Practical Problems in Voltammetry 3: Reference Electrodes for Voltammetry Adrian W. Bott Reference electrodes can give rise to a number of problems in Bioanalytical Systems West Lafayette, IN electrochemical experiments. However, a basic understanding of the 47906-1382 construction and operation of these electrodes can prevent such problems and can improve their reliability. In all electrochemical experiments, havior at low currents). Conse- sage of a current. One previously the reactions of interest occur at the quently, the interfacial potential of widely used reference electrode surface of the working electrode. the counter electrode in the two- that fulfills these criteria is the satu- Therefore, we are interested in con- electrode system discussed above rated calomel electrode (with a trolling the potential drop across varies as the current is passed large surface area mercury pool). the interface between the surface of through the cell. This problem is However, since the current passing the working electrode and the solu- overcome by using a three-elec- through the reference electrode in tion (i.e., the interfacial potential). trode system, in which the func- the three-electrode system is many However, it is impossible to control tions of the counter electrode are orders of magnitude lower than the or measure this interfacial potential divided between the reference and current that passes through the two- without placing another electrode auxiliary electrodes. This ensures electrode system, the requirements in the solution. Thus, two inter- that the potential between the work- for the reference electrode are less facial potentials must be consid- ing and reference electrodes is con- demanding; hence, smaller, more ered, neither of which can be meas- trolled and the current passes be- polarizable electrodes can be used. ured independently. Hence, one re- tween the working and auxiliary One aspect that is often over- quirement for this counter electrode electrodes. The current passing looked is the variation of the refer- is that its interfacial potential re- through the reference electrode is ence electrode potential with tem- mains constant, so that any changes further diminished by using a high- perature. Ideally, the potential in the cell potential produce identi- input-impedance operational ampli- should be temperature independent; cal changes in the working elec- fier for the reference electrode input. however, it typically changes by 0.5 trode interfacial potential. The requirements for the - 1 mV per degree Celsius. Conse- An electrode whose potential counter electrode of the two-elec- quently, precise potential measure- does not vary with the current is trode system include a high ex- ments require the use of a constant referred to as an ideal non-po- change current (fast electron trans- temperature apparatus. In addition, larizable electrode, and is charac- fer kinetics), a very large surface the temperature at which the meas- terized by a vertical region on a area (to lower the current density) urements were made should always current vs. potential plot (F1). and a high concentration of the spe- be reported. The absence of any However, there is no electrode that cies involved in the redox reaction, temperature control limits the accu- behaves in this way (although some such that the concentrations are not racy of the measurements to about approach ideal non-polarizable be- significantly changed by the pas- 5 - 10 mV (although this level of 64 Current Separations 14:2 (1995) 0' 0 RT 1 F1 precision may be acceptable for E=E+ ln γ nF - Current-potential some experiments). Cl curves for an ideal i non-polarizable elec- Two widely used aqueous ref- trode (dashed lines erence electrodes are the silver/sil- When quoting a redox poten- show the behavior of tial, it is important to be specific. electrodes that ap- ver chloride electrode and the satu- proach the ideal). For example, the standard redox rated calomel electrode (1). These 0 are now discussed in more detail. potential (E ) for the silver/silver chloride redox reaction at 25 oCis Silver/Silver Chloride Refer- +0.222 V (vs. NHE), whereas the E ence Electrode redox potential (E) for the BAS sil- ver/silver chloride reference elec- The redox process for this trode at this temperature is +0.196 electrode is: V(vs. NHE). The above equations show that variations in the chloride ion con- - - AgCl + e ⇔ Ag + Cl centration in the electrode change the redox potential. Since there is This electrode consists of a sil- generally a large chloride concen- ver wire, coated with silver chlo- tration gradient across the reference F2 ride, which is immersed in a solu- electrode frit, there is slow diffusion BAS silver/silver chlo- tion containing chloride ions. The of chloride ions from the reference ride reference elec- electrode solution into the sample trode. BAS RE-5 electrode (F2) uses an aqueous solution containing 3M so- solution; that is, the reference po- dium chloride (the use of sodium as tential will gradually change when the cation rather than potassium is used. discussed below). A porous Vycor There are some precautions frit is used for the junction between that can be taken to minimize the the reference electrode solution and reference potential drift. When the the sample solution. electrodes are made, the Vycor frit The potential E for any elec- is covered in plastic to prevent leak- trode is determined by the Nernst age. This plastic should be carefully equation, which relates E to the removed immediately upon receipt, standard potential E0 and the ac- and the Vycor frit should be im- tivities of the redox components (2) mersed in a 3M aqueous sodium (the standard potential is the poten- chloride solution (F3). The refer- tial of the electrode at unit activity ence electrode should also be re- under standard conditions). The moved from the electrochemical Nernst equation for the silver/silver cell and stored in this solution be- chloride electrode is tween experiments (this is particu- F3 larly important when using non- Reference Electrode aqueous solvent systems, for rea- Storage of the sil- 0 RT 1 ver/silver chloride ref- E=E + ln sons discussed below). Occasion- erence electrode. nF a - Cl ally, air bubbles will form in the so- (the activities of the solid silver and lution next to the Vycor frit; these silver chloride under standard con- should be removed by gently flick- ditions are unity). ing the end of the electrode. It is generally more convenient to consider concentrations rather Calomel Reference Electrode =γ - than activities(aCl-- Cl [Cl ]) , and the Nernst equation can be re- The redox process for this elec- written as follows: trode is: 0' RT 1 - - E=E + ln Hg2Cl2 +2e ⇔ 2Hg + 2Cl nF [Cl- ] where E0’ is the formal potential The BAS RE-2 saturated calo- and is related to the standard poten- mel electrode (SCE) is shown in tial by the equation: F4. One arm of the H-cell contains Current Separations 14:2 (1995) 65 F4 provided it is constant and repro- available commercially). The pre- BAS saturated calo- ducible. If there is any doubt that cipitation of electrolyte salts in- mel reference elec- trode. this is so, an internal reference creases the reference electrode im- (e.g., ferrocene) can be used; that pedance (vide infra) and changes Saturated is, the reference compound is added the liquid junction potential, which KCl solution to the sample solution at the end of causes the reference potential to the experiment, and its redox poten- change with time. Therefore, pro- tial is recorded. This approach can longed exposure to organic solvents also be used to compare redox po- should typically be avoided, and the KCI Crystals tentials measured in different sol- stability of the reference potential Calomel paste vent systems should be regularly checked (by us- Hg There has been much debate ing an internal reference or by com- over the use of aqueous reference paring with another reference elec- Pt wire electrodes such as the silver/silver trode). However, aqueous reference Porous Vycor chloride electrode and saturated electrodes can be used for bulk calomel electrode with non-aque- electrolysis experiments in non- ous solvent systems. One area of aqueous solvents, since a large mercury covered by a layer of mer- concern is the junction potentials overpotential is typically used and cury(II) chloride (calomel). This is across the salt bridge, which can the small potential drift that occurs in contact with a saturated solution range from tens to hundreds of mil- during the experiment should there- of potassium chloride. A porous livolts; however, as discussed fore have little effect (although the Vycor frit is again used for the above, such problems can be com- magnitude of the potential change junction between the reference pensated by the use of an internal should be checked after the experi- electrode solution and the sample reference. There are two more seri- ment). solution. ous problems; the precipitation of Another potentially serious The RE-2 electrode is provided electrolyte and the contamination problem that can occur is contami- as a kit requiring user assembly of the sample solution. nation of the sample solution by (N.B. the kit does NOT include any The electrolytes commonly components of the reference elec- mercury). Once assembled, the used in reference electrodes (so- trode solution (e.g., water and chlo- electrode should be stored with the dium and potassium chloride) are ride ions). In fact, many organomet- Vycor frit immersed in a saturated not very soluble in organic sol- allic compounds are highly reactive solution of potassium chloride to vents, and prolonged immersion of to water and cannot be exposed to maintain the chloride concentration aqueous reference electrodes in or- the small amounts of water that dif- in the reference electrode (vide su- ganic solvents can lead to precipita- fuse from the reference electrode pra).