10-63
10.9.2
Electrocapillary Measurement (ECM)
Electrocapillary Curve Measurement (ECM) is used to characterize mercury metal-
solution interfaces, examining macroscopic equilibrium properties such as surface
tension as a function of potential. Such measurements are useful in studies of the
structure of the interfacial double-layer, since the change in the surface tension with
potential provides information about the polarity and amount of excess surface charge
on the electrode (this is given by the slope of the electrocapillary curve). The
potential of zero charge (PZC) occurs at the maximum of the electrocapillary curve
(where the change of the surface tension with respect to the potential is zero). Many
properties of the solution-electrode interface are related to how far the potential is
from the PZC (in addition, the value of the PZC is sensitive to the presence of
chemical species in solution). The electrocapillary curve is also an important tool for
investigation of adsorption phenomena (it can also be used to define potential regions
where adsorption does not occur).
The most convenient way to measure the electrocapillary curve is to measure the
lifetime of a dropping mercury electrode (DME) at different potentials. The drop
lifetime, t
max
, is proportional to the surface tension,
γ
, the physical characteristics of
the mercury electrode (specifically, the capillary radius (r
c
) and the mercury flow rate
(m)) and gravity (g).
t
max
= (2
π
r
c
γ
)/mg
The classical technique for measuring drop lifetime is to use a stop-watch, and
measure the time for 10 drops. As well as being tedious and time-consuming, it is
prone to human error.
On the BAS 100B/W, the curve measurement is automated. The measurement of the
drop lifetime, the variation in potential and the plot of the electrocapillary curve are
controlled by the microprocessor. The resolution of the measurement is 3 msec (i.e.,
for a typical DME with 2-5 sec. dropping time 0.1% precision can be obtained).
Higher precision can be obtained by using multiple drop detection.
Drop detection in the BAS 100B/W is accomplished by superimposing a digitally
generated A.C. waveform on the applied D.C. potential (Figure 10-42) and measuring
the A.C. current response. Standard methods are used to sample and process the A.C.
current. The magnitude of the A.C. current depends on the size of the mercury drop,
so the fall of a drop is shown by a significant decrease in the A.C. current.
