Amperometry

1. AMPEROMETRY
SELINA SRAVANTHI
SAROJINI NAIDU VANITA PHARMACY MAHA VIDYALAYA,
TARNAKA, HYDERABAD

2. ◘ Polarography can be used as the basis of amperometric titration.
PRINCIPLE
◘ The potential applied between the polarisable electrode and the non-polarisable electrode
is kept constant.
◘ The diffusioncurrentis measured during the titration.
◘ During titration the concentration of the electro reducible ion changes and hence the
diffusion currentchanges.
◘ At the end point, there is a sharp change in the diffusion current (diffusion current vs
volume of titrant)
◘ The titration is performed between an electro reducible ion where at least the titrate or
titrant gives rise to diffusioncurrent.
◘ The potential selected for the titrations is at its limiting value (potential corresponds to the
point at which limiting current is reached.
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CONDITIONSFOR PERFORMING AMPEROMETRIC TITRATIONS
◘ The titrant or titrate or both must be electro-reducible.
◘ The potential applied should correspondto limiting current.
APPARATUSFOR AMPEROMETRIC TITRATION
◘ Same as polarographic instrumentation.
◘ Electrodes –
◘ Polarisable electrode – Dropping mercury electrode/ Rotating Platinum electrode.
◘ Non polarisable electrode – Saturated calomel electrode
◘ Galvanometer – measures current
◘ Automatic/ Manual Burette– To deliver titrant
◘ Stirrer – to mix the contentsof the titrant with that of titrate

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6. TYPESOF AMPEROMETRICTITRATION
◘ Titration of electro reducible ion vs non electro reducible ion
◘ Lead ion (Pb2+) vs Sulphate ion (SO4
2-)
◘ Titration of non electro reducible ion vs electro reducible ion
◘ Chloride ion (Cl-) vs Silver ion (Ag+)
◘ Titration of electro reducible ion vs electro reducible ion
◘ Lead ion (Pb2+) vs Dichromate ion (Cr2O7
2-)
◘ Redox titration where oxidant and reductantgive diffusioncurrent
◘ Ferric ion (Fe3+) vs Titanous ion (Ti2+)
◘ Dead stop end point technique
◘ Determination of water using Karl fischer technique
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Pb2+
SO4
2-
Pb2+
+ SO4
2-
→PbSO4 (s)
Ag+
+ Cl-
→AgCl (s)
Cl -
Ag+
Pb2+
+ Cr2O7
2-
→PbCrO4 (s)
Pb2+
Cr2O7
2-
Both oxidising agentandreducing
agentsshowdiffusion current

8. TYPES OF AMPEROMETRIC TITRATION
DEADSTOP ENDPOINT TECHNIQUE (BIAMPEROMETRY)
◘ Modification of the amperometric titration.
◘ Applicable only when oxidation-reduction system is involved before and after the end point.
◘ In this method, two similar Platinum electrodes are immersed in the titration cell.
◘ A small and constant voltage is applied to these two electrodes.
◘ An electrometric titration apparatus – two electrodes dipped in two different vessels and linked
through a salt bridge.
◘ The amount of oxidised form that undergoes reduction at cathode is equal to the amount of
reduced form that undergoes oxidation at anode.
◘ At this stage the electrodes are depolarised until either the oxidised or the reduced member are
utilised at the electrode.
◘ At the end point one electrode is depolarized as if the titrant did not undergo any redox (reversible)
system.
◘ The current at the endpoint becomes nearly or equal to zero and suddenly increases again.
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10. DETERMINATION OF WATER USING KARL FISCHER TECHNIQUE
◘ Karl Fischer titration is a widely used analytical method for quantifying watercontentin a variety of products.
ROH + SO2 + R’N → [R’NH]SO3R + H2O + I2 + 2R’N → 2[R’NH]I + [R’NH]SO4R
[alcohol] [base] [alkylsulfite salt] [water] [iodine] [hydroiodic acid salt] [alkylsulfate salt]
◘ The alcohol reacts with sulfur dioxide (SO2) and base to form an intermediate alkylsulfitesalt, which is then
oxidized by iodine to an alkylsulfate salt.
◘ This oxidation reaction consumes water.
◘ The reactive alcohol is typically methanol or 2-(2-Ethoxyethoxy)ethanol, alsoknown as diethylene glycol
monoethyl ether (DEGEE), or another suitable alcohol.
◘ Classic Karl Fisher reagentscontainedpyridine, a noxious carcinogen, as the base. The reagentsmost
frequently used today are pyridine-free and containimidazole or primary amines instead.
◘ Water and iodine are consumed in a 1:1 ratio in the abovereaction.
◘ Once all of the waterpresent is consumed, the presence of excess iodine is detected amperometrically by
the titrator’s indicatorelectrode (double platinumelectrode). That signals the end-point of the titration.
◘ The amount of water present in the sample is calculatedbased on the concentration of iodine in the Karl
Fisher titrating reagent (i.e., titer) and the amount of Karl Fisher Reagent consumed in the titration.
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12. ADVANTAGES OF AMPEROMETRIC TITRATIONS
◘ Both electroreducible and non reducible ions can be determined.
◘ Dilute solutions can be analysed.
◘ Reaction may be reversible or irreversible.
◘ Electrode characteristics need not be provided as they are constant throughout the titration.
◘ Incompletion of reaction at end point does not affect the results. Readings before and after the end
point are sufficient.
◘ Presence of impurities or other electrolytes does not affect the results.
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