Cooley's Cyclopaedia of Practical Receipts Volume Ii Part 243

The quant.i.tative determination of free sulphuretted hydrogen, or of a soluble sulphide in any solution, is conducted as follows:--The liquid to be tested is mixed with a small quant.i.ty of a cold solution of starch, made slightly acid with acetic acid. A solution of iodine of known strength, dissolved in pota.s.sic iodide is then added, until the liquid just begins to turn blue from the action of the excess of iodine on the starch. In this process the sulphuretted hydrogen converts the iodine into hydriodic acid, whilst sulphur is liberated.

Of course the quant.i.ty of sulphuretted hydrogen is calculated from the quant.i.ty of iodine employed. The reaction is--

2H_{2}S + 2I_{2} = 4HI + S_{2}.

The value of sulphuretted hydrogen as a reagent has already been alluded to. It throws down most of the metals from solutions of their salts in the form of insoluble sulphides; and each of the sulphides so produced in many cases being distinguished from the others by a special and characteristic colour. The sulphuretted hydrogen thus presents the metal in a form in which it can, in many instances, be easily and with certainty recognised.

Thus sulphide of lead is black, of a.r.s.enic yellow, of antimony orange, of manganese salmon colour, and of zinc white. By means of sulphuretted hydrogen, also, the chemist is enabled to separate the metals into groups.

For instance, from solutions containing certain metallic salts, sulphuretted hydrogen throws down the metals as sulphides, provided the solution has been previously made slightly acid. Copper, a.r.s.enic, tin, and cadmium, are some of the metals thrown down under these conditions.

The salts of iron, nickel, cobalt, and certain others, although they do not yield precipitates under like circ.u.mstances, are found to do so if their solutions are made alkaline instead of acid. Again, there are other salts, those of the alkalies and alkaline earths, which, when sulphuretted hydrogen is pa.s.sed through these solutions, give no precipitates either in acid or alkaline solutions. The chemist, therefore, in the course of an a.n.a.lysis, frequently avails himself of a knowledge of these facts to separate certain metals from each other.

=Hydrogen, Persulphide of.= _Syn._ HYDRIC PERSULPHIDE, HYDROGEN DISULPHIDE. To procure this substance, calcium disulphide (CaS_{2}) in solution is poured into hydrochloric acid diluted with twice its bulk of water. The solution being gently warmed, the persulphide subsides at the bottom as an oily fluid. Hydric persulphide has a great resemblance to hydric peroxide in qualities. It bleaches, and is decomposed, with violence, when brought into contact with the oxides of manganese and silver. It easily decomposes into sulphur and sulphuretted hydrogen.

=SULPHU'RIC ACID.= H_{2}SO_{4}. _Syn._ OIL OF VITRIOL, BRITISH O. OF V., VITRIOLIC ACID; ACIDUM SULPHURIc.u.m (B. P., Ph. L. & E.), ACIDUM SULPHURIc.u.m VENALE (Ph. D.), ACIDUM VITRIOLIc.u.m, L. This acid, in a concentrated form, was discovered by Basil Valentine towards the end of the 15th century. At first it was obtained by the distillation of green vitriol, but is now made by the oxidation of sulphurous anhydride, obtained by the combustion either of sulphur or of certain sulphides. In consequence of the growing demand for sulphur in the manufacture of gunpowder, ultramarine, and for the destruction of the vine parasites in the vineyards of France, Italy, and Spain, sulphuric acid is now seldom made by burning sulphur, but, with few exceptions, by roasting iron pyrites, or bisulphide of iron.

The following table will convey an idea of the enormous consumption of this mineral in vitriol making in England alone. The quant.i.ties given represent tons.

+-----+-----------------------------------------------------------+-------+ Pyrites from[212] Date.+-------+--------+--------+---------+--------+------+-------+ Sum Norway. Germany. Belgium. Portugal. Spain. Italy. Sundry Total. Places. +-----+-------+--------+--------+---------+--------+------+-------+-------+ 1862 4,975 6,817 9,860 53,296 33,717 ... 2,187 110,852 1863 6,736 15,409 12,059 109,180 33,213 ... 2,628 179,225 1864 16,087 12,751 7,069 118,489 15,529 ... 1,065 170,990 1865 22,229 14,727 2,121 137,787 16,393 ... 369 193,626 1866 38,262 21,574 4,006 165,993 11,910 ... 1,625 244,596 1867 77,895 34,592 2,299 105,556 50,222 ... 2,134 272,698 1868 63,007 41,559 ... 75,883 47,458 794 1,019 229,720 1869 63,091 13,983 ... 140,805 99,648 ... 2,420 319,947 1870 67,464 14,914 ... 174,459 150,990 ... 3,676 411,512 1871 74,416 12,809 ... 120,573 242,163 ... 4,581 454,542 1872 71,665 5,682 ... 180,329 257,429 ... 2,521 517,626 +-----+-------+--------+--------+---------+--------+------+-------+-------+

[Footnote 212: "Development of the Chemical Arts during the last Ten Years," by Dr A. W. Hofman ('Chemical News,' vol. xxv, 1879).]

Of the other sulphides employed in vitriol making may be mentioned galena, or native sulphide of lead, which, when roasted, is made to give up half its sulphur. The chief consumption of this mineral is in the Harz. Copper pyrites is also used in the Harz, as well as in Swansea and Glasgow.

Blende, or native sulphide of zinc, is also occasionally had recourse to.

In addition to the above sulphides, the vitriol maker in England, France, and Germany has lately largely availed himself of a compound known as 'Laming's mixture,' which is an impure oxide of iron that has been used in gas manufacture for the removal from the gas of the sulphur. Laming's mixture is consequently rich in this last element.

a.s.sociated with the pyrites in small quant.i.ties are various substances, some of which, becoming volatilised when the ore is burnt, enter the chambers with the mixed gases, and thus find their entrance into the acid, whilst others remain behind in the iron residue of impure ferric oxide, left on the hearth of the furnace after roasting. The former of these foreign bodies, which are found in most commercial acids, are described below under the section "Purification." Amongst the solid non-volatile matters, the extraction of which from the burnt iron has been found in many works to yield a profit, are zinc, copper, silver and thallium.

At Wolcrum, in Germany, the zinc which exists in the residue in the form of sulphate is extracted by lixiviation, and then treated with common salt, the reaction giving rise to the production of sulphate of soda and chloride of zinc. The soda obtained is sufficient to pay for the working of the operation, whilst a good profit is made by the sale of the large quant.i.ties of chloride of zinc which are thus yielded.[213]

[Footnote 213: Ibid.]

The copper, which in some residues is met with to the amount of 4 per cent., also pays for extraction, and is sold to the smelter. It is first converted into chloride, and then precipitated by iron. The silver is recovered by Claudet's process, which consists in precipitating it from a saline solution in which it is in the state of a soluble chloride, by iodide of pota.s.sium.

In the Widnes Copper Works the silver so extracted yields an annual profit of 3000.[214]

[Footnote 214: Ibid.]

Thallium is found in the fine dust caused by the combustion of the pyrites, which dust deposits in the flues between the furnace and the chambers. The metal is extracted from the dust by treating this latter with dilute sulphuric acid. The resulting sulphate is converted into chloride, and again reconverted alternately into sulphate and chloride several times, the sulphate last obtained being reduced by metallic zinc.[215]

[Footnote 215: Ibid.]

Selenium is also a frequent const.i.tuent in the flue dust. Some ores, after being subjected to roasting, yield iron capable of being worked. This is more particularly the case with the Spanish and Portuguese pyrites.

The following is an outline of the process by which sulphuric acid is obtained, and of the chemical changes which occur during its manufacture:

The sulphur or sulphide being placed on the hearth of the furnace, shown at A in the accompanying cut, when heated from below, soon takes fire, and combining with the oxygen of the atmospheric air, the admission of which into the furnace is regulated by an experienced workman, by the door shown in the plate, forms sulphurous anhydride. An iron pot, standing on the hearth of the furnace, contains a mixture of nitrate of soda and oil of vitriol, and this becoming heated by the burning sulphur, decomposition of the salt ensues, and fumes of nitric acid are given off. The sulphurous anhydride and nitric acid gases thus formed together with air are carried into large leaden chambers, standing on, and supported by, ma.s.sive frameworks of stout timber. Steam is admitted continuously by several jets (see plate) into these chambers, which are covered at the bottom with water to a depth of about three inches.

As soon as the mixed gases enter the chamber and come into contact with the steam, the sulphurous anhydride acts on the nitric acid, forming sulphuric acid, which falls into and is absorbed by the water on the floor of the chamber, and nitric oxide, which is liberated in the chamber.

The following equation will ill.u.s.trate the reaction:

2HNO_{3} + 3SO_{2} + 2H_{2}O = 3H_{2}SO_{4} + 2NO.

170 parts by weight of nitrate of soda are required to oxidise to sulphuric acid 96 parts of sulphur, whereas rarely more, and frequently less, than 5 parts of soda are required by the vitriol maker. This saving of material is effected by the function performed in the chamber by the nitric oxide resulting from the decomposition of the nitric acid.

The nitric oxide reacting upon the air in the chamber abstracts oxygen from it and becomes converted into nitric peroxide, thus:

2NO + O_{2} = 2NO_{2}.

Nitric peroxide is a very unstable compound, and directly it comes into contact with the fresh sulphurous anhydride entering the chamber, it oxidises it in the presence of water to sulphuric acid, thus:

2NO_{2} + 2SO_{2} + 2H_{2}O = 2H_{2}SO_{4} + 2NO.

This deportment of the nitric oxide being continuous, it will be seen it acts the part of a carrier of oxygen from the atmospheric air contained in the chamber to the sulphurous acid, and by so doing (theoretically) renders any further supply of nitrate of soda than that required to start the process unnecessary.

As soon as the water, or rather liquid acid on the floor of the leaden chambers, has acquired the sp. gr. of 135 to 150, it is drawn off, and concentrated by boiling in shallow leaden pans to the density of about 172, after which it is further concentrated in green-gla.s.s or platinum retorts, until the sp. gr. reaches 1842 to 1846. When of sp. gr. from 135 to 150 it is called chamber acid, and when of the last strength, is used in the manufacture of salt-cake, sulphate of ammonia, some kinds of manure, and nitric acid. Sulphuric acid of sp. gr. 1720 is mostly employed in the preparation of superphosphate of lime. After concentration to 1842 or 1846, the clear acid is put into large globular bottles of green gla.s.s (carboys), surrounded with straw and basket-work, and is sent into the market under the name of 'oil of vitriol.'

The leaden chambers in which the chemical changes take place, that result in the formation of the acid, vary greatly in dimensions in different works, being sometimes as much as 12 or 15 feet high, 15 or 20 wide, and from 150 to 300 feet long. They are mostly partially divided by incomplete leaden part.i.tions, known as curtains, so arranged on the roof and the floor as to cause the currents of mixed gases to come into collision, and thus cause their admixture. Where there are a number of small separate chambers they are connected by means of leaden tubes. A chamber having a capacity of 25,000 cubic feet will yield 10 tons of acid weekly.

The sheets of lead used in the construction of the chambers are united by fusion, or melting together of their edges. If cement were used it would be speedily attacked and destroyed by the acid and gaseous products.

The process for the manufacture of sulphuric acid above described, was devised in 1774 by a calico printer of Rouen, and improved by Chaptal.

In 1776 the first vitriol factory was set up at Prestonpans, by Dr Roebuck, of Birmingham, with whom originated the idea of the leaden chambers.

Various attempts have been at different times unsuccessfully made to supersede the old process. Of these we may mention:

1. The proposal to oxidise sulphurous acid by means of chlorine in the presence of steam.

2. Persoz's method to oxidise sulphurous acid by means of nitric acid, and to regenerate the nitric oxide resulting from the reduction of the acid by the oxygen of the air in the presence of steam.

3. _a_, by the decomposition of gypsum by superheated steam at a red heat; or _b_, by decomposing the gypsum by chloride of lead.

The failure of the above and other efforts has led to the chemist turning his attention to the elaboration and perfection of the old process, in the working of which considerable improvements have been introduced within the last ten or fifteen years; improvements resulting not only in a diminished cost of production, but in the manufacture of a purer, and therefore better acid.

The proper construction of the furnaces, ovens, and grates on which the firing of the sulphur or pyrites takes place, together with the flues, is an important condition in the manufacture of the acid; and to this end a great deal of scientific knowledge and experience have lately been applied with excellent effect. Of the many improvements in this direction for burning poor ores of pyrites is a contrivance much used in Germany, where the furnace on which it is carried out is known as Gerstenhofer's oven. It is shown in the accompanying drawing.

[Ill.u.s.tration]

The furnace is fitted inside with a number of little fire-clay projections, arranged as shown in the plate, in banks or terraces, the function of which is to prolong the exposure of the pyrites to heat. The furnace having been previously raised to a red heat, by means of a coal or wood fire (which is then extinguished), the pyrites are admitted into it through the hoppers (_a_). At the base of the hoppers are grooved iron rollers, which crush the lumps of ore as they enter the chambers, and by thus reducing their size, expose a larger amount of surface to the action of heat. The greater part of the sulphur of the pyrites is thus burnt off, as the lumps pa.s.s from terrace to terrace, the heat at the same time generated by their combustion being sufficient to keep up that of the furnace. A moderate blast of air is admitted at _c_, whilst the sulphurous acid formed ascends through _d_ into the leaden chambers, the spent pyrites falling out through the apertures at _c_.

Another improved furnace is Perret's, which is largely used in France. In this, small lumps of pyrites are placed on horizontal plates, and exposed to the hot gases generated in kilns below. The gases, on their way to the chambers, sweep over the pyrites and rob them of their sulphur.

The most important and noticeable improvement, however, of late years in sulphuric acid manufacture is that resulting from the addition to the plant of a contrivance of Gay-Lussac. Previous to this invention, the sulphuric acid of commerce, amongst other impurities, always contained appreciable quant.i.ties of certain oxides of nitrogen, the results of which were not only the contamination of the acid, but a waste of substances, which, properly utilised, are essential for the conversion of the sulphurous and sulphuric acid, and the loss of which leads to an increased consumption of nitrate of soda. Under the old method, these valuable oxides of nitrogen, which, with a large amount of nitrogen and a small quant.i.ty of oxygen, const.i.tuted the spent air of the last leaden chamber, were carried off into the air, and consequently lost. Now, instead of being allowed to diffuse into the atmosphere, they are made to pa.s.s through a tower or chamber (shown at C in the plate below) filled with c.o.ke, through which a thin stream of sulphuric acid is made to trickle. In pa.s.sing through the c.o.ke, therefore, the expiring spent gases come into contact with the sulphuric acid, to which they give up their oxides of nitrogen. From the tower (C) the acid flows into a cistern (D), whence it is pumped up to the top of another tower (E), either filled with c.o.ke, or arranged with inclined shelves, as shown in the plate. In this tower the acid meets with a current of hot sulphurous acid and air coming up from the furnace, which deprive it of the oxides of nitrogen, and the gaseous mixture enters the chambers, whilst the denitrafied acid flows off into a suitable reservoir.

[Ill.u.s.tration]