graphical record produced in *gas
chromatography.chromatography A technique for
analysing or separating mixtures of
gases, liquids, or dissolved sub-
stances. The original technique (in-
vented by the Russian botanist
Mikhail Tsvet in 1906) is a good ex-
ample of column chromatography. A
vertical glass tube is packed with an
adsorbing material, such as alumina.
The sample is poured into the col-
umn and continuously washed
through with a solvent (a process
known as elution). Different compo-
nents of the sample are adsorbed to
different extents and move down the
column at different rates. In Tsvet’s
original application, plant pigments
were used and these separated into
coloured bands in passing down the
column (hence the name chromatog-
raphy). The usual method is to collect
the liquid (the eluate) as it passes out
from the column in fractions.
In general, all types of chromatog-
raphy involve two distinct phases –
the stationary phase (the adsorbent
material in the column in the exam-
ple above) and the moving phase (the
solution in the example). The separa-
tion depends on competition for mol-
ecules of sample between the
moving phase and the stationary
phase. The form of column chro-
matography above is an example of
adsorption chromatography, in
which the sample molecules are ad-
sorbed on the alumina. In partition
chromatography, a liquid (e.g. water)
isÜrst absorbed by the stationary
phase and the moving phase is an
immiscible liquid. The separation is
then by *partition between the two
liquids. In ion-exchange chromatog-
raphy (see ion exchange), the process
involves competition between differ-
ent ions for ionic sites on the station-
ary phase. *GelÜltration is anotherchromatographic technique in which
the size of the sample molecules is
important.
See also affinity chromatog-
raphy; gas chromatography; high-
performance liquid chromatogra-
phy; paper chromatography; rf
value; thin-layer chromatography.chrome alum See potassium
chromium sulphate.chrome iron ore A mixed
iron–chromium oxide, FeO.Cr 2 O 3 ,
used to make ferrochromium for
chromium steels.chrome red A basic lead chromate,
PbO.PbCrO 4 , used as a red pigment.chrome yellow Lead chromate,
PbCrO 4 , used as a pigment.chromic acid A hypothetical acid,
H 2 CrO 4 , known only in chromate
salts.chromic anhydride See chro-
mium(vi) oxide.chromic compounds Compounds
containing chromium in a higher (+3
or +6) oxidation state; e.g. chromic
oxide is chromium(VI) oxide (CrO 3 ).chromite A spinel mineral,
FeCr 2 O 4 ; the principal ore of chro-
mium. It is black with a metallic
lustre and usually occurs in massive
form. It is a common constituent of
peridotites and serpentines. The
chief producing countries are
Turkey, South Africa, Russia, the
Philippines, and Zimbabwe.
chromium Symbol Cr. A hard sil-
very *transition element; a.n. 24;
r.a.m. 52.00; r.d. 7.19; m.p. 1857°C;
b.p. 2672°C. The main ore is
chromite (FeCr 2 O 4 ). The metal has a
body-centred-cubic structure. It is ex-
tracted by heating chromite with
sodium chromate, from which
chromium can be obtained by elec-
trolysis. Alternatively, chromite canchromatography 124c