SECTION II.. _the Chemistry of the Nitrate Bath._

The Chemistry of the Nitrate Bath.

The solution of Nitrate of Silver in which the plate coated with iodized Collodion is dipped, to form the layer of Iodide of Silver, is known technically as the Nitrate Bath. The chemistry of Nitrate of Silver has been explained at page 13, but there are some points relating to the properties of its aqueous solution which require a further notice.

Solubility of Iodide of Silver in the Nitrate Bath.--Aqueous solution of Nitrate of Silver may be mentioned in the list of solvents of Iodide of Silver. The proportion dissolved is in all cases small, but it increases with the strength of the solution. If no attention were paid to this point, and the precaution of previously saturating the Nitrate Bath with Iodide of Silver neglected, the film would be dissolved when left too long in the liquid.

This solvent power of Nitrate of Silver on the Iodide is well shown by taking the excited Collodion plate out of the Bath, and allowing it to dry spontaneously. The layer of Nitrate on the surface, becoming concentrated by evaporation, eats away the film, so as to produce a transparent, spotted appearance.

In the solution of Iodide of Silver by Nitrate of Silver a double salt is formed, which corresponds in properties to the double Iodide of Potassium and Silver in being decomposed by the addition of water. Consequently, in order to saturate a Bath with Iodide of Silver it is only necessary to dissolve the total weight of Nitrate of Silver in a small bulk of water, and to add to it a few grains of an Iodide; perfect solution takes place, and on subsequent dilution with the full amount of water, the excess of Iodide of Silver is precipitated in the form of a milky deposit.

Acid condition of Nitrate of Silver.--A solution of pure Nitrate of Silver is neutral to blue litmus-paper, but that prepared from the commercial Nitrate has usually an acid reaction; the crystals having been imperfectly drained from the acid mother-liquor in which they were formed. Hence, in making a new Bath it is often advisable not only to saturate it with Iodide of Silver, but to neutralize the free Nitric acid it contains.

There is also a peculiar condition of Nitrate of Silver crystallized from a solution of the metal in Nitric Acid, which renders it quite unfit for photographic purposes (see p. 101). It is thought to depend upon the presence of an oxide of Nitrogen, possibly of Nitrous Acid, and the remedy is to dry the crystals very strongly, or, better still, to fuse them at a moderate heat: mere neutralization with Carbonate of Soda does not suffice.

In melting Nitrate of Silver great care should be taken not to raise the heat so high as to decompose the salt, or a basic Nitrite will be formed, which affects the properties of the solution (p. 13): fused Nitrate of Silver ought, when cold, to be quite white, and to dissolve perfectly in water without leaving any residue. The only objection to the employment of Nitrate of Silver in this form is the facility with which it may be adulterated with Nitrates of Potash and Soda, the presence of which would lessen the available strength of the Bath.

The Nitrate Bath, although perfectly neutral when first prepared, may become acid by continued use, if Collodion containing much free Iodine be constantly employed. In that case a portion of Nitric Acid is liberated, thus:--

Nitrate of Silver + Iodine = Iodide of Silver + Nitric Acid + Oxygen.

When Collodion is iodized entirely with alkaline Iodides, it liberates Iodine by keeping; and hence the occasional use of Ammonia may be required to remove acidity from the Bath. But since the introduction of the Iodide of Cadmium, which preserves the Collodion nearly or quite colourless, the necessity for neutralizing Nitric Acid in the Bath has ceased.

Alkaline condition of the Bath.--By "alkalinity" of the Bath is meant a condition in which the blue tint is rapidly restored to reddened litmus-paper. This indicates that an Oxide of some kind is present in solution, which, by combining with the acid in the reddened paper, neutralizes it and removes the red colour.

If a small portion of caustic Potash or Ammonia be added to a strong solution of Nitrate of Silver, it produces a brown precipitate, which is Oxide of Silver.

Ammonia + Nitrate of Silver = Oxide of Silver + Nitrate Ammonia.

The solution however, from which the precipitate has separated, is not left in a neutral state, but possesses a faint alkaline reaction. Oxide of Silver and Carbonate of Silver are also abundantly soluble in water containing Nitrate of Ammonia; which salt is continually accumulating in the Bath when compounds of Ammonium are used for iodizing.

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.

Hyposulphites, Cyanides, and all fixing agents decompose Nitrate of Silver.

Organic matters, generally, reduce Nitrate of Silver, either with or without the aid of light. Grape Sugar, Albumen, Serum of Milk containing caseine, etc., blacken the Bath, even in the dark. Alcohol and Ether act more slowly, and produce no injurious effect unless the liquid is constantly exposed to light.

These facts indicate that the Nitrate Bath containing volatile organic matters must be preserved in a dark place; also that it should be kept exclusively for sensitizing the Collodion plates, and not used in floating papers intended for the printing process.

Changes in the Nitrate Bath by use.--The solution of Nitrate of Silver employed in exciting the Collodion film gradually decreases in strength, but not so quickly as the Bath used in sensitizing papers for printing. If the amount of Nitrate be allowed to fall as low as twenty grains to the ounce of water, the decomposition will be imperfect, and the film will be pale and blue, even with a highly iodized Collodion.

A gradual accumulation of Ether and Alcohol also takes place in the Bath after long use, in consequence of which the developing solutions flow less readily upon the Collodionized plates, and oily stains are apt to be produced.

Diminished sensitiveness of the Iodide film is sometimes traced to impurities in the Bath, when it is very old, and has been much used. These are probably of an organic nature and may often be partially removed by agitation with kaolin, or animal charcoal. The latter however is objectionable, being usually contaminated with Carbonate of Lime, which makes the Bath alkaline; or (in the case of purified animal charcoal) with traces of Hydrochloric Acid, which liberate Nitric Acid in the Bath. Even the kaolin may as a preliminary precaution be washed with dilute Acetic Acid to remove Carbonate of Lime if any should be present.