step-wise increase in current is ob-
tained. The height of each step is pro-
portional 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 ma-
terial that plane-polarizes unpolar-
ized 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 ab-
sorb light that is vibrating horizon-
tally – produced by reÛection from
horizontal surfaces – and thus reduce
glare.polar solventSee solvent.
pollutantAny substance, produced
and released into the environment as
a result of human activities, that has
damaging effects on living organ-
isms. Pollutants may be toxic sub-
stances (e.g. *pesticides) or natural
constituents of the atmosphere (e.g.
carbon dioxide) that are present in
excessive amounts. See pollution.pollutionAn undesirable change in
the physical, chemical, or biological
characteristics of the natural environ-
ment, brought about by man’s activi-
ties. It may be harmful to human or
nonhuman life. Pollution may affect
the soil, rivers, seas, or the atmos-
phere. There are two main classes
of *pollutants: those that are bio-
degradable(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 chlori-
nated hydrocarbons used as pesti-
cides), which eventually accumulate
in the environment and may be con-centrated in food chains. Other forms
of pollution in the environment in-
clude 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). Re-
cent pollution problems include the
disposal of radioactive waste; *acid
rain; *photochemical smog; increas-
ing levels of human waste; high lev-
els 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 wa-
ters by agricultural *fertilizers and
sewage efÛuent, causing eutrophica-
tion. 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 pollu-
tants in car exhausts.
poloniumSymbol Po. A rare radio-
active 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 oc-
curs in uranium ores to an extent of
about 100 micrograms per 1000 kilo-
grams. It has over 30 isotopes, more
than any other element. The longest-
lived isotope is polonium–209 (half-
life 103 years). Polonium has
attracted attention as a possible heat
source for spacecraft as the energy
released as it decays is 1.4 × 105
Jkg–1s–1. It was discovered by Marie
Curie in 1898 in a sample of pitch-
blende.
A- Information from the WebElements site
poly-PreÜx indicating a polymer,Polaroid 426p