ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 15

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 15
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 15 ELECTRODE MEASUREMENTS
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INDICATOR ELECTRODES
Chemically Inert Electrodes - Do not participate
in the reaction Examples Carbon Gold Platinum ITO
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INDICATOR ELECTRODES
Reactive Electrodes - Participate in the
reaction Examples Silver Copper Iron Zinc
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INDICATOR ELECTRODES
Two Classes of Indicator Electrodes - Metal
Electrodes - Surfaces on which redox reactions
take place Examples Platinum Silver
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INDICATOR ELECTRODES

• Two Classes of Indicator Electrodes
• - Ion-Selective Electrodes
• - Selectively binds one ion (no redox chemistry)
• Examples
• pH electrode
• Calcium (Ca2) electrode
• Chloride (Cl-) electrode

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DOUBLE-JUNCTION REFERENCE ELECTRODES
- With the use of reference electrodes - KCl
solution may slowly leak into solution through
the porous plug (salt bridge) - Cl- may
introduce errors (e.g. consumes Ag when reagent
is Ag solution) - Double-junction reference
electrode prevents direct leakage into reagent
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JUNCTION POTENTIAL
- When two dissimilar electrolyte solutions come
in contact - Potential difference develops at
the interface - Voltage is very small usually in
millivolts - Very common at the ends of salt
bridges - Observed voltage measurements may
include junction potential
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JUNCTION POTENTIAL
Eobserved Ecell Ejunction - A result of
unequal ion mobilities - K and Cl- have similar
mobilities - Reason why KCl is used in salt
bridges
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POTENTIOMETRY
- The use of voltage measurements for
quantification Direct Potentiometric Method -
Measures absolute potential (concentration) - A
metal in contact with a solution of its cation -
Associated with errors due to junction
potentials Examples - Silver wire for measuring
Ag - Potassium ion-selective electrode for
measuring K - pH electrode for measuring H
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POTENTIOMETRY
- The use of voltage measurements for
quantification Relative Potentiometric Method -
Measures changes in potential (concentration) -
Relatively precise and accurate Example -
Measuring changes in potential during titration
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ION-SELECTIVE ELECTRODES
- Responds preferentially to one species in
solution
Internal reference electrode
Filling solution
Ion-selective membrane
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ION-SELECTIVE ELECTRODES
- Selective (preferential) ion is C - Membrane
is made of poly(vinyl chloride) - Membrane is
impregnated with nonpolar liquid - Membrane
contains ligand L (ion-selective ionophore) -
Membrane contains the complex LC - Membrane
contains hydrophobic anion R- (ion exchanger)
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ION-SELECTIVE ELECTRODES
- C inside the electrode ? C outside the
electrode - Produces a potential difference
across the membrane
at 25 oC
- n is the charge on the selective ion (negative
for anions) n 1 for K n 2 for Ca2 n -2
for CO32-
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pH GLASS ELECTRODE
- The most widely used - Selective ion is H -
Glass membrane (bulb) consists of SiO4 - pH
changes by 1 when H changes by a factor of
10 - Potential difference is 0.05196 V when
H changes by a factor of 10 For a change in
pH from 3.00 to 6.00 (3.00 units) Potential
difference 3.00 x 0.05196 V 0.177
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pH GLASS ELECTRODE
Glass Electrode Response at 25 oC E constant
ß(0.05916)?pH ?pH pH difference between inside
and outside of glass bulb ß 1 (typically
0.98) (measured by calibrating electrode in
solutions of known pH) constant assymetry
potential
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pH GLASS ELECTRODE
Sources of Error - Standards used for
calibration - Junction potential - Equilibration
time - Alkaline (sodium error) - Temperature -
Strong acids - Response to H (hydration effect)
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COMPOUND ELECTRODE
- Electrode surrounded by a membrane - Membrane
isolates the analyte to which the electrode
responds Examples - Gas sensing electrodes NH3,
CO2, NOx, H2S, SO2 - Enzyme electrodes (highly
selective)
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ELECTROCHEMICAL METHODS
Applications - Biosensors (analyte
sensors) (Glucose sensors) - Chromatography
detectors - Solar energy storage systems -
Microelectronics - Electrocatalysis of fuel cells
and batteries
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ELECTROCHEMICAL METHODS
Electrogravimetric Analysis - Chemically inert
cathode with large surface area is used (in the
form of gauze) - Analyte is electroplated
(deposited) on a preweighed cathode - Cathode
is weighed again - Mass of analyte is determined
by difference Cu2(aq) 2e- ? Cu(s)
(deposited on cathode)
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ELECTROCHEMICAL METHODS
Coulometric Analysis - Amount of analyte is
determined from electron count - Electric
current and time required to generate product
are measured - Number of electrons is determined
from current and time - Number of moles of
analyte is determined from electron
count Reaction of I2 and H2S I2 H2S ? S(s)
2H 2I-
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ELECTROCHEMICAL METHODS
Three Electrode Cells - Reference electrode -
Working (indicator) electrode - Auxiliary
(counter) electrode - Current flows between
working and auxiliary electrodes - Voltage is
measured between working and reference electrodes
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ELECTROCHEMICAL METHODS
Amperometry - The electric current between the
pair of electrodes is measured - Voltage is
fixed - Current is proportional to the
concentration of analyte Biosensors (glucose
monitors)
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ELECTROCHEMICAL METHODS
Voltammetry - Voltage between two electrodes is
varied as current is measured -
Oxidation-reduction takes place at or near the
surface of the working electrode - Graph of
current versus potential is obtained (called
voltammogram) - Peak current is proportinal to
concentration of analyte
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ELECTROCHEMICAL METHODS
Voltammetry Polarography - Uses dropping-mercury
electrode Square Wave Voltammetry - Uses
waveform which consists of square
wave superimposed on a staircase
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ELECTROCHEMICAL METHODS
Voltammetry Stripping Voltammetry - Analyte is
concentrated into a drop of Hg by reduction -
Analyte is reoxidized by making potential more
positive - Current is measured during
oxidation Cyclic Voltammetry (CV) - Electrode
potential versus time is linear - Current versus
applied voltage gives a cyclic voltammogram
trace - Used to study electrochemical properties
of analytes