An alkaline Bath is perhaps of all conditions the one most fatal to success in photography. It leads to that universal darkening of the film on applying the developer to which the name of "fogging" has been given. Hence care must be used in adding to the Bath substances which tend to make it alkaline.
Collodion containing free Ammonia, often sold in the shops, gradually does so. The use of Potash, Carbonate of Soda, Chalk, or Marble, to remove free Nitric Acid from the Bath, has the same effect; and hence, when they are employed, a trace of Acetic acid must afterwards be added.
The mode of testing a bath for alkalinity is as follows:— a strip of porous blue litmus-paper is taken and held to the mouth of a bottle of glacial Acetic acid until it becomes reddened; it is then placed in the liquid to be examined and left for ten minutes or a quarter of an hour. If Oxide of Silver be present in solution, the original blue colour of the paper will slowly but gradually be restored.
Occasional formation of Acetate of Silver in the Nitrate Bath.—In preparing a new Bath, if the crystals of Nitrate of Silver are acid, it is usual to add an alkali in small quantity. This removes the Nitric Acid, but leaves the solution faintly alkaline. Acetic Acid is then dropped in, which, by combining with the Oxide of Silver, forms Acetate of Silver.
Acetate of Silver is not formed by the simple addition of Acetic Acid to the Bath, because its production under such circumstances would imply the liberation of Nitric Acid; but if an alkali be present to neutralize the Nitric Acid, then the double decomposition takes place, thus—
| Acetate of Ammonia | + | Nitrate of Silver | |
| = | Acetate of Silver | + | Nitrate of Ammonia. |
Acetate of Silver is a white flaky salt, sparingly soluble in water. It dissolves in the Bath only in small proportion, but yet sufficiently to affect the Photographic properties of the film (see p. [111] and [117]). The observance of the following simple rules will obviate its production in injurious quantity:—First, when it is required to remove free Nitric Acid from a bath not containing Acetic Acid a solution of Potash or Carbonate of Soda may be dropped in freely; but the liquid must be filtered before adding any Acetic Acid, otherwise the brown deposit of Oxide of Silver will be taken up by the Acetic Acid, and the Bath will be charged with Acetate of Silver. Secondly, in dealing with a Bath containing both Nitric and Acetic Acids, employ an alkali much diluted (Liquor Ammoniæ with 10 parts of Water), and add a single drop at a time, coating and trying a plate between each addition; the Nitric Acid will neutralize itself before the Acetic, and with care there will be no formation of Acetate of Silver in quantity.
Substances which decompose the Nitrate Bath.—Most of the common metals, having superior affinity for Oxygen, separate the Silver from a solution of the Nitrate; hence the Bath must be kept in glass, porcelain, or gutta-percha, and contact with Iron, Copper, Mercury, etc., must be avoided, or the liquid will be discoloured, and a black deposit of metallic Silver precipitated.
All developing agents, such as Gallic and Pyrogallic Acids, the Protosalts of Iron, etc., blacken the Nitrate Bath, and render it useless by reducing metallic Silver.
Chlorides, Iodides, and Bromides produce a deposit in the Bath; but the solution, although weakened, may again be used after passing through a filter.