INSTRUMENTATION Electrocapillary MeasurementsThe Lippmann Electrometer equilibrium electrochemI NicalSTUDYING adsorption at mercury by means of surface tension measurements, the use of the Lippmann electrometer is preferable to the use of drop weight techniques, particularly when slow adsorption processes are involved, as in the case of organic adsorbates. A recent improvement is described by Β. Ε. Conway and L. G. M. Gordon as a New Technique for Electrocapillary Mea surements Using the Lippmann Elec trometer, J. Electroanal. Chem., 15, 7 14 (1967). This study is important because the improvements have eliminated much of the tedium of earlier methods, afford somewhat better precision, and to quote the authors—"the procedures described allow the possibility of further develop ment towards a partially automated sys tem for electrocapillary measurements." I t may not be inappropriate to say something about the Lippmann elec trometer at this point because it is likely that few analysts other than those interested in interfacial electrochemis try are familiar with this 94-year-old device. They may be startled or even horrified by the suggestion that it is, among other things, a fairly sensitive (a few millivolts) null-potential indi cator. In modern practice, it is hope lessly outclassed by a digital voltmeter with print-out attachment costing a thousand times as much. The first comprehensive investigation of electrocapillarity was carried out by G. Lippmann, Ann. Phys. Chem., 149 [HI 546, (1873), who devised a sim ple apparatus for measuring simul taneously the boundary tension and the boundary phase potential between mer cury and various aqueous solutions. I n its simplest form, the Lippmann elec trometer consists of a mercury reservoir terminating in a fine capillary which is immersed in the solution. A mercury pool, or better, a standard reference electrode serves as the other pole. As progressively more negative potentials are applied to the mercury in the capil lary, the interfacial tension rises, reaches a maximum, and then decreases. The curve is an inverted parabola, more or less asymmetric. The electrocapillary Circle No. 44 on Readers' Service Card
BY RALPH H. MÜLLER
effect is noticed as a rise or fall of the interfacial meniscus. I t is best mea sured by a compensating pressure ap plied to the column to restore the meniscus to its original position. The position of the arm of the curve corre sponding to the smaller negative po tential is characteristic of the anion of the electrolyte and independent of the cation. The arm of the curve corre sponding to the larger negative poten tial is, similarly, found to be charac teristic of the cation and independent of the anion. At the maximum in the curve where d y/ θ v is zero the charge intensity in the mercury surface is zero, but the maximum in the curve does not occur when the phase boundary poten tial is zero. When the device is used primarily as an electrometer, it is de sirable to keep as far as possible from the maximum in the curve—i.e., near zero applied potential. As ordinarily used for interfacial tension measure ments, motion of the mercury meniscus is measured with a cathetometer and the restoring pressure is also measured similarly. The important contribution of Conway and Gordon has been im proved methods of obtaining the equiv alent information. After trying and re jecting photoelectric detection of menis cus motion, they chose an ac conduct ance method. In this approach the electrical resistance of the column of supporting electrolyte below the de scending column of mercury in the capillary, has been measured success fully. The ac component is a measure of displacement and is superimposed on the usual dc polarization of the elec trodes. Inasmuch as the total imped ance is comprised of the ohmic resist ance of the column of electrolyte as well as the double-layer capacitance of the very small area of the meniscus in the capillary, it becomes necessary to correct for the latter. This factor was solved by the ingenious inclusion of a variable inductance in the circuit. I n practice this inductance is chosen suffi ciently large that the variation of im pedance with dc potential (with the meniscus a t a given position) is made negligible. The circuit, as designed, had an impedance which varied with ac
frequency, due to the presence of the inductance and transformers in series with the double-layer capacitance of the mercury meniscus and the resist ance of the electrolyte column. The frequency chosen for the measurements was that for which the circuit was ob served to have maximum admittance (7-8 kHz) and the resulting sensitivity for the location of the meniscus was highest. All measurements were made at 7 kHz. The second improvement was the development of a direct reading mi crometer manometer to measure the pressure necessary for meniscus posi tion restoration. A U-tube manometer with 1-in. diameter columns of mercury on each side was provided with thermo stat water jackets. One side of the manometer was connected to the gas pressure assembly, the other limb was fitted with a single-ended micrometer capable of measuring a displacement of 0.001 cm with a 5-cm traverse. A stainless steel extension of the microm eter shaft was provided with a sharp 60° level tip to contact the mercury meniscus. The precise point of con tact was indicated by a simple series combination of a 1.2-v battery, 100-kn resistor, and microammoter. Throughout this investigation the new methods were compared with con ventional cathetometer techniques. As finally developed the average Δγ was only 0.03 dyne/cm with a substantial improvement of reproducibility over the cathetometry method. Interest continues a t a high level in electrocapillary phenomena and the literature is replete with reports of de tailed studies of electrode phenomena. What these researches will have to do with new and eminently practical meth ods of analysis remains to be seen. Heyrovsky's early interests were in electrocapillary phenomena at mercury surfaces and led him, in time to the in vention of the polarograph. I t seems quite certain that new and original ap plications of these various phenomena will be made. Some practices are in strumented to the point of diminishing returns. Real progress might well stem from some original approaches.
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