Polarography

 

ELECTRO  ANALYTICAL  METHODS: POLAROGRAPHY

☆Introduction :

1. Polarography is an accurate method for analysis of substances of various types of organic or inorganic, ionic or non-ionic in solution.

2. This method is constructed on measurement of relation between current and potential which established on continuous reproducing dropping mercury electrode, discovered by Heyrovesky in 1922. Reduction or Oxidation occurs at specific potential on this electrode. Each electrode shows concentration of reactant current concerned with reaction.

3. Polarography is one kind of electrolysis process. In this there is one non-polarised electrode and the other is polarised electrode, which can get various (different) values of electric potential.

4. To establish relation of electric current and concentration, polarographic method is used. This method is constructed on specific characteristics of current-voltage graphs obtained from electrolysis of solution. In this method qualitative analysis is performed with the help of half wave potential and also quantitative analysis can be performed with the help of diffusion current.

5. Advantage of this method is that qualitative and quantitative analysis can be performed at a same time (simultaneously).

If there are enough differences between characteristic potential values of components (constituents) in mixture, then from only one sample, analysis of each component can be performed instantly. If this sample is only 2 ml and concentration of substance is 10-3 to 10-6 M in it, though analysis can be performed. Analysis of only one sample can perform frequently in only few minutes or seconds Simultaneous analysis of about (approximately) 5 to 6 elements or components is possible.

6. In this electrolysis process, slowly voltage is increasing, then flowing electric-current in solution is measured. Thus, electric current-voltage graph is obtained. Since polarised electrode was dropping mercury electrode, this method was named polarography by Heyrovsky.

Kolthoff and Latinene measurement of electric current and voltage: This method is known as voltametry. The method in which by measuring electrolysis current at constant potential of indicator electrode for determination of concentration in solution is known as Amperometry.

7. Generally, in chemistry, Two types of electric cells are used.

 (1) Galvanic cell - In which chemical energy is converted in electrical energy.

(2) Electrolytic cell: In which electrical energy is converted into chemical energy. In this cell, electric current passes from outside in electrolytic solution (Applied voltage). This cell is important in polarography.

✰Decomposition potential (Ed)

.At this time, value of potential of anode and cathode, is called decomposition potential for corresponding (respective) electrode. Therefore, decomposition voltage is equal to addition of decomposition voltage of anode and cathode.

In short, to continue electrolysis reaction, to eliminate voltage of galvanic cell, applied voltage is given from outside, that is called decomposition voltage. Many times, it is called as reverse emf (back emf). Voltage produced in galvanic cell resist or oppose to applied voltage which is required to keep continue electrolysis.

Ed depends on following matters :

(1) Type of electrode 

(2) Type of electrolyte

(3) Concentration of electrolyte

(4) Surfaces effects

Example 

When electrolysis of aqueous H₂SO₄ is performed by using platinum electrode, in the beginning at slight voltage slight electric current passes but immediately it decreases and becomes zero. Now by increasing applied voltage in cell, there is no major increase in electric current-flow in galvanometer initially. But when value of applied voltage is 1.7 V to cell, Acidic water is electrolysed and bubbles of hydrogen and oxygen gas are produced at cathode and anode respectively. Therefore, in galvanometer, value of flowing electric current increases. This applied voltage is given to cell is called decomposition voltage. After this, if more voltage is increased, there is no major increase in electric current is observed.

Polarization: 

To perform electrolysis of electrolyte slight electric current passes (flows) in the beginning but immediately its value decreases. At this time cell is considered as polarised. The electrode whose potential depends on electric current, that electrode is called polarised.

Due to slight electrolysis, on electrode, products of electrolysis are collected. This product generates back emf which oppose or resist applied voltage and because of this polarization occurs on electrode.

Current will not flow cell until these products produced from polarization are not removed. But If by shaking solution or by adding such substances (e.g. Fe+2 and Fe+3 ions) which can be easily oxidised or reduced, then electrode reaction will start and therefore, there will be increase in current. This phenomena is called polarisation of electrode.

If there is no any change in rate of reaction on electrode it means that there is no change in electric current, if electrode adopts the value of potential given by outside then it is called polarised.

By changing total emf which is given to cell experimentally there is a big change in potential of polarised electrode. If only one electrode of a cell is polarised, then whatever change is applied on emf in similar manner there is change in electrode potential, while at any value of electric current, potential of non polarised electrode remains constant. Potential of this type of electrode is not changed by change in applied emf. There are two factors affecting polarization.

(1) Concentration polarization

(2) Over voltage or over potential

☆Concentration Polarization:

Positive ions should have meet near cathode equal to proportion of electric current. And therefore, at opposite of this electrode, in thin layer, number of positive ions decrease It means concentration of reactant (active positive ions) decrease concentration at the rest of whole solution, therefore, concentration polarization is produced It means cathode should have positive ions equal to proportion of electric current and when number of positive ions decrease, then, concentrate polarization is produced and electric current becomes low. It means, Either reduction process becomes slow or stop. Concentration polarisation is produced, when on surface of electrode reaching time of positive ions is less. At both electrode, in electrolytic cell, concentration of ions is different. Due to this situation, in electrolytic cell, concentration cell produced. Producing this concentration cell produces back emf.

This back emf oppose or resist to external applied voltage. Change in concentration (Difference in concentration) is removed by shaking solution. Diffusion of ions increases as increasing temperature and concentration polarisation decreases. Concentration polarisation is a basic principle of polarography. Surface area of electrode is more, polarization becomes less.

✰ Over voltage or Over potential:

Difference in decomposition voltage and voltage of reversible galvanic cell is called over voltage. In electrolysis potential is (given applied) more than theoretical Nernst potential with respect to one electrode is called over potential. Combined potential of both electrodes of complete cell is called over voltage. 

When concentration polarization is not produced, although in a cell, to occur a reaction voltage is applied more than theoretical. emf of galvanic cell is also less than theoretical value. Difference of this voltage is called over voltage. It is

produced due to slow electrochemical oxidation or reduction process on either a single or both electrodes. More time is required to get equilibrium when reaction is slow. To resist over voltage (to prevent over voltage) more energy is applied from outside. It means external emf is applied more.

Suppose that,

E = Value of experimental decomposition potential in electrolysis

and E, = Value of theoretical (reversible) decomposition potential of electrolysis.

where E, value, can be determined from nernst equation. We know that, over voltage is difference of Ea (decomposition voltage) and voltage of reversible galvanic cell (E).

For example, by using, Pb-electrode perform electrolysis of 1 N H₂SO₄ then E₁ = -1.67 V and E₁ = -1.23 V.

Total over voltage = Ed- Er

=1.67+1.23

=-0.44 V

Over voltage is always written in +ve. Total over voltage is equal to sum of cathodic over potential and anodic over potential.

Cathodic over potential is produced at cathode by reversible reduction while on anode, due to irreversible oxidation, anodic over voltage is produced.

Following factors are affecting over voltage:

(1) As temperature increases, over voltage decreases.

(2) As electric current density increases, over voltage increases.

(3) pH of solution.

(4) According to composition of electrode, over voltage changes.

e.g. For soft metals such as Sn, Zn, Hg its value is higher.

(5) Area of surface of electrode.

(6) Impurities in electrolytes.

Back E.M.F. (Back E.M.F.):

To keep electrolysis continue and to remove voltage of galvanic cell, applied voltage is given from outside, that is called decomposition potential, which is called back emf many times. Voltage produced from galvanic cell opposes or resists applied voltage.

✰Discussion of main points of polarography:

Active positive ions are those whose reduction process is to be studied. Here Cd2+ as positive ions are taken into consideration for discussion. Its concentration in solution is to be kept less or low (i.e. 10-1 M to 10-6 M).

All cations (Cd2+) in a solution (reach) moves towards cathode (DME) due to three forces.

(1) By Convection: Cd2+ ions reach on the surface of cathode mercury (Hg) by shaking solution and increasing temperature and density. But these factors can be controlled. Therefore, now, all cations of solution move towards DME (cathode) due to main two forces.

(2) Due to electrostatic attraction force which is produced by opposite electric charge on ion and electrode (ions having +ve charge while cathode (DME) having ve charge). It means by transference or migration current (i).

(3) Diffusion of particles arises due to the concentration gradient of the analyte between the surface of the electrode and bulk of the solution. This is called Diffusion current (i). Total

Electric current (i) = im+id According to theory (principle) Cd2+ ions should reach by diffusion only on surface of cathode (DME). It means, active positive ions (cations), must reach through

(by) diffusion force arising due to concentration difference at surface of electrode and bulk of the solution. Limiting current should be controlled by diffusion only. When positive ions move at surface of electrode by other factors, value of required limiting current for analysis becomes almost double. Hence, detection of elements or species in solution and Quantitative analysis can not be performed. Hence active ions should not reach at the surface of the electrode by transference or migration. These ions should stop going by this process.