Modern inorganic chemistry

428 THE ELEMENTS OF GROUPS IB AND IIB

Silver chloride is readily soluble in ammonia, the bromide less

readily and the iodide only slightly, forming the complex cation

[Ag(NH 3 ) 2 ]+. These halides also dissolve in potassium cyanide,

forming the linear complex anion [Ag(CN) 2 ]~ and in sodium

thiosulphate forming another complex anion, [Ag(S 2 O 3 ) 2 ]^3 ~.

All the silver halides are sensitive to light, decomposing eventually

to silver. In sunlight, silver chloride turns first violet and finally

black. The use of these compounds in photography depends on this

(see below). (All silver salts are, in fact photosensitive—the neck

of a silver nitrate bottle is black owing to a deposit of silver.)

Silver chloride is reduced to the metal by zinc. One of the methods

of recovering silver from "silver residues' depends on this. The

residue is first treated with concentrated hydrochloric acid and then

sulphuric acid and zinc added:

2AgCl + Zn -> 2Ag + 2C1" + Zn^2 +

Photography

It was known in the sixteenth century that silver salts were photo-
sensitive, but it was not until the beginning of the nineteenth
century, when Herschel found that silver chloride was soluble in
sodium thiosulphate, that photography became possible.
The plate or film of celluloid is coated with a colloidal gelatinised
solution when the unchanged bromide is dissolved to form a
chloride because of its greater sensitivity). During photographic
exposure, decomposition of the bromide occurs to form minute
particles of silver. These particles are too small to be seen by the
naked eye and are only detectable with the electron-microscope.
The number of such nuclei of decomposition in a given area of
plate or film depends on the intensity of light falling on the area.
When the film is developed (the developer being a reducing agent),
the unchanged silver bromide immediately surrounding these nuclei
is reduced to give a visible blackening of the film.
The film is now fixed by washing in sodium thiosulphate ('hypo')
solution when the unchanged bromide is dissolved to form the
complex ion

AgBr + 2S 2 Or - [Ag(S 203 ) 2 ]^3 - + Br~
The fixed plate is now a ^negative', for those patches on which
most light fell are black. The process is reversed in printing to
make the 'positive'—the printing paper having a covering of silver
chloride or bromide or a mixture of the two. This, in turn, is developed
and fixed as was the plate or film.