Polarography is an electroanalytical technique that involves measuring the current in a cell with a gradually increasing negative potential applied between a polarizable working electrode, usually a dropping mercury electrode, and a non-polarizable reference electrode. It can be used for both qualitative and quantitative analysis of electroreducible or electrooxidizable elements. The current-voltage curve obtained is called a polarogram, from which the half-wave potential and diffusion current can be determined for qualitative and quantitative analysis, respectively. Key components of the polarograph include the dropping mercury electrode, rotating platinum electrode, and reference electrode. Factors like concentration, temperature, viscosity, and capillary characteristics affect the diffusion current.

1. $elin@SNVPMV 1
SELINA SRAVANTHI
SAROJINI NAIDU VANITA PHARMACY MAHA VIDYALAYA,
TARNAKA, HYDERABAD, TELANAGANA

2. ‡ Its an electroanalytical technique
‡ It’s a method of instrumental analysis, which consists of measurement of potential
difference as current flows through solution.
‡ The value of current flowing through the cell at any applied voltage is measured
with the help of an instrument.
‡ Used for both qualitative and quantitative analysis of electro reducible or
oxidizable elements.
‡ The curve (current- voltage) obtained is called a polarogram.
‡ Instrument - Polarograph
$elin@SNVPMV 2

3. $elin@
SNVPMV
3
PRINCIPLE
‡ A gradually increasing negative potential (voltage) is
applied between a polarisable and non- polarisable
electrode.
‡ Corresponding current is measured.
‡ From the current voltage curve (sigmoid shape) –
Polarogram – both qualitative and quantitative
analysis is done.
‡ Half wave potential (𝑬 ൗ𝟏
𝟐
) is characteristic for every
element (qualitative)
‡ Diffusion current (id) is proportional to
concentration of the analyte (quantitative)

4. $elin@
SNVPMV
4
APPARATUS – DROPPING MERCURY ELECTRODE
‡ It is a polarisable electrode.
‡ Gradually increasing negative potential can be applied
‡ Consists of a fine capillary with a bore size of 20-50 μ is
connected to a mercury reservoir
‡ By adjusting the height of the reservoir, the drop time
can be adjusted
‡ Drop time is the time taken for a fresh droplet of
mercury to be formed from the capillary
‡ The size of the droplet depends on the bore size of the
capillary.
‡ Wire contacts are made inside the tubing where the
mercury flows through

5. $elin@SNVPMV 5

6. $elin@
SNVPMV
6
APPARATUS – ROTATING PLATINUM ELECTRODE
‡ The electrode includes a conductive disk (platinum wire) embedded in
an inert non-conductive polymer or resin (glass)
‡ Platinum wire is bent at the tip.
‡ It is attached to an electric motor that has very fine control of the
electrode's rotation rate (rotates at 600 rpm).
‡ The disk, like any working electrode, is generally made of a noble
metal (i.e., Platinum), however any conductive material (copper) can
be used based on specific needs.
‡ Wire contacts are made through mercury reservoir at the top such that
potential can be applied and current measured.
‡ When the electrode is rotated, the centrifugal force flings away the
solution from the centre.
‡ The solution flows up from the bulk to replace the boundary layer
(perpendicular to the electrode).
‡ Diffusion current is 20 times greater than DME.
‡ Simple to construct.
‡ Steady diffusion state is reached.
‡ Positive potential of up to +0.9V can be used.

7. $elin@
SNVPMV
7
APPARATUS – POLAROGRAPH
‡ Consists of polarisable electrode and a reference electrode (saturated
calomel electrode)- non polarisable.
‡ Sample cell in which the solution to be analysed is kept.
‡ Sample cell is made of glass and a tapering edge at bottom to hold the
mercury after droplets are formed.
‡ Capillary is dipped into the solution to be analysed
‡ By adjusting the height of the reservoir, the drop time can be adjusted
‡ Supporting electrolytes (KCl) is added in large quantities to eliminate
migration current.
‡ Oxygen present in the sample is removed by passing N2 or alkaline
pyrogallol solution
‡ Maxima suppressors are added.
‡ After initial and final potential is set current –voltage curve is
recorded.
‡ Half wave potential and diffusion current is determined from the
polarogram.

8. $elin@SNVPMV 8

9. $elin@
SNVPMV
9
TYPES OF CURRENT IN POLAROGRAPHY
‡ RESIDUAL CURRENT (ir): large condenser current + small Faradic current
‡ Condenser current – due to formation of Helmholtz double layer at mercury surface
‡ Faradic current – traces of impurities.
‡ MIGRATION CURRENT (im) : Due to migration of cations from the bulk of the solution towards
cathode irrespective of concentration gradient.
‡ Depends on the proportion of analyte of interest and supporting electrolyte.
‡ Migration current is eliminated by adding a large proportion of supporting electrolytes like KCl (50-100 times
the analyte (electro reducible ion).
‡ DIFFUSION CURRENT (id): Actual current due to the diffusion of electro reducible ion from the bulk
of the sample to the surface of the mercury droplet due to concentration gradient.
‡ LIMITING CURRENT (il): beyond a certain value the current reaches a steady value.
‡ The diffusion of ions = rate or reduction (due to surface saturation)

10. $elin@SNVPMV 10
ILKOVIC EQUATION
‡ Diffusion current at its limiting value
𝒊 𝒅 = 607 n C𝑫 ൗ𝟏
𝟐 𝒎 ൗ𝟐
𝟑 𝒕 ൗ𝟏
𝟔
Where
id = diffusion current due to electro reducible ion
n = no. of electron involved in the reduction of
one molecule.
C = conc. Expressed in mmol/L
D = diffusion co efficient of ions (cm2/sec)
m = wt. of mercury flowing through capillary
(mg/sec)
t = drop time in seconds (2-7 sec)

11. $elin@
SNVPMV
11
FACTORS AFFECTING DIFFUSION CURRENT
‡ CONCENTRATION :
‡ Concentration is directly proportional to diffusion current.
‡ TEMPERATURE:
‡ Diffusion current varies with temperature.
‡ VISCOSITY OF THE MEDIUM:
‡ Diffusion co-efficient depends on the viscosity of the medium.
‡ Gelatin at low concentration is used a maxima suppressor.
‡ CAPILLARY CHARACTERISTICS:
‡ The bore size of the capillary, drop time in seconds and the pressure
of mercury will affect the flow characteristics of the mercury droplet.
‡ PRESENCE OF MAXIMA SUPRESSOR:
POLAROGRAPHIC
MAXIMA
‡ In current voltage curve a
hump is seen in the absence
of maxima supressors.
‡ Not seen in dilute solutions
‡ Leads to error in determining
the half wave potential and
diffusion current.
‡ Suppressors like gelatin, dyes
(methyl red), surfactants
(Triton) are added.

12. $elin@SNVPMV 12
RELATIONSHIP BETWEEN POTENTIAL,
DIFFUSION CURRENT AND HALF WAVE
POTENTIAL
𝑬 𝒂𝒑𝒑 = 𝑬 ൗ𝟏
𝟐 +
𝟎.𝟎𝟓𝟗𝟏
𝒏
log
𝒊 𝒅−𝒊
𝒊
Where
Eapp = applied potential
𝑬 ൗ𝟏
𝟐 = Half wave potential
n = no. of electron involved in the reduction of one
molecule.
id = diffusion current
i = current at applied potential
HALF WAVE POTENTIAL (𝐄 ൗ𝟏
𝟐) – Qualitative parameter
‡ The potential found on the steeply rising (inflection) portion of the current- voltage curve and is
one half of the distance between residual and limiting current.
‡ Characteristic for a specific electro reducible ion or functional group.
‡ At this potential 50% of reduced form and 50% of oxidised form are present.

13. $elin@
SNVPMV
13
APPLICATIONS
‡ Used for estimation of cations and anions in the presence of interfereing ions.
‡ Determination of dissolved oxygen.
‡ For determination of trace metals and metal-containing drugs.
‡ To identify and quantify antiseptics and insecticides.
‡ To identify and quantify vitamins.
‡ To identify and quantify hormones.
‡ To identify and quantify antibiotics.
‡ Identification and quantifying of alkaloids.
‡ Blood serum and cancer diagnosis

14. $elin@SNVPMV 14

15. $elin@SNVPMV 15