Polarography The Dropping-Mercury Electrode

The primary element of polarography, the dropping mercury electrode, was introduced in science by the Czech physicist Bohumil Kucera (1874—1921) (Fig. 3.2.1) [344] in order to improve the Lippmann s method of surface tension measurement of polarized mercury [345]. 

In voltammetry as an analytical method based on measurement of the voltage-current curve we can distinguish between techniques with non-stationary and with stationary electrodes. Within the first group the technique at the dropping mercury electrode (dme), the so-called polarography, is by far the most important within the second group it is of particular significance to state whether and when the analyte is stirred. 

The gamma isomer of benzene hexachloride can also be determined by polarography (24, 0). The method is based on the fact that, under the conditions used, the gamma isomer is the only one of the five isomers that is reduced at the dropping mercury electrode. 

A modification of faradaic impedance measurement is a.c. polarography, where a small a.c. voltage is superimposed on the voltage polarizing the dropping mercury electrode (Fig. 5.181). 

Many of the experimental parameters for normal-pulse polarography are the same as with differential-pulse polarography. Differential-pulse polarography is a technique that uses a series of discrete potential steps rather than a linear potential ramp to optimize specific applications (130). Unlike normal-pulse polarography, each potential step has the same amplitude, whereas the return potential after each pulse is slightly negative of the potential prior to the step. In this manner, the total waveform applied to the dropping mercury electrode is very much like a combination of a linear ramp with a superimposed square wave. 

For many years, the study of the electrode—electrolyte interface and electrode kinetics was confined to the very reproducible mercury-aqueous system because of the availability of the dropping mercury electrode (DME) and development of polarography. Extensive leading work in this field was carried out by Heyrovsky, Frumkin, Grahame, and Randles. 

Equation (11) is also applicable as a good, or reasonably good, approximation to a number of techniques classified as d.c. voltammetry , in which the response to a perturbation is measured after a fixed time interval, tm. The diffusion layer thickness, 5/, will be a function of D, and tm and the nature of this function has to be deduced from the rigorous solution of the diffusion problem in combination with the appropriate initial and boundary conditions [21—23]. The best known example is d.c. polarography [11], where the d.c. current is measured at the dropping mercury electrode at a fixed time, tm, after the birth of a new drop as a function of the applied d.c. potential. The expressions for 5 pertaining to this and some other techniques are given in Table 1. 

In practice, differential pulse polarography is usually performed with the dropping mercury electrode. This means that appropriate expressions are needed for j F(tm ) and y F(f0), whereas for sufficiently small tp values, eqn. (50) for AyF(fp) remains valid. Some technical refinements, especially to reduce the effect of double-layer charging, have been described in the literature [51]. 

Moreover, it is difficult to find one s way in the overwhelming amount of literature on this subject because the major part of it is focussed on d.c. polarography and thus to the mass transfer problem at the dropping mercury electrode (DME). Neglecting the sphericity, the expansion of the drop has still to be accounted for in the diffusion equation for a species i. Equation (19b), which we have adopted thus far, should therefore be replaced by [11, 147]... 

Table VII is an index to the techniques employed. For the sake of brevity polarography is excluded from It, as pure water Is from Table VI and as the dropping mercury electrode is from Table VIII, but for each of the other fifty-odd techniques that are included in this volume an entry like...

Table VIM Is an index to the indicator electrodes employed It provides access to all of the data obtained with mercury-pool, carbon-paste, rotating disc, and each of the numerous other electrode materials and configurations represented in Table I, with the single exception of the dropping mercury electrode, which is omitted here for the same reason that polarography is omitted from Table VII.

There Is one null entry in this table, and that is the one for the dropping mercury electrode. This reflects the same fact that led to the omission of polarography from Table VII. There are some compounds in Table I for which no information obtained with a dropping mercury electrode is given in... 

Knoth et al. [48] studied the electrochemical behavior of omeprazole with the aid of the direct-current and differential-pulse polarography. Omeprazole was determined in Britton-Robinson buffers pH 7-9 up to a concentration of 10 5 M. The mechanism of the reduction process on the dropping mercury electrode is elucidated. With the consumption of two electrons and two protons, omeprazole will be reduced to 5-methoxy-2-[(3,5-dimethyl-4-methoxypyridin-2-yl)methylthio]-lH-benzimidazole which will be cleaved with the uptake of two further electrons and two protons into 4-methoxy-2,3,5-trimethyl pyridine and 2-mercapto-5-methoxybenz imidazole. 

Polarography has been an extremely valuable electroanalytical method, although its importance has declined in recent years. The life and work of Jaroslav Heyrovsky, the inventor of the technique, was reviewed in the Toronto symposium,94 and also on the occasion of the centenary of his birth.95,96 The 75th anniversary of the discovery of polarography occurred in 1997. On that occasion the son of the inventor gave an account of the studies on the dropping mercury electrode and on the polarization... 

As the later chapters indicate, a given question concerning a chemical system usually can be answered by any one of several electrochemical techniques. However, experience has demonstrated that there is a most convenient or reliable method for a specific kind of data. For example, polarography with a static or dropping-mercury electrode remains the most reliable electrochemical method for the quantitative determination of trace-metal ion concentrations. This is true for two reasons (1) the reproducibility of the dropping-mercury electrode is unsurpassed and (2) the reference literature for analysis by polarography surpasses that for any other electrochemical method by at least an order of magnitude. 

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