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 reversibly bind oxygen (as in haemoglobin 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 occurs 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 colouring agent in rocks; the largest deposits 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 introduced in Japan in 1912 by Masao
Takayama.
haemoerythrin A red iron-containing 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 composition.
haemoglobin One 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 globin protein), with
each chain folded to provide a binding site for a *haem group. Each of
the four haem groups binds one
oxygen molecule to form oxyhaemoglobin. Dissociation occurs in
oxygen-depleted tissues: oxygen is released and haemoglobin is reformed.
The haem groups also bind other inorganic molecules, including carbon
monoxide (to form carboxyhaemoglobin). In vertebrates, haemoglobin
is contained in the red blood cells
(erythrocytes).
haemoglobinic acid A 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 dissociate 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 haemoglobinic acid.
hafnium Symbol Hf. A silvery lustrous 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 extracted by formation of the chloride
and reduction by the Kroll process. It
is used in tungsten alloys in Ülaments
and electrodes and as a neutron absorber. The metal forms a passive
oxide layer in air. Most of its compounds are hafnium(IV) complexes;
less stable hafnium(III) complexes
also exist. The element was Ürst reported by Urbain in 1911, and its existence was Ünally established by
259
hafnium
h
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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 reversibly bind oxygen (as in haemoglobin 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 occurs 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 colouring agent in rocks; the largest deposits 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 introduced in Japan in 1912 by Masao
Takayama.
haemoerythrin A red iron-containing 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 composition.
haemoglobin One 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 globin protein), with
each chain folded to provide a binding site for a *haem group. Each of
the four haem groups binds one
oxygen molecule to form oxyhaemoglobin. Dissociation occurs in
oxygen-depleted tissues: oxygen is released and haemoglobin is reformed.
The haem groups also bind other inorganic molecules, including carbon
monoxide (to form carboxyhaemoglobin). In vertebrates, haemoglobin
is contained in the red blood cells
(erythrocytes).
haemoglobinic acid A 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 dissociate 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 haemoglobinic acid.
hafnium Symbol Hf. A silvery lustrous 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 extracted by formation of the chloride
and reduction by the Kroll process. It
is used in tungsten alloys in Ülaments
and electrodes and as a neutron absorber. The metal forms a passive
oxide layer in air. Most of its compounds are hafnium(IV) complexes;
less stable hafnium(III) complexes
also exist. The element was Ürst reported by Urbain in 1911, and its existence was Ünally established by
259
hafnium
h
www.AzShimi.ir www.AzShimi.com