Polarographic Curves

If — during this process — the Cu2+-concentration decreases, the mixed potential will shift along the cathodic partial current density curve (like a polarographic curve in this example) toward the equilibrium potential of the zinc amalgam, in case the amalgam reservior is large enough. 

FIGURE 7. Polarographic curves of (PhS02)2CHS02CH3(l dim) in DMF in the presence of Et4NBr 0.1 m curve a, response in neutral DMF curve b, obtained in the presence of an excess of Et4NOH (after Reference 42). 

Theoretical treatment of polarographic curves for the calculation of values of jo has been described [65Hey, 66Hey], for an overview see [94Gal], a further evaluation procedure has been described [6801d]. Experimental details, in particular of solid electrodes in combination with a rotating disk electrode have been reported elsewhere [84Guy]. (Data obtained with this method are labelled PP.)... 

The sensitivity and selectivity can be raised when recording as a function of potential not the current but its derivative with respect to potential. In this case a curve with maximum is obtained (Fig. 23.4) instead of the polarographic wave. The potential of the maximum corresponds to the half-wave potential in an ordinary polarographic curve, and the height of the maximum is proportional to the concentration of the substance being examined. A signal proportional to the derivative can be formed in polarographs with the aid of relatively simple electric circuitry. 

Shape of the polarographic curve. The kinetic theory of electrolysis (Section 3.2) for a redox system at a static inert electrode for partial and full exhaustion at the electrode under merely diffusion-controlled conditions leads, for ox + ne - red, to the relationship... 

As ksh in this instance is very small, then according to the Butler-Volmer formulation (eqn. 3.5) the reaction rate of the forward reaction, K — 8,he "F(E 0)/flr, even at E = E°, is also very low. Hence Etppl. must be appreciably more negative to reach the half-wave situation than for a reversible electrode process. Therefore, in the case of irreversibility, the polarographic curve is not only shifted to a more negative potential, but also the value of its slope is considerably less than in the case of reversibility (see Fig. 3.21). In... 

Majer65 in 1936 proposed measuring, instead of the entire polarographic curve, only the limiting current at a potential sufficiently high for that purpose if under these conditions one titrates metal ions such as Zn2+, Cd2+, Pb2+, Ni2+, Fe3+ and Bi3+ with EDTA66, one obtains a titration as depicted in Fig. 3.55 i, decreases to a very low value, in agreement with the stability constant of the EDTA-metal complex and the titration end-point is established by the intersection of the ij curves before and after that point correction of the i values for alteration of the solution volume by the titrant increments as in conductometric titration is recommended. 

Further, the operator must be able to choose the drop lifetime and the scan parameters, viz., the starting potential, direction (cathodic or anodic), rate and end potential, together with the sensitivity of the current measurement and the amplification in the ohmic cell resistance compensation circuit. Convenient additional facilities are (a) display of the polarogram on an oscilloscope, (b) delivery of hard copy of the polarograms on a chart recorder and (c) repeated recording of the polarographic curve for the same sample. 

Sluyters and coworkers [34] have studied the mechanism of Zn(II) reduction on DM E in NaCl04 solutions at different water activity (uw) using faradaic impedance method. Dqx and E p were determined from dc polarographic curves. Hydration numbers of Zn(Il) ion were estimated from the dependence of E[p on In Uw The obtained standard rate constant was changing with a NaCl04 concentration and the slope of the dependence of In k on potential was changing with potential (see Fig. 1). Therefore, the following mechanisms were proposed ... 

The curve is characterized by the peak height h, the peak potentials Fpc(red) and pa(ox) (or the half-peak potentials, which may be easier to measure exactly than the peak potential because of the somewhat flattened shape of the peak), and the peak height of the anodic peak. Further information may be obtained by semiintegration whereby a curve is obtained that resembles a polarographic curve and that contains all the information of the original data, not only peak heights and peak potentials. 

On polarographic curves one reduction wave is observed 14), the height of which decreases with increasing pH in the shape of a dissociation curve (Fig. 6). This wave is attributed to the reduction of the keto-form. Simultaneously with the decrease in the cathodic wave, an increase in an anodic wave, attributed to the oxidation of the ambidentate anion, can... 

Evidence for intermediates that have a suitable life-time, is often shown on polarographic curves by the appearance of a new wave at potentials more positive than the reactants or products. Intermediates are from their very nature, highly reactive species it is reasonable that species which are reactive in chemical reactions will also be reactive in electrochemical processes, and this is reflected by their rather positive half-wave potentials. 

When Ei differs sufficiently from E%, then with the change in pH two wave appear on polarographic curves. Furthermore, if n = n%, the change in the wave-heights is similar to that shown in Fig. 2. The difference between the thermodynamic and activation-controlled systems is that in the region in which the two waves are observed for the equilibrium case, described in Chapter 2.1, both waves are diffusion-controlled, whereas for the activation-controlled system discussed here, the more positive wave at i <0.15 ia possesses a kinetic character. 

Attempts to use polarography in the study of protolysis of phenyl-tropylium ions failed because the compounds react with metallic mercury and therefore give on polarographic curves a very positive wave, which arises from mercury dissolution and is unsuitable for quantitative treatment (57). 

For systems which involve rapidly established acid-base equilibria preceding the electrode process proper, the polarographic curves show a single wave the height of which is pH-independent. For the simple scheme (26) ... 

On polarographic curves two waves are observed the height of both waves increases with increasing pH (Fig. 23) and depends on buffer composition, as the dehydration is general base-catalysed. At more positive potentials, the bond C—X is cleaved in the form, where the polarizibility of this bond is enhanced by the vicinity of the free CO group. The half-wave potentials of the more negative wave are identical with... 

When dehydration occurs as a consecutive reaction, its effect on polarographic curves can be observed only, if the electrode process is reversible. In such cases, the consecutive reaction affects neither the wave-height nor the wave-shape, but causes a shift in the half-wave potentials. Such systems, apart from the oxidation of -aminophenol mentioned above, probably play a role in the oxidation of enediols, e.g. of ascorbic acid. It is assumed that the oxidation of ascorbic acid gives in a reversible step an unstable electroactive product, which is then transformed to electroinactive dehydroascorbic acid in a fast chemical reaction. Theoretical treatment predicted a dependence of the half-wave potential on drop-time, and this was confirmed, but the rate constant of the deactivation reaction cannot be determined from the shift of the half-wave potential, because the value of the true standard potential (at t — 0) is not accessible to measurement. 

Another system in which ring-formation has been considered to be manifested on polarographic curves is the reduction of pyridoxal (77, 80). The reduction wave of this compound changes with pH and the observed plot is similar to that shown in Fig. 22. This dependence can be explained either by hydration (as for other pyridine carboxaldehydes), or by hemiacetal formation. The same two interpretations can be applied to electronic spectra. A comparison with the behaviour of pyridoxal-5-phosphate can contribute to the solution of this problem. With this ester the formation of the hemiacetal form is impossible and practically no current decrease in acidic solutions can be observed. Hence it can be concluded that the decrease in the limiting current of pyridoxal is due to ring formation. Nevertheless, the possibility of some participation by a dehydration reaction cannot be completely excluded, for it is possible to assume that the introduction of a phosphoric acid residue into position 5 either shifts the equilibrium towards the dehydrated form or increases the rate of dehydration. 

Polarography can offer useful information for controlled potential electrolysis, in particular in the choice of a proper supporting electrolyte that would allow the best defined waves of the studied substance to be obtained, unaffected by electrolysis of other components of the solution. From polarographic curves, it is furthermore possible to obtain some... 

As the polarographic curve, furthermore, can be influenced by certain compounds or inhibitors, adsorption phenomena can complicate the interpretation of the curves, and catalytic waves may suggest further reductions than those found by macroelectrolysis, a certain caution must be exercised in evaluating the voltammetric data. In most cases, however, no complications arise, and with a little experience the differences mentioned above are not serious drawbacks, but are of value as the combination of polarography and macroelectrolysis then throws light on one or more of the steps in the reaction. 

Curve a indicates the dependence of the current on the cathode potential of the unreduced solution below —0.45 volt (SCE) no electrons are transferred, but between —0.45 and 0.60 volt the electrons gradually get enough energy to be transferred to the substrate. The polarographic curve is in this case distorted by a so-called maximum which may be of first or second order. Between —0.6 and — 0.85 volt the current is determined by the rate of transportation of substrate to the electrode surface, as the potential has a value where all molecules of 7 are reduced very quickly to 8, when they reach the... 

Fig. 10. Polarographic curves of 4-eyanopyridine. The curves start at -0.2 volt (SCE) 200 mvolt/mark. From Volke and Holubok.293...

The Half-Wave Potential.—In the preceding description of the analytical applications of the dropping mercury cathode it has been supposed that the nature of the reducible substance has been determined and that the position of the corresponding wave on the current-potential curv c is known. If the substance has not been previously identified, however, it is possible to do so by means of the polarographic curve. The reducible material is characterized by its half-wave potential this is the potential... 

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