Amperometric Titrations By Satish Pradhan Dnyanasadhana College, Thane. Department of Chemistry T.Y.B.Sc. Analytical Chemistry Paper-IV Sem-VI Amperometric Titrations By Dr.Bhagure G.R.

Contents 1.3 Amperometric titrations (04L) 1.3.1 Comparison between Amperometry and voltammetry. 1.3.2 Basic principle of amperometry 1.3.3 Rotating platinum electrode: Instrumentation, advantages and limitations. 1.3.4 Amperometric titrations: Examples and titration curves. 1.3.5 Applications, advantages and limitations

Rotating Platinum electrode Flange inward to inward to prevent mercuring from being thrown out Glass tubing Copper Wire Mercury reservoir Hole Stem to make electrical contact mercury reservoir Platinum Wire Direction of rotation

Diffusion current is directly proportional to concentration

Diffusion Current uA 20 40 60 80 100 Concentration

Principle of Amperometric titrations From polarogram it can be seen that , the diffusion current ( = limiting current - residual current) is proportional to the concentration of the electro-active material in the solution. If some of the electro-active material is removed by interaction with reagent, the diffusion current will decrease. This is the fundamental principle of amperometric titrations. The observed diffusion current at a suitable applied voltage is measured as a function of the volume of the titrant added. the end point is the point of intersection of two lines giving the change of current before and after the equivalence point. Some advantages of amperometric titrations may be mentioned:

Principle of Amperometric titrations Titrand Titrant Product Out of these three one is reducible

Titrand is reducible but Titrant and product not . Pb+2 Titrant SO4-2 Product The titration can be performed at fixed potential -0.8 Volt v/s saturated calomel

1. Titrand is reducible but titrant and product not: When solution containing Pb+2 ion is titrated against SO4-2 ion. A precipitate of PbSO4 is formed. The titration can be performed at fixed potential -0.8 Volt v/s saturated calomel electrode. As titration is proceeds concentration of Pb+2 ion decreases and diffusion current also decreases till it becomes minimum at equivalence point. The diffusion current remains constant beyond end point. The values of diffusion current is plotted against the volume of titrant added .The resulting titration curves is straight line leveling off at end point . The intersection of two extra plotted portions of the curves gives the end point.

Ex.Pb+2 v/s SO4-2 at constant potential -0.18 V (SCE) A) Ttrand is reducible but titrant and product not. Ex.Pb+2 v/s SO4-2 at constant potential -0.18 V (SCE) Diffusion current Equivalence point Volume of Titrant added

Titrant is reducible but Titrand and product not . Mg+2 Titrant 8- hydroxy quinoline . Product The titration can be performed At constant potential of -1.6 volt. v/s saturated calomel

2. Titrant is reducible but titrand and product not : When solution containing Mg+2 ion is titrated against with the reducible species such as 8- hydroxy quinoline . At constant potential of -1.6 volt. In this titration the current is steady till the end point because Mg+2 ion does not undergoes reduction. Beyond the end point the 8- hydroxy quinoline undergoes reduction. As its concentration increases diffusion current also increases.

Ex.Mg+2 v/s 8-Hydroxy quinoline -1.6 V(SCE) B) Titrant is reducible but titrand and product not Ex.Mg+2 v/s 8-Hydroxy quinoline -1.6 V(SCE) Diffusion current Equivalence point Volume of Titrant added

Titrand and titrant both are reducible but product not Pb+2 Titrant K2Cr2O7. Product The titration is performed at potential of -0.8 Volt v/s SCE

3. Titrand and titrant both are reducible but product not : When solution containing Pb+2 ion is titrated against K2Cr2O7. The titration is performed at potential of -0.8 Volt v/s SCE . Diffusion current is decreases due to removal of Pb+2 ion. The current is minimum at the end point. On further addition of the titrant the current once again increases. V shaped curve is obtained.

Volume of Titrant added Ttrand and titrant both are reducible but product not Ex.Pb+2 v/s K2Cr207 at constant potential -0.8 V (SCE) Diffusion current Equivalence point Volume of Titrant added

Advantages of Amperometric Titration 1 Method is accurate and equivalence point is obtained by means of graph 2 The electrode surface is continuously renewed ,current values are reproducible. 3 Amperometric titrations can be carried out at dilutions at which Potentiometry, or ordinary titrations method can not work 4 Readings near the end points are not of importance in this method , so precipitations titrations can be carried out.

Disadvantages of Amperometric Titration 1 Considerable time is required to remove dissolved oxygen before and during the titration 2 The titrations can be carried out at potentials more negative than 2 volts since hydrogen will be evolved. 3 Co precipitations can give inaccurate results.

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