The name is also used for the syn-
thetic compound, which is used as a
pigment.ortho-1.PreÜx indicating that a
benzene compound has two substi-
tuted groups in the 1,2 positions (i.e.
on adjacent carbon atoms). The ab-
breviation o- is used; for example
o-dichlorobenzene is 1,2-dichloro-
benzene. Compare meta-; para-.
2.PreÜx formerly used to indicate
the most hydrated form of an acid.
For example, phosphoric(V) acid,
H 3 PO 4 , was called orthophosphoric
acid to distinguish it from the lower
metaphosphoric acid, HPO 3 (which is
actually (HPO 3 )n). 3.PreÜx denoting
the form of diatomic molecules in
which nuclei have parallel spins, e.g.
orthohydrogen (see hydrogen). Com-
pare para-.orthoboric acidSee boric acid.orthoclaseSee feldspars.
orthoheliumA form of helium
once thought to exist as of one of
two species of the element. Because
the spectrum of atomic helium con-
sists of transitions between singlet
states (including transitions from the
ground state) and transitions be-
tween triplet states but not transi-
tions between singlet and triplet
states, early spectroscopists thought
that two species of helium existed.
The other form was called para-
helium.orthohydrogen See hydrogen.
orthophosphoric acidSee phos-
phoric(v) acid.
orthoplumbateSee plumbate.orthorhombicSee crystal system.
orthosilicateSee silicate.orthostannateSee stannate.
oscillating reaction (clock reac-
tion)A type of chemical reaction inwhich the concentrations of the
products and reactants change peri-
odically, either with time or with po-
sition in the reacting medium. Thus,
the concentration of a component
may increase with time to a maxi-
mum, decrease to a minimum, then
increase again, and so on, continuing
the oscillation over a period of time.
Systems are also known in which spi-
rals and other patterns spread
through the reacting medium,
demonstrating a periodic spatial vari-
ation. Oscillating chemical reactions
have certain features in common.
They all occur under conditions far
from chemical equilibrium and all in-
volve *autocatalysis, i.e. a product of
a reaction step acts as a catalyst for
that step. This autocatalysis drives
the oscillation by a process of posi-
tive feedback. Moreover, oscillating
chemical reactions are associated
with the phenomenon known as
bistability. In this, a reaction may be
in a steady-state condition, with reac-
tantsÛowing into a reaction zone
while products areÛowing out of it.
Under these conditions, the concen-
trations in the reaction zone may not
change with time, although the reac-
tion is not in a state of chemical equi-
librium. Bistable systems have two
possible stable steady states. Interac-
tion with an additional substance in
the reaction medium causes the sys-
tem to oscillate between the states as
the concentrations change. Oscillat-
ing chemical reactions are thought to
occur in a number of biochemical
processes. For example, they occur in
glycolysis, in which ATP is produced
by enzyme-catalysed reactions. They
are also known to regulate the
rhythm of the heartbeat. Most have
highly complex reaction mecha-
nisms. See also lotka–volterra mech-
anism; brusselator; oregonator;
chaotic reaction.ortho- 390o