THE ELEMENTS OF GROUPS IB AND MB 429
The formation of minute specks of silver when silver bromide
is exposed to light is known to be aided by the presence of gelatin,
which acts as a sensitiser. Very pure gelatin does not act in this way;
but ordinary gelatin contains a trace of sulphur; because of this, a
few sulphide ions, S^2 ~, are introduced into the silver bromide lattice,
which is made up of silver and bromide ions. Now a sulphide ion,
S^2 ~, must replace two bromide ions to keep the crystal electrically
neutral, but it only occupies the space of one. Hence a 'vacant
anion site', i.e. a "hole' is left in the crystal. When the crystal is
exposed to light, electrons are released from the crystal, and move
through it; and some of these, when they reach a "hole', become
"trapped'. When this happens, neighbouring silver ions unite with
these electrons, so forming a nucleus or speck containing a few
neutral silver atoms. These nuclei then grow when the silver bromide
is reduced by the developer, and form the dark patches of silver
where exposure has occurred.SILVER(I) NITRATE
Silver nitrate, the most common silver salt, is obtained by dissolving
the metal in nitric acid :
3Ag + 4HNO 3 -> 3AgNO 3 + 2H 2 O + NOT
Like all nitrates, it is soluble in water ; on heating it decomposes
evolving nitrogen dioxide and oxygen, but leaving the metal, and
not, as is usual with other nitrates, the oxide :2AgNO 3 -» 2Ag + 2NO 2 + O 2In ammoniacal solution (in which the ion [Ag(NH 3 ) 2 ] + is formed)
it is readily reduced to silver (see above) by many organic compounds.
The use of silver nitrate for marking clothes depends on its reduction
by the material to black silver. The reduction also occurs even when
the neutral solution comes in contact with the skin, and a black stain
is left. Thus solid silver nitrate rubbed on the skin leaves a black
deposit and so is used in surgery as a mild caustic— hence the old
name for silver nitrate of lunar caustic.
If ethyne is passed through an ammoniacal solution of silver
nitrate, there is a white precipitate of silver dicarbide (cf. copperf I)) :
2[Ag(NH 3 ) 2 ]+ -» Ag 2 (C=C)| + 2NH; -f 2NH 3
white
Silver nitrate is used extensively in qualitative and quantitative
analysis.