Suppose we titrate a solution of HCl against NaOH. Here, the used electrode whose potential depends upon the H+ ion concentration (e.g. Hydrogen, quinhydrone electrode, glass electrode) is placed in an HCl solution.
It is connected to the Reference electrode (calomel, Ag, AgCl electrode) to form a galvanic cell.
Hydrogen electrode used as H+ ion indicating electrode and saturated calomel electrode used as reference electrode, then the galvanic cell represents,
The Emf of the cell is given by,
If 100 ml 0.1 M HCl is to be titrated against 1M NaOH (titrant). So, the concentration of titrant (1M NaOH) is 5 to 10 times higher than that of the solution titrated. So, the volume change is as small as possible.
As titration proceeds, H+ ion concentration declines, which means the pH of the solution increases, and according to equation (1) emf of the cell increases.
No change in volume during the titration, and adding the first 9 ml of NaOH solution will give a difference of 0.0591 volts. If the addition of the following 0.90ml exact change. Thus the emf of the cell changes slowly at first but more and more rapid change as the endpoint approaches.
After the endpoint addition of NaOH, you produced a minimal change in the H+ ion concentration and little change in the emf of the cell.
Here, A plot of E against the volume of NaOH adds, shown in fig. (a) Emf of cell initially increases gradually and more after near the equivalent point. After the equivalent point, the Emf of the cell rises slightly on adding more NaOH.
Figure (a) Potentiometric titration curve obtained in acid-base titration.
Fig. (b) shows the change in the Emf with a change in volume (∆E/∆V) against the volume of NaOH. Here, the curve rises to a maximum at the equivalent point. A vertical line from the peak determines the volume at the equivalent point to the volume axis.
Figure (b) Determination of endpoint in acid-base titration.
