Electroanalytical Methods
description
Transcript of Electroanalytical Methods
CH 3001 Electroanalytical Methods and
Chemical Sensors
• Introduction
• Voltametry
• Potentiometry
• Coulometry and Electrogravimetry
• Conductimetry
• Electrochemical Sensors in brief
Mass transfer
M+n
mass transport occurs by:
1. diffusion = movement due to
concentration difference
2 migration of charged species
due to potential gradient
3. convection (mechanical
stirring or agitation)
Electrolysis
M+n
A-m
(-) (+)
anodeoxidationmeAA
cathodreductionMneMm
n
Faradic Current (Non-Charging Current)
M+n
e
(-) Current due to the transfer of electron from the electrode to the ion makes the ion to get reduced and the current produced by such processes is known as Faradic Current. This current obeys the Faraday’s law. This is also known as “non-charging current”.
Charging Current (Non-Faradic Current)
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
- -
- -
- -
- -
- -
- -
- -
- -
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- -
Met
alM+n
Built-up of electrical charge in the form of a double layer. Current due to the migration of ions through the double layer is known as “Charging Current”. This current does not follow the Faraday’s law. Therefore it is known as “Non-Faradic Current”.
Voltammetry
• Measurement of current as a function of potential.
• Measurement of current under condition of complete concentration polarization.
• A minimal consumption of analyte.
• Use of mercury as a electrode in this method is known as “Polarography”.
Voltammetry and Polarography
Voltammetry is the electrochemical technique in which the current at an electrode is monitored as a function of potential applied to that electrode.
The plot between current and the measured potential is known as “Voltamograme”.
In this technique, three-electrode-system is used.Working Electrode: Where the redox reaction takes place: usually-> Hg electrodeReference Electrode: Used to measure the potential of the working electrodeAuxiliary Electrode: Used to measure the current at the working electrode
A Modern Polarograph
Principle in Polarograpy
M+
Charging Reduction
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transferMassMM
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Current
Time
e
Faradaic Cureent
Charging Current
charged
Three ways of mass transfer when a potential is applied to the working
electrode:
1. Migration of ions under the influence of the electric field – Migration Current, imig
2. Convection current due to the movement of ions under the influence of hydrodynamic forces – Convection Current, icon
3. Diffusion of ions from higher to lower concentration – Diffusion Current, idif
inet = imig + icon + idif
Diffusion rate of the ions Concentration of the ions
diffusion current inet
DC Polarography
Here, the applied potential is DC voltage
The plot between current and the applied potential is known as a Polarogrph
il
EappE1/2
i
Powerful Variations of Voltametric Methods:
• AC Polarography • Normal-Pulse Polarography (NPP)• Differential Pulse Polarography
(DPP) • Anodic Stripping Voltammetry
(ASV) • Cyclic Voltametry (CV)
Faradic Current (Non-Charging Current)
M+n
e
(-) Current due to the transfer of electrons from the electrode to the ion makes the ion to get reduced and the current produced by such processes is known as Faradic Current. This current obeys the Faraday’s law. This is also known as Non-Charging Current.
Charging Current (Non-Faradic Current)
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
+ +
- - -
- -
- -
- -
- -
- -
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-
Met
alM+n
Built-up of electrical charge in the form of a double layer. Current due to the migration of ions through the double layer is known as charging current. This current does not follow the Faraday’s law. Therefore known as Non-Faradic Current.
Polarography Instrument
DC Polarograph
Oxygen Interferences
How to overcome oxygen interference
• Bubbling of solution with pure nitrogen gas.
• Bubbling should be done before the commencement of the analysis.
• Through out the analysis nitrogen atmosphere is maintained.
Advantages of DME (compared to planar electrodes)
• clean surface generated
• rapid achievement of constant current during drop growth
• remixing of solution when the drop falls
• high Hg overvoltage means even metals with high -ve E0 can be measured without H2 formation
Disadvantages of DME:
• Hg easily oxidized, limited use as anode (E< +0.4 V)
• 2Hg + 2Cl- Hg2Cl2 + 2e-
• nonfaradaic residual currents limit detection to >10-5 M
• cumbersome to use (toxic mercury)
• sometimes produce current maxima for unclear reasons (use maxima suppressor)
Polarographic Maxima
• sometimes produce current maxima
• Can be overcome by using maxima suppressor
Powerful improvement of polarography
• AC polarogrphy
• Normal pulse polarography
• Differential pulse polarography
• Stripping voltametry
• Cyclic voltametry
AC polarogrphy
• Instead of DC scan an AC scan is used.
Peaks - the selectivity is higher
Normal pulse polarographyIn Normal pulse voltammetry (polarography) - a potential wave is applied – an HME is used
The sensitivity is higher due to the absence of charging current
Differential pulse polarographyIn DifferentialDifferential pulse voltammetry (polarography) - a differential potential wave is applied – an HME is used
Both sensitivity and selectivity are higher due to the absence of charging current and peaks
Differential pulse polarography
pH (Glass Membrane) Electrodes
• One of the simpler ion-selective electrodes (ISE)
• Hydrogen Ion imparts a charge across a hydrated glass membrane
• Generally include an internal reference electrode (Ag/AgCl) and a separate Ag/AgCl electrode for sensing the charge imparted by the hydrogen ions
• Not as simple to use as you think!
constant pHconstant pH
External solutionExternal solution
glassglassmembrane-membrane-
hydratedhydrated(50 (50 m thick)m thick)
externalexternalreferencereferenceelectrodeelectrode(porous plug)(porous plug)
potentialpotentialdevelops acrossdevelops acrossmembrane duemembrane dueto pH differenceto pH difference
Combination Glass ElectrodeCombination Glass Electrode
solution levelsolution level
internalinternalreferencereferenceelectrodeelectrode
Commercial pH meter
3D network of silicate groups. There are sufficient cations within the interstices of this structure to balance the negative charge of silicate groups. Singly charged cations such as sodium are mobile in the lattice and are responsible for electrical conductance within the membrane.
Structure of membrane Glass
Hygroscopicity of Glass membrane
• Surface of the glass membrane must be hydrated to function as a pH electrode.
• It can lose pH sensitivity on dehydration, but is reversible and can be restored by soaking the electrode in water.
• The hydration involves an ion-exchange between singly charged cations in the glass lattice and protons from the solution.
GlH)solu(NaGlNa)solu(H
Electrical conduction across membranes
Conductance within the hydrated glass membrane involves the movement of Na+ and H+ ions.
Na+ - Charge carriers in the dry interiorH+ - Charge carriers in the gel interface
Gl)solu(HGlH
GlHGl)solu(H
2
1
The positions of these equilibrium depend on the H+ concentration on either side and charge on the glass surface giving potential. The potential difference between the two sides is known as the “Boundary Potential”.
The boundary potential
The potentials associated with each side (solution 1 and solution 2) E1 and E2
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The boundary potential depends only upon the hydrogen ion activity of the external
The asymmetry potential
• When glass electrode’s two sides are in contact with identical solutions, we expect zero potential
• However a small potential known as the “Asymmetric Potential” is encountered.
• This gradually changes with time.• Electrode must be calibrated against one or
more standards.
alnF
RTconstEE 21
Basic Nernst Equation of a pH Electrode
alnF
RTconstEE 21
depends on internal referencedepends on internal referenceelectrode and glass membraneelectrode and glass membranebehavior, behavior, which changes with timewhich changes with time
Must calibrate with a buffer!Must calibrate with a buffer!
How does the measured voltage, E, vary with pH?
EE
pHpH
E ’E ’Two point calibrationTwo point calibration
with bufferswith buffers
E = E’ – 0.0591E = E’ – 0.0591 pH pH
Calibration of pH meter
ISFETA “new”
pH electrode
What does 0.05916 mean?• It is a constant, if there is a
one-electron reaction• It can be considered as the
equivalent of a constant of 59.16 mV
• A pH meter is a high-impedance potentiometer (measures voltage)
• A pH change of “1” imparts a change in 59.16 mV to the potential recorded by the pH meter!
• 1 pH unit change= 59.16 mV
mV (relative readings)
pH
100.00 2.00
159.16 3.00
218.32 4.00
277.48 5.00
336.64 6.00
395.80 7.00
Errors in pH Measurement…1. Uncertainty in your buffer pH due to normal weighing, diluting
errors etc.
2. Junction Potential due to the salt bridge and differences in Junction Potentials over time due to contamination of the junction
3. Overcome by regular recalibration
4. Sodium Error will result in high concentration of sodium solutions. The sodium can also impart a charge across the glass membrane.
5. Acid Error (strong acids) can saturate or contaminate the membrane with hydrogen ions!
6. Equilibration Error is overcome by letting the electrode equilibrate with the solution
7. Dried out glass membrane (ruins electrode)
8. Temperature. Since temperature affects activities, it is best to have all solutions at the same, constant temperature!
9. Strong bases. Strongly basic solutions (>pH 12) will dissolve the glass membrane!
Sodium Electrode•The change in the composition of the glass membrane permits the determination of cations other than H+.
• By modification of the glass membrane, you can make an electrode that is more sensitive to Na+ compared to H3O+.
• The modification can be done by incorporating Al2O3 or B2O3.
Ion Selective Electrodes…
• Selectivity Coefficient– Defines how an ISE responds to the species of
interest versus some interfering species• Interferences cause a signal (voltage) to be imparted
on the electrode that is NOT the result of the ion or chemical species of interest
– You want the selectivity coefficient to be as SMALL as possible
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0.05916 constant E
analyte the is and ceinterferen the is where
to response electrode to response electrode
Examples of Electrodes / Interferences
Solid State Electrodes
• Uses a small amount of a doped crystal to transport charge from the solution to an inner electrode
• The inner (sensing) electrode can be a Ag/AgCl electrode, and a separate Ag/AgCl electrode can be present– Combination electrode
• You can use a separate reference electrode also.
• Typical equation (Fluoride Electrode):
• Use:– Prepare calibration and sample solutions to
similar ionic strengths and temperatures– Connect ISE and reference electrode to
potentiometer (pH meter in mV mode)– Record potentials of calibration solutions– Prepare calibration curve– Measure potential of sample solutions and
calculate fluoride activity!
)( log x 0.05916 x - constant E outside -FA
Liquid membrane electrodes
• Similar to pH electrode except the membrane is an organic polymer saturated with liquid ion exchanger.
• Interaction of this exchanger with target ions resulted in a potential across the membrane that can be measured.
Liquid Membrane ISE’s• Replace the solid
state crystal with a liquid ion-exchanger filled membrane
• Ions impart a charge across the membrane
• The membrane is designed to be SELECTIVE for the ion of interest…
Liquid membrane electrodes• The reservoir forces exchanger into
membrane.• The exchanger forms complexes with species
of interest.• This results in a concentration difference and
the resulting potential difference can be measured.
Gas Sensing Electrodes• Still considered “ion-
selective”• Works by the permeation
of gas across a selective membrane
• Also called compound electrodes
• The gas changes the pH inside the electrode (on the inside of the membrane) and this signal is proportional to the gas concentration.
Gas Sensing Electrodes
Enzyme Electrodes
Enzyme Electrodes
Enzyme Electrodes
EE transducer
Analyte signalrecognition
Enzyme Electrodes
EE transducer
Analyte No signalNo recognition
Potentiometric Titrations
ECell
VTitrantEnd Point
Potentiometric Titration
• How does the pH change during an acid-base neutralization reaction?
• Measure pH as the titrant is added.
pHpH
Volume of titrant (mL NaOH)Volume of titrant (mL NaOH)
HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO
pH = 7pH = 7
Equivalence point pH Equivalence point pH
How do we locate the equivalence point?How do we locate the equivalence point?
pHpH
Volume of titrant (mL NaOH)Volume of titrant (mL NaOH)
HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO
pH = 7pH = 7
Just acidJust acid
Just saltJust salt
Both acid & saltBoth acid & salt
xs NaOH & saltxs NaOH & salt
What is present at various points?What is present at various points?
pHpH
Volume of titrant (mL NaOH)Volume of titrant (mL NaOH)
HCl + NaOH HCl + NaOH NaCl + H NaCl + H22OO
The slope between each pair of data points.The slope between each pair of data points.
How is the slope changing?How is the slope changing?
increasing slopeincreasing slope
decreasing slopedecreasing slope
77
inflection pointinflection point - - changed changed from increasing to decreasingfrom increasing to decreasing
Using the derivative toUsing the derivative tolocate the equivalence pointlocate the equivalence point
0
2
4
6
8
10
12
14
16
0 5 10 15 20
volume of NaOH
pH
0
5
10
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The maximum of the derivative locates the equivalence point.The maximum of the derivative locates the equivalence point.
Electrogravimetry & coulometry Both are electrolysis processes
Constant potential electrolysis – potential of the
working electrode is held constant
Constant current electrolysis – current at the
working electrode is held constant
Complete electrolysis
Amount of material discharge – Electrogravimetry
Amount of electricity required - Coulometry
Electrogravimetry
• Mostly used in metal ion analysis
• Deposits on cathode as the metal
• Weight of the cathode before and after the complete electrolysis
• Electrodes used in the electrolysis must be inactive
Decomposition potentialIncrease of the potential at the working electrode leads to give a current at a certain potential.
Current
potentialDecomposition potential
Factors effecting decomposition potential
• Equilibrium potential• Ohmic potential• Overpotential –
activation and concentration
copaopopeqd EEEEE
Selectivity in electrolysis
Current
potentialE1 E1
Applied potential between E1 and E2 only one metal ion gets reduced.
The electrolysis under constant potential is selective.
The electrolysis under constant current is non-selective.
Selectivity in electrolysis cont...
Apparatus for electrogravimetry
Depolarization of electrodes
• As the current decreases, the potential needs to be increased in order to offset IR drop and then excess ions present in the solution may start to discharge. Under these circumstance, the electrode is said to be depolarized. This will lead to co-deposit and can start before the ion interested to complete the deposition. This will lead to interferences.
Interferences in ElectrolysisExample:
Mixture of Cu(II) and Pb(II)
Deposition of Cu(II) starts at 0.2 V. Due to the IR drop, in order to keep the current constant,potential should be increased.
In acidic solutions reduction of H+ occurs and deposition may not adhere to the electrode.
Depolarizers in electrolysis
• In order to avoid H2 evolution from cathode, we add nitrate ions into the solution.
OH3NHe8H10NO
H2
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2
• Here, the nitrate ion is called depolarizer which avoids the reduction of H+.
Electrolysis under constant current
• This is lack of selectivity and do not have any practical importance when a mixture of ions are present in the solution.
• However, if a single ion is present, this method is more advantages since the time necessary for complete electrolysis can be controlled.
Electrolysis under constant potential
• The potential of the working electrode can be controlled and kept at a constant value. However as the electrolysis is going on the current is continuously dropping (the rate of discharge is dropping).
• It takes a longer time for complete electrolysis, but however interferences can be avoided.
• Mixture of Cu(II) and Pb(II)
• The deposition potential for Cu(II) – 0.2 V
• The deposition potential for Pb(II) - -0.15 V
• By controlling the potential, Cu can be deposited first and then Pb can be determined by taking the weight of the cathode at each occasion.
Electrolysis under constant potential
The quantity of electrical charge
• Under constant current Q = it
• Under constant potential
• By measuring the amount of electricity one can calculate the amount of material discharge.
t
o
idtQ
Coulometry
• Two types of methods
• Constant potential coulometry - selective• Constant current coulometry- non-selective
• Coulometry is same as electrogravimetry but instead of measuring the weight of discharge material, here, the amount of electricity is measured which can be related to the amount of material according to Faraday’s law.
Coulometric Titration
• Here the titrant is electrically generated.
• Example: Analysis of As(III)
• The solution containing As(III) was added to excess I- and electrolysis carried out.
• The I2 produced will react with As(III)
I2e2I
e2)V(As)III(As
2