Cyclic voltammetry
Download as PPTX, PDF3 likes5,316 views
Cyclic voltammetry is an electroanalytical technique that measures current during redox reactions at an electrode. It involves scanning the potential of a working electrode versus a reference electrode and measuring the current. The potential is ramped from an initial value to a set switching potential and back to the initial value. This process is repeated in cycles. A cyclic voltammogram plots the current response of the working electrode versus the applied potential and provides information about redox potentials and reaction reversibility. Reversible reactions produce symmetrical peaks while irreversible reactions have wider separation between peaks. Cyclic voltammetry is useful for studying electrode reaction mechanisms and kinetics.
Cyclic voltammetry
- 1. CYCLIC VOLTAMMETRY S U B M I T T E D TO : - D R . M A N J U L ATA PA R I H A R SUBMITTED BY :- DALPAT SINGH
- 2. BASIC INTRODUCTION TO VOLTAMMETRY :- • Voltammetry is concerned with the study of voltage-current-time relationships during electrolysis carried out in a cell. In simple words, The technique commonly involves studying the influence changes in applied voltage on the current flowing in the cell. • It is used to determine substances in solution which can be reproducibly reduced or oxidized at an electrode surface. • The procedure normally involves the use of a cell with an of three electrodes - (1) a working electrode at which the electrolysis under investigation takes place. it should be completely polarizable so that the current which flows through the electrode is proportional to the concentration. (2) a reference electrode which is used to measure the potential of the working electrode. (3) an auxiliary electrode or counter current electrode which, together with the working electrode, carries the electrolysis current.
- 3. • The current-voltage graph which may be drawn is known as a voltammogram. • If we use dropping mercury electrode as working electrode and pool of mercury as auxiliary electrode then this technique is referred as Polarography. • Why modified voltammetry is introduced ? (1) Quantitative polarography is limited at best to solutions with electrolytes at concentrations greater than 10-5 M . (2) the half wave reduction potential difference between two ions must be at least 200mV if the reduction waves are to be separated. These limitations are largely due to the condenser current associated with the charging of each mercury drop as it forms. • Pulse polarography, rapid scan polarography , sinusoidal polarography and Cyclic voltammetry are types of modified voltammetry.
- 4. CYCLIC VOLTAMMETRY :- • Cyclic Voltammetry is a type of modified voltammetry in which a very fast scan is carried out in two directions , 0 to V and back down to 0. • In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time in cyclical phases. • Cyclic voltammetry takes the three electrode setup; when working electrode reaches a set potential, the ramp is inverted. This inversion can happen multiple times during a single experiment. • Normally this process is conducted using electrodes with a small surface area in unstirred solutions, producing a very small redox current. This means ohmic drop IR is small, even for the poorly conducting solutions.• The low capacitance completely eliminates any contribution to the diffusion current and allow fast scan rate ( up to 10KV s-1 ).
- 5. BASIC PRINCIPLES OF CYCLIC VOLTAMMETRY:- • The limiting current occurs because the reduction is limited by the rate at which ions reach the surface of the electrode. And there are three conditions to the limiting current – ilimit = id + iC + iM • Migration current :- electro active material reaches the surface of the electrode under the influence of an applied potential. Heyrovsky showed that the migration current can be practically eliminated if an indifferent electrolyte is added to the solution in a concentration so large that its ions carry essentially all the current and it doesn’t react with the electro active main species. • Convection current:- if ions are transported toward the electrode surface by mechanical means such as stirring in the solution then this will affect the limiting current. To eliminate this we use stationary solution system like dropping mercury electrode and we can also use hydrodynamic voltammetry in which the solution is in continual and constant motion.
- 6. • Diffusion current :- when an excess of supporting electrolyte is present in the unstirred solution then the electrical force on the reducible ions is nullified; this is because the migration current and the convection current is eliminated. So under this conditions the limiting current is almost solely a diffusion current. ilkovic gives this equation which govern the diffusion current :- id = 607nD1/2Cm2/3t1/6 Where id = the average diffusion current in microamperes n = no. of electrons consumed in the reduction of one molecule D = the diffusion coefficient of the substance C = concentration of analyte in mol L-1 m = the rate of flow of mercury from the DME in mgs-1 t = drop time in seconds • This equation is slightly temperature dependent. • Ilkovic equation neglects the effect of the curvature of mercury drop and this may be allowed for by multiplying the right hand side of the equation by (1+AD1/2t1/6m-1/3), where A is a constant having value 39. • The product m2/3t1/6 is important because it permits results with different capillaries.
- 7. • Residual current :- If a current voltage curve is determined for a solution containing ions with strongly negative reduction potential , a small current will flow before the decomposition of the solution begins. This current increases linearly with the applied potential. • Polarographic maxima :- the streaming movement of the diffusion layer is responsible for the current maximum. Current voltage curve exhibits maxima with DME so to eliminate it we use maximum suppressors like gelatin , dyestuff (fuchsine solution) etc. . these are the surface active substances and they form a absorbed layer on the aqueous side of the mercury solution interface which resists compression and prevent the streaming of movement of diffusion layer. • Half wave potential :- The potential at a polarized electrode obeys the Nernst equation and the concentrations of electro active species is directly related to the diffusion current.
- 9. • In cyclic voltammetry, the electrode potential ramps linearly versus time in cyclical phases. • The rate of voltage change over time during each of these phases is known as the experiment's scan rate (V/s). The potential is measured between the working electrode and the reference electrode, while the current is measured between the working electrode and the counter electrode. • Common materials for the working electrode include glassy carbon, platinum, and gold. These electrodes are generally encased in a rod of inert insulator with a disk exposed at one end. A regular working electrode has a radius within an order of magnitude of 1 mm. Having a controlled surface area with a well-defined shape is necessary for being able to interpret cyclic voltammetry results. • The counter electrode, also known as the auxiliary or second electrode, can be any material that conducts current easily and will not react with the bulk solution. • Electrolyte :- The electrolyte ensures good electrical conductivity . For aqueous solutions, many electrolytes are available, but typical ones are alkali metal salts of perchlorate and nitrate. In no aqueous solvents, the range of electrolytes is more limited, and a popular choice is tetrabutylammonium hexafluorophosphate. EXPERIMENTAL SETUP:-
- 10. • The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace. • Since the scan is in two directions, two curves are normally seen; a normal cathodic reduction wave and an anodic wave as the voltage reverses. Curves are equal in magnitude and approximately vertically aligned. • This indicates that the electrode process is a fast reversible Cyclic Voltammogram :-
- 11. CRITERIA FOR THE REVERSIBILITY OF ELECTROCHEMICAL REACTIONS :- • Electrochemical reversibility describes the rate at which the electron transfer occurs between the working electrode and the solution redox species. • Electrochemically reactions are of three types depending on their electrochemical behavior :- (1) Reversible electrochemical reactions :- A redox reaction can be termed as reversible electrochemical reaction, if species swiftly exchange electrons with the working electrode. This type of reaction can be recognized from a cyclic voltammogram by calculating the potential difference among the two peaks potential. The given equation concerns to an electrochemically reversible system: Where n = No. of electrons involved in the electrochemical reaction , Delta Ep = difference between the potential of anodic peak and cathodic peak
- 12. (2)Irreversible electrochemical reactions :-If there take place a very slow electron exchange between the working electrode and the redox specie then it is termed as irreversible electrochemical reaction. The separation of peak is greater than 0.058/n V in given formula (3) Quasi-reversible electrochemical reactions :- Quasi-reversible process exhibits intermediate behavior between irreversible and reversible process. In this process, both mass transfer and current transfer control the process. • we can also define it by the ratio of charge transfer to mass transfer. According to bard and faunlkner :- where Λ = electrochemical reversibility parameter k0 = electrochemical facility is a measure of the ease of electron exchange (Dfv)0.5 = mass transfer
- 13. THANK YOU