Dissociation Curve polarographic

Fig. 20. Comparison of polarographic (dotted) and potentiometrio (full) dissociation curves. [Polarographic dissociation curve corresponds to a dependence of ratio of wave-heights i/ilif on pH. The shift of polarographic p C -value when compared with potentio-metric pi results from recombination reaction A 4-B C.]...

To apply equation (3) for calculation of the equilibrium constant K waves Ia and ic must both be limited by diffusion. To prove this the current is measured under conditions when it is 15% or less of the total limiting current and its dependence on the mercury pressure is followed. A diffusion current must, under these conditions, show a linear dependence on the square root of the height of the mercury column. Whenever possible, polarographic dissociation curves should be compared with data on dissociation obtained by other methods, e.g. potentiometry, N.M.R. or spectrophotometry. In the latter case it is important to show that the species responsible for a given polarographic wave is identical with that responsible for the observed absorption peak. 

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... 

Methods of identification based on classical polarographic curves are discussed first. Kinetic currents often depend strongly on the pH of the supporting electrolyte, because the rates of the chemical reactions involved are pH-dependent. The dependence frequently possesses the shape of a dissociation curve as depicted on Fig. 20, or of a curve with a maximum. 

A direct nucleophilic attack of hydroxide ions on tropylium was rejected, because such reaction would result in a different shape of the polarographic dissociation curve. Using equation (25) rate constant A i = 2 X 10 sec M was computed (Zuman et al., 1961) and, using a titrimetrically determined value of the equilibrium constant also... 

Another common example of kinetic currents is observed with reducible acids and substances that are reduced in protonized and unprotonized forms (e.g. some keto-acids< and other acids,oximes< nitrones ) etc.). For such substances two waves are observed on the polarographic curve, the ratio of which changes with the pH-value of the solution in the shape of a dissociation curve (Figs. 11 and 12). The more positive wave corresponds to the acidic form, and the more negative one to the reduction of the corresponding base. However, the ratio of the two waves does not correspond to the equilibrium conditions in the bulk of the solution. The height of the more positive wave is considerably higher than it would be if it corresponded to the equilibrium concentration of the acidic form. Additional acidic form is formed around the electrode by recombination from the... 

Fig. 12. Comparison of titration (full line) and polarographic (dotted line) dissociation curves. The shift is due to the recombination reaction.

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