pHYSICAL cHEMISTRY - Iv

Experiment 2

Principle

Potentiometric titration is a volumetric method in which the potential between two electrodes is measured (referent and indicator electrode) as a function of the added reagent volume. potentiometric titration can be classified as acid-base, redox, precipitation and complexometric titrations. Potentiometric titrations provide more reliable data then data from titrations that use chemical indicators and are partially useful with colored or turbid solutions and for detecting the presence of unsuspected species.

In potentiometric titration generally, two electrodes are used, an indicator electrode (the glass electrode and metal ion indicator electrode) and reference electrode ( hydrogen, calomel and silver chloride electrodes). The indicator electrode from and half-cell with interested ions while reference electrode forms electrode the other cell. The overall electric potential is calculated by using equation

Ecell = Eind - E ref + Esol

Ecell is recorded at intervals as the titrant is added. Ecell depends on the on the concentration of the analysts Esol is potential drop over the test solution between the two electrodes.

Theory:

According to the Nernst equation, the potential of the electrode depends upon the concentration of the ionsreversible to the electrode and are in contact with it.

Ecell = E0 cell - RT / nF ln Q

The concentration of the ions can be changed by the neutralization ,precipitation or a redox reaction and hence electrode potential varies accordingly.The end point coinsides with a very rapid change in the potential of the indicator electrode.The potential change near equivalent point is rapid as compared to other points of the course of the titration.The end point can be determined accurately and precisely from the plot of E against V and plot of ∆E / ∆Vagainst V show in below figure.The end point can be obtained precisely from the by extrapolation of two linear branches.The shape of the curve depends on the type of titrations.

Materials required

Apparatus: Potentiometer, SCE and Pt-electrode,burette, beaker, wash bottle etc.

Chemicals: xN HCl ,0.1 N NaOH, Quinhydron powder, distilled water etc.

Procedure

Take 50 mL of xN HCl solution into a 150 mL beaker.Add about 0.5 g quinhydron powder and mix well.Titrate the solution with 0.1 N NaOH solution and measure the potential at each stage of addition.The addition should be carried out at the rate of 0.5 mL.Plot E against V and ∆E / ∆V against V .Determine the equivalence point by extrapolating the two linear lines and determine end point.

Observation table

Graph: Plot E against V and ∆E / ∆V against V

Calculations

1.Normality of Hydrochloric acid solution from graph (a)

HCl = NaOH

N1V1 = N2V2

N1 = 0.1 * V2 (from graph a) / 50

2. Normality of Hydrochloric acid solution from graph (b)

HCl = NaOH

N1V1 = N2V2

N1 = 0.1 * V2 (from graph b) / 50

Result

1.Concentration of given hydrochloric acid solution from graph a = __________ N

2..Concentration of given hydrochloric acid solution from graph b = __________ N

Reference Material

Practical Physical Chemistry Google EBOOK Link.

Questions

1.Give the Principle of Potentiometric Titration.

2.How instrumental method is advantageous over Classical method for determination of concentration of unknown solution ?