Silver, Hyposulphite of
Symbol, AgO S2O2. Atomic weight, 164.
This salt is fully described in Part I. page 129. For the properties of the soluble double salt of Hyposulphite of Silver and Hyposulphite of Soda, see page 43.
SUGAR OF MILK. See Milk.
SULPHURETTED HYDROGEN. See Hydrosulphuric Acid.
SULPHURIC ACID.
Symbol, SO3. Atomic weight, 40.
Sulphuric Acid may be formed by oxidizing Sulphur with boiling Nitric Acid; but this plan would be too expensive to be adopted on a large scale. The commercial process for the manufacture of Sulphuric Acid is exceedingly ingenious and beautiful, but it involves reactions which are too complicated to admit of a superficial explanation. The Sulphur is first burnt into gaseous Sulphurous Acid (SO2), and then by the agency of Binoxide of Nitrogen gas, an additional atom of Oxygen is imparted from the atmosphere, so as to convert the SO2 into SO3, or Sulphuric Acid.
Properties.—Anhydrous Sulphuric Acid is a white crystalline solid. The strongest liquid acid always contains one atom of water, which is closely associated with it, and cannot be driven off by the application of heat.
This mono-hydrated Sulphuric Acid, represented by the formula HO SO3, is a dense fluid, having a specific gravity of about 1·845; boils at 620°, and distils without decomposition. It is not volatile at common temperatures, and therefore does not fume in the same manner as Nitric or Hydrochloric Acid. The concentrated acid maybe cooled down even to zero without solidifying; but a weaker compound, containing twice the quantity of water, and termed glacial Sulphuric Acid, crystallizes at 40° Fahr. Sulphuric Acid is intensely acid and caustic, but it does not destroy the skin or dissolve metals so readily as Nitric Acid. It has an energetic attraction for water, and when the two are mixed, condensation ensues, and much heat is evolved; four parts of acid and one of water produce a temperature equal to that of boiling water. Mixed with aqueous Nitric Acid, it forms the compound know a as Nitro-Sulphuric Acid.
Sulphuric Acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It chars organic substances, by removing the elements of water, and converts Alcohol into Ether in a similar manner. The strength of a given sample of Sulphuric Acid may be calculated, nearly, from its specific gravity, and a Table is given by Dr. Ure for that purpose. (See Appendix.)
Impurities of Commercial Sulphuric Acid.—The liquid acid sold as Oil of Vitriol is tolerably constant in composition, and seems to be as well adapted for Photographic use as the pure Sulphuric Acid, which is far more expensive. The specific gravity should be about 1·836 at 60°. If a drop, evaporated upon Platinum-foil, gives a fixed residue, probably Bisulphate of Potash is present. A milkiness, on dilution, indicates Sulphate of Lead (see page 186).
Test for Sulphuric Acid.—If the presence of Sulphuric Acid, or a soluble Sulphate, be suspected in any liquid, it is tested for by adding a few drops of dilute solution of Chloride of Barium, or Nitrate of Baryta. A white precipitate, insoluble in Nitric Acid, indicates Sulphuric Acid. If the liquid to be tested is very acid, from Nitric or Hydrochloric Acid, it must be largely diluted before testing, or a crystalline precipitate will form, caused by the sparing solubility of the Chloride of Barium itself in acid solutions.
SULPHUROUS ACID.
Symbol, SO2. Atomic weight, 32.
This is a gaseous compound, formed by burning Sulphur in atmospheric air or Oxygen gas: also by heating Oil of Vitriol in contact with metallic Copper, or with Charcoal.
When an acid of any kind is added to Hyposulphite of Soda, Sulphurous Acid is formed as a product of the decomposition of Hyposulphurous Acid, but it afterwards disappears from the liquid by a secondary reaction, resulting in the production of Trithionate and Tetrathionate of Soda.
Properties.—Sulphurous Acid possesses a peculiar and suffocating odour, familiar to all in the fumes of burning Sulphur. It is a feeble acid, and escapes with effervescence, like Carbonic Acid, when its salts are treated with Oil of Vitriol. It is soluble in water.
TETRATHIONIC ACID.
Symbol, S4O5. Atomic weight, 104.
The chemistry of the Polythionic Acids and their salts will be found described in the First Part of this Work, page 157.
WATER.
Symbol, HO. Atomic weight, 9.
Water is an Oxide of Hydrogen, containing single atoms of each of the gases.
Distilled water is water which has been vaporized and again condensed; by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. Pure distilled water leaves no residue on evaporation, and should remain perfectly clear on the addition of Nitrate of Silver, even when exposed to the light; it should also be neutral to test-paper.
The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolours Nitrate of Silver, and is therefore injurious.
Rain-water, having undergone a natural process of distillation, is free from inorganic salts, but it usually contains a minute portion of Ammonia, which gives it an alkaline reaction to test-paper. It is very good for Photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tingeing it of a brown colour and imparting an unpleasant smell, it must be rejected.
Spring or River water, commonly known as “hard water,” usually contains Sulphate of Lime, and Carbonate of Lime dissolved in Carbonic Acid; also Chloride of Sodium in greater or less quantity. On boiling the water, the Carbonic Acid gas is evolved, and the greater part of the Carbonate of Lime (if any is present) deposits, forming an earthy incrustation on the boiler.
In testing water for Sulphates and Chlorides, acidify a portion with a few drops of pure Nitric Acid, free from Chlorine (if this is not at hand, use pure Acetic Acid); then divide it into two parts, and add to the first a dilute solution of Chloride of Barium, and to the second, Nitrate of Silver,—a milkiness indicates the presence of Sulphates in the first case or of Chlorides in the second. The Photographic Nitrate Bath cannot be used as a test, since the Iodide of Silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for Chloride of Silver.
Common hard water can often be used for making a Nitrate Bath when nothing better is at hand. The Chlorides it contains are precipitated by the Nitrate of Silver, leaving soluble Nitrates in solution, which are not injurious. The Carbonate of Lime, if any is present, neutralizes free Nitric Acid, rendering the Bath alkaline in the same manner as Carbonate of Soda. (See page 89.) Sulphate of Lime, usually present in well water, is said to exercise a retarding action upon the sensitive Silver Salts, but on this point the writer is unable to give certain information.
Hard water is not often sufficiently pure for the developing fluids. The Chloride of Sodium it contains decomposes the Nitrate of Silver upon the film, and the image cannot be brought out perfectly. The New River water, however, supplied to many parts of London, is almost free from Chlorides, and answers very well. In other cases a few drops of Nitrate of Silver solution may be added, to separate the Chlorine, taking care not to use a large excess.