SECTION II.. _the Reduction of Salts of Silver by Developing Agents._

The Reduction of Salts of Silver by Developing Agents.

The general theory of the reduction of metallic oxides having been explained, it may be desirable to enter more minutely into the exact nature of the process as applied to the compounds of Silver.

First, the Reduction of the Oxide of Silver will be taken, as the most simple illustration; then that of Salts of Silver formed by Oxygen-acids; and lastly, of the Chloride, Iodide, and Bromide of Silver containing no Oxygen.

Reduction of Oxide of Silver.--To illustrate this conveniently, the Oxide of Silver should be in a state of solution; water dissolves Oxide of Silver very sparingly, but it is freely soluble in Ammonia, forming the liquid known as Ammonio-Nitrate of Silver. If, therefore, a little of the Ammonio-Nitrate of Silver be placed in a test-tube, and solution of Sulphate of Iron be added to it, immediately it becomes discoloured, and a deposit settles to the bottom.

This deposit is metallic Silver, produced by the reducing agent appropriating to itself the Oxygen previously combined with the metal. As metallic Silver does not dissolve in Ammonia, the liquid becomes turbid, and the metal subsides in the form of a bulky precipitate.

Reduction of the Oxyacid Salts of Silver.--The term Oxyacid includes those salts which contain the Oxide of Silver intimately combined with Oxygen-acids; as e. g. the Nitrate of Silver, the Sulphate, the Acetate of Silver, etc.

These salts, soluble in water, are reduced by developing agents in the same manner as Oxide of Silver, but more slowly. The presence of an acid united with the base is a hindrance to the process and tends to keep the oxide in solution, especially when that acid is powerful in its affinities. To illustrate the effect of the acid constituent of the salt in retarding reduction, take two test-tubes, the one containing Ammonio-Nitrate, and the other ordinary Nitrate of Silver--a single drop of solution of Sulphate of Iron added to each will indicate an evident difference in the rapidity of deposition.

The precipitate of metallic Silver obtained by the action of reducing agents upon the Nitrate, varies much in colour and in general appearance. If Gallic or Pyrogallic Acid be employed, it is a black powder; whilst the salts of Iron, and especially the same with free Nitric Acid added, produce a sparkling precipitate, resembling what is termed frosted silver. Grape Sugar and many of the essential oils, such as the Oil of Cloves, etc., separate the metal from Ammonio-Nitrate of Silver in the form of a brilliant mirror film, and are often employed in silvering glass.

Silver precipitated by Gallic or Pyrogallic Acid does not appear to be free from organic matter, and probably contains also a small proportion of Oxygen.

In remarking upon these peculiarities in the molecular condition of precipitated Silver, it should be observed that the appearance of a metal whilst in mass is no indication of its colour when in the state of fine powder. Platinum and Iron, both bright metals, and susceptible of a high polish, are dull and intensely black when in a fine state of division; Gold is of a purple or yellowish brown; Mercury a dirty grey.

Reduction of the Hydracid Salts of Silver.--By the term Hydracid is meant Salts of Silver which contain no Oxygen or Oxygen-acids, but simply elements like Chlorine or Iodine combined with Silver. These elements are characterized by forming acids with Hydrogen, which acids are hence called Hydracids. Hydrochloric Acid (HCl) is an example; so also is Hydriodic Acid (HI).

The reduction of the Hydracid Salts requires to be discussed separately, because it is evidently different from that already described; the reducing agent tending only to absorb Oxygen, which is not present in these salts. The explanation is as follows: When a Chloride of a noble metal is reduced by a developer, an atom of water, composed of Oxygen and Hydrogen, takes a part in the reaction. The Oxygen of the water passes to the developer, the Hydrogen to the Chlorine.

To illustrate this, take a solution of Chloride of Gold, and add to it a little Sulphate of Iron. A yellow deposit of metallic Gold soon forms, and the supernatant liquid is found, by testing, to be acid from free Hydrochloric Acid. The following simple diagram, in which however the number of the atoms concerned is omitted, may assist the comprehension of the change.

Compound Atom of Compound Atom Atom of Chloride of Gold. of Water. Sulphate of Iron. ]

The symbol Au represents Gold, Cl Chlorine, H Hydrogen, and O Oxygen. Observe that the molecules H and O separate from each other and pass in opposite directions: the latter unites with the Sulphate of Iron; the former meets Cl, and produces Hydrochloric Acid (HCl), whilst the atom of Gold is left alone.

Hence there is no theoretical difficulty in supposing a reduction of Iodide of Silver by a developer, if we associate with the Iodide an atom of water to furnish the Oxygen. Unless the sensitive plate however has been exposed to the light, the reduction does not readily take place; nor can it be produced under any circumstances, with or without light, when the whole of the free Nitrate of Silver has been washed away from the plate. Pure Iodide of Silver is therefore unaffected by a developer, and the compound which blackens on the application of Sulphate of Iron or Pyrogallic acid is an Iodide with excess of Nitrate of Silver.

Compound Atom of Compound Atom of Atom of Iodide of Silver. Nitrate of Silver. Sulphate of Iron. ]

The mode in which a Salt of Silver, such as the Nitrate, soluble in water, may act in facilitating the reduction of Iodide of Silver, is shown in the preceding diagram, which corresponds closely with the last.

Notice that the compound atom of Nitrate of Silver contains a molecule of Oxygen for the developer, one of Silver (Ag) for the separated Iodine, and an atom of Nitric Acid (NO{5}), which is liberated, and takes no further part in the change.

The chain of chemical affinities is more complete in this diagram than in the last, where an atom of water only was present, the affinity of Iodine for Silver being greater than that of Iodine for Hydrogen. Hence it is possible that an excess of Nitrate of Silver may, by furnishing an elementary basis for which Iodine has an attraction, assist in drawing off that element, so to speak, from the original particle of Iodide of Silver touched by light.

The reader must not suppose from the remarks which have been made in this Section that images obtained by development consist invariably of pure metallic Silver. It can be shown that such is not the case,--that the process of reduction is in many cases suspended when a part only of the Oxygen has been removed; and hence results a subsalt similar to that produced by the direct action of light upon organic compounds of Silver, and differing in properties from metallic Silver. For further particulars see the Author's Photographic researches in the eighth Chapter.