and sodium hydrogencarbonate.
Basic buffers have a weak base and
a salt of the base (to provide the
conjugate acid). An example is ammonia solution with ammonium
chloride.
In an acidic buffer, for example,
molecules HA and ions A
– are present. When acid is added most of the
extra protons are removed by the
base:
A
– + H
+
→ HA
When base is added, most of the
extra hydroxide ions are removed by
reaction with undissociated acid:
OH
– + HA → A
– + H 2 O
Thus, the addition of acid or base
changes the pH very little. The hydrogen-ion concentration in a buffer
is given by the expression
K a = [H
+ ] = [A
– ]/[HA]
i.e. it depends on the ratio of conjugate base to acid. As this is not altered by dilution, the hydrogen-ion
concentration for a buffer does not
change much during dilution.
In the laboratory, buffers are used
to prepare solutions of known stable
pH. Natural buffers occur in living organisms, where the biochemical reactions are very sensitive to change
in pH. The main natural buffers are
H 2 CO 3 /HCO 3
– and H 2 PO 4
–
/HPO 4
2– .
Buffer solutions are also used in
medicine (e.g. in intravenous injections), in agriculture, and in many
industrial processes (e.g. dyeing,
fermentation processes, and the
food industry).
bumping Violent boiling of a liquid
caused by superheating so that bubbles form at a pressure above atmospheric pressure. It can be prevented
by putting pieces of porous pot in
the liquid to enable bubbles of
vapour to form at the normal boiling point.
buna rubber A type of synthetic
rubber based in polymerization of
butadiene (buta-1,3-diene). The name
comes from Bu (for butadiene) and
Na (for sodium, which was used as a
catalyst in the original polymerization reaction). An improved form,
known as Buna-S was developed by
copolymerizing butadiene with
styrene. In 1934, Buna-N was invented, in which the styrene was replaced by acrylonitrile, giving a
product with better oil resistance (see
nitrile rubber).
Bunsen, Robert Wilhelm
(1811–99) German chemist, who held
professorships at Kassel, Marburg,
and Heidelberg. His early researches
on arsenic-containing compounds
cost him an eye in an explosion. He
then turned to gas analysis and spectroscopy, enabling him and *Kirchhoff to discover the elements
*caesium (1860) and *rubidium
(1861). He also popularized the use of
the *Bunsen burner and developed
the *Bunsen cell.
Bunsen burner A laboratory gas
burner having a vertical metal tube
into which the gas is led, with a hole
in the side of the base of the tube to
admit air. The amount of air can be
regulated by a sleeve on the tube.
When no air is admitted the Ûame is
luminous and smoky. With air, it has
a faintly visible hot outer part (the
oxidizing part) and an inner blue
cone where combustion is incomplete (the cooler reducing part of the
Ûame). The device is named after
Robert *Bunsen, who used a similar
device (without a regulating sleeve)
in 1855.
Bunsen cell A *primary cell devised by Robert *Bunsen consisting
of a zinc cathode immersed in dilute
sulphuric acid and a carbon anode
immersed in concentrated nitric acid.
87
Bunsen cell
b
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Basic buffers have a weak base and
a salt of the base (to provide the
conjugate acid). An example is ammonia solution with ammonium
chloride.
In an acidic buffer, for example,
molecules HA and ions A
– are present. When acid is added most of the
extra protons are removed by the
base:
A
– + H
+
→ HA
When base is added, most of the
extra hydroxide ions are removed by
reaction with undissociated acid:
OH
– + HA → A
– + H 2 O
Thus, the addition of acid or base
changes the pH very little. The hydrogen-ion concentration in a buffer
is given by the expression
K a = [H
+ ] = [A
– ]/[HA]
i.e. it depends on the ratio of conjugate base to acid. As this is not altered by dilution, the hydrogen-ion
concentration for a buffer does not
change much during dilution.
In the laboratory, buffers are used
to prepare solutions of known stable
pH. Natural buffers occur in living organisms, where the biochemical reactions are very sensitive to change
in pH. The main natural buffers are
H 2 CO 3 /HCO 3
– and H 2 PO 4
–
/HPO 4
2– .
Buffer solutions are also used in
medicine (e.g. in intravenous injections), in agriculture, and in many
industrial processes (e.g. dyeing,
fermentation processes, and the
food industry).
bumping Violent boiling of a liquid
caused by superheating so that bubbles form at a pressure above atmospheric pressure. It can be prevented
by putting pieces of porous pot in
the liquid to enable bubbles of
vapour to form at the normal boiling point.
buna rubber A type of synthetic
rubber based in polymerization of
butadiene (buta-1,3-diene). The name
comes from Bu (for butadiene) and
Na (for sodium, which was used as a
catalyst in the original polymerization reaction). An improved form,
known as Buna-S was developed by
copolymerizing butadiene with
styrene. In 1934, Buna-N was invented, in which the styrene was replaced by acrylonitrile, giving a
product with better oil resistance (see
nitrile rubber).
Bunsen, Robert Wilhelm
(1811–99) German chemist, who held
professorships at Kassel, Marburg,
and Heidelberg. His early researches
on arsenic-containing compounds
cost him an eye in an explosion. He
then turned to gas analysis and spectroscopy, enabling him and *Kirchhoff to discover the elements
*caesium (1860) and *rubidium
(1861). He also popularized the use of
the *Bunsen burner and developed
the *Bunsen cell.
Bunsen burner A laboratory gas
burner having a vertical metal tube
into which the gas is led, with a hole
in the side of the base of the tube to
admit air. The amount of air can be
regulated by a sleeve on the tube.
When no air is admitted the Ûame is
luminous and smoky. With air, it has
a faintly visible hot outer part (the
oxidizing part) and an inner blue
cone where combustion is incomplete (the cooler reducing part of the
Ûame). The device is named after
Robert *Bunsen, who used a similar
device (without a regulating sleeve)
in 1855.
Bunsen cell A *primary cell devised by Robert *Bunsen consisting
of a zinc cathode immersed in dilute
sulphuric acid and a carbon anode
immersed in concentrated nitric acid.
87
Bunsen cell
b
www.AzShimi.ir www.AzShimi.com