Schematic Diagram of Manual Potentiometer

To measure the potential of an electrochemical cell under a condition of zero current we use a potentiometer. Shown here is a schematic diagram for a manual potentiometer, which consists of a power supply, an electrochemical cell with a working electrode (identified as “W”) and a counter electrode (identified as “C”), an ammeter for measuring the current passing through the electrochemical cell (identified as i), an adjustable, slide-wire resistor (identified as SW), and a tap key (T) for closing the circuit through the electrochemical cell.

Figure11.3

Using Ohm’s law, the current in the upper half of the circuit is

iup = EPS/Rab

where EPS and Rab are, respectively, the power supply’s potential and the resistance between points a and b of the slide-wire resistor. In a similar manner, the current in the lower half of the circuit is

ilower = Ecell/Rcb

where Ecell is the potential difference between the working electrode and the counter electrode, and Rcb is the resistance between the points c and b of the slide-wire resistor. When iup = ilower = 0, no current flows through the ammeter and the potential of the electrochemical cell is

Ecell = (Rcb/Rab) × EPS

To determine Ecell we momentarily press the tap key and observe the current at the ammeter. If the current is not zero, we adjust the slide wire resistor and remeasure the current, continuing this process until the current is zero. When the current is zero, we can calculate Ecell.

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