Difference between revisions of "Cyclic Voltammetry: Hints and Tips"

Revision as of 18:02, 23 March 2013

Safety

Purification and distillation of solvents should only be undertaken by an experienced chemist or laboratory technician. Incorrect procedures could lead to explosion or fire.

• All organic solvents, to a lesser or greater degree, are toxic, and most are flammable. General safety procedures include working in a well ventilated area (a fume hood is usually necessary), with protective clothing including rubber gloves and safety glasses. Adequate ventilation must be ensured about the distillation apparatus to prevent the buildup of flammable and toxic solvent vapours.

• Even if the solvent is relatively harmless, most organic solvents can penetrate the skin easily carrying potentially toxic solutes with them. Similarly most electrolytes used with organic solvents are toxic and/or skin irritants. Always familiarize yourself with the potential hazards by reading the MSDS (Materials Safety Data Sheets) available from the suppliers of solvents and electrolytes. Always assume that new compounds (for which no safety data may be available) are toxic and handle them with due care.

• Only use the drying agent indicated for that particular solvent. Do NOT mix drying agents.

• Solvents may be grossly wet and require pre-drying with a mild reagent (eg anhydrous sodium sulfate) before drying with highly reactive drying agents such as sodium wire or phosphorous pentoxide.

• Appropriate care should be exercised in the handling and disposal of reactive agents such as metal hydrides, phosphorous pentoxide, and sodium wire

• The constituent ions (both cations and anions) of electrolytes may be toxic and when dissolved in organic solvents they can be carried across the skin. In cases of accidental spillage, where a specific treatment is unknown, contaminated skin areas should be continuously flushed with water for at least several minutes.

Solvents

A suitable solvent must be chosen that meets several criteria. Obviously the analyte molecule of interest must have sufficient solubility (usually more than 0.0001 mol/L) to provide an adequate current signal. Also the solvent should not react with the analyte or its electrolysis products. It must also provide a potential window (the range between the cathodic to anodic potentials at which the solvent itself electrolyses) wide enough to see the redox processes of the analyte.

Unfortunately most organic solvents are poor electrical conductors and need relatively high concentrations (typically 0.1 mol/L) of an inert electrolyte added to facilitate the cyclic voltammetry experiment. In general the less polar the solvent the fewer electrolytes are available to will dissolve to the requisite concentration.

Water

Water is obviously a low toxicity, non-flammable solvent capable of dissolving many ionic and polar compounds. It also has the advantage of being a moderately good electrical conductor especially when used with an inert electrolyte added (often 0.1 mol/L KCl). If the analyte has acidic or basic functional groups then pH control of the solution is essential if repeatable results are to be obtained. Its main disadvantage, however, is the vast number of organic compounds that insoluble in it, and which must be studied using an organic solvent.

Organic Solvents

While it is almost always possible to find a solvent for any given organic compound one must also be aware that the solvent must be relatively inert under the oxidizing and reducing conditions to be used in a cyclic voltammetry experiment. It must be possible to purify the solvent to a high degree, as even a small percentage of an electroactive impurity would produce peaks in a cyclic voltammogram larger than those of the analyte (which is typically present in a concentration of between 0.0001 and 0.001 mol/L).

For guidelines on the purification of many solvents see “Purification of Laboratory Chemicals”, 4th edition, W. L. F. Armarego and D. D. Perrin, Butterworth-Heinemann, 1997, ISBN 0750637617.

Often anhydrous organic solvents are required because water sensitive compounds are being employed, or because the solvent itself may react with water at an electrode, or because water causes a reduction of the maximum, anodic or cathodic potentials that can be applied..

Solvents should be of at least AR (analytical reagent) grade otherwise they may be grossly wet, or otherwise need a preliminary purification. Further drying and purification is then usually necessary before the solvent can be used for cyclic voltammetry or other electroanalytical techniques.

Distillation of solvents should always take place using a short fractionating column filled with glass rings to prevent an aerosol of the boiling solvent being carried through the condensor. A dedicated still for each solvent is ideal.

Ethers (including tetrahydrofuran, diethyl ether, 1,4-dioxan, and 1,2-dimethoxyethane), as well as aromatic hydrocarbons (benzene, toluene, xylenes) can be first dried over sodium wire then distilled from freshly drawn sodium wire with a little benzophenone added to the distillation pot. A dark blue or purple color should develop and persist during reflux, for at least 10 minutes, which signals the presence of sodium benzophenone ketyl (a radical anion). The ketyl can only exist in the absence of water and oxygen. The anhydrous solvent can then be distilled. Excess sodium wire is destroyed by allowing the distillation pot to cool and cautiously adding absolute ethanol.

Dichloromethane, 1,2-dichloroethane, cyclohexane, or hexane, can be dried by distillation from either phosphorus pentoxide, P₂O₅, or from calcium hydride, CaH₂.

Dimethyl sulfoxide or dimethylformamide should be predried using a molecular sieve with a pore size less than 4 Å, followed by distillation at reduced pressure (10 – 20 mmHg).

Pre-dry acetonitrile with a molecular sieve, pore size less than 4 Å. Distil after reflux with a small amount of P₂O₅, (about 0.5% w/v) to remove the residual water. Avoid using too much P₂O₅ to prevent excessive formation of an orange polymeric material.

Supercritical Fluids

Supercritical fluids have been used as solvents for cyclic voltammetry. For an example see ‘Electrochemical investigations in liquid and supercritical 1,1,1,2-tetrafluoroethane (HFC 134a) and difluoromethane (HFC 32)’, Andrew P. Abbott, Christopher A. Eardley, John C. Harper, and Eric G. Hope, Journal of Electroanalytical Chemistry, 457, 1–4, 1998. In particular HFC 134a, with tetra-n-butyl ammonium tetrafluoroborate as electrolyte, was shown to be have an extraordinarily wide redox stability window of 9 V.

Liquid Electrolytes

The use of electrolytes that are liquid at ambient temperatures, often referred to as ionic liquids, has become more common in recent years, especially for use in battery technology. These materials can also be used as solvents for cyclic and other voltammetric techniques.

Inorganic Liquids

Cyclic voltammetry can also be performed in liquid ammonia, sulfur dioxide, hydrogen fluoride, etc, although obviously special reaction vessels must be constructed to withstand the high pressure and corrosive conditions employed.

Electrolytes

Some empirical rules for solubility of electrolytes in organic solvents are:

• chlorides, nitrates, tosylates and perchlorates (care! explosion hazard), are usually most soluble in alcohols;
• perchlorate, in the presence of potassium (or rubidium or cesium) ions, will give a precipitate of the metal perchlorate salt;
• electrolytes comprising large cations and anions will be relatively more soluble in non-polar solvents and less soluble in polar solvents;
• dimethylsulfoxide is often a good solvent for most electrolytes;
• fluoroborate and hexafluorophosphate salts exhibit particularly good solubility in acetone and acetonitrile;
• dichloromethane and similar solvents usually require tetra-n-butylammonium hexafluorophosphate, or other large anion/cation salt as an electrolyte.
• electrochemistry in toluene an be performed using liquid tetrabutylammonium tetrafluoroborate toluene solvate. See J. Chem. Soc. Chem. Commun, 323 (1985).

Use of Large Ions as Electrolytes

In general, larger cations and anions (with lower charge densities) produce salts that are more soluble in organic solvents, and so it is nearly always possible to find an electrolyte that will be suitable for a specific solvent. Common electrolytes are commercially available, but others will need to be prepared by the user. Purity of both commercial and home made electrolytes should be checked by performing a ‘blank’ voltammetric run at the same sensitivity setting of the potentiostat that is used when the analyte is present. Remember that ‘purity’ is a relative term — electrolyte and solvent that have been used satisfactorily with high concentrations of analyte at low potentiostat sensitivity settings, may prove to hopelessly contaminated when used with much lower analyte concentrations at very high potentiostat sensitivity settings.

Cations

Complex cations are typically subject to reduction at sufficiently large potential, oxidation is usually less of a problem. Remember that only a small proportion of the electrolyte needs to be electrolyzed to produce a signal that can interfere with the signal of the analyte. Some large cations that can be employed are shown in Table 1. Tetraalkylammonium salts are the most commonly used for organic solvent work because of their relatively low cost and because they are fairly resistant to reduction.

Anions

Large anions may be subject to reduction or oxidation. Some anions that can be employed as shown in Table 2 below. Perchlorate salts are a known explosion hazard and should be avoided wherever possible.

Note that while nitrate is often considered safe to use, it is still an oxidizing agent and should be handled with due caution. Its use as an electrolyte (at relatively high concentrations) in organic solvents is a potential explosion/fire hazard, especially if traces of acid are present, or if the solution is left to evaporate.

Synthesis of Some Tetraalkylammonium Electrolytes

Tetraethylammonium perchlorate, [(CH₃CH₂)₄N]ClO₄, M_r = 229.7

Acid–Bromide method: Tetraethylammonium bromide, M_r = 210.2, (100 g, 0.48 mol) is dissolved in water (100 mL) with slight warming, then 1.0 M perchloric acid (600 mL, 0.60 mol) is added. White crystals form immediately and are filtered off after cooling the mixture to below 5 °C. The crude product is washed with ice cold 1.0 M perchloric acid (100 mL) then recrystallised from 1.0 M perchloric acid (300 mL). The product is filtered off, washed first with ice cold 1.0 M perchloric acid (100 mL), then ice cold ethanol (200 mL). Recrystallisation from boiling ethanol (300 mL) with sufficient water to ensure complete dissolution (about 30 mL) gives the final product, which is washed with ice cold ethanol (200 mL). The product can be checked for purity by dissolving about 0.5 g in warm water (2 mL) and testing the pH of the solution (should be about 7), and for any reaction with silver nitrate (no precipitate should be visible). A further recrystallisation from ethanol may be necessary. The product is dried under vacuum (0.1 mm Hg), to yield 70.3 g (64%) of white needles. The solid compound should be treated as an oxidising agent and stored away from reducing (organic) materials.

Acid–Base method: Aqueous 1 M perchloric acid (250 mL, 0.25 mol) is added to a solution of 1 M tetraethylammonium hydroxide (250 mL, 0.25 mol). The mixture is adjusted to pH 7 (use a narrow range pH indicator paper, not litmus paper) by the addition of more acid or base solution as required, and stirred while cooling in an ice bath. The resulting precipitate is removed from the cold suspension by suction filtration and washed with cold water. The crude product can be recystallised from water and dried at 100°C for 24 h in vacuo. M.p. 351 – 352.5 °C with decomposition. The solid compound should be treated as an oxidising agent and stored away from reducing (organic) materials.

The purity of the product should be checked by cyclic voltammetry.

Tetra-n-butylammonium hexafluorophosphate, [n–Bu₄N]PF₆, M_r = 387.4

Acid–Bromide method: A solution of tetra-n-butylammonium bromide (100 g, 0.31 mol) in acetone (250 mL) is mixed with a solution of ammonium hexafluoro-phosphate (50 g, 0.4 mol) in acetone (350 mL). The resulting precipitate of ammonium bromide is removed by suction filtration. The filtrate is concentrated, with a rotary evaporator, to approximately 200 mL. Water is added to the acetone solution to precipitate tetra-n-butylammonium hexafluorophosphate (final volume approximately 2 L). The precipitate is removed by suction filtration, washed with water, and then redissolved in a solution of ammonium hexafluorophosphate (5 g, 0.04 mol) in acetone (200 mL) — more acetone can be added to complete dissolution if required. Addition of water (final volume approximately 2 L) causes the precipitation of the crude product which is removed by suction filtration and washed with water.

Acid–base method: An aqueous solution of 0.5 M hexafluorophosphoric acid (600 mL, 0.3 mol) is added to an aqueous 0.5 M solution of tetra-n-butyl-ammonium hydroxide (600 mL, 0.3 mol). The mixture is adjusted to pH 7 (use a narrow range pH indicator paper, not litmus paper) by the addition of more acid or base solution as required. The crude product is removed from the cooled mixture by suction filtration and washed with water.

Recrystallisation: The crude product, produced by either the acid–bromide or acid–base method, is recrystallized three or four times from a 3:1 mixture of ethanol/water and then dried for at least 24 h at 100°C in a vacuum oven. The yield is usually about 95 g. The purity of the product should be checked by cyclic voltammetry.

Tetra-n-butylammonium tetrafluoroborate, [n–Bu₄N]BF₄, M_r = 329.3

Acid–Bromide method: Aqueous fluoroboric acid (48%, 36 mL) is added to a solution of tetra-n-butylammonium bromide (84 g, 0.25 mol) in water (180 mL) and the mixture stirred for 1 minute. The resulting precipitate is removed by suction filtration and washed with water until the washings are no longer acid (use a narrow range pH paper). The crude product can be recystallised from ethyl acetate/cyclohexane. M.p. 162–162.5°C. The purity of the product should be checked by cyclic voltammetry.

Tetra-n-butylammonium fluoroborate toluene solvate, [n–Bu₄N]BF₄.3(C₆H₅CH₃)

Tetrabutylammonium fluoroborate is stirred in toluene between 22 – 25°C. A two phase mixture results. The lower layer has an approximate formula of [n-Bu₄N]BF₄.3(C₆H₅CH₃). (Below 22°C the solvent-free fluoroborate salt crystallises.) This layer an be separated and used, without further purification, for electrohemical work. Reference: J. Chem. Soc. Chemical Communications, 323, 1985.

Tetra-n-butylammonium triflate [n–Bu₄N]CF₃SO₃, M_r = 391.5

Triflic acid (trifluoromethanesulfonic acid), M_r = 150.1, (600 mL, 0.15 mol) is slowly added, with stirring, and cooling, to a commercially available 40% solution of tetrabutylammonium hydroxide (Mr = 259.5) (100 mL 0.15 mol) until the pH drops to 6.5 (use a narrow range indicator paper). Water is added if necessary to allow adequate stirring. The crude product is suspended in ice cold water and filtered 5 times to wash it, then dried and recrystallised from a mixture of dichloromethane and diethyl ether. Reference: Journal of Organic Chemistry, 37, 3968, 1972. The purity of the product should be checked by cyclic voltammetry.

Electrodes

Working Electrode

The working electrode is where the chemical reaction of interest takes place. Most commonly a disk electrode of 1 - 3 mm diameter is used that is embedded in a glass, Teflon, or PEEK (polyetheretherketone) body.

The disk material is usually made of an inert material, platinum, glassy carbon, and to a lesser extent gold, being the most commonly encountered materials which are polished to a mirror finish.

For glass body platinum or gold electrodes a soda glass is preferred to borosilicate glass as the thermal coefficient expansion more closely matches that of the metal (so that the disk material does not separate from the glass after repeated temperature changes). While the use of glass body electrode may be required for some experiments in difficult environments the greater robustness of Teflon or PEEK body electrodes make them usually preferred. Teflon offers only slightly better chemical inertness than PEEK, but suffers from 'cold flow' or 'creep'. That is the disk material can separate from a Teflon body more easily than a PEEK body. PEEK body working electrodes with a 1 mm diameter disk are offered by eDAQ:

• ET074 Glassy Carbon Disk Electrode
• ET075 Platinum Disk Electrode
• ET076 Gold Disk Electrode

Microelectrodes (sometimes referred to as ultramicroelectrodes) are usually considered to have disk diameters less than 25 microns. Thin platinum or gold wire electrodes have been made down to micron diameters. Carbon fiber (similar to glassy carbon) electrodes down to about 5 micron diameter have also been described..

The Mercury Electrode

At times it may be of use to perform cyclic voltammetry experiments in organic solvents using a mercury working electrode. Hanging mercury drop electrodes (HMDE’s) can be used but typically commercial models are expensive and all are cumbersome to set up, require periodic cleaning and maintenance, and require the use of relatively large amounts of elemental mercury. However they do have the advantage of being able to provide a clean mercury electrode by dislodgment of the old mercury drop and allowing a new drop to form, all at the press of a button, in any solvent you require. Alternatively mercury film electrodes (MFE’s) use much less mercury (often much less than 1%) than a HMDE, and are relatively low cost. The mercury is usually electrodeposited as a thin film on a glassy carbon, platinum, or carbon–fiber, support electrode. However, the mercury film must be removed and replated if it is fouled or oxidised. Some workers prepare the MFE by electrodeposition from aqueous mercuric ion solutions and then wash the electrode to replace the solvent. However it is also possible to electrodeposit mercury directly in suitable organic solvents: G. Alarnes-Varela, A.L. Suárez-Fernández, A. Costa-García, Electrochimica Acta, 44, 763–772, 1998.

Reference Electrode

Three-electrode potentiostats are designed so that no current passes through the reference electrode, thus it is not subject to any electrolysis reaction and maintains its integrity (and provides a constant potential) throughout the experiment. Unfortunately most reference electrodes are based on aqueous chemistry and are somewhat incompatible with the anhydrous immiscible organic solvents that are often used for cyclic voltammetry. With some organic solvents (eg acetonitrile) a silver wire/silver ion reference electrode can be constructed from a standard refillable electrode:

• Ag/AgCl miniature refillable electrode

At other times a simple platinum (or platinum coated wire) can be used as a pseudo-reference electrode and after the experiment ferrocene (which has a known E_1/2) is added to the reaction mixture to calibrate the system. A modern innovation is the leakless refernce electrode which uses a solid polymer junction to separate the inner aqueous filling solution of the electrode from the other reaction mixture, see

• ET069 5 mm diameter Leakless Ag/AgCl electrode
• ET072 2 mm diameter Leakless Ag/AgCl electrode

In a two electrode system (almost never used these days for cyclic voltammetry experiments) the reference and auxiliary electrode leads of three electrode potentiostat are connected to a single 'counter' electrode. In this case significant current can pass through the counter electrode which must be constructed to maintain a constant chemical environment for the lifetime of the experiment.

Auxiliary Electrode

The auxiliary electrode is often the 'forgotten' electrode in a cyclic voltammetry experiment. If an oxidation event is occurring at the working electrode then a reduction event must be occurring at the auxiliary electrode (and vice versa). Often the reaction at the auxiliary electrode is the electrolysis of the solvent or of an electrolyte component. Thus the auxilary electrode is required to be made of an inert material, and historically platinum wire has been used. While most metals can be used as electrodes for reduction processes relatively few are stable under oxidizing conditions, platinum being an obvious example. However several other metals are very oxidation resistant (often because they form a protective oxide coating). Molybdenum and titanium are in this category and make excellent auxiliary electrodes (and are much lower cost than platinum). In addition, platinum-coated titanium wire offers similar catalytic reactivity as pure platinum and these auxiliary electrodes are available from eDAQ:

• ET078 70 mm Titanium Wire Electrode
• ET086 150 mm Titanium Wire Electrode

Electrode Position

Electrode position is more critical in resistive organic solvents. Always position the electrodes so that the reference electrode tip is as close a possible to the working electrode tip. In particular avoid placing the auxiliary electrode between the working and reference electrodes.

iR Compensation

While a perfectly reversible electrode redox process ( for example the ferrocene/ferrocinium couple, or the hexacyano-Fe II/III couple) should exhibit a peak-to-peak separation of 69 mV at 25°C it is common to find that the observed separation can be in the order of 80, 100 mV, or even more. Resistive organic solvents show greater deviations than aqueous solutions. It can be minimised by using a high background electrolyte concentration and make sure that the reference electrode is close to the working electrode. The effect is due to the so called 'uncompensated resistance' or 'iR drop' between the reference and working electrodes. Some users like to employ iR compensation when performing cyclic voltammetry. However in many cases 'positive feedback' compensation is used - but this is only a rough approximate correction at best, as the exact iR drop will vary with applied potential and with the relatively large currents flowing during the charge transfer process. Current interrupt iR compensation will give better results as the iR drop is actually calculated at each potential in the potential staircase ramp, however most potentiostats do not provide this type of compensation, it is somewhat tricky for a non specialist to set correctly, and it cannot be used for fast scan rates.

One alternative is simply to do without iR compensation! Perform cyclic voltammetry, under the same condition you propose for your experiment, on ferrocene (organic solvents), or ferrocene carboxylic acid or hexacyanoferrate (aqueous solutions). The peak-to-peak separations you observe for these one electrode redox couples can be considered to the the minimum achievable with the solution and electrode arrangement that you are using. Thus if your analyte exhibits a redox couple with the same peak separation you can safely assume it is also a one electron reversible couple.

Reaction Vessel

The vessel chosen in which to perform the cyclic voltammetry experiment must be designed to hold the electrodes and also to allow nitrogen purging for oxygen-sensitive compounds or their electrolysis products. If you are characterising new compounds, which are often available only in milligram quantities, then a small volume vessel is also desirable. Even for routine or teaching experiments a small volume vessel cuts down on the amount of organic solvent and valuable electrolytes salts being used. Using 1 mm disk electrodes it is possible to perform cyclic voltammetry in vessels with only 1 - 3 mL of solvent. See

• ET080 3 mL Reaction vessel