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EFFECT OF WATER SUPPLY IN PROCESSING 153
So far as is known, any small traces of impurities left in the gelatin coating of motion picture negative or positive film after drying, by virtue of the presence of these impurities in the wash water, are not liable to seriously impair the keeping properties of the films over a period of four or five years. However, films which are to be kept for long periods of time should be finally washed in distilled water.
The Preparation of Dye Solutions
Many dyes are precipitated out of solution by calcium or magnesium salts and alum. The precipitation is not always immediate and may occur only after standing for a few days. The properties of dyes with respect to their rate of penetration into gelatin or the rate at which they are mordanted are affected considerably by the presence of metallic ions, or acids, or bases, so that in color photography or when using desensitizers impurities in the water are apt to produce anomalous results. Distilled water should be used whenever possible for preparing solutions of dyes.
Method of Purification of Water
Distillation. Distilled water should be used whenever possible for mixing solutions.
Boiling. Unless the water contains an excessive quantity of dissolved salts, it is sufficient usually to boil the water and allow it to settle. The supernatant portion then may be syphoned off or the solution filtered through fine muslin. Most colloidal vegetable and animal matter, comprising slimes and scums, coagulates on boiling and certain lime salts are changed to an insoluble condition and settle out. Dissolved extracts are not removed but dissolved gases are driven off by boiling.
Filtration. Various types of water filters are available commercially, but these do not remove dissolved salts or colloidal matter unless the water has been treated previously with a coagulant.
Chemical Treatment. The following methods of chemical purification may be adopted:
1. Potassium alum may be added in the proportion of 1 gram to 4 liters of water. This coagulates the slime which carries down any suspended particles and clears the solution rapidly. Dissolved salts are not removed by this method. The small percentage of alum introduced into the water has no harmful effect on the solution when subsequently used for mixing developers and fixing baths.
2. A solution of sodium oxalate may be added until no further precipitate forms. This method removes the calcium and magnesium salts and coagulates the slime though other dissolved salts are left in solution. Solutions of sodium phosphate and of sodium sulfite also may be used to precipitate calcium and magnesium.
3. Most of the commercial methods of water softening may be employed although such methods do not remove sodium and potassium salts. One of the most satisfactory consists in passing the water through a tank containing sodium aluminum silicate which is zeolite, and possesses the power of exchanging its sodium for the calcium and magnesium present in the water,