SULPHURIC ACID 



797 



it exists in the juices, and in animals in the Wood 

 and its derivates chiefly in the form of sulphates of 

 the alkalies ; while in the mineral kingdom it 

 occurs as gypsum (sulphate of lime), heavy spar 

 {sulphate of baryta), celestine (sulphate of stron- 

 tia), &c. It may be prepared on a small scale by 

 boiling sulphur in aqua regia or in nitric acid, 

 the sulphur becoming gradually oxidised into sul- 

 phuric acid. As a general rule, however, the 

 commercial acid is employed even for laboratory 

 experiments. In order to obtain the acid in a pure 

 form, suitable for medical use or medico-legal 

 analysis, it must be redistilled with sulphate of 

 ammonia in a retort containing a few slips of 

 platinum foil, the Hrst and last portions 1>eing 

 rejected. The distillation is attended with violent 

 concussions, partly owing to the high specific 

 gravity of the acid, and partly owing to its high 

 boiling-point, and this convulsive action is moder- 

 ated mechanically by the platinum slips. Sul- 

 phuric acid thus prepared according to the directions 

 of the British Pharmacopoeia may be regarded as 

 perfectly pure, presuming arsenic is not present. 

 Strong sulphuric acid has comparatively little 

 action on the metals except at a high temperature, 

 when it dissolves them, and at the same time 

 undergoes partial decomposition ; the metal being 

 oxidised by a portion of the acid which becomes 

 decomposed into oxygen and sulphurous acid, and 

 then uniting with a portion of undecomposed acid 

 to form a sulphate. Silver, copper, mercury, 

 arsenic, antimony, bismuth, tin, lead, and tel- 

 liirium are thus" acted on. Gold, platinum, 

 rhodium, and iridiiim are not affected by the acid 

 even at a boiling temperature. The more oxid- 

 isable metals, such as zinc, iron, nickel, and 

 manganese, are readily soluble in the dilute acid, 

 water being decomposed 

 and hydrogen liberated, 

 while the oxygen of the 

 water unites with the 

 metal ; and the metallic 

 oxide, at the moment of 

 its formation, combines 

 with the sulphuric acid 

 to form a sulphate. 



The sulphates or salts 

 formed by the combina- 

 tion of sulphuric acid 

 with a base are gener- 

 ally composed, as in the 

 case of green vitriol, 

 FeSO 4 7H s O, of 1 equiva- 

 lent of acid and 1 of 

 metallic oxide, with or 

 without water of crystal- 

 lisation. With the al- 

 kalies this acid also forms 



acid salts, as bisulphate of potash, and in a tew 

 cases copper, for example it forms basic salts. 

 The insoluble sulphates, such as that of baryta, 

 may be obtained by precipitating a soluble salt of 

 the base by a soluble sulphate ; thus, nitrate ot 

 baryta and sulphate of soda yield an insoluble sul- 

 phate of baryta and nitrate of soda, which remains 

 in solution. The soluble sulphates may be pre- 

 pared by dissolving the oxide or carbonate in 

 dilute sulphuric acid, in those cases in which the 

 metal itself is not readily attacked by the acid. 

 Sulphuric acid and the soluble sulphates are easily 

 detected by their yielding, with a solution of a 

 baryta salt, a white precipitate of sulphate of 

 baryta insoluble in acids. 



Sulphuric acid is prepared on a large scale by two 

 distinct processes-viz. by the distillation of green 

 sulphate of iron the original process of Basil 

 Valentine (15th century) ; and by the oxidation of 

 sulphurous acid through the agency of nitrous acid 



and hyponitric acid. The first process is chiefly 

 employed at Nordhausen in Germany. The sul- 

 phate of iron is distilled in earthen retorts, and 

 the acid passes over into a receiver containing a 

 little ordinary sulphuric acid, forming a brown 

 fuming oily liquid, of about sp. gr. 1-900. This 

 acid is known in commerce as Nordhausen acid, 

 and is chiefly used for dissolving indigo. 



The second method is that universally followed 

 in Great Britain, the germs of which were likewise 

 discovered by Valentine. He observed that when 

 the fumes of burning sulphur were collected under 

 a bell jar, slightly moistened with water, a small 

 quantity of liquid was deposited. This liquid, 

 which was simply sulphuric acid, on being concen- 

 trated from its solution by boiling was long sold 

 as oil of sulphur at prices as high as 2s. 6tl. per 

 ounce. About the year 1740 the French chemists 

 Lefevre and Lemery suggested that, by the use of 

 nitre along with the sulphur, the operation might 

 be conducted in close vessels, and a much greater 

 quantity of acid might be produced. This idea waa 

 acted on in England by Dr Ward, who established 

 works at Twickenham and Richmond, conducting 

 his manufacture by burning the mixed sulphur and 

 nitre in large stoppered glass receivers, into each 

 of which a small quantity of water was first intro- 

 duced. The substitution, in 1746, by Dr Roebuck 

 of Birmingham, of lead chambers in place of the 

 glass vessels may be regarded as essentially the 

 establishment of the process of manufacture followed 

 at the present day. 



The first stage in the manufacture of sulphuric 

 acid is the preparation of sulphurous acid by the 

 burning of sulphur or of iron pyrites. Previous to 

 the year 1838 Sicilian sulphur was almost exclu- 

 sively used in the manufacture, but in that year 



A sulphur-burner, or furnace; B, lead chamber, shown in section at B' ; C, steam-boiler; 

 D, leaden pan ; E, coke tower ; 8, steam-pipe ; n, nitre pot. 



the establishment of a monopoly of the sulphur 

 trade by the Sicilian government, and its conse- 

 quent increase in price, diverted the minds of 

 manufacturers to the employment of iron pyrites 

 (sulphide of iron). Iron pyrites is now much more 

 used than sulphur, and the only hindrance to its 

 universal adoption is the presence of foreign matter 

 in the pyrites, the most deleterious being arsenical 

 compounds ; and it has hitherto been found im- 

 practicable to free the sulphuric acid wholly from 

 the arsenious acid which renders it inapplicable for 

 many purposes. 



Wlien sulphur is the material used for producing 

 the sulphurous acid it is burned in an oven or 

 ' bifrner ' ( A) of brickwork, having a sole or bottom 

 of iron, termed the 'burner- plate.' Under this a 

 small fire is at first lighted, which is allowed to go 

 out after the sulphur lias ignited. A little above 

 the sulphur a small pot, called the nitre pot, n, is 

 either placed on a stand or hung from the roof. 



