OLTAMMETRY AND AMPEROMETRY

C9 V OLTAMMETRY AND AMPEROMETRY

Key Notes

Principles

Voltammetry is the study of the variation of current with applied potential in an electrolysis cell where the reactions are controlled by the diffusion of the sample species. The current is proportional to the concentration of the electroactive species and amperometric methods involve current measurement.

Instrumentation

The cell uses a working microelectrode, a reference electrode, and a counter electrode, and a controlled voltage supply.

Applications

These methods are used qualitatively to determine the nature of metal and organic species and their reactions, and quantitatively to measure trace levels of metals and organic compounds.

Related topics

Other topics in Section C.

Principles Voltammetric techniques involve the electrolysis of the solution to be analyzed using a controlled external power source and measuring the resultant current- potential or current time curves to obtain information about the solution.

The species to be determined undergoes oxidation or reduction at a

working electrode. The voltage between the working electrode and an auxil-

lary or counter electrode is controlled by the external circuitry in order to maintain a preselected potential difference at the working electrode, with respect to the reference electrode, as a function of time. A typical voltam- metric cell, shown in Figure 1, has a working electrode, reference electrode and an auxillary electrode and contains the solution to be analyzed. Often the solution is deaerated with nitrogen to prevent interference due to the reduc- tion reactions of oxygen.

If there is no reaction at the working electrode, the potential changes greatly for

a very small increase in current. A mercury drop electrode, for example, has a polarization range between +0.3 V and -2.7 V against the SCE and in the absence of oxygen so that many reactions that occur in that range may be studied.

By controlling the potential of the working electrode, a particular reaction may be selected. Suppose a cell has two inert, solid electrodes and a reference electrode, which dip into an aqueous solution containing copper ions.

In order to cause any reaction, the applied potential must exceed the decom- position potential. This may be calculated by considering the reactions at each electrode, and adding the extra potentials or overpotentials due to polarization effects at the electrodes and to the voltage needed to drive the current against the resistance of the solution. In this example, in order to drive the cell reaction,

a voltage greater than about –2.54 V must be applied.

C9 – Voltammetry and amperometry

Potentiostat

Nitrogen in Nitrogen out

Working electrode

Reference electrode Auxillary electrode

Sample solution

Stirrer

Fig. 1. Basic voltammetric cell.

The reaction that occurs is: Cu 2+ +H 2 O = Cu(s) + 1 ⁄ 2 O + 2H + 2 The copper is then deposited on the cathode and oxygen is evolved at the

anode. This is the basis for electrogravimetry, where the copper is completely deposited from solution and the increase in weight of the cathode determined. The analysis may be conducted using either a controlled potential or a controlled current.

Coulometric methods of analysis involve measuring the quantity of electricity in coulombs needed to convert the analyte to a different oxidation state. If the electrolysis occurs at 100% efficiency, Faraday’s laws may be applied and each 96485 C will bring about the reaction corresponding to 1 mole of electron transfer.

For example, using a silver anode, the passage of a current produces silver ions, which react with any chloride in the solution. Bromine and acids may also

be generated coulometrically. Polarographic methods employ a microelectrode, often a dropping mercury electrode (DME), as the working electrode, plus a reference electrode (SCE) and a mercury pool as auxillary electrode. The simplest potential-time regime, where the potential increases regularly (linear potential sweep dc voltammetry) is applied to the cell containing the analyte and a supporting electrolyte to carry the majority of the current.

In these methods, the transport of ions to the electrodes depends on three factors: diffusion, convection or stirring, and conduction. The effects of conduc- tion of the ion that reacts at the electrode is minimized by using a concentration of supporting electrolyte such as KCl about 50-times higher than that of the analyte. Stirring and convection are minimized. The resulting polarographic curve shows three regions.

(i) If a potential difference is applied across a cell and no reaction occurs, only

the residual current I r will flow.

(ii) If a reducible ion, say Cd 2+ , is present, it will migrate to the dropping mercury cathode. If the applied potential exceeds its decomposition poten- tial, E D , it will be reduced to the metal which dissolves in the mercury:

Hg Cd 2+ + 2e - = Cd(Hg)

Section C – Analytical reactions in solution

As the cadmium plates out, the layer around the electrode is depleted and more cadmium ions must diffuse in from outside through the diffusion layer of thickness d. This will cause a current,

I, to flow, which depends on the concentration gradient between the bulk solution and the surface. Eventually, the surface concentration becomes zero, and the limiting diffu- sion current is reached:

I d = constant (c(bulk))/d = k S (c(bulk)) The constant, k S , depends on the number of electrons transferred, the diffu-

sion coefficient of the ion in the solution, and the characteristics of the cathode.

(iii) If the potential is increased further, the current does not increase unless other reducible ions are present. These three regions are shown in Figure 2.

The potential difference,

E, across the cell at any stage is:

E=E SCE -E DME

or

SCE - (E Cd + (RT/2F) ln [(a(Cd 2+ , surface)/ a(Cd(Hg))] From the equations above, the concentrations may be substituted by the

E=E n

currents, since the concentration of reduced species in the mercury depends on the current

I and the diffusion constant in the amalgam, k A

I=k A ( c(Cd(Hg))

Cd + (RT/2F) ln (k A /k S )) + ( RT/2F) ln [(I d - I)/I] When

E=E n SCE - (E

1 ⁄ 2 I I = d , that is at the half-wave position, the DME has the half-wave

potential, E 1 ⁄ 2

E DME, 1 ⁄ 2 =E SCE - (E 1 ⁄ 2 + (RT/2F) ln [ 1 ⁄ 2 I d / 1 ⁄ 2 I d ]= E 1 ⁄ 2

The half-wave potential is usually quoted relative to the SCE, and, like the standard electrode potential, is characteristic of the electrode reaction. Typical values are shown in Table 1.

Limiting current

I ent, Curr

Residual current E 1/2

E/(V) Fig. 2. Current-voltage curve for 10 -4 M cadmium sulfate solution.

C9 – Voltammetry and amperometry

Table 1. Value of the half-wave potential Ion

E 1 ⁄ 2 /V with respect to the SCE in 0.1 M KCl

-2.14 C 6 H 5 NO 2 -0.22

Note that copper(II) is reduced in two stages.

Instrumentation For most voltammetric and amperometric methods, the instrumentation includes a working microelectrode, a reference electrode and an auxillary or counter electrode, together with electronic equipment to control the voltage and voltage sweep, plus a computer or recorder to collect data.

The earliest microelectrode used was the dropping mercury electrode (DME), where pure mercury flows through a fine capillary, either due to gravi- tational force, or by applied pressure. Drop times of a few seconds are usual. This electrode has the advantages that:

● the surface area is small and is constantly refreshed so that products of

electrolysis do not accumulate; ● mercury has a high overpotential for hydrogen formation, which allows the

reduction of other species. Other electrodes used are the static, or hanging mercury drop electrode,

where the drop is dislodged at a particular time and size, and solid micro- electrodes, such as platinum and glassy carbon, which may be incorporated into

a rotating disc electrode. Dissolved oxygen in the sample solution must be removed, since oxygen may

be reduced in two steps, giving waves that overlap with those of the sample. O 2 + 2H + + 2e - =H 2 O 2 E 1 ⁄ 2 = -0.05 V

E 1 ⁄ 2 = -0.9 V This is usually done by passing oxygen-free nitrogen through the sample

H 2 O 2 + 2H + + 2e - =H 2 O

solution during the experiment. Maxima on the waves are due to surface effects, and may be suppressed by adding a small amount of surface-active agents, such as gelatin or Triton-X100. Anodic stripping voltammetry is designed to measure trace amounts by preconcentrating them onto a suitable electrode. The experiment has two stages:

(i) The sample is electrolyzed onto a hanging mercury drop, or a mercury film deposited on a carbon electrode. By Faraday’s laws, (Topic C2), passing a current of

I amps for t seconds will produce a concentration c R in a mercury

film of thickness l, area A:

c R = It/nF lA Because the current is limited by diffusion:

I = mnFD c B A

where m is a mass transfer coefficient, D the diffusion coefficient and c B the

bulk concentration.

Section C – Analytical reactions in solution

(ii) The reduced species (that is, the metal) is then oxidized out of the film by making the electrode increasingly anodic. A peak appears on the current- potential plot, and the peak current can be shown to be:

I p = k(c B nt)

where the constant k includes the diffusion and other constants, and n is the rate of increase of the anodic potential. The peak potential at which an active species is oxidized is characteristic of that species, and is close to its half-wave potential.

Applications Polarographic techniques may be used in both qualitative and quantitative modes. Since the half-wave potential is characteristic of the particular reaction that is occurring at that potential, it is possible to identify the species involved. A simple case is shown in Figure 3 where a mixture of metal ions was analyzed. The two reduction waves for copper occur at –0.1 and –0.35 V, cadmium at –0.69, nickel at -1.10 and zinc at –1.35 V. This illustrates an analysis that may identify the species qualitatively and, by using a standard addition method, can also determine the ions quantitatively.

Organic substances may be determined either in an aqueous or a nonaqueous medium. For example, the concentration of nitrobenzene in commercial aniline may be found by studying the reaction:

5 2 + 4H + 4e - =C 6 H 5 NHOH + H 2 O The oxygen electrode is based on voltammetric principles and depends on

C 6 H NO

the diffusion and reduction of oxygen. It is also called the Clark sensor. The cell has a lead anode and a silver cathode set close together in an alkaline solution, often 1M KOH. At the anode, the reaction is

Pb(s) + 4OH - (aq) = PbO 2- 2 (aq) + 2H 2 O + 2e - The silver cathode is inert, unless oxygen or another reducible species can

diffuse to it. A semipermeable membrane through which only oxygen can diffuse surrounds the electrodes, and then the reduction reaction takes place.

O 2 (aq) + 2H O + 4e - = 4OH - 2

I ent, Curr

0 0.3 0.6 0.9 1.2 1.5 E (applied) (V)

Fig. 3. Polarogram of Cu 2 + , Cd 2 + , Ni 2 + and Zn 2 + ions at ~10 -4 M.

C9 – Voltammetry and amperometry

Since the current depends on the diffusion of the oxygen to the electrode from the external solution, and this diffusion is proportional to the concentration of oxygen in the external solution, this electrode may be used to measure dissolved oxygen.

Amperometric titrations are used to determine substances by measuring the limiting diffusion current of a species as a function of the volume of a reagent added to react with that species. Since I d is proportional to the concentration, it will decrease as a species is used up, or increase as the excess of a species becomes greater. For example, for the determination of Pb 2+ with Cr 2 O 7 2- :

4 + 2H At an applied potential of 0.0 V, and at pH4, dichromate is reduced, but Pb 2+ is

2Pb 2+ + Cr 2 O 7 2- +H 2 O = 2PbCrO

not, giving the graph shown in Figure 4. Applications of anodic stripping voltammetry, are chiefly for the determina- tion of trace amounts of amalgam-forming metals ( Fig. 5), while cathodic strip- ping voltammetry is used for determining species that form insoluble salts with mercury. The preconcentration stage allows determination in the concentration range 10 -6 to10 -8 M.

A) 8 µ I(

ent Curr 4

0 5 10 15 20 25 30 35 40 Volume dichromate added (ml)

Fig. 4. Amperometric titration of Pb(II) with dichromate at pH 4 and 0.0 V.

Fig. 5. Anodic stripping voltammogram for 7.5 ppb Cu(II), 2.6 ppb Pb(II) and trace amounts of Cd(II).

Section C – Analytical reactions in solution