Electrolytic methods, Chemistry tutorial

Introduction:

Electrolytic techniques comprise of the most precise and most sensitive instrumental methods. The analyte is oxidized or reduced at a suitable electrode and the amount of electricity comprised in the electrolysis is associated to the amount of analyte. Such techniques are best matched for large quantities of analyte, for example, millimole amount. Smaller amount of samples can at times be computed in such methods. Selectivity can simply be accomplished in electrolytic technique by suitable choice of electrolysis potential as various analyte encompass different potential at which they are oxidized or decreased.

Voltammetry:

Voltammetry is necessarily a current-voltage method in which the electrolysis is done on a micro scale by using micro working platinum or any appropriate electrode. This is a class of electro analytical chemistry and different industrial methods.

Fundamental principle of Voltammetry:

Voltammetry is basically the study of current as a function of applied potential. Such curves I = f(E) are termed as voltammograms. The potential is varied randomly either step by step or continuously, or the actual current value is evaluated as the dependent variable. The shape of curves based on the speed of potential variation (that is, nature of driving force) and on whether the solution is stirred or quiescent (that is, mass transfer). Most of the experiments control the potential (volts) of an electrode in contact by the analyte while measuring the resultant current (amperes).

1688_Voltammetric cell.jpg

Fig: Voltammetric cell

[Three-electrode setup: (1) working electrode; (2) auxiliary electrode; (3) reference electrode]

To conduct such an experiment needs at least two electrodes. The working electrode that makes contact with the analyte should apply the desired potential in a controlled manner and facilitate the transfer of charge to and from the analyte. A second electrode acts as the other half of the cell. This second electrode should encompass a recognized potential by which to gauge the potential of the working electrode, moreover it should balance the charge added or eliminated via the working electrode. Whereas this is a viable setup, it consists of a number of shortcomings. Most importantly, it is extremely difficult for an electrode to keep up a constant potential whereas passing current to counter redox events at the working electrode.  

To resolve this problem, the role of supplying electrons and referencing potential has been categorized between the two separate electrodes. The reference electrode is a half cell by a known reduction potential. Its only responsibility is to act as reference in measuring and controlling the working electrodes potential and at no point does it pass any current. The auxiliary electrode passes all the current required to balance the current noticed at the working electrode. To accomplish this current, the auxiliary will frequently swing to extreme potentials at the edges of the solvent window, where it oxidizes or decreases the solvent or supporting electrolyte. Such electrodes, the working, reference and auxiliary make up the modern three electrode system. In practice it can be very significant to encompass a working electrode with known dimensions and surface characteristics. As an outcome, it is very common to clean and polish working electrodes regularly. The auxiliary electrode can be nearly anything as long as it does not react by the bulk of the analyte solution and conducts well. The reference is the most complex of the three electrodes, there is a diversity of standards employed and it's worth investigating elsewhere. For the non-aqueous work, IUPAC proposes the use of the ferrocene or ferrocenium couple as an internal standard. In most of the Voltammetry experiments, a bulk electrolyte (as well termed as a supporting electrolyte) is employed to minimize the solution resistance. This is possible to run an experiment devoid of a bulk electrolyte; however the added resistance greatly decreases the precision of the results. 

Types of Voltammetry:

a) Linear sweep Voltammetry:

Voltammetric process in which the potential between the working electrode and a reference electrode is swept linearly in time whereas the current at a working electrode is evaluated is termed as linear sweep Voltammetry. Oxidation or reduction of species is registered as the peak or trough in the current signal at the potential at which the species starts to be oxidized or decreased.

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Fig: Linear potential sweep

b) Cyclic Voltammetry:

Cyclic Voltammetry is a kind of potentiodynamic electrochemical measurement. In a cyclic Voltammetry experiment, the working electrode potential is ramped linearly versus time similar to linear sweep Voltammetry. Cyclic Voltammetry takes the experiment a step further as compare to linear sweep Voltammetry that ends whenever it reaches a set potential. Whenever cyclic Voltammetry reaches a set potential, the working electrode's potential ramp is inverted. This inversion can occur multiple times throughout a single experiment. The current at the working electrode is plotted versus the applied voltage to provide the cyclic voltammogram trace. Cyclic Voltammetry is usually employed to study the electrochemical properties of the analyte in solution.

688_Cyclic voltammetry.jpg

Fig: Cyclic Voltammetry

Experimental setup:

The process employs a reference electrode, working electrode and counter electrode that in combination are at times termed to as a three-electrode setup. Electrolyte is generally added to the test solution to make sure sufficient conductivity. The combination of the solvent, electrolyte and specific working electrode material finds out the range of the potential.

Electrodes are static and sit in unstirred solutions throughout cyclic Voltammetry. This 'still' solution method yields in cyclic voltammeter's characteristic diffusion controlled peaks. This process as well allows a part of the analyte to remain after reduction or oxidation where it might display further redox activity. Stirring the solution between the cyclic Voltammetry traces is significant as to supply the electrode surface with fresh analyte for each and every new experiment. The solubility of an analyte can modify drastically by its overall charge. As cyclic Voltammetry generally modifies the charge of the analyte it is general for reduced or oxidized analyte to precipitate out to the electrode. This layering of analyte can insulate the electrode surface, display its own redox activity in following scans, or at the very least modify the electrode surface. For this and other reasons it is often essential to clean electrodes between scans.

The common materials for working electrodes comprise glassy carbon, platinum and gold. Such electrodes are usually encased in a rod of inert insulator by a disk exposed at one end. A regular working electrode consists of a radius in an order of magnitude of 1 mm. Having a controlled surface area by a defined shape is significant for interpreting the cyclic Voltammetry results.

To run the cyclic Voltammetry experiments at high scan rates a regular working electrode is inadequate. High scan rates make peaks with large currents and increased resistances that result in distortions. Ultra microelectrodes can be employed to decrease the resistance and current.

The counter electrode, as well termed as the auxiliary or second electrode, can be any material that conducts simply and won't react with the bulk solution. Reactions taking place at the counter electrode surface are insignificant as long as it carries on conducting current well. To maintain the experiential current the counter electrode will often oxidize or decrease the solvent or bulk electrolyte.

c) Adsorptive stripping Voltammetry:

Adsorptive stripping Voltammetry is identical to anodic stripping Voltammetry and cathodic stripping Voltammetry apart from that the pre-concentration step is not controlled via electrolysis. The pre-concentration step in adsorptive stripping Voltammetry is achieved through adsorption on the working electrode surface or through reactions by chemically modified electrodes.

d)  Differential Pulse Voltammetry:

Differential Pulse Voltammetry (that is, Differential Pulse Polarography or DPP) is frequently employed to make electrochemical measurements. This is considered as the derivative of linear sweep Voltammetry or staircase Voltammetry having a sequence of regular voltage pulses superimposed on the potential linear sweep or stair steps. The current is evaluated instantly before each and every potential change, and the current difference is plotted as a function of potential. By sampling the current just prior to the potential is changed, the effect of the charging current can be reduced.

By contrary, in normal pulse Voltammetry the current resultant from a sequence of ever larger potential pulses is compared by the current at a constant 'baseline' voltage. The other kind of pulse Voltammetry is square wave Voltammetry that can be considered a special kind of differential pulse Voltammetry in which equivalent time is spent at the potential of the ramped baseline and potential of the superimposed pulse. 

The system of this measurement is generally the similar as that of standard Voltammetry. The potential between the working electrode and the reference electrode is modified as a pulse from an initial potential to an interlevel potential and keeps at the interlevel potential for around 5 to 100 milliseconds; then it modifies to the final potential that is dissimilar from the initial potential. The pulse is repeated, changing the final potential and a constant difference is kept between the initial and the interlevel potential. The value of current between the working electrode and auxiliary electrode prior to and after the pulse are sampled and their differences are plotted against potential.

=> Uses:

Such measurements can be employed to study the redox properties of extremely small amounts of chemicals due to the following two characteristics:

i) In such measurements, the effect of the charging current can be minimized, therefore high sensitivity is accomplished.

 ii) Faradic current is extracted; as a result electrode reactions can be examined more accurately.

=> Characteristics:

Differential pulse Voltammetry consists of such characteristics:

i) Reversible reactions exhibit symmetrical peaks and irreversible reactions illustrate asymmetrical peaks.

ii) The peak potential is equivalent to E1/2r - E in reversible reactions, and the peak current is proportional to the concentration.

iii) The detection limit is around 10-8 M.

e) Cathodic stripping Voltammetry:

Cathodic stripping Voltammetry is the Voltammetric technique for quantitative determination of particular ionic species. This is identical to the trace analysis technique anodic stripping Voltammetry, apart from that for the plating step, the potential is held at an oxidizing potential, and the oxidized species are stripped from the electrode via sweeping the potential positively. This method is utilized for ionic species which form insoluble salts and will deposit on or close to the anodic, working electrode throughout deposition. The stripping step can be linear, staircase, square wave and pulse.

f) Anodic stripping Voltammetry:

The Anodic stripping Voltammetry is a Voltammetric technique for quantitative determination of particular ionic species. The analyte of interest is electroplated on the working electrode throughout a deposition step and oxidized from the electrode throughout the stripping step. The current is measured all through the stripping step. The oxidation of species is registered as a peak in the current signal at the potential at which the species starts to be oxidized. The stripping step can be linear, staircase, square-wave or pulse.

Electrochemical Cell Set-Up:

Anodic stripping Voltammetry generally incorporates three electrodes, a working electrode, auxiliary electrode (at times termed as the counter electrode) and reference electrode. The solution being analyzed generally consists of an electrolyte added to it. For most of the standard tests, the working electrode is a mercury film electrode. The mercury film makes an amalgam by the analyte of interest, which on oxidation yields in a sharp peak, enhancing resolution between analyte. The mercury film is made over a glassy carbon electrode. A mercury drop electrode has as well been employed for much the similar reasons. In cases where the analyte of interest consists of an oxidizing potential above that of mercury, or where a mercury electrode would be or else inappropriate, a solid, inert metal like gold, silver or platinum might as well be employed.

Anodic stripping Voltammetry generally incorporates four (4) steps if the working electrode is a mercury film or mercury drop electrode and the solution incorporates stirring. The solution is stirred throughout the first two steps at a repeatable rate. The first step is a cleaning step; in the cleaning step, the potential is held at a more oxidizing potential as compare to the analyte of interest for a period of time in order to completely eliminate it from the electrode. In the second step, the potential is held at a lower potential, low enough to decrease the analyte and deposit it on the electrode. After the second step, the stirring is stopped, and the electrode is kept at the lower potential. The main purpose of this third step is to let the deposited material to distribute more uniformly in the mercury. Whenever a solid inert electrode is employed, this step is unnecessary. The last step comprises increasing the working electrode to a higher potential (that is, anodic) and stripping (that is, oxidizing) the analyte. As the analyte is oxidized, it gives off electrons which are evaluated as a current.

Stripping analysis is the analytical method which comprises (a) Pre-concentration of a metal stage to a solid electrode surface or into Hg (liquid) at negative potentials and (b) selective oxidation of each and every metal phase species throughout an anodic potential sweep.

Stripping analysis consists of the given properties: Very sensitive and reproducible (RSD < 5%) process for trace metal ion analysis in the aqueous media. Concentration limits of detection for many metals are in the low ppb to high pptrange (S/N = 3) and this compares favorably by AAS or ICP analysis.

Field deployable instrumentation which is economical.

Around 12 to 15 metal ions can be examined for by this procedure.

The stripping peak currents and peak widths are the function of size, coverage and distribution of the metal phase on the electrode surface (that is, Hg or alternate)

Sensitivity:

The Anodic stripping Voltammetry can detect μg/l concentrations of analyte. This process consists of an excellent detection limit (usually 10-9 - 10-10 M)

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Fig: Anodic stripping Voltammetry

A: Cleaning step, B: Electroplating step, C: Equilibration step, D: Stripping step

Amperometry:

Amperometry is the application of Voltammetric measurements at a fixed potential to detect modifications in currents as the function of concentration of electroactive species.

Amperometry is the principle identical to Voltammetric, refers to a class of titration in which the equivalence point is found out via measurement of the electric current generated via the titration reaction. This is a form of quantitative analysis.

Chronoamperometry is the electrochemical method in which the potential of the potential of the working electrode is stepped and the resultant current from faradic techniques taking place at the electrode is monitored as a function of time. Limited information regarding the identity of the electrolyzed species can be acquired from the ratio of the peak oxidation current versus the peak reduction current. Though, as by all pulsed methods, Chronoamperometry produces high charging currents that decay exponentially by time as any RC circuit. The Faradic current-which is because of electron transfer events and is most frequently the current component of interest-decays as illustrated in the Cottrell equation. In most of the electrochemical cells this decay is much slower as compare to the charging decay--cells without supporting electrolyte are notable exceptions.

As the current is integrated over relatively longer time intervals, Chronoamperometry provides a better signal to noise ratio in comparison to other Amperometry method.

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Fig: Double-pulsed Chronoamperometry waveform showing integrated region for charge determination

Anthracene in deoxygenated dimethylformamide (DMF) will be decreased (An + e- → An-) at the electrode surface which is at a certain negative potential. The reduction will be diffusion-limited, thus causing the current to drop in time (that is, proportional to the diffusion gradient which is made by diffusion).

We can do this experiment various times increasing electrode potentials from low to high. (In between the experiments, the solution must be stirred.) Whenever we measure the current i(t) at a certain fixed time point 'τ' after applying the voltage, we will observe that at a certain moment the current i(τ) doesn't increase anymore; we have reached the mass-transfer-limited region. This signifies that anthracene arrives as fast as diffusion can bring it to the electrode.

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