1. Dr. Rasha Hanafi
Lecture 8 Non- potentiometric methods of analysis: Amperometry and Voltammetry (Polarography)
Dr. Rasha Hanafi
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

2. Dr. Rasha Hanafi
Learning outcomes
By the end of this session, the student should be able to:
Use amperometry for determination of vital signs.
Identify fundamentals of Polarography as a subtype of voltammetry.
Determine voltage/ current changes in a polarographic wave.
Do quantitative analysis using polarography
Calibration curve method.
Standard addition method.
Describe experimental set up and applications of polarography
Determine advantages and limitations of polarography.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

3. Classification of electroanalytical methods
Electroanalytical methods may be based on the measurement of either:
Current at a fixed potential.
Potential at a fixed current.
Today’ s lecture
Amperometry
I α conc.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

4. 1. Amperometry current α conc. at fixed voltage
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

5. Electrochemical reaction on the electrode surface
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.
electrons
Enzyme
Reduced Mediator
Oxidized Mediator
Glucose
Gluconolactone
electrode
Fe3+
Fe2+
Electrochemical reaction on the electrode surface
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

6. The current due to glucose equals:
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

7. 2. What is Voltammetry? Polarography?
Dr. Rasha Hanafi
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.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

8. 3. The Polarographic experiment
Dr. Rasha Hanafi
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).
Current Measurement is taken
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

9. Base line of residual current
Dr. Rasha Hanafi
4. Current  Voltage Relationship (i-E curve) in Polarography
Half-wave potential characteristic for each electroactive species
il limiting current plateau
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
i (A)
E vs SCE
Electrode becomes more and more –ve and capable of reducing Cd2+
Cd e Cd
Current starts to be registered at the electrode
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 E½
id = il - ir
Working electrode isn’t yet capable of reducing Cd2+  only small residual current (ir) flows through the electrode
Base line of residual current
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.
Diffusion current, id
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

10. I =Limiting Current = diffusion current  [C]0
Dr. Rasha Hanafi
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
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.
bulk
Electrode surface
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

11. Dr. Rasha Hanafi
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.
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 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)
E1/2
Near 1.2 V, current increases rapidly (reduction of H+ to H2)
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

12. 6. Quantitative analysis: The Ilkovic Equation
Dr. Rasha Hanafi
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

13. 7. Don’ts in the experiment.
Dr. Rasha Hanafi
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

14. 8. Quantitative Analysis, what to do?
Dr. Rasha Hanafi
8. Quantitative Analysis, what to do?
1. Calibration curve method
A polarographic run is carried out separately for each standard solution and the diffusion current is calculated (id1, id2, …..)
A working curve of diffusion currents against concentrations of standards (obey Ilkovic Eq.) is plotted.
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.
id1
id(unk)
Concentration, mM id
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

15. 9. Quantitative Analysis Using Polarography, how to calculate?
Dr. Rasha Hanafi
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).
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)
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

16. 10. Applications of polarography
Dr. Rasha Hanafi
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.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

17. 11. Is Polarography an advantageous type of Voltammetry?
Dr. Rasha Hanafi
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

18. 11. High hydrogen overvoltage?
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

19. 12. Cyclic Voltammetry (CV) Irreversible reaction
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.
forward
reversed
1st cycle
2nd cycle
Initial potential
final potential
Reversible reaction
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.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016

20. Dr. Rasha Hanafi
References
“Principles of instrumental analysis, 5th ed. by Skoog, Holler, Nieman” Chapter 22 and Chapter 24.
“Quantitative Chemical Analysis, 6th ed. Daniel Harris” Chapter 17.
Lecture of “Non- potentiometric methods of analysis ” by Dr. Raafat Aly, GUC, spring 2010.
Dr. Rasha Hanafi, GUC
Lecture 8- PHCM662-SS2016