Principal of polarography
♦An electrochemical technique that involves the measurement of current as a function of time-dependent potential.
♦ The term polarography is applied to the current-voltage curves when the dropping mercury electrode is used as an indicator electrode.
♦ This method comes under the category called voltammetry.
♦ polarography: It is an electrochemical method of analysis based on current flow. Resulting from the electrolysis of solution a polarizable microelectrode as a function of the applied voltage.
♦ The nature of the graph (also known as a polarographic wave ) gives an idea about the nature and amount of material present in the solution.
♦ Polarographic: It is a graph.
♦ Polarogram: It is an instrument for separation.
♦ When a chemical reaction takes place at an electrode. the potential will be no longer at equilibrium, due to current flow through the electrochemical cell which causes a change in electrode potential and this process is known as polarization.
♦ If polarization is taking place at anode it is termed anodic polarization and if it is at cathode it is cathodic polarization.
Type of polarization:
1) Activation polarization:
2) Concentration polarization:
3) Resistance polarization:
♦When the dropping mercury electrode (DME) is held in the mass transfer controlled region of electrolysis current will be flowing during the light time of the single drop of Hg. This current know as diffusion limiting current.
♦A lifetime of the drop, drop diameter and weight of drop are important factors that are considered in one because they affect the flow of current.
8 Measurement of diffusion current-controlled developed on a relationship which acts as a backbone for developing a mathematical relationship for DME
When: n = number of e-
id = diffusion Limited current.
e = cone, a profile that is a concentration under boundary condition it is Co is concentration.
D = diffusion coefficient.
The rate of mercury flow from the reservoir of DME depends on the mass and density of mercury.
We can also calculate the weight of the drop at any time.
ro = radius of drop which given by
Area of a drop is given by,
♦ At any time expansion of drop cause stretching of existing diffusion layer which in turn makes the diffusion layer thinner than earlier.
♦ This stretching effect leads to an increase in concentration gradient at the electrode surface and a larger current will be floe. It would be the same as if the effective diffusion coefficient were (7/3 ).
♦ Hence we will multiply by ( 7/3 ) 1/2 we get a new equation.
♦ After evaluation of constants in equation we can rewrite as.
id = diffusion current,
t= drop time in ‘8’
Do = diffusion coefficient in cm2/s
m = mass of Hg drop in mg/s
Co=conc. of ions in mol/cm3
Dropping mercury electrode : ( DME )
♦The DME cell consists of 3 electrodes that are immersed in the solution to be analyzed.
1. A DME act as a working electrode where the actual reaction of interest takes place.
2. A reference electrode usually a calomel electrode which gives a stable potential with known potential with which the potential of the working electrode is measured.
3. 3rd electrode is a counter electrode that is made up of chemically inert conductive material like platinum or graphite.
♦ Different materials like platinum, gold, silver, mercury can be used as a working electrode, but mercury has certain advantages which include.
1. Mercury is liquid.
2. Each mercury dropped gives a new clean, and reproducible surface.
3. The reproductive geometry of each drop with time.
4. Large activation potential for ‘H’ gas previous reaction which has occurred on the earlier drop.
6. Diffusion equilibrium is achieved rapidly.
♦ In DME: mercury drop grows continuously as mercury flow from the reservoir under inefficient gravity.
consist of dropping funnel for mercury reservoir ( about 100 ml capacity ).
It is connected to the capillary tube of the fine bore.
The connection is made either by using heavy walled rubber tubing of about 80 cm long by neoprene tubing.
The effective glass capillary tube has a length of about 5-10 cm and a bore diameter of about 0.05mm.
the outer diameter of the capillary tube maybe 6-8 mm.
The tip of the capillary is acted fine and horizontal.
The overall adjust5ment of DME is such that with a pressure of about 50 an of mercury procedure a drop weighting about 6-10 mg in a full time. at 3-6 seconds.
The DME is fitted to the cell and the cell is mounted in the heavy stand on a platform free from vibration.
The instrument part consists of a standard power supply from a battery.
Sample solution placed in a beater ( glass ).
It is situated at such that time 3-6 sec.
Supporting electrolyte i.e KCL migrating current eliminate.
KCI solution diffusion current not measured the only diffusion.
O 2 removed and N2 passed because the curve of O2 is added.
Maxima suppressor added in required cone range.
Diffusion current and cone of analysis.
Hence only diffusion current is measured and is indicated by a galvanometer.
Potential difference to be applied between the anode and cathode which is indicated by voltameter.
Rotating platinum electrode:
Advantages over DME:
1. The electrode is simple to construct.
2. The rotating platinum microelectrode can be used at a positive potential of up to 9 units. Whereas DME is working in a pot range of -2 volt to +0.4 volt. Therefore scope is very wide.
3. The diffusion current ( which is proportional to the concentration of ions reduced at the cathode ) is 20 times larger than in the case of DME. The large current obtained allows a small cone of metal ions to be measured with greater accuracy. Hence the use of rotating platinum microelectrode makes the amperometric method more sensitive and accurate.
♦Glass tubing -length – 15 to 20 cm.
Diameter – 6mm.
♦the platinum wire extends from 5-6 mm from the glass tubing wall.
♦Hole in glass tubing for electrical contact with a platinum wire.
♦This electrode mounted with a shaft of the motor and rotated constantly with the motor. ( speed is about 600 rpm ).
♦wp attached which filled with mercury 80.
♦Get electrical contact between the rotatory platinum electrode and mercury.
♦ Saturated calomel electrode ( SCE ) as the reference electrode.
♦ Micro-butter = is used for adding the titrant in experimental use.
♦Flask / Beaker contains the experimental solution.
♦An anode is connected with the power supply.
♦A known volume of experimental solution is added in the cell/ beaker/ flask. and solution first deoxygenated with the help of N2 gas for 15 to 20 min.
♦A suitable supporting electrolyte KCI is added.
♦The reference electrode acts as an anode.
♦Platinum mercury electrode acts as a cathode.
♦ By moving sliding contact, J on a potentiometer.
♦ Wire x y potential applied to the cell is so adjusted in any value at the platue of the polarographic view-position is constant.
♦ And current passing through the cell is measured after each addition of sample by micro burette.
♦ Sensitive galvanometer is used to measure the current.
♦ The additional titration instruments, discusses the experimental solution to a certain extent, due to the dilution of the solution the observed current is smaller hence the observed current is multiplied by the correction factor,
correction factor= V+v / v
V = Volume of origin solution.
v = volume of instalment of titrant
♦ to minimize the effect of dilution the titration should be at least 20 times more concentrated than the solution being titrated. As the volume of titrant added is small therefore micro burette is used to ensure correct addition.
♦ The diffusion current values are found after each addition of the titrant. The graph is plotted. Id v/s volm of the titrant.
♦ From the graph, the endpoint of the titration and subsequently the cone of reducible species present in the solution a be found cut by using,
N1 x V1=N2 x V2
Application of polarography:
1. Determination of Inorganic compound :
The ionic solution of various elements can be determined directly. The mixtures of ions can be determined if half-wave potentials of the separated by 0.2 v.
2. Determination of an organic compound:
The estimation of sugars has been carried out in the range of 10-3 to 10-2 M, using pH 2.3 and supporting electrolyte hydrazine sulfate polarogrophically.
3. Determination of dissolved oxygen;
The determination of dissolved oxygen in an aqueous solution or in organic solvents can be carried out successfully with the help of polarography oxygen waves are measured, not sweet out oxygen with nitrogen.
4. Studies of the complex :
Study of the composition of complexes if the simple metal ion and complex of that metal ion in the same oxidation state give reversible electrode process and involve the same number of the electrode.
5. Determination of plant content is obtained by extraction.
6. Mixtures of ions determine.
7. Determination of a large number of alloys.
8. Determination of electro-reducible/ electro- oxidizable functional groups like ketone, aldehyde, nitrate.
9. Determination of organic metallic compound.
10. Determination of medicine like epinephrine, nor-procaine, phenytoin.
11. To study the replication rate of micro-organisms.