Equivalent Circuit for a Cell with Liquid Junction

1 Equivalent Circuit for a Cell with Liquid Junction 

Our task is to measure the EMF of an electrochemical cell as accurately as possible. The measuring apparatus chosen should indicate only the most electrochemically meaningful precision. Fig. 32 illustrates the measuring circuit of an ion-selective in- 

Equivalent circuit for an electrochemical cell with liquid junction 

A 01 is the Galvani potential at the phase boundary between two different metals, for example at the contact point between the common silver wire shunt element and the copper instrument lead. Ignoring the surface potentials (see Ax Chap. 1.1), the EMF at such a contact point is known as the contact Volta potential. These can be highly temperature dependent, as demonstrated by the analogous thermocouples. The Ag/Cu contact has a temperature coefficient of 10 mV/°C, the Cu/tin solder contact 10 mV/°C, and the Cu/CuO contact an observable coefficient of 1 mV/°C. This last contact may arise if corroded copper plugs are used. In precise measurements striving for accuracies and sensitivities 0.1 mV, clean contacts are a must, and temperature variations should be avoided as much as possible. 

A 03 is the equilibrium Galvani potential at the inner phase boundary of the ion-selective membrane. As for A 02, this will only be constant when the activity of the corresponding ion taking part in this electrode reaction (the measured ion) is constant in the inner solution. The resistance of the active phase, Ry[, is often the largest of all resistances in the circuit. As such it determines the error in the EMF measurement due to the /jn potential drop, along with the circuit current /m j determined by the input resistance of the voltmeter. 

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