Transcript Chapter 23

Chapter 23
Potentiometry
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.
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
Reference Electrodes
Reference Electrodes
Calomel Electrodes:
Reference Electrodes
Silver/Silver Chloride Electrodes:
Metallic Indicator Electrodes
There are four types of metallic indicator
electrodes:
1. Electrodes of the first kind.
2. Electrodes of the second kind.
3. Electrodes of the third kind.
4. Redox electrodes.
Metallic Indicator Electrodes
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
Membrane Indicator
Electrodes
Glass Electrodes:
Membrane Indicator
Electrodes
Glass Electrodes: Potential
 The boundary potential.
 The potential of the internal Ag/AgCl
reference electrode.
 A small asymmetry potential.
Membrane Indicator
Electrodes
 Alkaline
Error
 Selectivity Coefficients
 Acid Error
Membrane Indicator
Electrodes
Crystalline Membrane Electrodes:
Membrane Indicator
Electrodes
Fluoride Electrode:
Membrane Indicator
Electrodes
Liquid Membrane Electrodes:
 Cation exchangers
 Anion exchangers
 Neutral macrocyclic compounds, which
selectively complex certain cations
Membrane Indicator
Electrodes
Ion-Selective Field-Effect
Transistors (ISFETs)
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.
Molecular-Selective Electrode
Systems
Gas-Sensing Probes:
Instruments for Measuring
Cell Potentials
 Direct-Reading
Instruments
 Commercial Instruments
 Utility
 General-purpose
 Expanded-scale
 Research
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
Potentiometric Titrations
Schematic
representatio
n of an
automatic
potentiometri
c titrator
devised by
Lingane in
1948.
References
http://ull.chemistry.uakron.edu/analytical/Potenti
ometry/
http://chem.ch.huji.ac.il/~eugeniik/instruments/el
ectrochemical/potentiometric_titrators.htm
http://www.fz-juelich.de/isg/sensorik/bcs-isfete.html
http://www.chemistry.msu.edu/courses/cem333/
Chapter23,potentiometry.PDF