a cofactor or prosthetic group to
form the haemoproteins. These are
haemoglobin, myoglobin, and the
cytochromes. Essentially, haem
comprises a porphyrin with its four
nitrogen atoms holding the iron(II)
atom as a chelate. This iron can re-
versibly bind oxygen (as in haemo-
globin and myoglobin) or (as in the
cytochromes) conduct electrons by
conversion between the iron(II) and
iron(III) series.
haematin test (Teichmann test) A
test for blood using the presence of
characteristic haematin crystals. It
was introduced in 1853 by Ludwig
Teichmann.
haematite A mineral form of
iron(III) oxide, Fe 2 O 3. It is the most
important ore of iron and usually oc-
curs in two main forms: as a massive
red kidney-shaped ore (kidney ore)
and as grey to black metallic crystals
known as specular iron ore.
Haematite is the major red colour-
ing agent in rocks; the largest de-
posits are of sedimentary origin. In
industry haematite is also used as a
polishing agent (jeweller’s rouge) and
in paints.
haemochromogen test
(Takayama test) A test used to
conÜrm the presence of blood. It
is a microcrystal test exploiting
the characteristic appearance of
haemochromogen crystals observed
under a microscope. The rest was in-
troduced in Japan in 1912 by Masao
Takayama.
haemoerythrinA red iron-contain-
ing respiratory pigment that occurs
in the blood of annelids and some
other invertebrates. Its structure is
essentially the same as that of
*haemoglobin except the prosthetic
group has a different chemical com-
position.
haemoglobinOne of a group of
globular proteins occurring widely in
animals as oxygen carriers in blood.
Vertebrate haemoglobin comprises
two pairs of polypeptide chains,
known as α-chains and β-chains
(forming the globinprotein), with
each chain folded to provide a bind-
ing site for a *haem group. Each of
the four haem groups binds one
oxygen molecule to form oxy-
haemoglobin. Dissociation occurs in
oxygen-depleted tissues: oxygen is re-
leased and haemoglobin is reformed.
The haem groups also bind other in-
organic molecules, including carbon
monoxide (to form carboxyhaemo-
globin). In vertebrates, haemoglobin
is contained in the red blood cells
(erythrocytes).haemoglobinic acidA very weak
acid formed inside red blood cells
when hydrogen ions combine with
haemoglobin. The presence of the
hydrogen ions, which are produced
by the dissociation of carbonic acid,
encourages oxyhaemoglobin to disso-
ciate into haemoglobin and oxygen.
The oxygen diffuses into the tissue
cells and the haemoglobin acts as a
*buffer for the excess hydrogen ions,
which it takes up to form haemoglo-
binic acid.hafniumSymbol Hf. A silvery lus-
trous metallic *transition element;
a.n. 72; r.a.m. 178.49; r.d. 13.3; m.p.
2227 ± 20 °C; b.p. 4602°C. The element
is found with zirconium and is ex-
tracted by formation of the chloride
and reduction by the Kroll process. It
is used in tungsten alloys inÜlaments
and electrodes and as a neutron ab-
sorber. The metal forms a passive
oxide layer in air. Most of its com-
pounds are hafnium(IV) complexes;
less stable hafnium(III) complexes
also exist. The element wasÜrst re-
ported by Urbain in 1911, and its ex-
istence wasÜnally established by259 hafnium
h