Poggendorf compensation method

The EMF of a galvanic cell is a thermodynamic equilibrium quatity. Thus, the potential of a cell must be measured under equilibrium conditions, i.e. without current flow. The measured EMF must be compensated by a known external potential difference. The measurement of the EMF of a cell is thus based on determination of a potential difference that exactly compensates the measured potential difference so that no current passes. This is easily achieved by the Poggendorf compensation method (see Fig. 3.13). 

In the null-point instruments use is made of the well-known compensating method according to Poggendorf, by which the emf of the cell under test is compared with that of a standard cell. The circuit diagram of such a method54 is illustrated in Fig. 2.14. 

It is an attractive feature of potentiometry that the equipment is rather inexpensive and simple one needs a reference electrode, an indicator electrode and a voltagemeasuring instrument with high input impedance. The potential measurement has to be accomplished with as low a current as possible because otherwise the potential of both electrodes would change and falsify the result. In the past, a widespread method was the use of the so-called Poggendorf compensation circuit. In most cases today, amplifier circuits with an input impedance up to 10 2 are used. The key element for potentiometry is the indicator electrode. Currently, ion-selective electrodes are commercially available for more than 20 different ions and almost all kinds of titrations (acid-base, redox, precipitation and complex titrations) can be indicated. In the following, some indicator electrodes and the origin of the electrode potentials will be described. 

In contrast to ion-selective electrodes, electrodes of the first and second kind, as well as redox electrodes, usually exhibit exchange current densities Zq > 10" A/cm. Such electrodes can easily tolerate currents of around 10 A. For this reason, not nearly as much importance has been attributed to the circuit current with earlier electrochemical techniques. Many textbooks mention the compensation method of Poggendorf in this connection. In this method the EMF to be measured is balanced by an equally large opposing voltage to avoid too large a circuit current. Figure 34 illustrates this... 

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