Determination of strength of HCl potentiometrically

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 Eox Q = E oxQ - 0.0591[H + ] Eox Q = E oxQ + 0.0591pH Eox Q = -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