An alkali which was often used with pyrogallol in the early days of photography, but which is rarely used nowadays, is ammonia. Nitrogen combines with three times its volume of hydrogen to form a gas, NH3. This gas is known as ammonia and is very soluble in water, its solution being strongly alkaline. Ammonia combines directly with acids to form salts which are similar to the salts of sodium and potassium. Thus with hydrochloric acid it forms ammonium chloride, which is similar to sodium chloride and potassium chloride:

NH3 + H Cl = NH4 Cl Ammonia Hydrochloric Ammonium Acid Chloride.

Ammonia is a somewhat weaker alkali than soda or potash but stronger than the carbonates. For use in development it has the disadvantage that being used in the form of a solution of a gas its strength is somewhat uncertain and variable, the ammonia escaping from the solution. Also, it is a solvent of silver bromide and tends to produce colored fogs which are not so easily produced with other alkalis.

Eastman Portrait Film Negative, Artura Print By Pasquale S. Culotta Baltimore, Md.

Eastman Portrait Film Negative, Artura Print By Pasquale S. Culotta Baltimore, Md.

Ammonia Solution is commercially prepared from the ammoniacal liquor obtained in the distillation of coal for coal gas. The liquor is neutralized with sulphuric acid, the ammonium sulphate crystallized out, and the ammonia gas liberated from the sulphate with lime and led into water, in which it dissolves. The solution is usually free from impurities.

Ammonia solutions are prepared commercially in two strengths, "ammonia water," containing 10% of ammonia gas by weight and having a specific gravity of .96, and "stronger ammonia water" containing 28 % of ammonia by weight and having a specific gravity of .90.

The alkalis generally used for photographic work are not the caustic alkalis but the carbonates, which are salts of carbonic acid, H2CO3. Carbonic acid is a very weak acid, so that in solution the carbonates are not neutral but alkaline because of the predominance of the strong base over the weak acid, the carbonate being, to some extent, split up into the bicarbonates or acid carbonates and the caustic alkali. The use of a carbonate in development therefore represents a sort of reservoir of alkali, only a small amount of alkali being present at any time, but more being generated by dissociation of the carbonate as it is used up. If instead of using carbonate we were to use for development a solution containing a proportional amount of caustic alkali, we should have only a small amount of alkali present, and it would soon be exhausted. The use of carbonate therefore enables us to employ a small concentration of alkali and yet to keep that concentration nearly constant during use.

When a salt is dissolved in water at a high temperature until no more will dissolve and then the solution is allowed to cool, the salt will generally be deposited in crystals; sometimes, as in the case of silver nitrate, the crystals consist of the pure substance, but more often each part of the salt combines with one or more parts of water to form the crystals. This combined water is called "water of crystallization." Thus, crystals of sodium carbonate formed from a cool solution contain ten parts of water to one of carbonate, and their combination should be written Na2CO3 • 10 H2O

What is called in the last paragraph a "part" of sodium carbonate, Na2CO3, will weigh 106 units, while a "part" of water, H2O, weighs 18 units, so that the crystals of sodium carbonate contain 106 parts by weight of sodium carbonate and 180 parts by weight of water, and consequently crystallized sodium carbonate contains only 37% of dry sodium carbonate. If sodium carbonate is crystallized from a hot solution only one part of water is combined in the crystals with each part of sodium carbonate so that they have the composition Na2CO3 • H2O and contain 85% of dry carbonate. Sodium carbonate containing ten parts of water!of crystallization loses nine of them by drying in the air and breaks up, forming the compound with one part of water. This last part of water is only removed with difficulty by heating in the air, when the dry carbonate is formed, containing only a small residual amount of water and about 90% carbonate.

When exposed to the air chemicals often either absorb or give up water. Those which absorb water are said to be "hygroscopic," and if they absorb so much that they dissolve and form a solution they are said to be "deliquescent." Chemicals which give up water to the air, so that the crystals break down and become covered with powder, are called "efflorescent."

Sodium Carbonate comes on the market in three forms:

Crystals with ten parts of water, Na2CO3 • 10 H2O containing 37% of the carbonate; Crystals with one part of water, Na2CO3 • H2O, containing 85 % of the carbonate; Dry powder containing 98% of the carbonate.

The carbonate is made by treatment of salt solution with ammonia and carbon dioxide which reacts with the salt to produce sodium bicarbonate, NaH CO3. The bicarbonate is heated and half of the carbonic acid is driven off, producing crude sodium carbonate, which at this stage is known as "soda ash." This is then dissolved in water and crystals of "sal soda," containing ten parts of water, are produced. From this a crystalline salt with either ten or one parts of water is prepared for photographic use, but owing to the uncertainty of the composition of these crystals it is better to prepare the pure dry carbonate. This is obtained by heating the pure bicarbonate which can be precipitated from a solution of sal soda by means of carbon dioxide gas. When the bicarbonate is heated in the air, half of the carbonic acid is driven off, and sodium carbonate, Na2CO3, is produced according to the equation: