.

Preparation. The sodium salt is prepared by boiling together sulphur and sodium sulphite:

NagSOS = NagSgOg.

It is also formed when sulphur dioxide is passed into a solution

of sodium sulphide, the reaction taking place in two stages according to the following:

(1) 3SO2 + 2Na2S = 2Na2SO + 3S.

(2) Na2SO + S = Na,S2O3

On adding iodine to a solution of sodium sulphide and sulphite, the thiosulphate is formed as follows:

NagS Na,SO, + I2 = Na2S2O3 + 2NaI.

When sodium, or calcium, pentasulphide is exposed to the air, it absorbs oxygen and is converted into thiosulphate :

In case the calcium salt is formed, it may be converted into the sodium salt by the addition of sodium carbonate. The lime used in purifying illuminating gas, and known as gas lime, contains calcium pentasulphide and hyposulphite. This is exposed to the air, whereby the sulphide is converted into thiosulphate, the mass is lixivated with water, and the solution, by double decomposition with sodium carbonate, gives calcium carbonate and sodium thiosulphate, as follows:

CaS2O3 + Na2CO3 = CaCO3 + Na2S2O3.

This and the ball soda, the waste from the manufacture of sodium carbonate, are the sources of the commercial sodium thiosulphate. Another method, which is sometimes employed on a small scale, consists in taking three parts of dried sodium carbonate and one part of sulphur, heating together in a porcelain dish to the fusing point of the sulphur, and stirring to facilitate contact with the air. Sodium sulphide is first formed, which by contact with the air is oxidized to sulphite; this is then dissolved in water and boiled with more sulphur, forming the thiosulphate according to the above reactions.

Properties. Sodium thiosulphate occurs in large, colorless, transparent, monoclinic prisms or plates, efflorescent in dry air, odorless, having a cooling, somewhat bitter and sulphurous taste, and a neutral or faintly alkaline reaction. It is soluble in 1.5 parts of water at 15°, and in 0.5 part of boiling water, in the latter case with partial decomposition. The salt is insoluble in alcohol. When rapidly heated it melts at 50°, and on increasing the temperature to 100° it loses all its water (36.3 per cent.). The aqueous solution dissolves oxide or chloride of silver, and discharges the color of iodine solution and iodized starch.

When a solution of thiosulphate is treated with an acid, sulphur dioxide is given off and sulphur is precipitated:

=

Na2S2O3 + 2HCI 2NaCl + SO2+S+ H2O.

This reaction serves to distinguish the thiosulphates from the sulphites.

DITHIONIC ACID, H2SO

Preparation. When sulphur dioxide is passed into water in which manganese dioxide is suspended, the following reaction takes place :

MnO2SO2 = MnSO8

Some manganous sulphate is formed at the same time. The solution is next treated with barium sulphide :

On adding sulphuric acid to this, the dithionic acid is obtained in solution:

BaSO+ H2SO4

=

BaSO4 + H2S2O8

The solution may be concentrated in a vacuum over sulphuric acid, until it reaches the specific gravity 1.347, when further evaporation resolves it into sulphur dioxide and sulphuric acid:

Dithionic acid is also formed when a dilute solution of iodine in potassium iodide is added to a solution of acid sodium sulphite:

The dithionates are not so easily oxidized as the thiosulphates. On heating, however, they are decomposed into sulphur dioxide and sulphite. They do not precipitate sulphur on the addition of hydrochloric acid and heating; this distinguishes them from the thiosulphates.

TRITHIONIC ACID, H2SO ̧

Preparation. When sublimed sulphur and a strong solution of acid potassium sulphite are digested at a temperature of from 50° to 60°, the potassium salt of trithionic acid is formed, as follows:

6KHSO + S2

=

2K2S3O8+ K2S2O3 + 3H,C.

When a concentrated solution of potassium thiosulphate is saturated with sulphur dioxide, potassium trithionate is formed:

On adding iodine to a solution of sodium thiosulphate and sulphite, sodium trithionate is produced:

Na2S2O+Na,SO, + I = Na,S,O + 2NaI.

When the potassium salt, obtained in the second method, is treated with hydrofluosilicic acid the free acid is produced :

K2SO + SiF4.2HF

=

H2S3O+ SiF4.2KF.

This diluted acid allows of only moderate concentration in a vacuum, for it readily decomposes into sulphur, sulphur dioxide, and sulphuric acid. The potassium salt is the only one which is well known.

TETRATHIONIC ACID, H2SO ̧

Preparation. The sodium salt of this acid is formed when iodine is added to an aqueous solution of sodium thiosulphate:

The free acid may be prepared by carefully decomposing barium tetrathionate with diluted sulphuric acid. The diluted acid may be boiled, but on concentrating decomposition takes place, as follows:

When hydrogen sulphide is passed into a solution of sulphur dioxide, pentathionic acid results, with separation of sulphur :

5H2S 5SO2 = H2SO8+ 4H2O + 5S.

The solution is milky, and is best cleared by digesting with metallic copper. Any copper which is dissolved is precipitated by hydrogen sulphide.

The solution may be concentrated to a specific gravity of 1.6, when it commences to evolve sulphur dioxide.

COMPOUNDS OF SULPHUR AND OXYGEN WITH
CHLORINE AND BROMINE.

THIONYL CHLORIDE, SOCIĄ.

This compound is prepared by passing dry sulphur dioxide over phosphorus pentachloride :

It is a colorless liquid, with a pungent odor, and fumes in contact with air. It boils at 78°, and has a specific gravity at o° of 1.675. When brought in contact with water it is decomposed, as follows:

SOCl2 + 2H2O = H2SO3 + 2HCl.

SULPHURYL-HYDROXYL-CHLORIDE, OR CHLORSULPHONIC

ACID, SO¿(OH)Cl.

This compound is formed by the action of hydrochloric acid on sulphur trioxide:

It may also be obtained by distilling a mixture of sulphuric acid and phosphorus oxychloride:

It is a colorless, fuming liquid, having a specific gravity of 1.766 at 18°, and boiling at 158°. Water decomposes it with violence, according to the following reaction:

SO2(OH)Cl + H2O = H2SO4 + HCl.

SULPHURYL CHLORIDE, SOCI.

This substance may be prepared by direct union of sulphur dioxide and chlorine in sunlight.

It is a colorless, fuming liquid, with a suffocating odor. It boils at 70°, and has a specific gravity of 1.659 at 20°. Water decomposes it into sulphuric and hydrochloric acids.

SULPHURYL BROMIDE, SOBг.

This compound is similar to the sulphuryl chloride, and may be considered as derived from sulphuric acid by the replacement of the two hydroxy Is by bromine. It is prepared by the direct union of sulphur dioxide and bromine in sunlight:

History. Selenium was discovered in 1817 by Berzelius, in the deposits from sulphuric acid chambers, and named from ɛjn, the moon.

Occurrence. It is widely distributed in small quantities associated with sulphur.

Preparation. This element is most conveniently prepared from lead chamber deposits. The crude material is mixed with sufficient sulphuric acid to make a paste, heated to the boiling point, and treated with nitric acid from time to time, until the red color disappears. The solution, which now contains selenic acid, H2SO, is, when cold, poured off from the sediment and saturated with sulphur dioxide, when selenium separates out as a red powder.

Thus prepared it contains lead, and sometimes other metals, from which it may be purified by distillation or by fusing with potassium nitrate and sodium carbonate, forming sodium selenate, from which it is precipitated as a red powder by hydrochloric acid. Another method is to evaporate the solution of selenic acid, obtained as above, to remove nitric acid, and then boil with concentrated hydrochloric acid, by which the selenic acid is reduced to selenous acid, as follows: