Cyclic Voltammetry: Hints and Tips

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 predrying 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.

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 is repeatable results are to be obtained.

Organic Solvents

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 reduces the maximu, 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.

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 E–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 E–2. 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.

Complex anions for cyclic voltammetry

Anion	Formula	Mr	Comments
nitrate	Cell 2	Cell 3
perchlorate	Cell B	Cell C
triflate	Cell B	Cell C
methanesulfonate	Cell B	Cell C
tosylate	Cell B	Cell C
trifluoroacetate	Cell B	Cell C
tetrafluoroborate	Cell B	Cell C
tetraphenylborate	Cell B	Cell C
hexaffluorophosphate	Cell B	Cell C

Anion Formula Mr Comments nitrate NO3– 62.00 Potential explosion hazard. perchlorate ClO4– 99.45 Explosion hazard, easily reduced. triflate CF3SO3– 149.1 methanesulfonate CH3SO3– 95.09 tosylate CH3C6H4SO3– 171.2 trifluoroacetate CF3COO– 113.0 tetrafluoroborate BF4– 86.80 May hydrolyze to HF. tetraphenylborate B(C6H5)4– 319.2 hexafluorophosphate PF6– 145.0 May hydrolyze to HF.

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.

The Mercury Electrode

At times it may be of use to perform cyclic voltammetry experiments in organic solvents using a mercury working electrode. Mercury dropping electrodes (MDE’s) can be used but commercial models are expensive, often 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. Mercury film electrodes (MFE’s) use much less mercury (often much less than 1%) than a MDE, and are relatively low cost. The mercury is usually electrodeposited as a thin film on a glassy carbon, 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.