Chemistry Reference
In-Depth Information
nation of Cu(I) and Cu(II) concentrations in styrene. Electrochemical
methods to determine copper ion concentrations in aqueous solution or in
organic solutions with a higher dielectric constant than styrene have been
described before
9-12
.
Electrochemical measurements are useful for determining concentra-
tions of electroactive species in solution. Playing the role of solvent, the
monomer studied in this chapter is styrene. One of its most remarkable
characteristics is the low dielectric constant (e=2.43 at 298.0 K) compared
with that of water (e=78 at 298.0 K). A solvent with a low dielectric con-
stant is a highly resistive medium, in which voltammetric measurements are
not evident. Voltammetric measurements in styrene as solvent have not
been described before. Papers describing an electrochemical method for the
determination of styrene in more polar organic solvents can be found in the
literature
13-17
.
Voltammetric measurements in highly resistive media became possible
by using ultramicro electrodes, which should have dimensions in the range
of micrometers or less. One of their most claimed advantages is that the
electrode processes are commonly associated with low currents in the range
of nano- or picoamperes. As a result, the ohmic 'IR'drop, even in organic
solvents with high resistance, can be kept sufficiently low and voltammet-
ric experiments can be performed. Furthermore, ultramicro electrodes have
an excellent signal ratio of Faraday-to-background current and enable low
concentrations of electroactive species to be determined in high-resistance
media. The ability of measuring in low conducting media opens up new per-
spectives, particularly for electroanalytical purposes in monitoring polymer
reactions
18
.
12.2.2 Determination of solution resistance
In Fig. 12.1, a Nyquist (Fig. 12.1a) and a Bode (Fig. 12.1b) plot are shown,
obtained by electrochemical impedance spectroscopy (EIS) using a stan-
dard conductivity cell, in a styrene solution with different concentrations of
tetra hexyl amino phosphate (THAP). From the semi-circle (Fig. 12.1a) and
the wide range of frequencies where only resistive effects are observed (Fig.
12.1b, range where phase-angle shift is near zero), it can be deduced that
the electrical equivalent circuit consists of a resistor and a capacitor in par-
allel. However, the semi-circle for pure styrene could not be obtained
experimentally (and is not shown in the figure) because of an excessively
high solution resistance. As expected, this resistance drops by adding THAP
and was obtained from Fig. 12.1a through the diameter of the semi-circle,
which is equal to the resistance of the solution in the conductivity cell. This
resistance can also be obtained from Fig. 12.1b in the plateau of the log
Z
vs. log
f
plot, if, in the same region of frequencies, no phase-angle shift
