Page 35 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
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resistant air electrodes, often gold plated and protected with, e.g.,= a Teflon
or Permaflon (FEP Teflon) layer.= This minimizes the drift of the surface
potential of the vibrating electrodes and ensures stable and reproducible
results. 34,37
Preparation of an uncontaminated surface of an aqueous solution is
very difficult.= Even minute traces of adsorbable organic impurities
strongly influence the surface potential of water.= Cleaning of the aqueous
surface (e.g.,=by siphoning off the surface layer) is usually necessary, while
for organic solutions it is usuallynot needed. 34
It is now possible to construct a processor setup that combines good
reproducibilityand a low time constant with a theoretical sensitivity of
±0.05mV; for about a 0.5-mm air gap, changes in E equal to ±0.5 maybe
detectable. Of course, it is easier for solid than for liquid surfaces.
Recently scanning Kelvin probes and microprobes have been devel-
oped as high-resolution surface analysis devices.= They permit the inves-
tigation of the lateral distribution of woÀ functions or the surface
potentials of various phases. 38–43 Up to now, they were used for instance,
for determinating the corrosion potential profiles of metal and semicon-
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ductors under verythin films of electrolytic solution and creating surface
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potential maps of Langmuir-Blodgett films of charged ionomer–poly–
mer blends. 43
The static capacitor method is in principle the true original Kelvin
technique.= In this method, the tendency for charge to flow from the
capacitor through the external circuit connecting the capacitator plates is
detected, and then a backing-off potential is rapidlyapplied until charge
ceasestoflow. 44–47
A theoretical approach that permits common discussion of the ioniz-
ing gap, as well as dynamic and static condenser methods, has been
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described. The dynamic condenser method gives probably= the most
reliable results.= However, the ionizing probe method appears to be experi -
mentallysimpler for investigation of liquid surfaces, in particular, owing
to the delicate mechanical and vibrational or signal measuring problems
involved with the condenser methods.= It has been shown that the two
techniques give similar results. 25
The principle of the jet method, which also utilizes a condenser, was
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originated by Kenrick and improved by Randles and later by McTigue
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et al. It may be summarized as follows: A jet of one liquid is directed
down the axis of a tube, the inner surface of which is covered bya stream
of the second liquid. If the reference electrodes are the same and the outer