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

Revision as of 10:52, 25 March 2013

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.

Also be aware of the capabilities of your potentiostat. Some potentiostats have large current and/or voltage capability and are capable of causing electrical shock. Always operate the potentiostat in accordance with the manufacturer's instructions.

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.

**Table 1. Complex cations**
Cation	Formula	M_r	Comments
tetramethylammonium	[N(CH₃)₄]⁺	74.15
tetraethylammonium	[N(CH₂CH₃)₄]⁺	130.3
tetra-n-butylammonium	[N(CH₂CH₂CH₂CH₃)₄]⁺	242.5	PF₆^– salt used in dichloromethane
tetraphenylphosphonium	[P(C₆H₅)₄]⁺	339.4	Easily reduced. Hydrolyzed by hydroxide.
benzyltriphenylphosphonium	[P(CH₂C₆H₅)(C₆H₅)₃]⁺	353.4	Easily reduced. Hydrolyzed by hydroxide.
bis(triphenylphosphino)imminium	[(C₆H₅)₃P=N=P(C₆H₅)₃]⁺	538.6
tetraphenylarsonium	[As(C₆H₅)₄]⁺	383.3
(18-crown-6)potassium	[K(C₁₂H₂₄O₆)]⁺	303.4	Can be prepared in situ from potassium salts.
(dibenzo-18-crown-6)potassium	[K(C₂₀H₂₄O₆)]⁺	399.5	Can be prepared in situ from potassium salts. Soluble in benzene, toluene etc.

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.

**Table 2. Complex anions**
Anion	Formula	M_r	Comments
nitrate	NO₃^–	62.00	Potential explosion/fire hazard
perchlorate	ClO₄^–	99.45	Explosion hazard
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^–	312.9
hexafluorophosphate	PF₆^–	145.0	May hydrolyze to HF

Synthesis of Some Tetraalkylammonium Electrolytes

While many of these salts are commercially avialable it is still often much cheaper to make them when larger quantities are required (as is often the case when a lab has several workers using them as electrolytes). The commercially available salts often need recystallisation to increase their purity to an acceptable level for cyclic voltammetry, which further increases their effective cost.

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 tetra-n-butyl-ammonium 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:

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

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, tungsten, 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:

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 the cyclic voltammogram of a perfectly reversible one-electron redox process ( for example the ferrocene/ferrocinium couple, or the hexacyano-Fe II/III couple) should exhibit a theoretical peak-to-peak separation of 57 mV at 25°C it is common to find that the experimentally observed separation can be in the order of 80, 100 mV, or even more. Resistive organic solvents show greater deviations than aqueous solutions. This separation 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 many 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 conditions 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-to-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 electrolyte 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

It is often useful to have the reaction vessel equipped with a magnetic spin bar so that the solution can be stirred between scans.

Troubleshooting

When unexpected results occur it may not be obvious as to the cause of the problem. However a few simple tests can cave an indication as to the location of the fault.

Many potentiostats are fitted with a 'dummy cell', often an internal resistor with a value of a few kohm to several megohm. If this is the case, use the dummy cell mode and apply an appropriate potential. If the resulting current signal obeys Ohm's Law, i = E/R, then it is likely that the potentiostat itself is functioning correctly.

Next, attach a resistor of appropriate value (usually a few kohm to several megohm) to the electrode leads if the potentiostat: working electrode lead to one end of the resistor, and reference and auxiliary leads to the other end. Apply a potential and check the current signal. If the current signal obeys Ohm's Law, i = E/R, (for example 1 V applied across a 1 megohm resistor would produce 1 microamp) then it is likely that the electrode leads are in good condition, and the problem may be with the electrodes or the chemistry in the reaction vessel.

A variant on this latter procedure is to replace the resistor with a voltmeter. Most voltmeters have an internal resistance of 1 - 10 megohm, and so would produce a current of 1 - 0.1 µA if a 1 V potential were applied to them. Connect the working electrode lead of the potentiostat to red voltmeter input, and the reference and auxiliary leads to the black (reference) voltmeter input. The voltmeter will display the actual applied potential so that you can check that the potentiostat is actually applying the correct value. If a negative value is seen instead of a positive value (or vice vera) the you need to switch the polarity of your potentiostat.

Noise

The current signal is susceptible to electrical interference. Usually noise levels on the current signal should be less than 1 µA peak-to-peak. Levels much above this indicate a potential problem or a large noise source nearby. Often the noise is picked up by a relatively high impedance from the reaction mixture or the electrodes themselves. A faulty broken reference electrode can give rise to excessive noise.

Nearby equipment (especially equipment that has a heavy current requirement) may cause interferece. Try turning nearby equipment off and see if the signal noise disappears.

Mains powered equipment usually relies on the earthing of the power socket to also provide good grounding to the potentiostat (and its shielding), However what constitutes a safe electrical earthing does not always provide good instrumental grounding and noise may actually be picked up on the earthing. Power sockets also also grouped in various power circuits distributed throughout the laboratory building and each circuit usually has its own earthing that goes to a common ground. This means that moving the apparatus to a different power socket on a different circuit may improve your signal.

In most cases, currents signals of a microampere or greater can be measured without use of a Faraday cage, or other special shielding. It is usual to find that 90% or more of 'noise' is due to the pick up of mains hum (50 or 60 Hz) so that a 10 Hz filter is very effective at reducing most noise. However remember that a low pass filter can also distort you peak shapes. As a rough rule of thumb you can use a 10 Hz filter with scan rates up to 100 mV/s with no discernable peak shape distortion. In fact it is usually recommended to use a 10 Hz filter setting for all cyclic voltammetry experiments using a scan rate of 100 mV/s or less.

If the current signal is sub-microampere, or if you are employing fast scan rates then you may have no choice but to place your reaction vessel inside a Faraday cage (a cage or box of a conductive material electrically connected to earth). A home-made Faraday cage using an old biscuit tin, or a wooden frame covered with copper mesh, will often work just as well as an expensive commercial product, but remember that the cage must be earthed. All AC powered devices (including an AC powered potentiostat) should stay outside the Faraday cage. Many potentiostat employ shielded lead wires which are passed through a small hole in the wall of the Faraday cage. If your potentiostat has unshielded lead wires consider shortening them as much as possible to avoid unnecessary noise pickup. Note that eDAQ Potentiostats have a conveniently placed 4 mm binding post on their back panel for the grounding of a Faraday cage.

Desktop computers, and old CRT (cathode ray tube) monitors are often sources of high frequency noise. Laptop computers and LCD displays are usually much quieter. It is normally good practice to position the computer and monitor away from the reaction vessel.

To check for noise emanating from nearby equipment, connect a high value resistor (1 megohm or more) to the potentiostat lead wires, working electrode lead to one end of the resistor, and reference and auxiliary leads to the other end. Examine the current signal (any potential even zero volts can be used) on a ranges of 10 microampere or less, so that you can see the signal noise. Make sure all low pass filters are turned off. As you move the resistor about, for example near power leads and computers, you may find noisy 'hot spots'.

Hum/Sinusoidal noise

Sometimes a sinusoidal interference is seen on the current signal. This is almost always due to an aliasing of mains interference (at 50 or 60 Hz) against the sampling frequency and sampling period. If the scan rate is less than 100 mV/s then use a 10 Hz low pass filter setting on the current signal. This will remove almost all the mains interference without distorting the peak shapes in the voltammogram. A similar effect can be achieved by making the sampling period an integral number of mans cycles (i.e. a multiple of 20 ms at 50 Hz, and 16²/₃ ms at 60 Hz, while a multiple of 100 ms works for both 50 and 60 Hz), however this also requires a relatively slow scan rate.

Drift

Excessive current signal drift can indicate a faulty connection to the electrodes, or a faulty reference electrode. Also check the surface of the working electrode in case there has been some sort of byproduct being electrodeposited.

Decreasing peak intensity

The peak height might be seen to be slowly decreasing scan after scan. Even if the scanning is stopped and the solution stirred, and the scanning restarted, the peaks are still seen to be small. In this case check the working electrode for the buildup of an electrodeposited material which may be acting as an insulating layer.

Zero Signal

A zero current signal usually indicates an incomplete circuit. Check that the working electrode is connected. Also check the working electrode with a multimeter/ohmmeter making sure that there are at most only a few ohms between the electrical contact at the top of the electrode and its tip.

Peaks in Strange Positions

If the peaks you see are drifting to or have moved to potentials that seem to be wrong then the reference electrode is probably faulty. Try a new electrode, or cleaning the old electrode.

Peaks at Opposite Polarity

If you find your redox couples appearing with a negative E_1/2 instead of a positive value (or vice versa) then your potentiostat is probably operating with the wrong polarity. Most people doing a cyclic voltammetry experiment want a more negative potential to indicate more reducing conditions at the working electrode. However the potentiostat design may use the reverse convention wjere a negative potential means that the working electroide is more positive (ie oxidising) than the reference. Some older style potentiostats have a polarity reversal switch to invert the polarity. Read you potentiostat manual carefully to determine its polarity convention. You can also use a voltmeter to check the applied potential. Connect the working electrode lead of the potentiostat to red voltmeter input, and the reference and auxiliary leads to the black (reference) voltmeter input. The voltmeter will display the actual applied potential so that you can check that the potentiostat is actually applying the correct value.

Current Signal Overload

If the current signal goes off scale check that the electrodes (and their contacts and clips) are not touching one another. If the potentiostat dummy cell behaves well, but when the lead wires are connected to a external resistor a current overload occurs then suspect a short circuit in the lead wire connector, or an electrical short to a cable shield.

Potential Overload

If the potentiostat indicates a potential overload (or 'out of compliance' situation) then check that the auxiliary electrode it still connected. If you are using particularly resistive organic solvents and the auxiliary electrode is distant from the working electrode the it is possible that your potentiostat does not have sufficient compliance potential (ie the potential between the working and auxiliary electrodes) to achieve the requested applied potential (i.e. the potential between the working and reference electrodes). Consider bringing the auxiliary electrode closer to the working electrode, increasing the background electrolyte concentration, or changing the solvent.

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 | NO<sub>3</sub><sup>–</sup> || 62.00 ||  Potential explosion/fire hazard
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