Amperometry and Biamperometry

The Detailed Theory and instrumentation of Both Amperometry and Biamperometric analysis is given with Titration curves and Applications.
Medha Thakur (M.Sc Chemistry)

1. AMPEROMETRy and Biamperometry Presented by : Medha Thakur MSc Chemistry, S.N.D.T university

2. Contents AMPEROMETRY Introduction Principle Instrumentation Titration Cell Indicator Electrode Experimental Condition Titration Procedure & Curves Advantages BIAMPEROMETRY Introduction Instrumentation Titration Curves Application

3. INTRODUCTION Amperometry is a general term for techniques, whereby the concentration of an electroactive substance is measured by the current which results from its reaction at an electrode. Amperometric titration are those in which amperometry is used to follow concentration changes and thus to detect equivalent points of titrations.

4. PRINCIPLE the limiting current in polarography is independent of the applied voltage impressed upon a D.M.E. (or other indicator) is proportional to the concentration of the electroactive material in solution. This is the Fundamental principle of amperometric titration. the end point is the point of intersection of the two lines, giving the change of current before and after the equivalence point. the current which passes through the titration cell between an indicator electrode (e.g. DME) and the appropriate reference electrode (e.g. S.C.E.) is measure as a function of the volume of the titrating solution.

5. INSTRUMENTATION It is relatively simple and consist of the following : an emf source such as a battery or a power supply a potential regulating device, a microammeter or galvanometer, an electrolysis and titration cell, indicator electrodes, reference electrode and salt bridge, a titrant delivery device such as burette or a micro burette and stirrer. The electrical circuit is similar to that in polarogrphs. The only difference is in the cell.

6. 1) TITRATION CELL An excellent and inexpensive titration cell consist of a Pyrex glass 100 cc flat or round bottomed flask as shown in fig. The burette , N 2 inlet tube , DME and the salt bridge are fitted into the neck by means of a rubber stopper . The salt bridge is connected to a reference electrode (SCE).

7. 2) INDICATOR ELECTRODES Normally DME is used as the indicator electrode However it cannot be used at high positive potentials (above 0.4v w.r.t. S.C.E.) because of oxidation of Hg. Hence above +0.4v upto 1.1v a rotating platinum micro electrode is used . The diffusion current is not steady if a stationary Pt electrode is used.

8. EXPERIMENTAL CONDITION cell is maintained at a fairly constant temperature during the titration . It is advantageous to store the reagent beneath an atmosphere of inert gas. not necessary if the regent solution has concentration 10-20 times the concentration. If the analyte is electro-reducible , supporting electrolyte is added to eliminate the migration current. If the reagent is electro-reducible & the analyte not, then addition of supporting electrolysis is not required A maxima suppressor may be added. If the polarographic characteristic of the analyte & the reagent are not known.

9. TITRATION PROCEDURE The general procedure is as follows: A known volume of the solution under test is placed in the titration cell. which is then assembled with the burette, DME, N 2 inlets etc. The electrical connections are completed (DME at cathode and SCE or Hg pool as anode) and the dissolved O 2 is removed by passing pure N 2 for 15 min. The voltage is adjusted to the desired value & the initial id value is noted. A known volume of reagent is run from a semi- microburette , N 2 bubbled in for 2 min. The current & burette reading are both noted. The procedure is repeated until sufficient readings have been obtained to permit the end point to be determined as the intersection of the two linear parts of the graph.

10. TITRATION CURVES fig. A, B & C are most common types of curves encountered in amperometric titration. The slight rounding off in the vicinity of the equivalence point is due to the solubility of the ppt. For each amperometric titration the applied voltage is adjusted to the value between X and Y as shown in fig. A’-C ’.

11. Curves In fig. A , only the material being titrated is reduced and gives a diffusion current [For e.g. Pb [II] ions titrated oxalate or sulphate ions] the current decrease upto the end point. After the end point the current is fairly constant.

12. Continue In fig. B , Reagent is reduced, analyte is not. [e.g. sulphate ions titrated with Ba [II] or Pb [II] ions]. Thus current is constant upto the end point but rises steadily after the end point.

13. Continue .. In fig. C , both analyte and the titrating reagent are reduced and give diffusion currents and a sharp V shaped curve is obtained. [ for e.g. Pb ions titrated with dichromate ions, Ni [II] with dimethyl glyoxime ] Thus current decreases upto the end point & then rises steadily after the end point.

14. ADVANTAGES The titration can usually be avoided out rapidly , since the end point is found graphically. can be carried out at dilutions[10-4M] at which visual or potentiometric titrations no longer yield accurate results. Foreign salts may frequently be present without interference, usually added as the supporting electrolyte in order to eliminate migration current. The results of the titrations are independent of the characteristic of capillary. temperature need not be known, it is kept constant during the titration.

15. BIAMPEROMETRY UNIT-1

16. INTRODUCTION These is a modification of the amperometric method . In this method, two identical stationary microelectrodes, immersed in a well stirred solution of the sample are used, instead of the SCE and the DME. A small potential [say 0.01 to 0.1v] is applied between the two electrodes Reversible oxidation to reduction system be present either before or after the end point. also sometimes called as the dead stop end point technique.

17. INSTRUMENTATION A simple apparatus suitable for biamperometry is shown in fig. is A pair of identical microelectrode (E&F) are used 3v battery or 2v accumulator, M is a micro ammeter, P is potentiometer to control the potential The potential is set between 10-100 mv .

18. TITRATION CURVES fig. a,b,c the three types of titration curves that are commonly encountered. Curve in fig. a is observed when both the reactants are reversible w.r.t. the electrode curves . in fig. b & c are obtained when only one of the reactant exhibits reversible behavior. The titration of Fe [II] with Ce [IV] is an e.g. of fully reversible system.

19. APPLICATIONS Bimperometric titration method has been widely applied to titrations involving I 2 , it is also useful with reagents such as Br 2, Ti [III], Ce [IV]. An important use is the titration of H 2 O with KFR. The principle advantage of the method is its simplicity. No reference electrode is require & the only instrument needed is a simple voltage divider.