400 THE TRANSITION ELEMENTS
depends o^ nou. for example in a motor car. However, if iron is in
coii tact with a less reactive metal the iron corrodes. This problem
is encountered when a ktin can' is scratched. If it is necessary to join
iron to a less reactive metal, to prevent corrosion of the iron, a
sacrificial anode must be added. Thus, for example, large pieces of
magnesium are bolted to ships to prevent corrosion of the iron
propeller shaft which is bolted to a brass propeller.
Oxygen
(air)
Figure 13.7. Corrosion oj iron in contact with zinc and a drop of water
Rusting can be prevented by painting or coating with a con-
tinuous layer of another metal which does not itself corrode rapidly,
for example zinc or tin. More recently, steel has been coated with
plastics by electrophonetic decomposition from an emulsion of the
plastic.
TESTS FOR IRON
Reagent
Ammonia or sodium
hydroxide (hydroxyl
ions)
Potassium hexacyano-
ferrate(II). K 4 Fe(CN) 6
Potassium hexacyano-
ferrate(III), K 3 FefCN) 6
Potassium thiocyanate,
KCNS
Green precipitate.
turns brown on
exposure to air
White precipitate,
rapidly turning blue
Dark blue precipitate
(Turnbull's blue)
No coloration*
IrondH]
Red-brown precipitate
Prussian blue precipitate
Reddish-brown colora-
tion (no precipitate)
Blood red coloration
- This test is extremely sensitive and usually sufficient feme ions are present in an
iron(II) salt to give some coloration. The blood red colour appears to be due to a
complex.
