This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Dried - Na2CO3.
White dry powder containing about 80% sodium carbonate. Used the same as sodium carbonate crystals.
Anhydrous - Na2CO3.
White powder; being from 99 to 100% pure sodium carbonate. This is the purest and best form of sodium carbonate in photographic use.
(See Sodium Hydrate.)
Chloride of Sodium, Common Salt. White, cubical crystals or crystalline powder. Soluble in 2.7 parts water, insoluble in alcohol. The solution is neutral to litmus paper. Used in some toning baths to assist in giving uniform tones to the prints. Also employed in the salting of albumen paper, etc.
NaC6H7O7 + H2O.
White crystals or granular powder. Soluble in 1.1 parts water, slightly soluble in alcohol. Used as a restrainer.
(See Sodium Bichromate.)
NaOH + H2O - II, 420. Caustic Soda, Sodium Hydroxide. White, deliquescent flakes, plates or sticks; acrid caustic taste. Soluble in 1.8 parts boiling water. Keep in well-stoppered bottles. Used sometimes as an alkaline in developing solutions.
(See Sodium Hydrate.)
Na2S2O3 + 5H2O - II; IV; V. Hyposulphite of Soda, " Hypo," Sodium Thiosulphate. White, transparent crystals. Cooling, with bitter after-taste. Soluble in 0.65 parts water; insoluble in alcohol. Permanent in air at ordinary temperatures, but liquefies at 122o Fahr. in its own water of crystallization. For this reason it should be kept in a cool place. Use. The principal use of sodium hyposulphite in photography is as a fixing agent, because it readily dissolves the haloid silver salts. For PLATES a solution of I part sodium hyposulphite in 4 parts water is recommended, while for DEVELOPING PAPERS 1 part sodium hyposulphite in 16 parts water should be employed. An acid bath, however, is to be recommended for the fixing of developing papers, as such a bath hardens the film on the paper and tends to do away with yellow stains, which sometimes will occur in the case of a plain hypo bath if the developer is not rinsed off the print sufficiently before placing it in the fixing bath. For PRINTING-OUT PAPERS the hypo bath should test from 15o to 19o hydrometer. When prints are placed in the hypo bath for fixing they should be constantly moved, so as to insure thorough fixation. Then as thoroughly remove the unused silver salts. When a silver print is fixed in sodium hyposulphite two salts are formed. Ex. The silver print contains silver chloride, and the sodium hyposulphite dissolves this, forming silver sodium hyposulphite and sodium chloride, according to the following formula:
AgCl + Na2S2O3 == AgNaS2O3 + NaCl
The double salt of silver hyposulphite and sodium is practically insoluble in water, but is soluble in sodium hyposulphite. For this reason it is important that the plates and papers remain in the fixing bath double the length of time required to visibly remove the unused silver salts. Upon this rests the permanency of the image. The plates should be thoroughly washed in many changes of water, or in running water, for at least one hour, in order to thoroughly remove all chemicals. Hypo eliminators have been recommended for the purpose of removing the excess of soluble salts, but washing in water is the best hypo eliminator. If sulphuric acid is added to a solution of sodium hyposulphite, sulphur will be liberated from the hypo. For this reason the fixing bath for papers should be alkaline; otherwise sulphur stains giving yellowish whites will result.