V.S.MURALIDHARAN CSIR EMERITUS SCIENTIST

V.S.MURALIDHARAN CSIR EMERITUS SCIENTIST
VOLATMMETRY V.S.MURALIDHARAN CSIR EMERITUS SCIENTIST

Electro analytical Methods
Quantity measured in parentheses. I = current, E = potential, R = resistance, G = conductance, Q = quantity of charge, t = time, vol = volume of a standard solution, wt = weight of an electrodeposited species

PRINCIPLE APPLICATIONS QUALIT-ATIVE INFORM-ATION
METHOD MEASUREMENT PRINCIPLE APPLICATIONS QUALIT-ATIVE INFORM-ATION DESIRED MINIMUM SAMPLE SIZE DETECTION LIMIT COMMENTS Voltammetry (Polarography) (amperometric titrations) (chronoamperometry) Current as a function of voltage at a polarized electrode Quantitative analysis of electrochemically reducible organic or inorganic material Reversibility of reaction 100 mg –3 ppm 10 mg A large number of voltage programs may be used. Pulse Polarography and Differential Pulse Polarography improve detection limits. Potentiometry (potentiometric titration) (chronopotentiometry) Potential at 0 current Quantitative analysis of ions in solutions, pH. Defined by electrode (e.g., F-, Cl-, Ca2+) ppm Measures activity rather than concentration. Conductimetry (conductometric titrations) Resistance or conductance at inert electrodes Quantification of an ionized species, titrations Little qualitative identification information Commonly used as a detector for ion chromatography. Coulometry Current and time as number of Faradays Exhaustive electrolysis g High precision possible. Anodic Stripping Voltammetry (Electrodeposition) Weight Quantitative trace analysis of electrochemically reducible metals that form amalgams with mercury Oxidation potential permits identification of metal. g 10 ng Electro deposition step provides improved detection limits over normal voltammetry.

Reactions which affect working range for polarisable electrodes
Solvent 2 H+ + 2 e - H2 E° = V O2 + 4 H+ + 4 e >2 H2O E° = V O2 + 2 H+ + 2 e ---- > H2O2 E° = V Electrode Hg e --- Hg E° = V Hg2Cl2 + 2 e ---2 Hg + 2 Cl – E° = V Pt e --- Pt E° = V VSMuralidharan

Working Range for Polarizable Electrodes
SCE = V VSMuralidharan

Summary of Voltammetric Techniques
VSMuralidharan 25/11/1439

POLAROGRAPHY V.S.MURALIDHARAN V.S.MURALIDHARAN 8/6/2018

Voltammetric Methods Historical Electrolysis at DME -1920’s
Usually 3-electrode cells Measurement of current that results from the application of potential. Different voltammetric techniques are distinguished primarily by the potential function that is applied to the working electrode and by the material used as the working electrode. V.S.MURALIDHARAN 8/6/2018

Polarography uses mercury droplet electrode that is regularly renewed during analysis.
Applications:Metal ions (especially heavy metal pollutants) - high sensitivity.Organic species able to be oxidized or reduced at electrodes: quinones, reducing sugars and derivatives, thiol and disulphide compounds, oxidation cofactors (coenzymes etc), vitamins, pharmaceuticals.Alternative when spectroscopic methods fail. V.S.MURALIDHARAN 8/6/2018

History Jaroslav Heyrovský was the inventor of the polarographic method, and the father of electroanalytical chemistry, for which he was the recipient of the Nobel Prize. His contribution to electroanalytical chemistry can not be overestimated. All modern voltammetric methods used now in electroanalytical chemistry originate from polarography. On February 10, 1922, the "polarograph" was born as Heyrovský recorded the current-voltage curve for a solution of 1 M NaOH. Heyrovský correctly interpreted the current increase between -1.9 and V as being due to deposition of Na+ ions, forming an amalgam. V.S.MURALIDHARAN 8/6/2018

Typical polarographic curves (dependence of current I on the voltage E applied to the electrodes; the small oscillations indicate the slow dropping of mercury): lower curve - the supporting solution of ammonium chloride and hydroxide containing small amounts of cadmium, zinc and manganese, upper curve - the same after addition of small amount of thallium. Swedish king Gustav Adolf VI awards the Nobel Prize to Heyrovský in Stockholm on V.S.MURALIDHARAN 8/6/2018

Linear sweep Polarography
Potential Ramps Linear sweep Polarography In order to derive the the current response one must account for the variation of drop area with time: A = 4(3mt/4d)2/3 = 0.85(mt)2/3 Density of drop Mass flow rate of drop using Cottrell Equation i(t) = nFACD1/2/ 1/2t1/2 V.S.MURALIDHARAN 8/6/2018

We also replace D by 7/3D to account for the compression of the diffusion layer by the expanding drop Giving the Ilkovich Equation: id = 708nD1/2m2/3t1/6C I has units of Amps when D is in cm2s-1,m is in g/s and t is in seconds. C is in mol/cm3 This expression gives the current at the end of the drop life. The average current is obtained by integrating the current over this time period iav = 607nD1/2m2/3t1/6C V.S.MURALIDHARAN 8/6/2018

E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple)
The diffusion current is determined by subtracting away the residual currentFurther improvements can be made by reducing charging currents E1/2 = E0 + RT/nF log (DR/Do)1/2 (reversible couple) Usually D’s are similar so half wave potential is similar to formal potential. Also potential is independent of concentration and can therefore be used as a diagnostic of identity of analytes. For example V.S.MURALIDHARAN 8/6/2018

Good buffering reqd in organic polarography
Organic reductions often involve hydrogen ions R + nH+ + ne  RHn Good buffering reqd in organic polarography Metal Complexes MLp + ne + Hg  M(Hg) + pL Difference between half-wave potential for complexed and uncomplexed metal ion is given by: E1/2(c) - E1/2(free) =RT/nF ln Kd - RT/nF p ln [L] +RT/nF ln (D free/D (c))1/2 Stoichiometry can thus be determined by plotting E1/2 versus [L] Also possible to improve resolution between neighbouring waves by carefully choosing ligand and concentration V.S.MURALIDHARAN 8/6/2018

Irreversible systems Heyrovsky-Ilkovich Equation
Describes wave shape for reversible systems (with fast electron transfer kinetics) E = E1/2 + RT/nF ln((id - i)/i) Plot E vs log((id - i)/i) gives straight line of slope 0.059/n Convenient way to get n Intercept is half wave potential Irreversible systems The waves are more “drawn out” than for reversible systems Limiting currents still show a linear function of concentration Shape of polarogram is given by: E = E0 + RT/nF ln (1.35kf((id - i)/i)(t/D)1/2) transfer coefficient forward rate constant V.S.MURALIDHARAN 8/6/2018

Charging Currents : first look
Drop acts as capacitor Double layer Since potential change during drop life is very small we can neglect charging changing with potential charging thus depends on time and electrode area ic = dq/dt = (E-Epzc)Cdl dA/dt but A = 4(3mt/4d)2/3 = 0.85(mt)2/3 ic = (E-Epzc) Cdl m2/3t-1/3 so I(total) = id + ic = kt1/6 + k’t-1/3 V.S.MURALIDHARAN 8/6/2018

Charging current sets limit of detection Various ways of reducing it
e.g., Sample current at end of drop life (TAST polarography) V.S.MURALIDHARAN 8/6/2018

VOLTAMMETRY A.) Comparison of Voltammetry to Other Electrochemical Methods 1.) Voltammetry: electrochemical method in which information about an analyte is obtained by measuring current (i) as a function of applied potential - only a small amount of sample (analyte) is used Instrumentation – Three electrodes in solution containing analyte Working electrode: microelectrode whose potential is varied with time Reference electrode: potential remains constant (Ag/AgCl electrode or calomel) Counter electrode: Hg or Pt that completes circuit, conducts e- from signal source through solution to the working electrode Supporting electrolyte: excess of nonreactive electrolyte (alkali metal) to conduct current

Potentiostat Counter electrode carries most of the current
Reference electrode must be physically close to working electrode Virtually no current flows between working and reference electrodeaccurate potential measurement VSMuralidharan vsm

Apply Linear Potential with Time
Observe Current Changes with Applied Potential

B.) Theory of Voltammetry
1.) Excitation Source: potential set by instrument (working electrode) - establishes concentration of Reduced and Oxidized Species at electrode based on Nernst Equation: - reaction at the surface of the electrode Eelectrode = E log (aP)p(aQ)q … 0.0592 (aR)r(aS)s … n Apply Potential

1) Differences from Other Electrochemical Methods
a) Potentiometry: measure potential of sample or system at or near zero current. voltammetry – measure current as a change in potential b) Coulometry: use up all of analyte in process of measurement at fixed current or potential voltammetry – use only small amount of analyte while vary potential 2) Voltammetry first reported in 1922 by Czech Chemist Jaroslav Heyrovsky (polarography). Later given Nobel Prize for method.

Current is just measure of rate at which species can be brought to electrode surface
Two methods: Stirred - hydrodynamic voltammetry Unstirred - polarography (dropping Hg electrode) Three transport mechanisms: (i) migration – movement of ions through solution by electrostatic attraction to charged electrode (ii) convection – mechanical motion of the solution as a result of stirring or flow (iii) diffusion – motion of a species caused by a concentration gradient

Mox + e- » Mred If E appl = Eo: Eappl = E0 - log [Mox]s = [Mred]s
At Electrodes Surface: Eappl = Eo log Mox + e- » Mred 0.0592 [Mred]s at surface of electrode n [Mox]s Applied potential If E appl = Eo: Eappl = E log [Mox]s = [Mred]s 0.0592 [Mred]s [Mox]s n

ˆ [Mred]s >> [Mox]s
Apply Potential E << Eo If Eappl << Eo: Eappl = E log ˆ [Mred]s >> [Mox]s 0.0592 [Mred]s n [Mox]s

2.) Current generated at electrode by this process is proportional to concentration at
surface, which in turn is equal to the bulk concentration For a planar electrode: measured current (i) = nFADA ( ) where: n = number of electrons in ½ cell reaction F = Faraday’s constant A = electrode area (cm2) D = diffusion coefficient (cm2/s) of A (oxidant) = slope of curve between CMox,bulk and CMox,s dCA dx dCA dx dx dCA

As time increases, push banding further and further out.
Results in a decrease in current with time until reach point where convection of analyte takes over and diffusion no longer a rate-limiting process.

- largest slope (highest current) will occur if:
Thickness of Diffusion Layer (d): i = (cox, bulk – cox,s) - largest slope (highest current) will occur if: Eappl << Eo (cox,s . 0) then i = (cox, bulk – 0) where: k = so: i = kcox,bulk therefore: current is proportional to bulk concentration - also, as solution is stirred, d decreases and i increases nFADox d nFADox d nFADox d

Voltammetry Apparatus
Various working microelectrodes used: hanging mercury drop (depicted), glassy carbon, etc. Typically small volume (1-10 mL) VSMuralidharan 25/11/1439

E½ at ½ i Limiting current Related to concentration
3.) Combining Potential and Current Together Limiting current Related to concentration E½ at ½ i Half-wave potential : E1/2 = E0 - Eref E0 = -0.5+SCE for Mn+ + me- » M(n-m)+

Potential applied on the working electrode is usually swept over (i. e
Potential applied on the working electrode is usually swept over (i.e. scan) a pre-defined range of applied potential 0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte Electrode become more and more reducing and capable of reducing Cd2+ Cd e- Cd Current starts to be registered at the electrode Current at the working electrode continue to rise as the electrode become more reducing and more Cd2+ around the electrode are being reduced. Diffusion of Cd2+ does not limit the current yet All Cd2+ around the electrode has already 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 i (A) E½ Working electrode is no yet capable of reducing Cd2+  only small residual current flow through the electrode id Base line of residual current -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 V vs SCE

Effect of sweep rate ? Why peak comes?

Voltammetry Slow E scan at constant fixed electrode surface, or
Fast E scan at DME during the lifetime of a single drop Negligible bulk electrolysis of solution VSMuralidharan 25/11/1439

How to calculate peak currents
How to calculate peak currents? Current is a function of concentration gradient FOR SECOND PEAK

CURRENT FUNCTION FOR A REVERSIBLE PROCESS
mv mv

The second term in the right hand side is for spherical diffusion
The second term in the right hand side is for spherical diffusion. This correction is small when the electrode radius is large Then

Cyclic Voltammetry 1) Method used to look at mechanisms of redox reactions in solution 2) Looks at i vs. E response of small, stationary electrode in unstirred solution using triangular waveform for excitation Cyclic voltammogram

Cyclic Voltammetry (CV)
Excitation Cyclic Voltammetry (CV) E2 Eapp, V forward reverse Important parameters: Epa and Epc ipc and iac E’ DE = |Epa - Epc| E1 Time, s Response Epa Eapp, V I, A E1 E2 Epc R - ne- = O 25/11/1439 VSMuralidharan

Analysis of an Example K3Fe(CN)6
Starting at an initial voltage (A), the potential is scanned in the negative direction. At B, the potential has become negative enough to start a cathodic current between the species, reducing the analyte at the working electrode. The reaction continues at the electrode until most of the species has been reduced, peaking the cathodic current at (C). The current then decays for the rest of the forward scan until the potential scan is reversed (D). The scan in the positive direction proceeds similarly to that of the negative scan. The cathodic current continues to slowly decay until the potential reaches a point to start the oxidation of the analyte (E). The anodic current is then measured as the concentration of the reduced species is significantly diminished (F). The anodic current then decays from this peak, and the potential completes its cycle. 8/6/2018 V.S.MURALIDHARAN

Cyclic Votammetry Continuous, cyclic sweeping of potential
Especially useful for reversible reactions Can provide information about transient electroactive species VSMuralidharan 25/11/1439

‚ ipc – cathodic peak current
Start at E >> E0 Mox + ne- » Mred - in forward scan, as E approaches E0 get current due to Mox + ne- » Mred driven by Nernst equation ‚ concentrations made to meet Nernst equation at surface eventually reach i max < solution not stirred, so d grows with time and see decrease in i max - in reverse scan see less current as potential increase until reduction no longer occurs < then reverse reaction takes place (if reversible reaction) < important parameters ‚ Epc – cathodic peak potential ‚ Epa – anodic peak potential ‚ ipc – cathodic peak current ‚ ipa – anodic peak potential < ipc . ipa < d(Epa – Epc) = /n, where n = number of electrons in reaction < E0 = midpoint of Epa  Epc

Voltammetric Parameters
The peak potential, is offset about 28 mV at 25°C The peak current is proportional to the square root of the scan rate VSMuralidharan 25/11/1439

For Nernstian CV DEp = |Epa - Epc| = 59/n mV at 250C
independent of n Eo = (Epa + Epc)/2 Ipc/Ipa = 1 VSMuralidharan 25/11/1439

For Nernstian Process Potential excitation controls [R]/[O] as in Nernst equation: Eapp = E /n log [R]/[O] if Eapp > E0, [O] ___ [R] and ox occurs if Eapp < E0, [O] ___ [R] and red occurs i.e., potential excitation CONTROLS [R]/[O] VSMuralidharan 25/11/1439

Quasi-reversible or Irreversible
Ep > 59 mV and Ep increases with increasing  iR can mascarade as QR system Irreversible: chemically - no return wave slow ET - 2 waves do not overlap VSMuralidharan 25/11/1439

Criteria for Nernstian Process
Ep independent of scan rate ip  1/2 (diffusion controlled) Ipc/Ipa = 1 (chemically reversible) VSMuralidharan

Some schemes E A + e = B EE A + e = B; B + e = C; 2B = A + C
EC A1 + e = B1; B1 = B2 EC’ A + e = B; B + P = A + Q EC2 A + e = B; 2B = B2 CE Y = A; A + e = B ECE A1 + e = B1; B1 = B2; B2 + e = C2; B1 + B2 = A1 + C2 v.s.muralidharan 8/6/2018

Adsorption Phenomena Non-specifically adsorbed Specifically adsorbed
No close-range interaction with electrode Chemical identity of species not important Specifically adsorbed Specific short-range interactions important Chemical identity of species important VSMuralidharan 50

Applications of CV Many functional group are not reducible so we can derivatize these groups convert them into electroactive groups by chemical modification EXAMPLES: alcohols + chromic acid = aldehyde group phenyl + nitration = nitro group 25/11/1439 VSMuralidharan 51

CV and Adsorption If electroactive adsorbed species:
Ep = Eo - (RT/nF) ln (bo/bR) ip = (n2F2/4RT) A o*  If ideal Nernstian, Epa = Epc and Ep/2 = 90.6 mV/n at 250C 90 mV I Eapp VSMuralidharan 52

UME’s Radial vs. Planar Diffusion
Radial Diffusion Redox wave: sigmoidal shape Iss = 4nFrDoCo* Iss scan rate independent  DoCo* Planar Diffusion Redox wave: normal shape Ip  1/2  Do1/2 C 25/11/1439 VSMuralidharan 53

EXAMPLE 1: UME’s in Sol-Gels
Learn Do from CA Obtain Co*from slow scan rate CV (Iss) 25/11/1439 VSMuralidharan 54

EXAMPLE 2: No Electrolyte!
20 mV/s 20 mV/s [S2Mo18O62]4- + e- = [S2Mo18O62]5- + e- = [S2Mo18O62]6- BAS 100-A 3-electrode cell: GC macrodisk/Pt wire/ Pt wire ACN with no electrolyte 100 mV/s vsm VSMuralidharan 55

Ultra micro electrodes
Fast scan rates 30 V/s planar diffusion Slow scan rates 5 mV/s radial diffusion Fe3+ 0.1 m 0.1 m 25/11/1439 VSMuralidharan 56

EXAMPLE 2: Oxidation of Cysteine at BDD
25/11/1439 VSMuralidharan 57

Stripping Analysis or Stripping Voltammetry
Cathodic (CSV) Good for anions and oxyanions Anodic (ASV) Good for metal cations 25/11/1439 VSMuralidharan 58

Stripping Voltammetry - Steps
1. Deposition 2. Concentration 3. Equilibration 4. Stripping 25/11/1439 VSMuralidharan 59

Example of ASV: Determination of Pb at HDME
Deposition (cathodic) reduce Pb2+ Stir (maximize convection) Concentrate analyte Stop stirring = equilibration/rest period Scan E in anodic sense and record voltammogram oxidize analyte (so redissolution occurs) Eapp I Ip Pb  Pb2+ + 2e- 25/11/1439 VSMuralidharan 60

Stripping Voltammetry - Quantitation
Ip  Co* Concentrations obtained using either Standard addition Calibration curve 25/11/1439 VSMuralidharan 61

HDME ASV Usually study M with Eo more negative than Hg
Ex: Cd2+, Cu2+, Zn2+, Pb2+ Study M with Eo more positive than Hg at GC Ex: Ag+, Au+, Hg Can analyze mixture with DEo  100 mV 25/11/1439 VSMuralidharan 62

CSV Anodic deposition Equilibrate (stop stirring)
Form insoluble, oxidized Hg salt of analyte anion Stir (maximize convection) Equilibrate (stop stirring) Scan potential in opposite sense (cathodic) Reducing salt/film and forming soluble anion Record voltammogram 25/11/1439 VSMuralidharan 63

Stripping Voltammetry
For cations of amalgam-forming metals and anions of sparingly soluble Hg salts First step is electrochemical deposition (preconcentration) at the Hg electrode Second step is anodic or cathodic potential sweep, in which peak currents are measured Extremely sensitive (nM or ppb range) depending on preconcentration time 25/11/1439 VSMuralidharan 64

vsmuralidharan

Anodic Stripping Voltammagram
VSMuralidharan 66

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Questions? audience VSM

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