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Chapter 23. Potentiometry. Reference Electrodes.

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Chapter 23 l.jpg

Chapter 23

Potentiometry


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Reference Electrodes

A reference is an electrode that has the half-cell potential known, constant, and completely insensitive to the composition of the solution under study. In conjunction with this reference is the indicator or working electrode, whose response depends upon the analyte concentration.


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Reference Electrodes

Ideal Reference Electrode:

  • Is reversible and obeys the Nernst equation

  • Exhibits a potential that is constant with time

  • Returns to its original potential after being subjected to small currents

  • Exhibits little hysteresis with temperature cycling



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Reference Electrodes

Calomel Electrodes:


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Reference Electrodes

Silver/Silver Chloride Electrodes:


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Metallic Indicator Electrodes

There are four types of metallic indicator electrodes:

  • Electrodes of the first kind.

  • Electrodes of the second kind.

  • Electrodes of the third kind.

  • Redox electrodes.



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Membrane Indicator Electrodes

Properties:

  • Minimal solubility. A necessary property of an ion-selective medium is that its solubility in analyte solutions approaches zero.

  • Electrical conductivity. A membrane must exhibit some electrical conductivity, albeit small. Generally, this conduction takes the form of migration of singly charged ions within the membrane.

  • Selective reactivity with the analyte. A membrane or some species contained within the membrane matrix must be capable of selectively binding the analyte ion. Three types of binding:

    Ion-exchange

    Crystallization

    Complexation



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Membrane Indicator Electrodes

Glass Electrodes: Potential

  • The boundary potential.

  • The potential of the internal Ag/AgCl reference electrode.

  • A small asymmetry potential.


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Membrane Indicator Electrodes

  • Alkaline Error

  • Selectivity Coefficients

  • Acid Error


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Membrane Indicator Electrodes

Crystalline Membrane Electrodes:


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Membrane Indicator Electrodes

Fluoride Electrode:


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Membrane Indicator Electrodes

Liquid Membrane Electrodes:

  • Cation exchangers

  • Anion exchangers

  • Neutral macrocyclic compounds, which selectively complex certain cations




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Molecular-Selective Electrode Systems

Gas-Sensing Probes:

  • Microporous materials- manufactured from hydrophobic polymers that have a porosity of about 70% and a pore size of less than 1m, and are about 0.1mm thick.

  • Homogeneous films- solid polymeric substances through which the analyte gas passes by dissolving in the membrane, diffusing, and then desolvating into the internal solution. They are usually thinner than microporous in order to hasten the transfer of gas and thus the rate of response of the system.



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Instruments for Measuring Cell Potentials

  • Direct-Reading Instruments

  • Commercial Instruments

    • Utility

    • General-purpose

    • Expanded-scale

    • Research


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Direct Potentiometric Measurements

  • The Sign Convention and Equations for Direct Potentiometry

  • The Electrode Calibration Method

    • Inherent Error in the Electrode

    • Activity Versus Concentration

  • Calibration Curves for Concentration Measurement

  • Standard Addition Method

  • Potentiometric pH Measurements with a Glass Electrodes

    • Summary of Errors Affecting pH Measurements with the Glass Electrode

    • The Operational Definition of pH


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Potentiometric Titrations

Schematic representation of an automatic potentiometric titrator devised by Lingane in 1948.


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References

http://ull.chemistry.uakron.edu/analytical/Potentiometry/

http://chem.ch.huji.ac.il/~eugeniik/instruments/electrochemical/potentiometric_titrators.htm

http://www.fz-juelich.de/isg/sensorik/bcs-isfet-e.html

http://www.chemistry.msu.edu/courses/cem333/Chapter23,potentiometry.PDF


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