Introduction to Polarography

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Questions and Answers

The working electrode is often a drop suspended from the end of a ______ tube.

capillary

The DME stands for ______ mercury electrode.

dropping

The optimum interval between drops for most analysis is between ______ to 5 seconds.

2

The vast majority of reactions studied with the mercury electrode are ______.

<p>reductions</p> Signup and view all the answers

A mercury electrode is not very useful for performing ______, because Hg is too easily oxidized.

<p>oxidations</p> Signup and view all the answers

The polarographic apparatus consists of a polarizable electrode (DME) and a ______ electrode.

<p>reference</p> Signup and view all the answers

Supporting electrolytes like potassium chloride are added to the sample solution to eliminate ______ current.

<p>migration</p> Signup and view all the answers

The height of the mercury reservoir is adjusted so that drop time of about ______ to 7 seconds is set.

<p>2</p> Signup and view all the answers

The polarographic technique is used for the qualitative or quantitative analysis of electroreducible or oxidisable ______.

<p>elements</p> Signup and view all the answers

The principle of polarography involves applying a gradually increasing negative ______ between two electrodes.

<p>potential</p> Signup and view all the answers

The dropping mercury electrode is an example of a ______ electrode used in polarography.

<p>polarisable</p> Signup and view all the answers

The point of inflection in the current-voltage curve is known as the ______-wave potential.

<p>Half</p> Signup and view all the answers

The diffusion current measured in polarography is proportional to the concentration of the specific ______.

<p>compound</p> Signup and view all the answers

The linear relationship between the diffusion current and the concentration of electroactive species is represented by the ______ equation.

<p>Ilkovic</p> Signup and view all the answers

In polarography, even low concentrations of samples can be detected, ranging from ______ M to ______ M.

<p>10-3; 10-4</p> Signup and view all the answers

Mercury is used as a working ______ in polarography due to its liquid state.

<p>electrode</p> Signup and view all the answers

An increase of current above the limiting value in the form of a maximum is known as polarographic ______.

<p>maxima</p> Signup and view all the answers

Maximum suppressors, such as gelatin, help eliminate polarographic ______.

<p>maxima</p> Signup and view all the answers

The ______ current is the sum of the relatively larger condenser current and a very small Faradic current.

<p>residual</p> Signup and view all the answers

The migration current is due to migration of cations from the bulk of the solution towards the ______.

<p>cathode</p> Signup and view all the answers

Diffusion current is caused by the actual diffusion of electroreducible ions from the bulk of the sample to the surface of the ______ droplet.

<p>mercury</p> Signup and view all the answers

The diffusion current varies inversely with the ______ of the medium.

<p>viscosity</p> Signup and view all the answers

The concentration of the electroreducible ions has a ______ relationship with diffusion current.

<p>direct</p> Signup and view all the answers

The ______ current reaches a steady value known as the limiting current beyond a certain potential.

<p>limiting</p> Signup and view all the answers

Flashcards

Polarography

An electrochemical technique that measures current flow between two electrodes in a solution as the applied voltage increases gradually. This allows for the determination of the concentration and nature of a solute.

Dropping Mercury Electrode (DME)

A polarizable electrode used in polarography where mercury drops continuously from a capillary tube, providing a fresh surface for each measurement.

Non-Polarizable Electrode

A non-polarizable electrode used in polarography that maintains a stable potential even with varying current. It is often a saturated calomel electrode.

Half-wave Potential

The point on the polarogram where the current reaches half of its maximum value. It is specific to the analyte and allows qualitative identification of substances.

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Diffusion Current

The current measured at the plateau of the polarogram. It is directly proportional to the concentration of the analyte, enabling quantitative analysis.

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Ilkovic Equation

The equation that describes the relationship between diffusion current and the influencing factors, including the number of electrons involved, diffusion coefficient, concentration, mercury flow rate, and drop time.

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Electroactive Species

The ability of a substance to be reduced or oxidized at the electrode surface, making it measurable by polarography.

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Advantages of Polarography

Organic and inorganic samples can be analyzed, even at low concentrations, with rapid results. It allows for both qualitative and quantitative analysis of mixtures without separation.

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Static Mercury Drop Electrode (DSME)

A type of mercury electrode where the mercury drop is held stationary at the end of the capillary tube. This ensures a constant surface area of mercury for a longer time.

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Hanging Mercury Drop Electrode (HMDE)

A type of mercury electrode where a single mercury drop hangs from the end of a capillary tube. This electrode is useful for studying the kinetics of electrode reactions.

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Overpotential

The tendency of a material to resist oxidation. A high overpotential means the material is resistant to oxidation.

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Working Electrode

The electrode used in polarography where a potential is applied between it and a reference electrode, causing a current to flow.

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Reference Electrode

A type of electrode used in polarography that remains at a constant potential, providing a reference point for measuring the potential of the working electrode.

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Migration Current

The process of separating the ions in a solution based on their ability to migrate in an electric field.

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Supporting Electrolyte

A substance added to a solution to increase its conductivity and reduce the migration current.

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Polarographic Maxima

An increased current exceeding the limiting value, often observed on polarographic waves, indicated by a peak in the current-voltage curve.

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What issue does polarographic maxima cause?

A phenomenon that can interfere with accurate measurements in polarography, obscuring the true half-wave potential and diffusion current.

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What are maximum suppressors?

Substances added to polarography experiments to suppress polarographic maxima, enhancing measurement accuracy.

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What is condenser current?

Current attributed to the flow of electricity through the double layer formed at the mercury electrode surface. It's significant in polarography due to the constantly renewed mercury drop.

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What is migration current?

The current that arises from the movement of ions in the solution towards the electrode. In contrast to diffusion current, it's influenced by the electric field, not just the concentration gradient.

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What is diffusion current?

The current that arises due to the diffusion of electroreducible ions from the bulk solution towards the surface of the mercury droplet.

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What is the limiting current?

The steady-state current value reached at a specific potential, indicating that the rate of diffusion of analyte to the electrode equals the rate of its reduction.

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What factors affect diffusion current?

Concentration, temperature, viscosity, and capillary characteristics.

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Study Notes

Introduction to Polarography

  • Polarographic technique analyzes electroreducible or oxidizable elements/groups.
  • It's an electromechanical technique, measuring current between electrodes in a solution while increasing voltage.
  • This helps determine solute concentration and nature.
  • Invented by Jaroslav Heyrovský.

Principle of Polarography

  • Polarography involves applying a gradually increasing negative potential between a polarizable and non-polarizable electrode.
  • Polarizable electrode: Dropping mercury electrode
  • Non-polarizable electrode: Saturated calomel electrode
  • The current is measured.
  • The resulting current-voltage curve (sigmoid shape) allows qualitative and quantitative analysis.
  • The technique is called polarography.
  • The instrument used is a polarograph, and the curve recorded is a polarogram.

Polarographic Measurements

  • Measurements are governed by the Ilkovic Equation.
  • Ilkovic Equation: The relationship between diffusion current (id) and the concentration of electroactive species.
  • Formula for Ilkovic Equation: id = 607 nCD1/2 m2/3 t1/6
    • id = diffusion current in microamperes
    • n = number of electrons required per molecule
    • C = concentration
    • D = diffusion coefficient
    • m = mass of mercury flow
    • t = drop time

Advantages of Polarography

  • Analysis of organic and inorganic samples.
  • Detection of even low concentrations (10-3 M to 10-4 M).
  • Rapid analysis.
  • Enables easy qualitative and quantitative analysis.
  • Mixture analysis without separation.

Mercury Electrodes

  • Mercury is a liquid electrode, often a drop suspended from a capillary tube.
  • Types:
    • Hanging mercury drop electrode (HMDE)
    • Dropping mercury electrode (DME)
    • Static mercury drop electrode (SMDE)
  • DME is commonly used; Mercury drops form at the end of a capillary tube due to gravity.

Why Dropping Mercury Electrode?

  • Reproducible current-potential data due to continuous fresh surface exposure.
  • Potential depends less on surface conditions of other electrodes (like Pt).
  • Solvent reduction does not interfere with analyte reduction at high overpotential.

Problems with Mercury Electrode

  • Mercury is easily oxidized, thus unsuitable for oxidations.
  • Use is limited to potentials of less than 0.4 V (vs SCE), above which mercury dissolves.
  • Hydrogen liberation occurs below -1.8V
  • Capillary maintenance is challenging due to clogging.

Method of Analysis

  • Polarographic apparatus comprises a polarizable electrode (DME) and a reference electrode (SCE).
  • A potential range of 0 to -3 V can be applied between electrodes.
  • An analysis cell, typically made of glass, is used to hold the sample.
  • The height of the mercury reservoir is adjusted to set a drop time of around 2-7 seconds.

Supporting Electrolytes & Maximum Suppressors in Polarography

  • Supporting electrolytes (like KCl) remove migration current
  • Oxygen in the sample is eliminated with nitrogen or alkaline pyrogallol.
  • Maximum suppressors are added (like gelatin, dyes, surfactants).
  • Initial and final potentials are set in the instrument.
  • Current-voltage curves (polarograms) are recorded.
  • The polarogram helps determine half-wave potential and diffusion current and performs qualitative and quantitative analysis.

Polarographic Maxima

  • An increase in current above expected values is a polarographic maximum, often noticed on polarographic waves.
  • Maxima often originate from catalytic discharge of ions, or increased depolarizer transport, causing a streaming action
  • Maxima lead to errors in determining half-wave potential and diffusion current.

Elimination of Polarographic Maxima

  • Maximum suppressors (gelatin, dyes, surfactants), which increase viscosity, help eliminate maxima.

Different Types of Current in Polarography

  • Residual current: The sum of condenser current and small Faradic current (due to impurities).
  • Migration current: Caused by cation migration towards the cathode, irrespective of concentration gradient.
  • Diffusion current: Resulting from electroreducible ions diffusing to the mercury drop surface due to the concentration gradient.
  • Limiting current: A steady current value beyond a certain potential; diffusion rate equals reduction rate.

Factors Affecting Diffusion Current

  • Concentration: Directly proportional to diffusion current.
  • Temperature: Directly proportional to diffusion current.
  • Viscosity: Diffusion coefficient depends on viscosity, thus affecting diffusion current.
  • Capillary characteristics: Capillary bore size, drop time, and mercury pressure impact diffusion current.
  • Presence of Maximum Suppressors: Maximum suppressors influence diffusion current.

Pharmaceutical Applications

  • Used for analysis of dissolved oxygen, peroxides, trace metals, metal-containing drugs, antiseptics, insecticides, vitamins, hormones, antibiotics, alkaloids, blood serum, and cancer diagnostics.

Advantages of Polarography

  • Small sample volume required.
  • Analysis of turbid and colored solutions is possible.
  • Applicable to substances not electrochemically active (indirect analysis).
  • No prior separation of components required.
  • High sensitivity for trace elements and toxic impurities.
  • High speed analysis is beneficial for quality control (QC).

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