The Limiting Curve

Polarography is a particular case of general electro analytical technique known as voltametry. These methods are used to study the decomposition and quantitative analysis of solution. Principle: – It is a rapid and accurate technique in which electrolysis of experimental solution is carried out by using one electrode of comparatively large surface area (i. e. non-polarisable electrode) and the other electrode having a very small area (polarisable electrode). The non polarisable electrode is SCE used as reference electrode.

Alternatively Hg pool can be used where the polarisable electrode consist of dropping mercury electrode (DME) During electrolysis potential difference is applied to the cell and it is kept on increasing and corresponding current is measured. Then graph of current Vs voltage is plotted. The change in current with applied voltage gives concentration of species while the voltage at which the greater change occurs is an indicator of its identity. Thus polarogram given both quantitative and qualitative information about the composition of the solution subjected to electrolysis in polarographic cell.

* Decomposition Potential and Overvoltage: – When a solution of metal salt is subjected to electrolysis the metal ions migrate towards cathode due to electrostatic attraction and undergo reduction at the cathode. Mn+ + e- M “The minimum potential that must be applied to a cell to bring about continuous electrolytic decomposition is called decomposition potential of the electrolyte. ” It is found that experimentally observed decomposition potential is higher than the theoretical value, the difference between two is called “polarization voltage” the cell and the electrodes are then said to be polarized.

Polarization is the behavior of the electrode which makes the potential of the electrode to differ from the theoretical value for the same electrode as calculated by Nernst equation. Polarization is supposed to be due to a “back emf” brought about by either the collection of the products at the electrode. “When a cell is polarized, it is necessary to apply an external potential which is greater than that calculated. This excess potential is called overvoltage. ” Overvoltage depends upon various factors like concentration, temperature, nature of substance discharged, the surface character of the electrodes and current density.

* Advantages of D M E: – i)Its surface is reproducible, smooth and continuously renewed. This eliminates passivity or poisoning effect. ii)Mercury posses the property of forming amalgams with many metals and therefore lowers their reduction potential. iii)It can be employed over the range of + 0. 4 to – 2. 0V with reference to the saturated calomel electrode where almost all metal ions reduced. iv)The diffusion current attains the steady value after each change of applied voltage. v)Since the area of the electrode is very small, the actual current passing will also be small.

The concentration change in the bulk of the experiment solution is therefore negligible. vi)Since the electrode is continuously renewed, series of reducible species can be estimated in one solution. * Drawbacks: – i)Since the drop is growing it represents a constantly changing area. Hence current passing through the cell increases as the drop grow and drop to zero as the drop breaks. ii)Mercury can be easily oxidized. iii)The capillary may be easily plugged. iv) Liquid used is mercury which is costly and poisonous. * Instrumentation and Procedure for D. C. Polarography: –

A simple polarographic set up is shown in figure. This was designed by Heyrovsky. In this apparatus there is a dropping mercury electrode which consist of a mercury reservoir from which mercury drops down as small drops at the rate of an dropping every 3 – 6 seconds through a capillary. These drops the cathode of the cell in which the anode is a large pool of mercury. Since its area is larger, it is incapable of getting polarized and its potential therefore assumed constant. The reaction taking place at DME will therefore almost completely control the polarization of the cell.

Cathode (DME) and anode (large pool of Hg) are connected to the negative and positive terminals resp. of a battery. The applied voltage can be changed by adjusting the sliding contact (K) along the potentiometric wire AB. G. is a galvanometer which measures current strength and ‘s’ is a shunt for adjusting the sensitivity of the galvanometer. The experimental solution is placed in vessel (V) and at the bottom of this vessel is the pool of mercury. There is an arrangement for blowing N2 gas through the sample which removes dissolved oxygen from the sample.

* Working: – When mercury drops fall into the Soln. and the potential of the DME is adjusted to the reduction potential of the metal ion in the solution these metal ions get reduced to the metal which dissolves in mercury drop to form an amalgam. Mn+ + ne- ? M/Hg The reducible component (Mn+) reduced at the surface of DME comes from the region of the solution adjacent to mercury surface this will cause a zone of low Concn near the surface of the drop whereas Concn in the bulk of the solution will remain unaltered and therefore higher.

As a result of this a concentration gradient is establish between depleted concentration and the Concn of the bulk. In polarography, the species from solution can flow towards DME by two process. 1) Concentration difference 2) Electrical potential difference. In polarography, conditions are so established that the movement of ions to the DME is controlled by concentration gradient alone. In order to neutralized the effect of potential gradient a large excess of salt which is not reduced at the applied potential is added to the electrolyte solution.

This salt is called “Indiffent electrolyte” or “supporting electrolyte”. A supporting electrolyte is one which conducts the current but does not react with material under investigation, not at the electrodes within the potential range studied. KCl is generally used for this purpose. It is suppose that ions of supporting electrolyte move to DME due to potential gradient and accumulate there until their interaction produces potential equal in magnitude or opposite in sign to the applied potential, metal ions (Mn+) under these conditions move to the DME under the influence of concentration gradient alone.

For this purpose the concentration of supporting electrolyte must be at least 100 times that of the experimental electrolyte. Also since reducible ions migrate to the cathode under the influence of concentration gradient alone, experimental solution must not be stirred. When applied voltage is slowly increased and the current is recorded, a high graph will be obtained as shown in figure. The ‘s’ shape curve obtained is called polarographic wave or polarogram

It can be shown from the figure that at A, the potential applied is insufficient to reduce the metal ions in the Soln small current that flows is known as residual current and is carried by supporting electrolyte and impurities present in the sample. As cathode potential is made more and more negative and reaches a value sufficient to reduce the metal ions the reduction of metal ions begins and the current also begins to increase. At point B potential of the electrode becomes equal to the decomposition potential of Mn+ ion. The reduction of metal ions increase and the current also increases along the curve BC.

This continues until at a particular potential ions which diffuse to the cathode are reduced. At the point C, current no longer increases linearly with applied voltage but reaches a steady limiting value at the point D. After this no increasing in current is observed at higher cathode potential. Thus the current corresponding to the curve CD is known as limiting current. “The difference between residual current and limiting current is called diffusion current”. The height of the curve is called wave height, gives diffusion current. * Factor Affecting The Limiting Curve: –

The factors which affect the current voltage curve are as follows: – 1) Residual Current. 2) Migration Current. 3) Diffusion Current. 1)Residual Current: – When a current voltage curve is determined of a solution containing ions with a strongly negative reduction potential (e. g. K+ ions) “A small current will flow before the decomposition of solution begins, this current increases almost linearly with the applied voltage this is called Residual Current”. Consider a potassium chloride solution, the potassium ions in it will be attracted to the drop.

They are not reduced to K atoms but remains too close to mercury surface forming the electrical double layer which will produce a current. The effect is like charging up a condenser when the drop falls off, a new drops forms and a new condenser is formed up causes a continuous flow of electric current which increase as the potential of the drop is increased. In the case of electrolytes containing traces of impurities a small faradic current also combines the condenser current. Thus, Residual Current = Faradic Current + Condenser Current. 2. Migration Current: –

“The migration current is the current required for the migration of charged particles due to the diffusion of the particles in the electric field. ” The electro attractive material reaches the surface of the electrode by the process. (a) Migration of charged particles by electrical potential difference and (b) Diffusion of particles. The current required for above two processes is called Migration Current. Heyrovsky proved that the migration current can be almost eliminated if a supporting electrolyte is added to the Soln. Suppose a solution contains 0.

1M KCl and 0. 01 M Co2+ ions the current is carried through the cell by all the ions present. The fraction of total current carried by each ion depends upon relative concentration compared with other ions and transport number. In present case about 90% of the current will be transported to the cathode by the K+ ions ,if concentration of K+ ion is increased to more than 99% of the total cations present the relative current carried by other cations are reduced practically to zero. Thus all the currents through the cell will be transport by potassium ions. 3.

Diffusion Current: – “The diffusion current ‘id’ which is the difference of limiting current and residual current,” The diffusion Current is directly proportional to the concentration of the substance being reduced or oxidized at the dropping electrode. When dropping mercury electrode is used it is observed that if a solution of a substance is taken, then in the bulk of the solution concentration of ions remain constant. The fresh amounts of ions are transferred to the electrode surface due to the diffusion which is proportional to the difference of concentration.

This difference in Concn has been found to be equal to the Concn of ions in the bulk of solution. Then limiting current becomes equal to diffusion current. D. Ilkovic derived an equation for the diffusion current id and the equation is known as the Ilkovic equation. [id = 607 n ] Where id = diffusion current in MA. 607 = a Combination of numerical constant n = number of electrons involved in the reduction process at the cathode D = diffusion coefficient of the metal ion in cm2s-1 m = Rate of flow of mercury from the capillary in mgs-1 t = drop time in sec c = concentration of metal ion in milli mole dm-3