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3. Voltammetry

Voltammetry: is a technique which concerned with the study of voltage-current-time relationships during electrolysis in a cell. The technique commonly involves the influence of the change in applied potential (voltage) on the current flowing in the cell, but in some cases, the variation of current with time may be investigated.

Electrolysis: is the occurrence of chemical reaction under the influence of an electromotive force at electrode immersed in solution.As many of other instrumental methods, the substance to be determined is exposed to source of excitation (potential) to obtain a response (the resulting variation of current with potential which called voltammogram).

Voltammetry is based upon the measurement of a current that develops in an electrochemical cell under conditions of complete concentration polarization.

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(Potentiometric measurements are made at currents that approach zero and where polarization is absent.)Furthermore, in voltammetry a minimal consumption of analyte takes place, whereas in electrogravimetry and coulometry essentially all of the analyte is converted to another state

Potential applied to the electrochemical cell is supplied by an apparatus called potentiostat. ·

Electrochemical cell for voltammetric study.It is usually composed of three electrodes as shown in figure 1-Working (indicator) electrode: at which the electrolysis of substance under investigation takes place, this electrode is usually small in size and polarized i.e. it adopts the potential externally imposed on it e.g. dropping mercuric electrode (DME), solid electrodes like gold, platinum and carbon.2-Reference electrode: Which is used to measure the potential of working electrode. This electrode is larger in size

Instrumental analysis Dr. Hisham E Abdellatef

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relative to working electrode and is none polarized (depolarized) i.e. it retains a constant potential e.g. saturated calomel electrode (SCE), Ag/AgCI.

3- Counter (auxiliary) electrode: which together with the working electrode carries the electrolysis current. It can be made from any metal, but its presence must not affect on the oxidation or reduction of the reactive substance, so it usually put in separate compartment inside the cell (put in sintered glass tube). It made from gold or platinum or other metals. In some cases only two electrodes are used (working and reference) e.g. in complete aqueous medium where the resistance for the current flow is very small, but usually three electrode cell is used

Solutions: redox couple + solvent + supporting electrolyte

Supporting electrolyte: it’s a solution of indifferent electrolyte, which by itself don't oxidized or reduced at working electrode at the selected potential range. It has several rules:

1- it raises the conductivity of the solution,2- It carries the bulk of the current so prevent the

migration of electroactive materials to the working electrode.


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3- It may control or (buffer) the level of hydrogen ion activity in solution.

4- Also it may associate with the electroactive solute in the complexing of the metal ions by legends.

eg. KNO3, NaCl, Na3PO4, buffer or mixture of buffer andorganic solvents. If the medium is completely non aqueous example when the experiment is done in dimethyl formamide DMF, tetralkylammonium salts are used e.g. tetramethyl ammonium perchlorate (CH3).·,NCIO4, The concentration of supporting electrolyte must be at least 100 times larger than the concentration of the electro-active substance. Before carrying electrolysis this supporting electrolyte must be purged by a steam of inert gas e.g. nitrogen or argon to remove oxygen.

Voltammetric Techniques:• Polarography • Differential Pulse• Square Wave

Voltammetry• Normal Pulse

• Cyclic Voltammetry • Sampled DC• LSV • Stripping Analysis

POLAROGRAPHY

Polarography: is the exception case of voltammetry, where the working electrode is the dropping mercuric


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electrode (DME) (as cathode) and the mercuric pool is the reference electrode (as anode). It is usually used for determination of the reduction side. The plot of the applied potential vs corresponding current is called polarogram and the apparatus is called a polarograph. When an external electromotive force (e.m.f.) is applied to a cell that containing solution of substance that can be reduced e.g.CdCl2, the following reaction will occur.

Cd+2 + 2e + Hg ⇄ Cd(Hg)From A to B, practically no current will pass through the cell. At B, when the potential of the working electrode (DME.) is equal to the deposition potential of the metallic cadmium, the current suddenly increases and the working electrode becomes depolarized by the cadmium ion. At point C, the current no longer increase linearly with the applied potential but approaches a steady limiting value.At point D no increase in current is observed at higher cathodic potential unless a


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second compound able to depolarize the working electrode, present in the solution. The polarogram is characterized by different parameters:1- Residual current: at small applied potential only a residual current flow in the cell which caused by: _

a- The presence of impurities in the supporting electrolyte or sample.

b- The charging current, which is the current that required for the charging of the so called electrical double layer at the surface of mercuric electrode.

2- Limiting current: is the region on the polarogram in which the current after increased sharply, becomes essentially independent on the applied potential.3- Diffusion current: it is the difference between limiting and residual currents. It is directly proportional to the concentration of the - reactive substance. Half wave potential (E½): is the potential at which the current is equal to one half the diffusion current. It is a characteristic of the nature of the reactive material, independent on


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concentration, depend on pH of the medium, type of supporting electrolyte and type of electrode.

Decomposition potential: is the potential at which the substance begins to oxidize or reduce. `Factors affecting electrode reaction rate and currentThe following factors affect on the electrode reaction rate:

1- Mass transfer.2- Electron transfer at the electrode surface.3- Chemical reactions such as protonations or

dimerizations, or catalytic decomposition on the electrode surface.

4- Other surface reactions, such as adsorption, desorption, or crystallization.

Mass transfer: which is the movement of material from one location in solution to another arises from the following different sources:a- Migration; which is the movement of a charged bodies under the influence of difference inthe electrical field.b- Diffusion: which is the movement of species under the influence of difference in concentration (concentration gradient).

c- Convection: which is the movement of species under the influence of stirring or hydrodynamic transport.


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In most cases of voltammetry and polarography, the measurements are carried out in quiet solution at fixed temperature, so there is no effect of convection. The use of high concentration of supporting electrolyte neglect the effect of migration. So diffusion is the only mode of mass transfer that controlled the electrode reaction rate.

Diffusion currentWhen the potential applied to a polarographic cell exceeds the decomposition potential of an electrolyte species, its concentration at the surface of the electrode (d.m.e.) is immediately diminished. A concentration gradient is therefore established and more ofthat species diffused from the bulk solution to the electrode surface (Fick's low of diffusion). The resulting current flow is proportional to the rats of diffusion.

I α (C-Co) or i=K(C—Co)where C and Co are the concentration of the electroactive species in the bulk solution and at the surface of electrode , respectively. By increasing the applied voltage, reduction occurs more rapidly and (Co) becomes zero and the concentration gradient reaches a maximum. At this point, the rate of the diffusion and the current flowing in the cell reaches a limiting value. llkovic Equation


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It is the equation that relates the parameters that determine the magnitude of the diffusion current obtained with a dropping rnercuric electrode.

Id (average) = 607 n D1/2m2/3t1/6C whereid = is the average diffusion current in microamper. n = is the number of electrons consumed in the reduction of one molecule of the electroactive species.D = is the diffusion coefficient of the reducible or oxidizible substance express as Cm2/s. M = is the rate of flow of mercury from the dropping mercuric electrode express in mg/s. T = is the drop time in s.C = is the concentration in m mol/1.The constant 607 is a combination of natural constants including the Faraday's constant, it is slightly temperature-dependent and this value (607) at 25°C.

Advantages of dropping mercuric electrode (DME)1- Its surface is reproducible, smooth and continuously

renewed, this eliminate the poisoning effect.2- Mercury forms amalgams (solid solution) with many

metals. 3- The diffusion current assumed a steady value

immediately after each change of applied potential and is reproducible.


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4- The large hydrogen over-potential of mercury renders possible deposition of substance that difficult to reduce e.g. alkaline metal e.g. Al+3 and M+2. .

5- The surface area can be calculated from the weight of the drop.

Disadvantage of dropping mercuric electrode:1- At potential more positive than +0.4V vs SCE, mercury

e dissolve and producing polarographic wave which masks the other waves of other oxidizable species e in the solution, so d.m.e can be used only for the analysis of reducible or easily oxidizeble substance.

2- The capillary is very small in size so easy to block and so all process stopped.

3- The presence of impurities in the mercury cause blockage of the capillary results in malfunction of the electrode.

Substances determined by polarographyi- Polarographic analysis of Inorganic compoundsMost metal ions are reducible at DME, and multicomponent mixtures can often be analysed by selecting an appropriate supporting electrolyte so that the half-wave potentials of two ions are differed by about -0.2 V vs SCE or by using complexing agents by taking the advantage of complexing ability of the metal ions.


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Based on this, polarography is used predominantly for trace metal analysis of alloys, ultra-pure metals, minerals/metallurgy, environmental analysis (air, water, soil and sea water contaminants), foodstuffs, beverages and body-fluids, toxicology and clinical analysis. Reducible anions such as BrO3, IO3-, Cr2O72-and NO2- can also be determined using well buffered solutions. ii. Polarographic analysis of organic compounds This technique is used in organic chemistry for qualitative and quantitative analysis and structure determinations. Most of the organic compounds are insoluble in pure aqueous medium and also in mercury to form amalgam. Therefore, the solvent in which the organic compound and its electrode product is soluble is added to the supporting electrolyte. These solvents include various alcohols or ketones, dimethyl formamide, acetonitrile, ethylene diamine and others. The commonly used supporting electrolytes which are easily mixed with organic solvents are various quaternary ammonium salts such as tetrabutyl ammonium iodide. Some of the organic functional groups that are reducible at DME :


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Application of polarography (voltammetry)A-Qualitative application1- This technique can be used for the determination of

equilibrium constant and reaction rate. 2- Elucidation of and reaction mechanism coupled with

the actual electrode process. 3- Study the reversibility of the reaction 4- This technique is useful for stability studies and study

the connection between polarographic properties and physiological activities of drugs.

5- Polarography (volmmmetry) can be used in connection with spectroelectrochemistry (SEC)to show the change in the spectrum of the substance after the application of potential. Also this technique is used to study some p application phenomena like adsorption.

B- Quantitative application ,The polarographic technique can be used in the determination of concentration range from 10-5M to 10-2M


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and employ volumes Less than 1 ml. two or more electroactive ions may be determined successively if their half-wave potentials differ by at least 0.4 volt for single charged ions and 0.2volt for doubly charged ions ’ provided that the ions are present in approximately equal concentration if the concentration differ considerably, the difference between the half-wave potential must be larger

1. Polrography (voltametry) can be used for determination of the concentration of the electreactive compounds by direct measurements of the diffusion current.

2. It can be used in combination with other technique e.gt high performance liquid chromatography (HPLC) to determine the concentration of substances presents in minute amounts especially in fluids.

3. Also it can be used for determination of the concentration at certain potential and this called amperometry. Amperometric titration is the determination of the relationship between the volume of titrant and the corresponding current of substance at constant potential provided that both reactant and reagent are electrcochemically active or only one is active.

Amperometric titration Instrumental analysis Dr. Hisham E Abdellatef

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Polarography can be used for determination of the concentration at certain potential and this called amperometry. Amperometric titration is the determination of the relationship between the volume of titrant and the corresponding current of substance at constant potential provided that both reactant and reagent are electrochemically active or only one is active, so there are several types of curves appear in the figure

1. If the reactant is active while the titrant is inactive e.g. titration of Pb+2 with SO-4

2. If the reactant is inactive while the titrant is active e.g. titration of Mg2+ with 8-hydroxyquinoline.

3. If both the reactant and the titrant are active e. g the titrant of Pb+2 and CrO4

2-.


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Evaluation of quantitative results: a- Wave height – concentration plots.

The wave heights are measured for a series of different of the substance under investigation. The polarogram of the unknown is measured exactly as the standard, and the concentration is read from the graph.

Measurement of wave-heights: With a well defined polarographic wave where the limiting current plateau is parallel to the residual current curve, the measurement of the diffusion current is relatively simple. A reliable procedure is illustrated in figure.

b- Internal standards or Pilot ion method:


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This method is based on the fact that the relative diffusion current constants I are independent of the particular capillary used, provided the nature and concentrationOf supporting electrolyte and the temperature are kept constant. Hence , upon determining the relative wave height or diffusion current of the unknown ion and with some standard or pilot ion added to the solution in known amount and comparing these with the ratio for known amounts of the same two ions. The main requirement for such an ion (singly charged) is that its half wave potential should differ by at least 0.# Volts from the unknown or any other ion in the solution with which it might interfere.From the Ilkovic equation , I d = 607 n D 1/2 m 2/3 t 1/6 c ;Now , for the pilot ion : I d1= 607 n1D1

1/2 m 2/3 t 1/6 c1 ;Now for test ion : I d2= 607 n2 D2

1/2m2/3 t 1/6 c2 ; now,

I d1=607n1D1½m2/3 t1 /6c1

I d2=607n2D2½m2 /3 t1 /6c2

If I = 607 n D ½ thenI d1 = I1 c 1I d2 I2 c 2The ratio I1 /I2 is known as the pilot ion ratio (R) and is independent of the capillary characteristics so that:I d1 = c 1 .R


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I d2 c 2

c- Methods of standard addition: The Polarogram of the test solution of known volume is initially recorded then an accurately measured quantity of a standard solution of the substance is added and a second polarogram is obtained. The original concentration of the substance can be calculated from the increase in the diffusion current from the following formula:

C 1 = c 2 I 2 v 2 v 1(I 2- I )+I 2 v 2

Where,

c= concentration,I = diffusion currentv=volumec 1 = test solutionc 2 = standard solution

Advantages of the polarographic technique for analysis of pharmaceutical compounds

1- Only small volumes of samples are necessary. 2- Turbid and colored solutions, which make other

methods Impossible, can be analyzed.3- The usefulness of the method is not restricted to

electrochemically active compounds but the field of application is extended by indirect method.

4- For the analysis of pharmaceutical products, it is sufficient simply to dilute the liquid sample e.g. injection solution-with the supporting electrolyte or to dissolve tables in such a solution. The insoluble parts of the tablet or additives do not usually influence the


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polarographic curves. When present, they should be left to settle after which aliquot simple may be taken from the supernatant liquid; The separation of the exicipients in many cases is not necessary and this simplifies the preparation of the samples.

5- The sensitivity of voltammetiy (polarography) is sufficient high to enable the determination of; small amount of substance or traces of perhaps toxic impurities.

Stripping voltammetry In this method, a Hanging Mercury Drop Electrode (HMDE) is used, the potentiostat is set a fixed value (0.2-0.4 V) more negative than the highest reduction potential encountered among the reducible ions, then electrolysis will occur, deposition of metals will take place on the HMDE cathode and amalgam formation will take place. The reducible ion may be transferred to the mercury cathode, this operation is referred as a concentration step, and the metals become concentrated into the relatively small volume of the mercury drop. The cell is allowed to stand for 30 seconds. The potentiostat is then caused to make a voltage sweep in reverse. This means that a gradually increasing positive potential is applied to the HMDE, which is now the anode of the cell. As the potential approaches the oxidation potential of one of the metals dissolved in the mercury, then ions of


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that metal pass into solution from the amalgam and the current increases rapidly and attains a maximum value when the potential has a value approximating to the appropriate oxidation potential. The metal is said to be stripped from the amalgam and the curve shows a peak! The curve is termed Stripping voltammogram The technique can be used to measure concentrations in the range 10-8- 10-9M that is suitable for the determination of trace metal impurities, analysis of semiconductor materials, investigation of pollution problems

Anodic Stripping Voltammogram of 0.3

uM Pb2+ in 0.1 M KNO3 with 20 uM Hg2+,

Cyclic voltammetry Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. In typical cyclic voltammetry, a solution component is electrolyzed (oxidized or reduced) by placing the solution in contact with an electrode surface and then making that surface sufficiently positive or negative in voltage to force electron transfer. In simple cases, the surface is started at a particular voltage with respect to a reference half-cell such as calomel or Ag/AgCl, the electrode voltage is changed to a higher or lower voltage at a linear rate and finally, the voltage is changed back to the original value at the same


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linear rate. When the surface becomes sufficiently negative or positive, a solution species may gain electrons from the surface or transfer electrons to the surface. This results in a measurable current in the electrode circuitry. The result of cyclic voltammetry obtained in form of cycle between current and potential, potential on X axis and voltage on Y axis.

Figure shows a generic voltammogram. A voltammogram explains the reversibility of the redox couple.

Epc= Peak cathodic potential; Ipc= Peak cathodic current; Epa= Peak anodic potential; Ipa=Peak anodic currentThe peak potential of the anodic sweep, Epa and the peak potential for cathodic peak, Epc, can be directly read from the program, and the difference between them, ∆Epeak, can be calculated. If redox couple is reversible, then the relationship,

n ∆ Epeak= 59mV (1.1)In addition, the ratio of the anodic peak current to the cathodic peak current is given by:


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ipa / ipc = 1 (1.2)The formal potential Eo, for a reversible redox couple is easily determined as the average of the two peak potentials as follows.

Eo=(Epa+Epc) / 2 (1.3)Quantitative information regarding analyte concentration can be obtained from the voltammogram using Randles- Sevcik equation. This equation specifies the peak current, ip (anodic and cathodic), in terms of analyte concentration ,C.

ip= 0.4463 n FAC (n F v D/ R T)1/2 (1.4)where, n=no. of electrons appearing in the half-reaction for the redox couple v=rate at which potential is swept

F=Faradays constant (96485 C/mol),A=electrode area (cm2)R=universal gas constant (8.314 J/mol K)T=absolute temperature (K)D= analyte’s diffusion coefficient (cm2/sec)

If temperature is assumed to be 250C (298.15 K), the eqn. can be written as:

Ip = (2.687 x 105) n3/2 v1/2 D1/2 A C

Where the constant is understood to have units (2.687 x 105 C mol-1 V-1/2).During the forward scan the Fe(III) get reduced as :

Fe (III) + e-

Fe(II)


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Similarly during the reverse potential the oxidation of the Fe (II) takes place to (III) which can be shown as:

Fe (III) -

Fe(II) + e-

The redox reaction therefore can be used as an indication of major analytical tool for the determination of trace elements which are electro active in nature.


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