are the absorption intensities for
right- and left-circularly polarized
light, respectively. One application of
CD spectroscopy is to determine the
conÜgurations of complexes of transi-
tion metals. If the CD spectra of simi-
lar transition metal complexes
(including similarity of geometry) are
taken, the features of their CD spec-
tra are also similar.
CESee capillary electrophoresis.
celestine A mineral form of stron-
tium sulphate, SrSO 4.
cell1.A system in which two elec-
trodes are in contact with an elec-
trolyte. The electrodes are metal or
carbon plates or rods or, in some
cases, liquid metals (e.g. mercury). In
an electrolytic cell a current from
an outside source is passed through
the electrolyte to produce chemical
change (see electrolysis). In a
voltaic cell, spontaneous reactions
between the electrodes and elec-
trolyte(s) produce a potential differ-
ence between the two electrodes.
Voltaic cells can be regarded as
made up of two *half cells, each
composed of an electrode in contact
with an electrolyte. For instance, a
zinc rod dipped in zinc sulphate solu-
tion is a Zn|Zn2+half cell. In such a
system zinc atoms dissolve as zinc
ions, leaving a negative charge on
the electrode
Zn(s) →Zn2+(aq) + 2eThe solution of zinc continues until
the charge build-up is sufÜcient to
prevent further ionization. There is
then a potential difference between
the zinc rod and its solution. This
cannot be measured directly, since
measurement would involve making
contact with the electrolyte, thereby
introducing another half cell (see
electrode potential). A rod of cop-
per in copper sulphate solution com-
prises another half cell. In this case
the spontaneous reaction is one in
which copper ions in solution take
electrons from the electrode and are
deposited on the electrode as copper
atoms. In this case, the copper ac-
quires a positive charge.
The two half cells can be con-
nected by using a porous pot for the
liquid junction (as in the *Daniell
cell) or by using a salt bridge. The re-
sulting cell can then supply current
if the electrodes are connected
through an external circuit. The cell
is written
Zn(s)|Zn2+(aq)|Cu2+(aq)|Cu
E = 1.10 V
Here, E is the e.m.f. of the cell equal
to the potential of the right-hand
electrode minus that of the left-hand
electrode for zero current. Note that
‘right’ and ‘left’ refer to the cell as
written. Thus, the cell could be writ-
ten
Cu(s)|Cu2+(aq)|Zn2+(aq)|Zn(s)
E = –1.10 V
The overall reaction for the cell is
Zn(s) + Cu2+(aq) →Cu(s) +
Zn2+(aq)
This is the direction in which the cell
reaction occurs for a positive e.m.f.
The cell above is a simple example
of a chemical cell; i.e. one in which
the e.m.f. is produced by a chemical
difference. Concentration cells are
cells in which the e.m.f. is caused by
a difference of concentration. This
may be a difference in concentration
of the electrolyte in the two half
cells. Alternatively, it may be an elec-
trode concentration difference (e.g.
different concentrations of metal in
an amalgam, or different pressures of
gas in two gas electrodes). Cells are
also classiÜed into cells without
transport(having a single electrolyte)
and with transport (having a liquid
junction across which ions are trans-
ferred). Various types of voltaic cell109 cell
c