Understanding Polarography in Electrochemical Analysis

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Faculty of Pharmacy
Minia University
 
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the working microelectrode is the dropping
mercury electrode (DME) and
the counter electrode is a mercury pool.
Convection effects can be minimized by
using unstirred solutions during current
measurements
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Nitrogen inlet
Analyte solution
Mercury pool
Nitrogen outlet
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I
d
 = 607 nD
1/2
C m
2/3
 t
1/6
i
d
 = diffusion current, 
A
n = number of electrons
D = diffusion coefficient of analyte, cm
2
 s
-1
C = concentration of analyte, mM
m = mass of mercury flowing through capillary tip,
mg s
-1
t  = drop life, s
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High overpotential for the reduction of H
3
O
+
Reproducible dropping action
Formation of an amalgam with several
metals
Electrode 
surface is continuously renewed.
Easy calculation of surface area. 
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Mercury is easily oxidized.
Low sensitivity (10
-5
 M).
Difficulties in its use
Mercury toxicity.
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Polarographic
maximum
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The maximum may be reduced or completely removed
by adding a maximum suppressor.
This is usually a simple gelatinous material (for example
Triton X-100 or gelatin) that greatly slows down the
migration of analyte ions to the surface of the electrode.
The accumulation of analyte ions is greatly reduced and
the maximum in the polarogram is suppressed.
Care must be taken to avoid adding excess of these
maxima suppressor, because they can change the
solution viscosity and reduce the magnitude of the
diffusion current.
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The first wave
O
2
 + 2H
+
 + 2 e
-
 
 H
2
O
2
 (in acidic medium)
O
2
 + 2H
2
O + 2 e
-
 
 2H
2
O
2
 (neutral/alkaline)
The second wave
H
2
O
2
 + 2H
+
 + 2 e
-
 
 2H
2
O (in acidic medium)
H
2
O
2
            + 2 e
-
 
 2OH
-
 (in alkaline medium)
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Physically by bubbling inert gas.
Chemically by addition of hydrazine
N
2
H
4
 + O
2
 
N
2
 + 2H
2
O
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Most metal ions (reduced)
Mixtures of electro-active ions may
 
E
1/2
 >= 0.4 Volt for singly charged ions
               0.2 Volt for doubly charged ions
Inorganic anions.
Dissolved oxygen in solution.
Oxidation states of a metal in solution.
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Most of the reducible organic substances
(aldehyde, ketons, acids, nitro , etc. )
Easily oxidizable organic substances
Notes:
Careful selection of solvent
pH should be controlled.
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:
-
must be able to dissolve a supporting electrolyte and
the analyte,
should not be reduced easily, and
if necessary be buffered.
Further, it must conduct electricity.
The most popular being water.
To dissolve organic compounds in water, a second
solvent, such as ethanol, acetone, acetonitrile, acetic
acid or dioxane, may first be mixed with water.
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The 
pH
 of the solution should be control with a buffer.
This is because of the reduction of organic substances usually
involves the reduction of a hydrogen on the molecule.
In this case, the buffer serves as the supporting electrolyte.
The usual choice for the analysis of simple organic
pharmaceuticals would be 
an aqueous alcoholic (e.g. 80%
buffer-20% methanol
) solution containing a simple salt as
supporting electrolyte (e.g. at least 
0.01 M potassium chloride
)
Simple buffer solutions are effective over narrow pH ranges, but
mixed buffer systems, e.g. Britton-Robinson buffer, are more
versatile when the polarographic 
behaviour
 of an organic species
is be fully investigated.
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Care must be taken to ensure that
no ions in the electrolyte interact with the analyte
pH changes do not affect its stability, and that
the use of aqueous/alcoholic solvents does not
cause changes in
the E
1/2 
of the electroactive species,
the limiting current values, or
the reduction mechanism involved, which might
lead to errors in the analysis.
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.
Imines
   
Dienes``
  
Oxime
Alkynes
  
Nittiles
  
Ketones
Diazocompound.
 
Aldehydes
Nitroso compound.
 
Halides
  
Sulphones
Thiocyanates
  
Sulphonium salts
 
Heterocycles
Nitro compounds
 
Organometalics
Dizunium salts
 
Aromatic carboxylic acids
- The products of polarographic reduction may not be the
same as if the reduction were attempted chemically,
- The reason is that the electrode kinetics limit the
progress of multi-stage reactions.
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Nitrofurantoin, Metronidazole and Tinidazole
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 solvent for reduction  
of
Nitrazepam 
in 20% methanol-0.1M
hydrochloric acid,
This gives peaks at -0.11 V and -
0.67 V vs silver-silver chloride.
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It isexample for the use of
chemical derivatisation in
polarography
he analysis of atropine by d.c.
polarography may be quoted, in
0.1 M tetrabutyl ammonium
perchlorate in acetonitrile can be
used as solvent.
Nitration 
of 
atropine can be effected by reaction at room
temperature with 10% potassium nitrate in concentrated
sulphuric acid, and a sensitivity of 200 ng atropine ml-L
of 1 M sodium hydroxide supporting electrolyte with dpp
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Explore the realm of polarography, a type of voltammetry utilizing the dropping mercury electrode (DME) and mercury pool. Learn about classical polarographs, the Ilkovic equation, advantages and disadvantages of mercury drop electrodes, and how to minimize polarographic maxima in electrochemical analysis.


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  1. Electrochemical methods of analysis Bulk methods Interfacial methods Dynamic methods Static methods Conductimetry Controlled current controlled potential Potentiometry Controlled current coulometry Variable potential Fixed potential Controlled potential coulometry Amperometry Voltammetry Hydrodynamic voltammetry Polarography

  2. Definition Polarography is a type of voltammetry in which the working microelectrode is the dropping mercury electrode (DME) and the counter electrode is a mercury pool. Convection effects can be minimized by using unstirred solutions during current measurements

  3. Classical polarograph Variable resistor DME Reference electrode Nitrogen outlet Nitrogen inlet Analyte solution Mercury pool

  4. Dropping Mercury Electrode (DME)

  5. Drop life and observed current

  6. Polarogram

  7. Ilkovic equation Id= 607 nD1/2C m2/3t1/6 id= diffusion current, A n = number of electrons D = diffusion coefficient of analyte, cm2s-1 C = concentration of analyte, mM m = mass of mercury flowing through capillary tip, mg s-1 t = drop life, s

  8. Advantages of mercury drop electrode High overpotential for the reduction of H3O+ Reproducible dropping action Formation of an amalgam with several metals Electrode surface is continuously renewed. Easy calculation of surface area.

  9. Disadvantages Mercury is easily oxidized. Low sensitivity (10-5M). Difficulties in its use Mercury toxicity.

  10. Polarographic maxima Polarographic maximum

  11. Minimization of polarographic maxima The maximum may be reduced or completely removed by adding a maximum suppressor. This is usually a simple gelatinous material (for example Triton X-100 or gelatin) that greatly slows down the migration of analyte ions to the surface of the electrode. The accumulation of analyte ions is greatly reduced and the maximum in the polarogram is suppressed. Care must be taken to avoid adding excess of these maxima suppressor, because they can change the solution viscosity and reduce the magnitude of the diffusion current.

  12. Oxygen waves

  13. Origin of oxygen waves The first wave O2+ 2H++ 2 e- H2O2(in acidic medium) O2+ 2H2O + 2 e- 2H2O2(neutral/alkaline) The second wave H2O2+ 2H++ 2 e- 2H2O (in acidic medium) H2O2 + 2 e- 2OH-(in alkaline medium)

  14. Removal of oxygen waves Physically by bubbling inert gas. Chemically by addition of hydrazine N2H4+ O2 N2+ 2H2O

  15. APPLICATIONS OF POLAROGRAPHY Inorganic applications Most metal ions (reduced) Mixtures of electro-active ions may E1/2>= 0.4 Volt for singly charged ions 0.2 Volt for doubly charged ions Inorganic anions. Dissolved oxygen in solution. Oxidation states of a metal in solution.

  16. APPLICATIONS OF POLAROGRAPHY Organic applications Most of the reducible organic substances (aldehyde, ketons, acids, nitro , etc. ) Easily oxidizable organic substances Notes: Careful selection of solvent pH should be controlled.

  17. Selection of the solvent solvent :- must be able to dissolve a supporting electrolyte and the analyte, should not be reduced easily, and if necessary be buffered. Further, it must conduct electricity. The most popular being water. To dissolve organic compounds in water, a second solvent, such as ethanol, acetone, acetonitrile, acetic acid or dioxane, may first be mixed with water.

  18. pH control of the solution The pH of the solution should be control with a buffer. This is because of the reduction of organic substances usually involves the reduction of a hydrogen on the molecule. In this case, the buffer serves as the supporting electrolyte. The usual choice for the analysis of simple organic pharmaceuticals would be an aqueous alcoholic (e.g. 80% buffer-20% methanol) solution containing a simple salt as supporting electrolyte (e.g. at least 0.01 M potassium chloride) Simple buffer solutions are effective over narrow pH ranges, but mixed buffer systems, e.g. Britton-Robinson buffer, are more versatile when the polarographic behaviour of an organic species is be fully investigated.

  19. Careful choice of buffer Care must be taken to ensure that no ions in the electrolyte interact with the analyte pH changes do not affect its stability, and that the use of aqueous/alcoholic solvents does not cause changes in the E1/2 of the electroactive species, the limiting current values, or the reduction mechanism involved, which might lead to errors in the analysis.

  20. Some functional groups reducible in polarography. Imines Alkynes Diazocompound. Nitroso compound. Halides Thiocyanates Nitro compounds Dizunium salts Dienes`` Nittiles Aldehydes Oxime Ketones Sulphones Heterocycles Sulphonium salts Organometalics Aromatic carboxylic acids - The products of polarographic reduction may not be the same as if the reduction were attempted chemically, - The reason is that the electrode kinetics limit the progress of multi-stage reactions.

  21. Nitrofurans and nitroimidazoles Nitrofurantoin, Metronidazole and Tinidazole

  22. Benzodiazepines determination solvent for reduction of Nitrazepam in 20% methanol-0.1M hydrochloric acid, This gives peaks at -0.11 V and - 0.67 V vs silver-silver chloride.

  23. Atropine determination It isexample for the use of chemical derivatisation in polarography he analysis of atropine by d.c. polarography may be quoted, in 0.1 M tetrabutyl ammonium perchlorate in acetonitrile can be used as solvent. Nitration of atropine can be effected by reaction at room temperature with 10% potassium nitrate in concentrated sulphuric acid, and a sensitivity of 200 ng atropine ml-L of 1 M sodium hydroxide supporting electrolyte with dpp

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