graphical record produced in *gas
chromatography.
chromatography A technique for
analysing or separating mixtures of
gases, liquids, or dissolved substances. The original technique (invented by the Russian botanist
Mikhail Tsvet in 1906) is a good example of column chromatography. A
vertical glass tube is packed with an
adsorbing material, such as alumina.
The sample is poured into the column and continuously washed
through with a solvent (a process
known as elution). Different components 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 chromatography). The usual method is to collect
the liquid (the eluate) as it passes out
from the column in fractions.
In general, all types of chromatography involve two distinct phases –
the stationary phase (the adsorbent
material in the column in the example above) and the moving phase (the
solution in the example). The separation depends on competition for molecules of sample between the
moving phase and the stationary
phase. The form of column chromatography above is an example of
adsorption chromatography, in
which the sample molecules are adsorbed on the alumina. In partition
chromatography, a liquid (e.g. water)
is Ürst 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 chromatography (see ion exchange), the process
involves competition between different ions for ionic sites on the stationary phase. *Gel Ültration is another
chromatographic technique in which
the size of the sample molecules is
important.
See also affinity chromatography; gas chromatography; highperformance liquid chromatography; paper chromatography; r f
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 chromium(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 chromium. 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 silvery *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 extracted by heating chromite with
sodium chromate, from which
chromium can be obtained by electrolysis. Alternatively, chromite can
chromatography
124
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chromatography.
chromatography A technique for
analysing or separating mixtures of
gases, liquids, or dissolved substances. The original technique (invented by the Russian botanist
Mikhail Tsvet in 1906) is a good example of column chromatography. A
vertical glass tube is packed with an
adsorbing material, such as alumina.
The sample is poured into the column and continuously washed
through with a solvent (a process
known as elution). Different components 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 chromatography). The usual method is to collect
the liquid (the eluate) as it passes out
from the column in fractions.
In general, all types of chromatography involve two distinct phases –
the stationary phase (the adsorbent
material in the column in the example above) and the moving phase (the
solution in the example). The separation depends on competition for molecules of sample between the
moving phase and the stationary
phase. The form of column chromatography above is an example of
adsorption chromatography, in
which the sample molecules are adsorbed on the alumina. In partition
chromatography, a liquid (e.g. water)
is Ürst 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 chromatography (see ion exchange), the process
involves competition between different ions for ionic sites on the stationary phase. *Gel Ültration is another
chromatographic technique in which
the size of the sample molecules is
important.
See also affinity chromatography; gas chromatography; highperformance liquid chromatography; paper chromatography; r f
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 chromium(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 chromium. 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 silvery *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 extracted by heating chromite with
sodium chromate, from which
chromium can be obtained by electrolysis. Alternatively, chromite can
chromatography
124
c
www.AzShimi.ir www.AzShimi.com