Amperometric Polarographic Measurements

Polarographic methods can be used to estimate the equivalence point of a reaction, provided that at least one of the participants or products of the titration is oxidized or reduced at the microelectrode. When the potential applied across the two electrodes is maintained at some constant value, the current may be measured and plotted against the volume of the titrant, thus, the term amperometric titration. In the case of working electrode-reference electrode pair, the potential of the indicator electrode is maintained at a constant value with respect to a reference electrode, measuring a limiting current, which is proportional to the concentration of one or more of the reactants or products of the titration. 

Several analytical methods will differentiate the "free" (hydrated) metal ions from dissolved complexed metal ions. These methods include specific ion electrodes, polarographic, and other amperometric and voltammetric methods and various types of spectroscopy (see Section 7-10). Specific ion electrodes only respond to the free metal ion for which they are "specific." To determine the relative amounts of complexed and uncomplexed metal ion in a solution, we can use a "wet chemical" method to measure the total concentration of "free + complexed" ions, and then an ion-specific electrode to determine the free metal ion concentration (activity). Care must be taken to eliminate interferences that may affect these measurements. We deduce the concentration of the "complexed ions" by the difference between these two measurements. For example, in the EDTA titration method for hardness, free and complexed calcium and magnesium ion s are measured. 

The ODR method relies on amperometric or polarographic techniques to measure the oxygen concentration of the soil pores. The amperometric techniques imply the measurement of current. The polarographic techniques imply that the electrode at which reduction of reactants occurs is in a polarized condition. Under polarized conditions, the concentration of reactants at the electrode surface is low, whereas the concentration of products is high. Under these conditions, the amount of current that the electrode will pass is directly proportional to the flux of reactant to the electrode surface. These principles are used in the ODR method. 

Finally, when amperometric measurements are carried out the decision must be taken automatically which set of the currents samples represents the diffusion current only. If the current samples are going to be used for a polarographic record the same principle is employed. Current samples for which the exponent value suggests any electrochemical complication are rejected. In the past much attention was given to studies of the i-t curves on a single drop as well as how the surface active substances influence the shapes of the curves. Worth noticing in this respect, according to the authors opinion, are a number of studies which sum up the more important developments. ... 

In indirect methods, as mentioned in Chapter IV, polarographic-ally inactive substances are transformed into compounds showing waves on polarographic curves, or concentration changes of a polarographically active substance which reacts with the electro-inactive compound to be determined, are measured. Finally, polarometric (amperometric) titrations can be included in this group of analytical methods. 

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