pHYSICAL cHEMISTRY - IiI

Experiment 5

Aim of the Experiment

To determine the strength of the halides in a mixture using standard silver nitrate solution.

Principle

A mixture of iodide and chloride may be titrated with AgNO3 solution potentiometrically using a silver electrode. The first point of the inflection is the equivalence point of iodide ions and the second is that for chlorine ion reaction. The indicator electrode must be reversible to the halides and silver electrode serves this purpose. The following cell is constructed,

Pt, Hg/Hg2Cl2(S), KCl// Ag+ / Ag

(E1) (E)

Eobs = E – E1

= E 0 + RT / F *log[Ag + ] − E1

Therefore when AgNO3 is added to the system, e.g. KCl, KI etc., AgCl and AgI is formed which in turn dissociates to produce Ag+ ions and this becomes reversible to the silver electrode.

Materials required

Silver electrode, Standard calomel electrode, Potentiometer etc.

Procedure

The cell is set up and connections are made carefully, calomel electrode is used as the other half cell along with ammonium nitrate salt bridge. At the beginning of the titration calomel electrode is positive, while the silver electrode dipping in the mixture of halides is negative. When all the iodide ions have been precipitated as AgI, the first equivalent point is noted and the electrode terminals are reversed indicated by change in sign of electrode potential such that the silver electrode becomes positive when the titration is being continued. 20 or 30 ml of the mixture of the halides is taken in a beaker and the clean silver electrode is immersed in the solution. The e.m.f reading are taken as the volume of silver nitrate added. The addition causes the e.m.f to change and consequently the e.m.f changes are measured. The addition of silver nitrate is carefully done such that the equivalence points are clearly found out. The volume of silver nitrate is plotted in a graph against the e.m.f and the first derivative graph of Eobs vs volume of silver nitrate is made. Two maxima are observed, one for the iodide and the other for the chloride ion. Knowing the strength of silver nitrate, the strength of halides can be calculated.

Calculations

From graph,

1. Concentration of Iodide ion in Halide mixture

M1V1 = M2V2

M1 = 0.01 * V2 (from graph) / 30

Strength of iodide ion in solution : M1 * molecular wt. of KI

2.Concentration of Chloride ion in Halide mixture

M3V3 = M4V4

M1 = 0.01 * V4 (from graph) / 30

Strength of Chloride ion in solution : M3 * molecular wt. of KCL

Result

Strength of iodide ion in the mixture = M

Strength of chloride ion in the mixture = M

Conclusion

Based on this experiment, we can conclude that potentiometric titration using a system of a electrolytic cell can be used to analyze the concentration and identify species of halides present in any sample.

Reference Material

1. Practical Physical Chemistry Google EBOOK Link.

Questions

1.Give the applications of Potentiometric titration.

2.What is precipitation titration?