Measurement of Electromotive Force Chemistry Notes
Measurement of Electromotive Force :
→ Poggendorff complementary principle is used for measurement of cell potential i.e., e.m.f. of cell. According to this, if a potential is applied by any other cell (standard westen cell) in opposite direction, which is equal to cell potential of a cell then current flow will stop in circuit. In such condition, potential difference between two electrodes is electromotive force of that cell. Potentiometer is used in this measurement. Circuit is established according to following figure :
→ AB is a wire of Pt-Ir alloy having one metere length of same horizontal cross-section which is stretched on meter scale. The Pt-Ir alloy is used in it due to following properties :
- It is hard metal so horizontal cross-section is fixed from rubbing of sliding contact.
- Its resistance is high so it is comfortable in potential balance.
→ C is a lead storage battery of fixed electromotive force whose electromotive force is higher then e.m.f. of the cell. X is that unknown cell whose e.m.f. is determined by which any one cell can be taken in circuit. The e.m.f. of Cis distributed uniformly on which ABwire on establishing circuit.
→ So, the value of potnetial drop is constant on moving from A to B when unknown cell X is taken in circuit which opposes e.m.f. of C because both these are working in opposite direction to each other. On moving sliding contact on AB wire, D is obtained in galvanometer (G) at zero deflection.
→ In this position potential drop of cell till AD length willbe equivalent to total potential of unknown cell X. So, the potential of X cell (Ex) will be directly proportional to AD length of wire. When standard westen cell (S) is taken in circuit and zero deflection position is also determined for this cell. Let it is D. So, potential of Scell (Es) will be proportional to AD length of wire. Hence,
The e.m.f. of standard westen cell is certain. So, the cell potential or e.m.f. of unknown cell X can be measured easily.