2NaHCO3 = Na2CO3 + CO2 + H2O

Sodium Bicarbonate = Sodium Carbonate + Carbon Dioxide + Water.

The exact amount of heating is very important. If it is not done for sufficient time there will be a large amount of bicarbonate left in the product, and bicarbonate is practically useless as an alkali in photography. On the other hand, if heating is continued too long, caustic soda will be produced. In the preparation of photographic carbonate the heating should be continued so that the material is almost pure sodium carbonate containing practically no bicarbonate but is very slightly on the alkaline side. Much caustic soda would be fatal, but it is better to have a trace of caustic soda than bicarbonate. The preparation of carbonate of soda is a matter to which the greatest attention is given by the Eastman Kodak Company, and the Kodak Tested Carbonate is specially prepared to meet the needs of the photographer.

Potassium Carbonate, also known as Salts of Tartar, is sometimes used instead of sodium carbonate as a component of developers. It has the advantage of greater solubility and is a stronger alkali than the sodium compound. It is prepared in a manner similar to sodium carbonate as a dry powder, but it absorbs water very readily and must therefore be kept in well sealed bottles.

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

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

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

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

Owing to the fact that developers are necessarily substances which have a great affinity for oxygen and that the air contains oxygen, developing solutions containing only the developing agent and alkali would be rapidly spoiled from oxidation by the air. In order to make the developer keep there is added to the developing solution, in addition to the reducing agent, an alkali and some sulphite of soda. Sulphite of soda has a very strong affinity for oxygen, being easily oxidized to sulphate of soda so that it protects the developer from the oxygen of the air, thus acting as a "preservative." This action of the sulphite is very easily seen with the pyro developer. The oxidation product of pyrogallol is yellow, and this oxidation product which is formed in development is deposited in the film along with the silver, so that if we use a pyro developer without sulphite we shall get a very yellow negative, the image consisting partly of silver and partly of the oxidized pyrogallol. If we use sulphite in the developer, the image will be much less yellow because the pyrogallol will be prevented from oxidizing, the sulphite being oxidized instead, and finally if we add a great deal of sulphite, we shall get almost as blue an image as with Elon, the oxidation product of which is not colored.

Sodium Sulphite is prepared by blowing sulphur dioxide gas into a solution of carbonate of soda. When sulphite is crystallized from the cooled solution it forms crystals containing seven parts of water to one of sulphite, of the composition Na2SO3 • 7H2O which contain, when pure, 50% of dry sulphite. These crystals give up water when kept in the air and form a white powder on the surface. Since sulphite when exposed to the air has a tendency to oxidize to the sulphate and as the sulphate is not a preservative, it is well to view with suspicion sulphite which has effloresced to a great extent. A quick rinse in cold water will remove the white powder from the crystals.

Sulphite free from water is produced by two methods; by drying the crystals, which produces what is called the "desiccated" salt, containing about 95% of pure sulphite, and by precipitation from hot solutions which gives a compound generally called "anhydrous" sulphite, and which contains as much as 98% of sulphite.

Eastman Tested Sulphite is the desiccated salt, and is prepared in a very pure state, almost free from sulphate. If prepared in this way as a dry powder the sulphite will keep well for a long time.

Sodium forms a number of compounds with sulphurous acid in addition to sodium sulphite itself. Thus we have sodium acid sulphite or bisulphite, NaHSO3, which maybe regarded as a compound of sodium sulphite with sulphurous acid:

Na,SO3 + H2SO3 = 2NaHSO3 Sodium Sulphurous Bisulphite

Sulphite Acid of Soda

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

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

Again we have sodium metabi-sulphite, which is a compound of sodium sulphite with sulphur dioxide:

Na2SO3 + SO2 = Na2S2O8 Sodium Sulphur Sodium

Sulphite Dioxide Metabisulphite

These acid sulphites are very similar in their properties and probably form the same solution when dissolved in water.

Potassium Metabisulphite is often used as a preservative. It forms good crystals and is convenient in use, but is very costly in comparison with sodium bisulphite.

Sodium Metabisulphite has been introduced to take the place of the potassium salt owing to the scarcity of potash.

Sodium Bisulphite, when pure, is a white salt which has acid reaction, often containing a slight excess of sulphur dioxide. Since sodium sulphite is an alkaline salt owing to the predominance of the strong base, soda, over the weak sulphurous acid, a neutral solution can be produced by adding a small amount of bisulphite to sulphite, and this neutral sulphite solution has found extensive application as a preservative for a pyro-soda developer. Bisulphite is used very largely as a preservative for fixing baths, supplying both the sulphite and the acid necessary.

It is difficult to prepare bisulphite free from iron, and any iron in the bisulphite produces a dark color when used for making up a pyro solution.

Recently, the Eastman Kodak Company have succeeded in obtaining an entirely satisfactory bisulphite and have listed it among their Tested Chemicals.