of the perovskite structure include
CaTiO3, BaTiO3, and SrTiO3.
A
- An interactive version of the structure
peroxides A group of inorganic
compounds that contain the O 2 2–ion.
They are notionally derived from hy-
drogen peroxide, H 2 O 2 , but these
ions do not exist in aqueous solution
due to extremely rapid hydrolysis to
OH–.
A
- Information about IUPAC nomenclature
peroxodisulphuric acid See per-
oxosulphuric(vi) acid.
peroxomonosulphuric(VI) acid
See peroxosulphuric(vi) acid.
peroxosulphuric(VI) acid The
term commonly refers to peroxo-
monosulphuric(VI) acid, H 2 SO 5 ,
which is also called permonosul-
phuric(VI) acid and Caro’s acid. It is a
crystalline compound made by the
action of hydrogen peroxide on con-
centrated sulphuric acid. It decom-
poses in water and the crystals
decompose, with melting, above
45 °C. The compound peroxodisul-
phuric acid, H 2 S 2 O 8 , also exists (for-
merly called perdisulphuric acid). It is
made by the high-current electrolysis
of sulphate solutions. It decomposes
at 65°C (with melting) and is hydrol-
ysed in water to give the mono acid
and sulphuric acid. Both peroxo acids
are very powerful oxidizing agents.
See also sulphuric acid (for structural
formulas).
persistent Describing a pesticide
or other pollutant that is not readily
broken down and can persist for long
periods, causing damage in the envi-
ronment. For example, the herbi-
cides Paraquat and DDT can persist
in the soil for many years after their
application.
persistent organic pollutant
(POP)A toxic substance that is not
biodegradable and persists in the en-
vironment. POPs can concentrate as
they move up the food chain and are
generally deleterious to health. In
2001, an international conference in
Sweden resulted in an agreement
(the Stockholm Convention) to reduce
or eliminate the 12 POPs of greatest
concern, and to add other chemicals
to the list in the future. The 12 chem-
icals were the pesticides aldrin,
chlordane, *DDT, dieldrin, endrin,
heptachlor, mirex, and toxaphene,
together with the industrial chemical
hexachlorobenzene and the groups
*polychlorinated biphenyls, *diox-
ins, and *furans.
A
•OfÜcial Stockholm Convention sitePerspexTradename for a form of
*polymethylmethacrylate.perturbation theoryA method
used in calculations in both classical
physics (e.g. planetary orbits) and
quantum mechanics (e.g. atomic
structure), in which the system is di-
vided into a part that is exactly calcu-
lable and a small term, which
prevents the whole system from
being exactly calculable. The tech-
nique of perturbation theory enables
the effects of the small term to be
calculated by an inÜnite series (which
in general is an asymptotic series).
Each term in the series is a ‘correc-
tion term’ to the solutions of the ex-
actly calculable system. In classical
physics, perturbation theory can be
used for calculating planetary orbits.
In quantum mechanics, it can be
used to calculate the energy levels in
molecules.PESSee photoelectron spec-
troscopy.PESMSee photoelectron spectro-
microscopy.405 PESM
p