348 GROUP VII: THE HALOGENS
whilst calcium and potassium bromates(V) are used in the malting
industry to suppress root formation after germination of barley.
Bromine is sometimes used in place of chlorine for sterilising water.IODINE
Iodine as such finds few uses but a solution in alcohol and water,
also containing potassium iodide ('tincture of iodine^5 ) was com-
monly used as an antiseptic for cuts and wounds, but had rather an
irritant action. lodoform (triiodomethane), CHI 3 , is also an anti-
septic, but newer compounds of iodine are now in use. Silver iodide,
like silver bromide, is extensively used in the photographic industry.TESTS FOR HALIDES
TESTS FOR FLUORIDE
Most fluorine-containing compounds can be reduced to the fluoride
ion, F~, which can be detected by the tests given below.- The action of concentrated sulphuric acid liberates hydrogen
fluoride, which attacks glass, forming silicon tetrafluoride; the latter
is hydrolysed to "silicic acid' by water, which therefore becomes
turbid. - Addition of calcium nitrate solution to a fluoride gives a white
precipitate of calcium fluoride, CaF 2. If the latter is precipitated
slowly, it can be filtered off and weighed to estimate the fluoride.
Fluoride can also be determined by the addition of sodium chloride
and lead nitrate which precipitate lead chlorofluoride, PbClF. This
is filtered off and weighed.
TESTS FOR CHLORIDE
Most chlorine-containing compounds can be converted to give
chloride ions, for example covalent chlorides by hydrolysis, chlorates
by reduction. The chloride ion is then tested for thus:
- Addition of silver nitrate to a solution of a chloride in dilute
nitric acid gives a white precipitate of silver chloride, AgCl, soluble
in ammonia solution. This test may be used for gravimetric or
volumetric estimation of chloride; the silver chloride can be filtered
off, dried and weighed, or the chloride titrated with standard silver
nitrate using potassium chromate(VI) or fluorescein as indicator.