Transcript Lecture 8

Lecture 8
Non- potentiometric methods of analysis:
Amperometry and Voltammetry (Polarography)
Dr. Rasha Hanafi
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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Learning outcomes
By the end of this session, the student should be able to:
1.Use amperometry for determination of vital signs.
2.Identify fundamentals of Polarography as a subtype of voltammetry.
3. Determine voltage/ current changes in a polarographic wave.
4. Do quantitative analysis using polarography
– Calibration curve method.
– Standard addition method.
5. Describe experimental set up and applications of polarography
6. Determine advantages and limitations of polarography.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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Electroanalytical methods may be based on the measurement of
either:
1. Current at a fixed potential.
2. Potential at a fixed current.
Today’ s lecture
Amperometry
I α conc.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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1. Amperometry
 In this technique, the electrical current that passes between a pair of electrodes in an
electrolysis reaction is measured at constant applied potential.
 One of the reactants is the intended analyte, and the measured current is proportional
to the concentration of analyte.
current α conc.
at fixed voltage
Example: Blood Glucose monitor
 Diabetics need to monitor
their blood glucose level several
times a day. They may use a glucose monitor
consisting of a disposal test strip
 The patient normally adds one drop of
blood on the test strip and inserts it into the meter
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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Test Strip
• Working electrode 1 is coated with the enzyme glucose oxidase and a mediator.
• The enzyme catalyzes the oxidation of glucose in the presence of the mediator which
acts as oxidizing agent.
Glucose
Gluconolactone
Enzyme
Oxidized Mediator
Fe3+
Fe2+
electrons
Dr. Rasha Hanafi, GUC
Reduced Mediator
electrode
Electrochemical reaction on the electrode surface
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• The mediator transports e- between the analyte (glucose) and the electrode.
• The mediator consumed in the previous reaction is then regenerated at the working
electrode
• Thus the current at the working electrode is proportional to the concentration of
ferrocene, which, in turn, is proportional to the concentration of glucose in blood.
 Problem:
Other species such as ascorbic acid, uric acid and
acetaminophen found in blood can be oxidized at
the same potential required to oxidize the
mediator (dimethylferrocine).
 Correction:
The test strip has a second electrode coated with
the mediator, but not with glucose oxidase. Thus,
interfering species that are oxidized at electrode 1
are also oxidized at electrode 2.
The current due to glucose equals:
(current at electrode 1 – current at electrode 2)
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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2. What is Voltammetry? Polarography?
• Voltammetry is a collection of techniques in which the relationship between current and voltage
(i-E curve) is observed during the electrochemical process.
• The most important electrode in Voltammetry is the working electrode which could be made
from a variety of materials including: mercury, platinum, gold, silver, glassy carbon, nickel and
palladium.
The polarograph
What is Polarography?
 When voltammetry is conducted with a dropping
mercury electrode (DME) it is polarography.
 The dispenser suspends one drop of mercury
(working electrode) from the bottom of the
capillary.
 After current and voltage are measured, the drop
is mechanically removed.
 Then a fresh drop is suspended and the next
measurement is made.
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Lecture 8- PHCM662-SS2016
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3. The Polarographic experiment
Voltage Scan
 Potential is varied with time.
 After each new drop of Hg is dispensed, the
voltage is made more negative by 4 mV
(Staircase voltage ramp).
Current during the life time of the drop
 The current oscillates permanently
between a minimum and maximum value.
 This behavior is caused by the non-constant
electrode surface area. The current rises as
the drop surface area grows, and decreases
as the drop falls and then rises again.
 Current is measured during the last 17 ms
of the life of each Hg drop (sampled
Current
Measurement is
taken
current).
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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4. Current  Voltage Relationship (i-E curve) in Polarography
Half-wave potential characteristic for each electroactive species
il limiting current plateau
Electrode becomes more and more –ve
and capable of reducing Cd2+
Cd2+ + 2e-
i (A)
All Cd2+ at the electrode surface has
been reduced. Current at the electrode
becomes limited by the diffusion rate
of Cd2+ from the bulk solution to the
electrode. Thus, current stops rising
and levels off at a plateau
Cd
Current starts to be registered at the
electrode
E½
id = i l - ir
Working electrode isn’t
yet capable of reducing
Cd2+  only small
Current at the working electrode
continue to rise as more Cd2+
around the electrode is being
reduced. Diffusion of Cd2+ does
not limit the current yet.
residual current (ir)
flows through the
electrode
Diffusion
current, id
Base line of residual
current
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
E vs SCE
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Lecture 8- PHCM662-SS2016
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4.Current  Voltage Relationship (i-E curve) in Polarography, cont.
• The rate at which analyte diffuses from bulk solution to the surface of the electrode
is proportional to the concentration difference between the two regions
Current (I)  rate of diffusion  [C]0 – [C]s
bulk
Electrode surface
• At sufficiently high potential, the rate of the reaction at the electrode surface is so
fast that the analyte is reduced or oxidized as soon as they reach the electrode
surface. Thus [C]s ~ 0
I =Limiting Current = diffusion current  [C]0

• The limiting current is called the diffusion current because it is controlled only by
the rate at which analyte can diffuse to the electrode.
• The dependence of the measured diffusion current on bulk concentration alone is
the basis for quantitative analysis.
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Lecture 8- PHCM662-SS2016
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5. The Polarogram
• Half wave potential, E1/2 is the potential
at which half of the maximum current is
reached, id/2, and is characteristic of a
given analyte in a given medium and thus
can be used for qualitative analysis.
Near 1.2 V, current increases
rapidly (reduction of H+ to H2)
• For quantitative analysis, the diffusion
current, id , (interval between residual
current and limiting current in the plateau
region) is proportional to concentration of
analyte. id is measured from the base line
E1/2
recorded without analyte.
• Residual Current is due to reduction
reactions of impurities in solution and
charging of Hg drop (non-Faradeic
current/non- redox current)
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Lecture 8- PHCM662-SS2016
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6. Quantitative analysis: The Ilkovic Equation
It relates the diffusion current, id, the concentration of the analyte and the characteristics of Hg
drop :
id = 607 n D1/2 m2/3 t1/6 C
id = k C
id : max value of the diffusion current in the life of the drop
n : number of electrons involved
C : concentration of ion in bulk, mmol/L
D : diffusion coefficient of the ion (cm2/s) [temperature dependent]
m : flow rate of Hg in (mg/s)
t : drop life time (s)
• Ilkovic equation is applicable in the given form only if the migration of the analyte ions (mass
transport of ions due to the electric field) is suppressed by addition of inert supporting
electrolyte (usually 1 M KCl).
• In this case diffusion remains essentially the only kind of mass transport of analyte ions in the
redox process. Note that polarographic measurements are conducted in unstirred solution (no
convection current).
• Furthermore, the supporting electrolyte enhances conductivity of the solution.
• The supporting electrolyte may also contain buffer to adjust the pH at a suitable value and
chelating agents to mask the interfering ions.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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7. Don’ts in the experiment.
• Dissolved oxygen must be removed from the polarographic cell because O2 gives two
polarographic waves when it is reduced first to H2O2 and then to water. These waves give rise to
huge diffusion currents which might interfere in the analysis.
• Typically, N2, as inert gas, is bubbled through the analyte solution to remove O2. The liquid
shouldn’t be purged with N2 during measurement because we don’t want convection of the
analyte to the electrode to happen.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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8. Quantitative Analysis, what to do?
1. Calibration curve method
a) A polarographic run is carried out separately for each standard solution and the diffusion
current is calculated (id1, id2, …..)
b) A working curve of diffusion currents against concentrations of standards (obey Ilkovic Eq.)
is plotted.
c) A separate polarographic wave is done for the analyte and the analyte diffusion current is
calculated and compared to the calibration curve to get the concentration of the analyte.
id
id(unk)
i d1
Concentration, mM
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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9. Quantitative Analysis Using Polarography, how to
calculate?
2. Standard Addition method
• Spike a small volume (Vs) of standard solution of known concentration (Cs) into original
volume of sample (Vu) of unknown concentration.
• Measure first the current for the original sample, i, then the current for the spiked sample, i’.
• Calculate the concentration of the analyte using the Ilkovic equation (i = k c).
k cu
i

k (cs ( dil.)  c u ( dil.) )
i
cu
i

cs ( dil.)  c u ( dil.)
i
where cu is concentration of the unknown.
• Note: In the above equation we substitute for diluted concentrations of unknown and
standard as follows:
(Cu)diluted = Cu Vu /(Vu + Vs) & (Cs)diluted = Cs Vs /(Vu + Vs)
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Lecture 8- PHCM662-SS2016
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10. Applications of polarography
Polarography can be used for the determination of many inorganic ions than can be reduced in
the range of +0.4 and -1.2 V. Mainly most of the transition metals, lanthanides and actinides
can be determined by polarography. It can be also used for quantitative analysis of a wide
variety of electroactive organic functional groups. It is possible to analyze simultaneously a
mixture of 3 or 4 electroactive ions.
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Lecture 8- PHCM662-SS2016
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11. Is Polarography an advantageous type of Voltammetry?
1. Advantages of DME
 A new electrode surface forms every few seconds preventing buildup of impurities
or reaction products (fresh, clean, and renewed surface ).
  Metallic products dissolve or amalgamate with Hg leaving surface clean.
Cd2+ + 2e-(Hg)
Cd (Hg)
   Hg has high hydrogen overvoltage i.e., H2 does not evolve on Hg
surface unless a very high negative potential (-1.2 V) is reached and thus H+ will
not interfere in the determination of many ions.
2. Disadvantages of DME
 Hg is easily oxidized which prevents the use of more positive (oxidizing)
potentials. Hg can not be used for determination of species that can be oxidized
above +0.4 V.
  Hg is highly toxic and has a measurable vapor pressure.
   Hg must be highly pure.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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11. High hydrogen overvoltage?
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Lecture 8- PHCM662-SS2016
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12. Cyclic Voltammetry (CV)

In CV, the current in the cell is measured as a function of potential.

The potential of an electrode in solution is linearly cycled from a starting potential to
the final potential and back to the starting potential. This process, in turn, cycles the
redox reaction. Multiple cycles can take place.
final potential
1st cycle
Initial potential
Reversible
reaction
2nd cycle
Irreversible
reaction
 The initial portion of the cyclic voltammogram,
beginning at t , shows a cathodic wave.
 Instead of leveling off at the top of the wave, the current
decreases as the potential is increased further.
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Lecture 8- PHCM662-SS2016
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References
1. “Principles of instrumental analysis, 5th ed. by Skoog, Holler,
Nieman” Chapter 22 and Chapter 24.
2. “Quantitative Chemical Analysis, 6th ed. Daniel Harris”
Chapter 17.
3. Lecture of “Non- potentiometric methods of analysis ” by Dr.
Raafat Aly, GUC, spring 2010.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016
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