Chapter II Vocabulary of Photographic Chemicals
ACETIC ACID.
Symbol, C4H3O3 + HO. Atomic weight, 60.
Acetic Acid is a product of the oxidation of Alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air; but if a portion of yeast, or Nitrogenous organic matter of any kind, be added, it soon acts as a ferment, and causes the spirit to unite with oxygen derived from the atmosphere, and so to become sour from formation of Acetic Acid, or “vinegar.”
Acetic Acid is also produced on a large scale by heating wood in close vessels: a substance distils over which is Acetic Acid contaminated with empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much used in commerce.
The most concentrated Acetic Acid may be obtained by neutralizing common vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda so formed; this Acetate of Soda is then distilled with Sulphuric Acid, which removes the Soda and liberates Acetic Acid: the Acetic Acid being volatile, distils over, and may be condensed.
Properties of Acetic Acid.—The strongest acid contains only a single atom of water; it is sold under the name of “Glacial Acetic Acid,” so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pungent odour and a density nearly corresponding to that of water; the specific gravity of Acetic Acid however is no test of its real strength, which can only be estimated by analysis.
The commercial Glacial Acetic Acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and Hydrochloric Acids are also common impurities. They are injurious in Photographic Processes, from their property of precipitating Nitrate of Silver. To detect them proceed as follows:—dissolve a small crystal of Nitrate of Silver in a few drops of water, and add to it about half a drachm of the Glacial Acid; the mixture should remain quite clear even when exposed to the light. Hydrochloric and Sulphurous Acid produce a white deposit of Chloride or Sulphite of Silver; and if Aldehyde or volatile tarry matter be present in the Acetic Acid, the mixture with Nitrate of Silver, although clear at first, becomes discoloured by the action of light.
Glacial Acetic Acid sometimes has a smell of garlic. In this state it probably contains an organic Sulphur Acid, and is unfit for use.
Many employ a cheaper form of Acetic Acid, sold by druggists as “Beaufoy’s” acid; it should be of the strength of the Acetic Acid fortiss. of the London Pharmacopœia, containing 30 per cent, real acid. It will be advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other impurities, before use.
ACETATE OF SILVER. See Silver, Acetate of.
ALBUMEN.
Albumen is an organic principle found both in the animal and vegetable kingdom. Its properties are best studied in the white of egg, which is a very pure form of Albumen.
Albumen is capable of existing in two states; in one of which it is soluble, in the other insoluble, in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as Potash or Ammonia.
Soluble Albumen may be converted into the insoluble form in the following ways:—
1. By the application of heat.—A moderately strong solution of Albumen becomes opalescent and coagulates on being heated to about 150° Fahrenheit, but a temperature of 212° is required if the liquid is very dilute. A layer of dried Albumen cannot easily be coagulated by the mere application of heat.
2. By addition of strong acids.—Nitric Acid coagulates Albumen perfectly without the aid of heat. Acetic Acid however acts differently, appearing to enter into combination with the Albumen, and forming a compound soluble in warm water acidified by Acetic Acid.
3. By the action of metallic salts.—Many of the salts of the metals coagulate Albumen completely. Nitrate of Silver does so; also the Bichloride of Mercury. Ammoniacal Oxide of Silver however does not coagulate Albumen.
The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemical compound of the animal matter with Protoxide of Silver. This substance, which has been termed Albuminate of Silver, is soluble in Ammonia and Hyposulphite of Soda; but after exposure to light, or heating in a current of Hydrogen gas, it assumes a brick-red colour, being probably reduced to the condition of an organic compound of a Suboxide of Silver. It is then almost insoluble in Ammonia, but enough dissolves to tinge the liquid wine-red. The red coloration of solution of Nitrate of Silver employed in sensitizing the Albuminized photographic paper is probably produced by the same compound, although, often referred to the presence of Sulphuret of Silver.
Albumen also combines with Lime and Baryta. When Chloride of Barium is used with Albumen, a white precipitate of this kind usually forms.
Chemical composition of Albumen.—Albumen belongs to the Nitrogenous class of organic substances (see page 325). It also contains small quantities of Sulphur and Phosphorus.
ALCOHOL.
Symbol, C4H6O2. Atomic weight, 46.
Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the Alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver; a portion of the vapour of water however passes over with the Alcohol, and dilutes it to a certain extent, forming what is termed “Spirits of Wine.” Much of this water may be removed by redistillation from Carbonate of Potash, in the manner described at page 196 of this work; but in order to render the Alcohol thoroughly anhydrous, it is necessary to employ Quicklime, which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong Alcohol of ·823, and the two are distilled together.
Properties of Alcohol.—Pure anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour of water, and becomes diluted by exposure to damp air; boils at 173° Fahr. It has never been frozen.
Alcohol distilled from Carbonate of Potash has a specific gravity of ·815 to ·823, and contains 90 to 93 per cent, of real spirit.
The specific gravity of ordinary rectified Spirits of Wine is usually about ·840, and it contains 80 to 83 per cent, of absolute Alcohol.
AMMONIA.
Symbol, NH3 or NH4O. Atomic weight, 17.
The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains one atom of Nitrogen combined with three of Hydrogen: these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is Ammonia.
Properties of Ammonia.—Ammoniacal gas is soluble in water to a large extent; the solution possessing those properties which are termed alkaline (see page 308). Ammonia however differs from the other alkalies in one important particular—it is volatile: hence the original colour of turmeric-paper affected by Ammonia is restored on the application of heat. Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is converted into Carbonate of Ammonia; it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Ammonia often contains Chloride of Ammonium, recognized by the white precipitate given by Nitrate of Silver after acidifying with pure Nitric Acid.
The strength of commercial Ammonia varies greatly; that sold for pharmaceutical purposes under the name of Liquor Ammoniæ, contains about 10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes with the proportion of Ammonia present, the Liquor Ammoniæ being usually about ·936.
Ammonia, although forming a large class of salts, appears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally are protoxides of metals, as already shown at page 308, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The following remarks may perhaps tend somewhat to elucidate the difficulty:—
Theory of Ammonium.—This theory supposes the existence of a substance possessing the properties of a metal, but differing from metallic bodies generally in being compound in structure: the formula assigned to it is NH4, one atom of Nitrogen united with four of Hydrogen. This hypothetical metal is termed “Ammonium;” and Ammonia, associated with an atom of water, may be viewed as its Oxide, for NH3 + HO plainly equals NH4O. Thus, as Potash is the Oxide of Potassium, so Ammonia is the Oxide of Ammonium.
The composition of the salts of Ammonia is on this view assimilated to those of the alkalies proper. Thus, Sulphate of Ammonia is a Sulphate of the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride of Ammonium, etc.
AMMONIO-NITRATE OF SILVER. See Silver, Ammonio-Nitrate of.
AQUA-REGIA. See Nitro-Hydrochloric Acid.
BARYTA, NITRATE OF. See Nitrate of Baryta.
BICHLORIDE OF MERCURY. See Mercury, Bichloride of.
BROMINE.
Symbol, Br. Atomic weight, 78.
This elementary substance is obtained from the uncrystallizable residuum of sea-water, termed bittern. It exists in the water in very minute proportion, combined with Magnesium in the form of a soluble Bromide of Magnesium.
Properties.—Bromine is a deep reddish-brown liquid of a disagreeable odour, and fuming strongly at common temperatures; sparingly soluble in water (1 part in 23, Löwig), but more abundantly so in Alcohol, and especially in Ether. It is very heavy, having a specific gravity of 3·0.
Bromine is closely analogous to Chlorine and Iodine in its chemical properties. It stands on the list intermediately between the two; its affinities being stronger than those of Iodine, but weaker than Chlorine (see Chlorine).
It forms a large class of salts, of which the Bromides of Potassium, Cadmium, and Silver are the most familiar to Photographers.
BROMIDE OF POTASSIUM.
Symbol, KBr. Atomic weight, 118.
Bromide of Potassium is prepared by adding Bromine to Caustic Potash, and heating the product, which is a mixture of Bromide of Potassium and Bromate of Potash, to redness, in order to drive off the Oxygen from the latter salt. It crystallizes in anhydrous cubes, like the Chloride and Iodide of Potassium; it is easily soluble in water, but more sparingly so in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric Acid.
BROMIDE OF SILVER. See Silver, Bromide of.
CARBONATE OF SODA.
Symbol, NaO CO2 + 10 Aq.
This salt was formerly obtained from the ashes of seaweeds, but is now more economically manufactured on a large scale from common salt. The Chloride of Sodium is first converted into Sulphate of Soda, and afterwards the Sulphate into Carbonate of Soda.
Properties.—The perfect crystals contain ten atoms of water, which are driven off by the application of heat, leaving a white powder—the anhydrous Carbonate. Common Washing Soda is a neutral Carbonate, contaminated to a certain extent with Chloride of Sodium and Sulphate of Soda. The Carbonate used for effervescing draughts is either a Bicarbonate with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent, of real alkali; it is therefore nearly double as strong as the washing Carbonate, which contains about 22 per cent, of Soda. Carbonate of Soda is soluble in twice its weight of water at 60°, the solution being strongly alkaline.
CARBONATE OF POTASH. See Potash, Carbonate of.
CASEINE. See Milk.
CHARCOAL, ANIMAL.
Animal Charcoal is obtained by heating animal substances, such as bones, dried blood, horns, etc., to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of Carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of Carbonate and Phosphate of Lime, the former of which produces alkalinity in reacting upon Nitrate of Silver (see p. 89). Animal Charcoal is freed from these earthy salts by repeated digestion in Hydrochloric Acid; but unless very carefully washed it is apt to retain an acid reaction, and so to liberate free Nitric Acid when added to solution of Nitrate of Silver.
Properties.—Animal Charcoal, when pure, consists, solely of Carbon, and burns away in the air without leaving any residue: it is remarkable for its property of decolorizing solutions; the organic colouring substance being separated, but not actually destroyed, as it is by Chlorine employed as a bleaching agent. This power of absorbing colouring matter is not possessed in an equal degree by all varieties of Charcoal, but is in great measure peculiar to those derived from the animal kingdom.
CHINA CLAY, OR KAOLIN.
This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It consists of the Silicate of Alumina,—that is, of Silicic Acid or Flint, which is an Oxide of Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is perfectly insoluble in water and acids, and produces no decomposition in solution of Nitrate of Silver. It is employed by Photographers to decolorize solutions of Nitrate of Silver which have become brown from the action of Albumen or other organic matters.
Commercial Kaolin may contain chalk, in which state it produces alkalinity in solution of Nitrate of Silver. The impurity, detected by its effervescence with acids, is removed by washing the Kaolin in diluted vinegar and subsequently in water.
CHLORINE.
Symbol, Cl. Atomic weight, 36.
Chlorine is a chemical element found abundantly in nature, combined with metallic Sodium in the form of Chloride of Sodium, or Sea-salt.
Preparation.—By distilling common Salt with Sulphuric Acid, Sulphate of Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine combined with Hydrogen; by the action of nascent Oxygen (see Oxygen), the Hydrogen may be removed in the form of water, and the Chlorine left alone.
Properties.—Chlorine is a greenish-yellow gas, of a pungent and suffocating odour; soluble to a considerable extent in water, the solution possessing the odour and colour of the gas. It is nearly 2½ times as heavy as a corresponding bulk of atmospheric air.
Chemical properties.—Chlorine belongs to a small natural group of elements which contains also Bromine, Iodine, and Fluorine. They are characterized by having a strong affinity for Hydrogen, and also for the metals; but are comparatively indifferent to Oxygen. Many metallic substances actually undergo combustion when projected into an atmosphere of Chlorine, the union between the two taking place with extreme violence. The characteristic bleaching properties of Chlorine gas are explained in the same manner:—Hydrogen is removed from the organic substance, and in that way the structure is broken up and the colour destroyed.
Chlorine is more powerful in its affinities than either Bromine or Iodine. The salts formed by these three elements are closely analogous in composition and often in properties. Those of the Alkalies, Alkaline Earths, and many of the Metals, are soluble in water; but the Silver salts are insoluble; the Lead salts sparingly so.
The combinations of Chlorine, Bromine, Iodine, and Fluorine, with Hydrogen, are acids, and neutralize Alkalies in the usual, manner, with formation of Alkaline Chloride and water (see page 311).
The test by which the presence of Chlorine is detected, either free or in combination with bases, is Nitrate of Silver; it gives a white curdy precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble in Ammonia. The solution of Nitrate of Silver employed as the test must not contain Iodide of Silver, as this compound is precipitated by dilution.
CHLORIDE OF AMMONIUM.
Symbol, NH4Cl. Atomic weight, 54.
This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in commerce in the form of colourless and translucent masses, which are procured by sublimation, the dry salt being volatile when strongly heated. It dissolves in an equal weight of boiling, or in three parts of cold water. It contains more Chlorine in proportion to the weight used than Chloride of Sodium, the atomic weights of the two being as 54 to 60.
CHLORIDE OF BARIUM.
Symbol, BaCl + 2 HO. Atomic weight, 123.
Barium is a metallic element very closely allied to Calcium, the elementary basis of Lime. The Chloride of Barium is commonly employed as a test for Sulphuric Acid, with which it forms an insoluble precipitate of Sulphate of Baryta. It is also said to affect the colour of the Photographic image when used in preparing Positive paper, which may possibly be due to a chemical combination of Baryta with Albumen; but it must be remembered that this Chloride, from its high atomic weight, contains less Chlorine than the alkaline Chlorides (see page 124).
Properties of Chloride of Barium.—Chloride of Barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leaving the anhydrous Chloride.
CHLORIDE OF GOLD. See Gold, Chloride of.
CHLORIDE OF SODIUM.
Symbol, NaCl. Atomic weight, 60.
Common Salt exists abundantly in nature, both in the form of solid rock-salt and dissolved in the waters of the ocean.
Properties of the pure Salt.—Fusible without decomposition at low redness, but sublimes at higher temperatures; the melted salt concretes into a hard white mass on cooling. Nearly insoluble in absolute alcohol, but dissolves in minute quantity in rectified spirit. Soluble in three parts of water, both hot and cold. Crystallizes in cubes, which are anhydrous.
Impurities of Common Salt.—Table Salt often contains large quantities of the Chlorides of Magnesium and Calcium, which, being deliquescent, produce a dampness by absorption of atmospheric moisture: Sulphate of Soda is also commonly present. The salt may be purified by repeated re-crystallization, but it is more simple to prepare the pure compound directly, by neutralizing Hydrochloric Acid with Carbonate of Soda.
CHLORIDE OF SILVER. See Silver, Chloride of.