Chem 3253 Electrochemistry April 7, 2004

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Chem 3253ElectrochemistryApril 7, 2004
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Four Electroanalytical Methods

• 1) Potentiometric
• 2) Voltammetric, Polarographic, Amperometric
• 3) Electrolysis (including electrogravimetric
  and coulometric)
• 4) Conductiometric
• We will consider the details of each of these
• separately.

3
Factors in using the Glass Electrode

• 1) The electrical resistance of the glass
  membrane is very high
• ( 100 M? ), so special high input resistance
  voltmeters
• (electrometers with Rinput 106 M ? ), are
  required to obtain
• accurate potential readings.
• 2) In use a small error, known as the asymmetry
  potential
• develops across the membrane so that frequent re-
• standardization of the electrode is required.
  Standardization
• consists of placing the electrode in a buffer of
  known pH and
• then electrically adjusting the meter to give the
  reading of that
• buffer.

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Factors in using the Glass Electrode

• 3) The composition and structure of the glass
  membrane affects
• the selectivity of the glass membrane electrode.
  Membrane
• electrodes are not exclusive in their response to
  a single ion,
• but respond to other ions of similar size and
  charge, so the
• potential may be a combination of the presence of
  several ions.
• The response for a desired ion A to the response
  to B is given
• by the selectivity coefficient k B/A . Small
  values of k B/A,
• such as 0.001 are desirable. This means that at
  equal
• concentrations of A and B, only 1/1000 of the
  potential
• comes from B. A typical glass electrode for pH
  measurements
• may have appreciable Na ion error, especially at
  high pH (when
• H is low). Low sodium error electrodes are
  available when
• needed.

5
Glass Electrodes
6
The Ion-exchange occurring across the glass
membrane that establishes the electrical
potential.
7
Ion-Selective Electrodes (ISE)

• In addition to the glass electrode for pH
  measurements, there
• are a wide range of membrane electrodes for the
  measurement
• of specific ions. Although sometimes called
  ion-specific
• electrodes one must realize that they are really
  selective for
• definite ions, but have the same limitations of
  selectivity as the
• Glass electrodes discussed earlier. They should
  really be
• termed as ion-selective electrodes (ISE). All of
  the types of
• membranes described earlier are used in their
  construction.

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Polymer membrane ISE, similar to the Calcium ISE
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Solid crystal membrane ISE the Flouride electrode
10
Calibration curve for the F? ISE other
electrodes might have different y-intercept
values. The theoretical slope is 59.16/n where n
is the charge of the ion.
11
Examples of Solid state ISEs
12
Ion Selective Electrodes

• At the present time the following Ion-selective
• electrodes have been developed
• NH4, Ca2, Mg2, K, Cl -, S-2, F -, ClO4-,
  NO3-, Br -, I -, CN -, Ag, Cu2, Pb2, Cd2
• in addition to these, by using permeable
• membranes over glass electrodes that measure
• the amounts of various gases or of specific
• metabolites have been developed, such as the
• CO2, urea, or glucose electrodes.

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A secondary membrane electrode here CO2
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The measurement of Electrode Potentials

• The measurement of the potential of ion-selective
  electrodes is
• accomplished by making an electrochemical cell
  consisting of
• the ion-selective electrode and a suitable
  reference electrode.
• Emeasured Eindicator Ereference.
• Since the potential of the reference electrode is
  known,
• Eindicator Ecell Ereference .
• There are three different ways that
  potentiometric
• measurements may be done in the laboratory
• 1 - Direct
• 2 - Electrode Calibration
• 3 - Standard Addition

15
The measurement of Electrode Potentials

• 1 - Direct the measured potential is related to
  the concentration of the analyte by the Nernst
  equation, the 0.05916/n pA, where pA is the log
  A. Although this method is straight forward and
  simple, it does not account for matrix effects,
  i.e., the effect of other ions, etc in the
  sample. This method is useful for continuous
  monitoring and is thus used in the analysis of
  industrial and natural processes such as effuate
  streams.

16
The measurement of Electrode Potentials

• 2 - Electrode Calibration electrode response is
  calibrated against solutions of known analyte.
  Often a plot is constructed of Emeasured vs pA
  and used as both a check of range of linearity of
  the response and to use as a calibration curve.

17
The measurement of Electrode Potentials

• 3 - Standard Addition in this method the
  electrode is used to first measure the unknown
  sample and then a small known amount of the
  analyte is added and the measurement taken. The
  incremental change in potential was caused by the
  known addition. This then allows the analyst to
  calculate the concentration of the analyte in the
  original sample. This technique is best to assure
  that the ionic strength of the solution (and
  therefore the activity) is not causing a
  significant error. The following relationship is
  useful in the calculation of the concentration of
  the analyte using the standard addition method
• A S X VS / (VA VS) X 10 - n (?E1 -
  ?E2)/0.05916 - VA
• where A is the analyte, S the standard solution,
  V volume, and n number of electrons.

18
The measurement of Electrode Potentials

• 3 - Standard Addition in this method the
  electrode is used to first measure the unknown
  sample and then a small known amount of the
  analyte is added and the measurement taken. The
  incremental change in potential was caused by the
  known addition. This then allows the analyst to
  calculate the concentration of the analyte in the
  original sample. This technique is best to assure
  that the ionic strength of the solution (and
  therefore the activity) is not causing a
  significant error. The following relationship is
  useful in the calculation of the concentration of
  the analyte using the standard addition method
• A S X VS / (VA VS) X 10 - n (?E1 -
  ?E2)/0.05916 - VA
• where A is the analyte, S the standard solution,
  V volume, and n number of electrons.
• Sample Standard Addition Problem
• The wastewater from an industrial processing
  plant was routinely analyzed for lead as required
  by EPA.
• A Pb ion-selective electrode and a SCE reference
  were placed in the sample. The potential
  difference was found to be 0.118 volts. 5.00 mL
  of a 0.00600 M solution of Pb2 standard solution
  was added to the above sample and the measurement
  repeated the potential difference was found to
  be 0.109 volts. What is the approximate
  concentration of Pb in the wastewater? Express
  your final answer in ppm.
• Solution
• One simply substitutes into the previous given
  equation
• Pb2 0.00600 X 5.00 / (50.0 5.00) X 10
  -2-0.118 (-0.109) / 0.05916 50.0
• Pb2 0.0300 / 55.0 X 10 0.304 50.0
• Pb2 0.0300 / 110.7 50.0 0.0300 /
  60.7 4.94 e-4 M 4.94 e-4 mmol/mL
• Ppm m g/g m g/mL (for dilute solutions)
• Atomic mass Pb 207.2 g/mol 207.2 mg/mmol
• Mass of Pb (4.94 e-4 mmol/mL X 207.2 mg/mmol)
  0.1023 mg/mL
• 0.1023 mg/mL X 1000 m g/mg 102.3 ppm
• Because of the loss of significant digits by
  subtraction -0.118 (-0.109) 0.009, the
  analyst can only report 1 significant digit, 100
  ppm (1 X 10 2)
• The response is a logarithmic function of
  concentration (Nernstian), i.e., the potential
  changes (0.05916 volts/n) per power of ten in
  concentration. (_at_ 25oC)

19
The measurement of Electrode Potentials

• 3 - Standard Addition in this method the
  electrode is used to first measure the unknown
  sample and then a small known amount of the
  analyte is added and the measurement taken. The
  incremental change in potential was caused by the
  known addition. This then allows the analyst to
  calculate the concentration of the analyte in the
  original sample. This technique is best to assure
  that the ionic strength of the solution (and
  therefore the activity) is not causing a
  significant error. The following relationship is
  useful in the calculation of the concentration of
  the analyte using the standard addition method
• A S X VS / (VA VS) X 10 - n (?E1 -
  ?E2)/0.05916 - VA
• where A is the analyte, S the standard solution,
  V volume, and n number of electrons.

20
The measurement of Electrode Potentials

• 2 - Electrode Calibration electrode response is
  calibrated against solutions of known analyte.
  Often a plot is constructed of Emeasured vs pA
  and used as both a check of range of linearity of
  the response and to use as a calibration curve.
• Standard Addition in this method the electrode
  is used to first measure the unknown sample and
  then a small known amount of the analyte is added
  and the measurement taken. The incremental change
  in potential was caused by the known addition.
  This then allows the analyst to calculate the
  concentration of the analyte in the original
  sample. This technique is best to assure that the
  ionic strength of the solution (and therefore the
  activity) is not causing a significant error. The
  following relationship is useful in the
  calculation of the concentration of the analyte
  using the standard addition method
• A S X VS / (VA VS) X 10 - n (?E1 -
  ?E2)/0.05916 - VA
• where A is the analyte, S the standard solution,
  V volume, and n number of electrons.
• Sample Standard Addition Problem
• The wastewater from an industrial processing
  plant was routinely analyzed for lead as required
  by EPA.
• A Pb ion-selective electrode and a SCE reference
  were placed in the sample. The potential
  difference was found to be 0.118 volts. 5.00 mL
  of a 0.00600 M solution of Pb2 standard solution
  was added to the above sample and the measurement
  repeated the potential difference was found to
  be 0.109 volts. What is the approximate
  concentration of Pb in the wastewater? Express
  your final answer in ppm.
• Solution
• One simply substitutes into the previous given
  equation
• Pb2 0.00600 X 5.00 / (50.0 5.00) X 10
  -2-0.118 (-0.109) / 0.05916 50.0
• Pb2 0.0300 / 55.0 X 10 0.304 50.0
• Pb2 0.0300 / 110.7 50.0 0.0300 /
  60.7 4.94 e-4 M 4.94 e-4 mmol/mL
• Ppm m g/g m g/mL (for dilute solutions)
• Atomic mass Pb 207.2 g/mol 207.2 mg/mmol
• Mass of Pb (4.94 e-4 mmol/mL X 207.2 mg/mmol)
  0.1023 mg/mL
• 0.1023 mg/mL X 1000 m g/mg 102.3 ppm
• Because of the loss of significant digits by
  subtraction -0.118 (-0.109) 0.009, the
  analyst can only report 1 significant digit, 100
  ppm (1 X 10 2)

21
The measurement of Electrode Potentials

• 2 - Electrode Calibration electrode response is
  calibrated against solutions of known analyte.
  Often a plot is constructed of Emeasured vs pA
  and used as both a check of range of linearity of
  the response and to use as a calibration curve.

22
The measurement of Electrode Potentials

• 1 - Direct the measured potential is related to
  the concentration of the analyte by the Nernst
  equation, the 0.05916/n pA, where pA is the log
  A. Although this method is straight forward and
  simple, it does not account for matrix effects,
  i.e., the effect of other ions, etc in the
  sample. This method is useful for continuous
  monitoring and is thus used in the analysis of
  industrial and natural processes such as effuate
  streams.
• Electrode Calibration electrode response is
  calibrated against solutions of known analyte.
  Often a plot is constructed of Emeasured vs pA
  and used as both a check of range of linearity of
  the response and to use as a calibration curve.
• Standard Addition in this method the electrode
  is used to first measure the unknown sample and
  then a small known amount of the analyte is added
  and the measurement taken. The incremental change
  in potential was caused by the known addition.
  This then allows the analyst to calculate the
  concentration of the analyte in the original
  sample. This technique is best to assure that the
  ionic strength of the solution (and therefore the
  activity) is not causing a significant error. The
  following relationship is useful in the
  calculation of the concentration of the analyte
  using the standard addition method
• A S X VS / (VA VS) X 10 - n (?E1 -
  ?E2)/0.05916 - VA
• where A is the analyte, S the standard solution,
  V volume, and n number of electrons.
• Sample Standard Addition Problem
• The wastewater from an industrial processing
  plant was routinely analyzed for lead as required
  by EPA.
• A Pb ion-selective electrode and a SCE reference
  were placed in the sample. The potential
  difference was found to be 0.118 volts. 5.00 mL
  of a 0.00600 M solution of Pb2 standard solution
  was added to the above sample and the measurement
  repeated the potential difference was found to
  be 0.109 volts. What is the approximate
  concentration of Pb in the wastewater? Express
  your final answer in ppm.
• Solution
• One simply substitutes into the previous given
  equation
• Pb2 0.00600 X 5.00 / (50.0 5.00) X 10
  -2-0.118 (-0.109) / 0.05916 50.0
• Pb2 0.0300 / 55.0 X 10 0.304 50.0
• Pb2 0.0300 / 110.7 50.0 0.0300 /
  60.7 4.94 e-4 M 4.94 e-4 mmol/mL
• Ppm m g/g m g/mL (for dilute solutions)
• Atomic mass Pb 207.2 g/mol 207.2 mg/mmol
• Mass of Pb (4.94 e-4 mmol/mL X 207.2 mg/mmol)
  0.1023 mg/mL
• 0.1023 mg/mL X 1000 m g/mg 102.3 ppm
• Because of the loss of significant digits by
  subtraction -0.118 (-0.109) 0.009, the
  analyst can only report 1 significant digit, 100
  ppm (1 X 10 2)

23
The measurement of Electrode Potentials

• The measurement of the potential of ion-selective
  electrodes is
• accomplished by making an electrochemical cell
  consisting of
• the ion-selective electrode and a suitable
  reference electrode.
• Emeasured Eindicator Ereference.
• Since the potential of the reference electrode is
  known,
• Eindicator Ecell Ereference .
• There are three different ways that
  potentiometric
• measurements may be done in the laboratory
• 1 - Direct the measured potential is related to
  the concentration of the analyte by the Nernst
  equation, the 0.05916/n pA, where pA is the log
  A. Although this method is straight forward and
  simple, it does not account for matrix effects,
  i.e., the effect of other ions, etc in the
  sample. This method is useful for continuous
  monitoring and is thus used in the analysis of
  industrial and natural processes such as effuate
  streams.
• Electrode Calibration electrode response is
  calibrated against solutions of known analyte.
  Often a plot is constructed of Emeasured vs pA
  and used as both a check of range of linearity of
  the response and to use as a calibration curve.
• Standard Addition in this method the electrode
  is used to first measure the unknown sample and
  then a small known amount of the analyte is added
  and the measurement taken. The incremental change
  in potential was caused by the known addition.
  This then allows the analyst to calculate the
  concentration of the analyte in the original
  sample. This technique is best to assure that the
  ionic strength of the solution (and therefore the
  activity) is not causing a significant error. The
  following relationship is useful in the
  calculation of the concentration of the analyte
  using the standard addition method
• A S X VS / (VA VS) X 10 - n (?E1 -
  ?E2)/0.05916 - VA
• where A is the analyte, S the standard solution,
  V volume, and n number of electrons.
• Sample Standard Addition Problem
• The wastewater from an industrial processing
  plant was routinely analyzed for lead as required
  by EPA.
• A Pb ion-selective electrode and a SCE reference
  were placed in the sample. The potential
  difference was found to be 0.118 volts. 5.00 mL
  of a 0.00600 M solution of Pb2 standard solution
  was added to the above sample and the measurement
  repeated the potential difference was found to
  be 0.109 volts. What is the approximate
  concentration of Pb in the wastewater? Express
  your final answer in ppm.
• Solution
• One simply substitutes into the previous given
  equation