step-wise increase in current is obtained. The height of each step is proportional to the concentration of the
component. The technique is useful
for detecting trace amounts of metals
and for the investigation of solvated
complexes.
Polaroid A doubly refracting material that plane-polarizes unpolarized light passed through it. It
consists of a plastic sheet strained in
a manner that makes it birefringent
by aligning its molecules. Sunglasses
incorporating a Polaroid material absorb light that is vibrating horizontally – produced by reÛection from
horizontal surfaces – and thus reduce
glare.
polar solvent See solvent.
pollutant Any substance, produced
and released into the environment as
a result of human activities, that has
damaging effects on living organisms. Pollutants may be toxic substances (e.g. *pesticides) or natural
constituents of the atmosphere (e.g.
carbon dioxide) that are present in
excessive amounts. See pollution.
pollution An undesirable change in
the physical, chemical, or biological
characteristics of the natural environment, brought about by man’s activities. It may be harmful to human or
nonhuman life. Pollution may affect
the soil, rivers, seas, or the atmosphere. There are two main classes
of *pollutants: those that are biodegradable (e.g. sewage), i.e. can
be rendered harmless by natural
processes and need therefore cause
no permanent harm if adequately
dispersed or treated; and those that
are nonbiodegradable (e.g. *heavy
metals (such as lead) in industrial
efÛuents and *DDT and other chlorinated hydrocarbons used as pesticides), which eventually accumulate
in the environment and may be concentrated in food chains. Other forms
of pollution in the environment include noise (e.g. from jet aircraft,
trafÜc, and industrial processes) and
thermal pollution (e.g. the release of
excessive waste heat into lakes or
rivers causing harm to wildlife). Recent pollution problems include the
disposal of radioactive waste; *acid
rain; *photochemical smog; increasing levels of human waste; high levels of carbon dioxide and other
greenhouse gases in the atmosphere
(see greenhouse effect); damage to
the *ozone layer by nitrogen oxides,
*chloroÛuorocarbons (CFCs), and
*halons; and pollution of inland waters by agricultural *fertilizers and
sewage efÛuent, causing eutrophication. Attempts to contain or prevent
pollution include strict regulations
concerning factory emissions, the
use of smokeless fuels, the banning
of certain pesticides, the increasing
use of lead-free petrol, restrictions on
the use of chloroÛuorocarbons, and
the introduction, in some countries,
of catalytic converters to cut pollutants in car exhausts.
polonium Symbol Po. A rare radioactive metallic element of group 16
(formerly VIB) of the periodic table;
a.n. 84; r.a.m. 210; r.d. 9.32; m.p.
254°C; b.p. 962°C. The element occurs in uranium ores to an extent of
about 100 micrograms per 1000 kilograms. It has over 30 isotopes, more
than any other element. The longestlived isotope is polonium–209 (halflife 103 years). Polonium has
attracted attention as a possible heat
source for spacecraft as the energy
released as it decays is 1.4 × 10
5
J kg
–1 s
–1 . It was discovered by Marie
Curie in 1898 in a sample of pitchblende.
A
• Information from the WebElements site
poly- PreÜx indicating a polymer,
Polaroid
426
p
www.AzShimi.ir www.AzShimi.com
component. The technique is useful
for detecting trace amounts of metals
and for the investigation of solvated
complexes.
Polaroid A doubly refracting material that plane-polarizes unpolarized light passed through it. It
consists of a plastic sheet strained in
a manner that makes it birefringent
by aligning its molecules. Sunglasses
incorporating a Polaroid material absorb light that is vibrating horizontally – produced by reÛection from
horizontal surfaces – and thus reduce
glare.
polar solvent See solvent.
pollutant Any substance, produced
and released into the environment as
a result of human activities, that has
damaging effects on living organisms. Pollutants may be toxic substances (e.g. *pesticides) or natural
constituents of the atmosphere (e.g.
carbon dioxide) that are present in
excessive amounts. See pollution.
pollution An undesirable change in
the physical, chemical, or biological
characteristics of the natural environment, brought about by man’s activities. It may be harmful to human or
nonhuman life. Pollution may affect
the soil, rivers, seas, or the atmosphere. There are two main classes
of *pollutants: those that are biodegradable (e.g. sewage), i.e. can
be rendered harmless by natural
processes and need therefore cause
no permanent harm if adequately
dispersed or treated; and those that
are nonbiodegradable (e.g. *heavy
metals (such as lead) in industrial
efÛuents and *DDT and other chlorinated hydrocarbons used as pesticides), which eventually accumulate
in the environment and may be concentrated in food chains. Other forms
of pollution in the environment include noise (e.g. from jet aircraft,
trafÜc, and industrial processes) and
thermal pollution (e.g. the release of
excessive waste heat into lakes or
rivers causing harm to wildlife). Recent pollution problems include the
disposal of radioactive waste; *acid
rain; *photochemical smog; increasing levels of human waste; high levels of carbon dioxide and other
greenhouse gases in the atmosphere
(see greenhouse effect); damage to
the *ozone layer by nitrogen oxides,
*chloroÛuorocarbons (CFCs), and
*halons; and pollution of inland waters by agricultural *fertilizers and
sewage efÛuent, causing eutrophication. Attempts to contain or prevent
pollution include strict regulations
concerning factory emissions, the
use of smokeless fuels, the banning
of certain pesticides, the increasing
use of lead-free petrol, restrictions on
the use of chloroÛuorocarbons, and
the introduction, in some countries,
of catalytic converters to cut pollutants in car exhausts.
polonium Symbol Po. A rare radioactive metallic element of group 16
(formerly VIB) of the periodic table;
a.n. 84; r.a.m. 210; r.d. 9.32; m.p.
254°C; b.p. 962°C. The element occurs in uranium ores to an extent of
about 100 micrograms per 1000 kilograms. It has over 30 isotopes, more
than any other element. The longestlived isotope is polonium–209 (halflife 103 years). Polonium has
attracted attention as a possible heat
source for spacecraft as the energy
released as it decays is 1.4 × 10
5
J kg
–1 s
–1 . It was discovered by Marie
Curie in 1898 in a sample of pitchblende.
A
• Information from the WebElements site
poly- PreÜx indicating a polymer,
Polaroid
426
p
www.AzShimi.ir www.AzShimi.com