Polarography

 Chapter 14

Polarography 

Introduction 

• The polarographic method of analysis has been developed by Jaroslav. Heyrovsky, a Czech chemist in 1922, who received Nobel prize in chemistry in 1952. Since then, this method has acquired immense importance in analytical chemistry for determination of metal ions and other electroactive organic compounds.

• The term polarography is applied to the current voltage curves when dropping mercury electrode is used as an indicator electrode. This method comes under the category called as voltammetry, which is a general term used for all current voltage methods using micro- electrodes.

• Polarography is essentially an electrolysis on a microscale for the solution of substances in a concentration range of 10"6 to 1 0"2 M. The method is based upon study of the current- voltage relationship by using polarographic apparatus.

Principle 

• It will be seen that initially there is a seq small gradual rise in the current between A and B. It is known as residual current.

• Though no electrolysis process takes place before the applied potentialupto B, no current should flow through the galvanometer. The small background current which is usually observed is knowns residual current. It is sum of faradic current (if) and condenser current.

The Polarographic Apparatus

• There are two main units of polarographic apparatus, a polarographic cell and an electrical instrument. Polarographic cells are of numerous types and different forms are available commercially. Simplest one consists of a beaker type of glass cell with a lid. A large pool of mercury at the bottom of cell has an electrical connection. This mercury electrode acts as an anode terminal. The lid has glass bend tubes for passing nitrogen gas and tip of dropping mercury electrode.

A solution to be analyses along with solution of ground-inert electrolyte and maxima suppressor is placed in the cell. The anode and cathode are joined to the positive and negative terminals of a potentiometer slide wire. This enables a variable potential difference to be applied between the anode and cathode which is indicated by a voltmeter (V). The current flowing through the cell is indicated by the galvanometer (G). Air from the cell is driven off by passing a current of nitrogen before the measurement of current voltage is made.

Practical Aspects of Polarography 

The Dropping Mercury Electrode: As stated earlier dropping mercury electrode (DME) is used as an indicator electrode for its unique nature

Advantages of DME: 

(i) It provides a smooth, fresh surface for the reaction.
(ii) Each drop remains unaffected and does not become contaminated by the deposited metal.
(iii) Mercury forms amalgam with most metals.
(iv) Mercury has a high hydrogen overvoltage.
(V)  Diffusion equilibrium is readily established at mercury-solution interface

Disadvantages of DME: 

(i) Surface area of a drop of mercury is never constant.
(ii) Applied voltage produces changes in surface tension and hence change in drop
size.
(iii) Mercury has limited applications in analysis of more positive potential range,
(iv) Mercury is poisonous so care should be taken in its handling.

Measurement of Wave heights 

• For the quantitative evaluation of polarogram precise measurement of the height of each wave need be carried out. With the smooth polarographic wave the residual current plateau and limiting current plateau are almost parallel and measurement of diffusion current is simple. Subtracting the residual current from the limiting current by extrapolation of lines gives the measure of diffusion current. For measurement of wave height of diffusion current and extrapolation method as shown in Fig. 14.7 is carried out. As shown in the Fig. 14.7 the lines are constructed on the graph of current voltage to find a point F for determining half wave potential. The height G for the diffusion current is also determined for quantitative work.

Quantitative Polarographic Methods 

There are various methods by which quantitative estimation of drugs, metal ions etc. is made. Some such methods are described as under:

1. Direct comparison method: In the direct comparison method measurements of current of a standard solution of test ion with that of unknown or test ion is carried under the same condition of experiment. Then, using the Ilkovic equation, thdiffusion current quotient id/C can be found out. The height of unknown wave when divided by quotient gives the concentration of unknown (test) ion. For accuracy of this method, the conditions employed in experiment should be the same.

2.Calibration curve method: Various concentrations of standard solutions are analyzed for determining their diffusion current. A plot of diffusion current vs. concentration is plotted. Diffusion current of unknown (test) sample is determined under the same conditions of experimentation of standard solution and the concentration of unknown is found out from the graph. This method gives accurate results provided same conditions are kept with respect to capillary characteristic, temperature, flow rate of mercury, concentration of maxima suppressors.

Recent Advances in Pyrography 

In recent years, some significant and useful advances in polarographic techniques have been reported. Some such advances are given below:

1.   Alternating current polarography: This method has been developed in an attempt to          have more rapid and accurate determination of half wave potential. The method                enables estimation of small concentration of ion in presence of other reducible                   substances. In this technique alternating current is supplied to the electrode.

2.  Oscillographic polarography: This method utilises alternating current coupled with            oscillograph. The method is adapted for analysis of pharmaceutical products like               antibiotics,vitamins, hormones from various materials.