Mercury electrode drop frequency

Fig. 1.5 Barker square-wave polarogram of electrode reaction (1.1) on dropping mercury electrode. P = frequency = 225 Hz, amplitude = 30 mV, drop life time = 1 s,...

Fig. 23. The in-phase (/LF) an< quadrature (Qlf ) component of the demodulation response pertaining to the tris-oxalato Fe(III)/ tris-oxalato Fe(II) electrode reaction at the dropping mercury electrode in aqueous IMK2C2O4 + 0.05 M H2C2O4 solution. High frequency 100 kHz low frequency 170 Hz, = 0.21 A cm-2 Fe(III) concentration 1 mM. The thin solid lines represent the demodulation response in the absence of the redox couple [72].

Damping — Diminishing of the amplitude of oscillations. Electrical damping of the current oscillations caused by the -> dropping mercury electrode was used in -> polarography. Damping is also used to diminish higher-frequency contributions to noise however,... 

For the application of this equation one has to measure surface energy (surface tension) and its dependence on electrode potential at constant activities of the different components. Precise measurements are restricted to liquid metals like mercury or gallium and their alloys. Classical experiments were made with the Lippmann electrocapillary meter. The measurement of the drop time or the drop frequency of a dropping mercury electrode is easier. 

Rapid square-wave (SQW). Five square-wave oscillations with a frequency of around 125 Hz are superimposed on the voltage ramp during the last 40 ms of controlled drop growth - with a dropping mercury electrode the drop surface is then constant. The oscillation amplitude can be pre-selected. Measurements are performed in the second, third, and fourth square-wave oscillation the current is integrated over 2 ms at the end of the first and the end of the second half of each oscillation. The three differences... 

A further method relies on the fact that the natural drop time of a dropping mercury electrode is proportional to the interfacial tension [18]. Again, drop birth can be detected electrically by the sudden change in impedance, so the method is easily automated. Unlike the other methods there is no adequate theory describing the mechanism of drop detachment, so the proportionality constant is again obtained by calibration with a solution of known properties. The method is extremely sensitive to vibrations and impurities, and consequently it is difficult to obtain results better than 1%. Also as a dropping mercury electrode is used, the system is dynamic and may not be in equilibrium if the rate of adsorption is slow. Similarly, capacitance measurements will be frequency dependent if adsorption is slow compared with the period of a.c. perturbation, and this provides a useful check of whether equilibrium is obtained. 

Instruments for both these methods are commercially available, or can be made using standard operational amplifier techniques [1]. An instrument incorporating two p.s.d.s. with synchronisation for the dropping mercury electrode has been described by de Levie Husovsky [2]. Both of these techniques are convenient in that they give analogue displays of the measured quantities. By using a swept-frequency oscillator, both quantities can be recorded as a function of frequency on an X-V recorder. 

The quantification of AA in tropical fruit juices has been performed in a BIA system where a mercury electrode was used as an amperometric sensor [41], During electrochemical oxidation of AA, the capillary of the mercury drop electrode was adapted in an inverted position of the electrochemical cell in order to obtain stable and reproducible signals. Using a working potential of -1-0.23 V (vs. Ag/AgCl), it was possible to reach a frequency of 300 injections per hour. 

The dropping mercury electrode with adjustable vibrating frequency of 5 Hz up to a record-high value of 1 kHz was then made and studied in detail ([172, 173], p. 76) (cf. Fig. 9.17). This was achieved using wider glass capillaries which were drawn out at the end. With conventional flow rates of mercury (m 0.002 g/s), this electrode made it possible to lower the time of contact U of the mercxury with the solution to so low values like in case of the Heyrovsky s streaming mercury... 

This is shown in Fig. 2.19. The relationships between A fJ, and nE v, and nIEE do not depend on electrode size [28-30]. So, if nE vi = 25 mV and nAF, = —5 mV, the relationship (2.21) is AWp = 0.465 + 0.45p [26]. If the frequency is high and a hanging mercury drop electrode is used, the spherical effect is usually negh-gible (p < 10 ). However, the influence of sphericity must be taken into consideration under most other conditions, and generally at microelectrodes. The net peak current is a linear function of the square-root of frequency Alp/nFAc QD I =... 

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