A titration in which endpoint is determined by measuring potential of Indicator electrode is known as potentiometric titration

1. To determine strength of given
HCI solution potentiometrically
By
Dr Bhavana P Khobragade
RDIK and NKD College Badnera

2. Potentiometric Titration
• The titration in which end point is determine
by measuring potential of an Indicator
electrode is known as potentiometric titration.
There are two electrodes used in
Potentiometric Titration
• An indicator electrode is one whose potential
changes as a result of change in concentration
of ions with which it is reversible

3. In acid base titration, an electrode whose potential depend on
concentration of H+ ions is selected. Some commonly used
indicator electrodes use for such a titration are hydrogen
electrode, quinhydrone electrode, and glass electrode. For
routine potentio- metric titration, quinhydrone electrode is
preferred because it is easy to set up and establishes
equilibrium very quickly.
The potential of quinhydrone electrode is given by
EoxQ = E0
oxQ - 0.0591[H+]
EoxQ = E0
oxQ + 0.0591pH
EoxQ = -0.69 + 0.0591pH

4. In general practice, a small quantity of quinhydrone is
added to a solution whose potential is to be measured
and a platinum electrode is dipped in the solution. When
coupled with calomel electrode, a cell is formed
Hg | Hg2Cl2,KCI || H+unknown conc. Q| QH2| Pt.
whose potential is given by
Ecell = E ox LHE –Eox RHE
(-0.24) –(-0.69 + 0.0591pH)
= 0.45-0.0591pH
pH = (0.45-Ecell)/0.0591
Thus the change in pH reflects is changing e.m.f. When
NaOH is added initially e.m.f. changes very slowly and
rapidly at the equivalence point and then again very slowly
as shown in the fig.`

5. A more sensitive and satisfactory method of finding the end point
is a plot E/V against volume. The maximum in the curve gives
the end point of titration.

6. Apparatus : Potentiometer, calomel electrode,
platinum electrode, 100 ml glass beaker, pipette,
burette, stirrer etc.

7. Chemicals
0.1 M HCI, 0.1 M NaOH, quinhydrone, oxalic acid etc.
.

8. Procedure
• Using oxalic acid determine normality of NaOH solution.
• Standardize the potentiometer with std. cell.
• Pipette out 10 ml 0.1 M HCl in a 100 ml glass beaker.
• Then add 30 ml distilled water and a pinch of quinhydrone.
• Immerse into it platinum and calomel electrodes at list 1 cm
deep below the surface of the solution.
• Connect the two electrodes to a potentiometer and measure
the e.m.f.
• Add 0.5 ml NaOH successively each time and measure e.m.f.
• Note point where sudden change in e.m.f. (e.m.f. change > 20
mV)
• Continue the titration for more 5-6 readings after the e.m.f.
has started decreasing.

9. Sr. No. Volume of oxalic acid
taken (cm3)
Volume of NaOH
required(cm3)
01 10ml 10.5ml
02 10ml 10ml
03 10ml 10ml
Observations : Weight of oxalic acid dissolved = ........... g
Table 1: Standardization of NaOH
1. Determine Molarity of NaOH
Oxalic acid NaOH
N1V1 = N2V2
10ml X 10ml = N2 X10ml(End Point)
N2 = 0.1N

10. End Point = 8.5

11. • Graph : Plot a graph between the e.m.f. of the
cell and the volume of alkali added. The point
of inflection in the curve gives the end point.
Also plot E/V against volume of alkali
added. The maximum of the curves gives the
end point.

12. Calculations :
End point from the graph = 8.5 ml.
NaOH HCl
N2V2 = N3V3
0.1 X 8.5 = N3 X 10
Therefore N3 = 0.085 N HCl
Strength = 0.085 X strength of HCl (36.5)
= 3.1 gm/lit
Result : The strength of given HCl solution is found to be 3.1 gm/lit

13. Precautions :
1. Care must be taken that glass rod does not
hit electrodes during stirring.
2. While performing titration, add NaOH in
small portions near the end point.

14. Thanks