Reference Electrodes Line Up
Reference electrodes are widely used as:
- electrochemical measurements (CV, LSV, DPV, etc.)
- electrochemical devices (detectors for HPLC, biosensor, etc.)
Various kinds of them such as aqueous, non-aqueous, calomel and special-constructing types are available.
- Ag/AgCl type [Aqueous reference electrodes]
- Ag/Ag+ type [Non Aqueous reference electrodes]
- Calomel Standard reference electrode
- Reference electrode for alkaline solution
- Reversible Hydrogen Electrode
The reference of the potential in electrochemical measurement is based on the reference electrode, so it is a very important electrode.
SFTec offers a wide line of reference electrodes, as reference electrode for measurement in aqueous solution, non-aqueous solution, alkaline solution, and reversible hydrogen electrode. Also, the reference electrode has the standard size, which makes possible to fit in all of our accessories.
Furthermore, to complete the line up, ALS screw type electrodes are available, which could be used in: Cross flow cell, Radial flow cell, EQCM flow cell and SEC-Spectroelectrochemical flow cell.
Reference Electrodes Technical notes
This is a basic content about the types of reference electrodes used for electrochemical measurement, their uses, and selection methods, for beginners in electrochemical measurement.
In electrochemistry, a specific potential is applied upon a working electrode in order to let a specific reaction occur. The potential of a working electrode should be always exactly specified. That is the reason requiring a reference electrode as reference of the potential.
The reference electrode consists of:
- metal electrode (platinum, gold, mercury, silver, carbon etc.)
- internal electrolyte solution
- a frit (ceramic, Vycor etc.).
Typically a small glass tube (sometimes tubular Teflon for anti-corrosion) is used as the container for these three components. The frit is located at the end of the tube for electric contact with test solution. Requirement of a reference electrode is a long time stability in constant indication of potential. For that, electric current should not be allowed to flow through the reference electrode. Since the current flow may cause a drift of potential, a reference electrode is desired to be connected to a high impedance circuit. Potentiostat has a reference electrode connecting terminal which is high input-impedance terminal. Hence, current flow is strictly limited through a reference electrode.
Although only connecting a platinum wire, which is immersed in a test solution, to the reference terminal of potentiostat makes electric contact with the test solution, so called pseudo-reference electrode, but that could not guarantee the stability or reproducibility of potential measurement. It might be worthy to note that reproducible measurement is possible only by connecting a reliable reference electrode.
Although electric current does not flow through reference electrode, internal impedance of reference electrode itself is better as small as possible. As potentiostat is an automatic control system, involving a high impedance component in the feedback loop results in decrease of response rate (in another words, narrowing of band-width), then it destabilizes to oscillation. It is necessary to avoid the formation of deposit in the frit located in the end of reference electrode, which might result in extremely high impedance of the reference electrode.
Standard hydrogen electrode (SHE) is a primary standard of potential, but it is not necessarily convenient for daily use because of troublesome handling of hydrogen gas. Instead of SHE, as secondary reference electrodes, saturated calomel electrode (SCE), silver-silver chloride electrode (SSCE) and reversible hydrogen electrode (RHE) etc are well known and used.
Why does a reference electrode indicate a constant potential ?
The reason is that a redox reaction on the electrode surface occurs reversibly and equilibrates all the time as shown below.
SHE H2 ⇔ 2H+ + 2e- (0V)
SCE Hg + Cl- ⇔ 1/2Hg2Cl2 + e- (+0.24V)
SSCE Ag + Cl- ⇔ AgCl + e- (+0.20V)
The electrode, which is able to consist of a redox pair in equilibrium is called a reference electrode. The potential of a reference electrode is not able to be changed externally so that it is called non-polarizable electrode. On the contrary, platinum, gold and carbon, of which potential are able to be changed externally are called polarizable electrodes and are used as working or counter (auxiliary) electrodes.
As the relative potentials among the various reference electrodes are fixed and well-known, it is of significant meaning that the comparison between experimental data obtained in even if different places and times is able if the reference electrode used is specified.
A reference electrode is used as in equilibrium so that the net-current in circuit connected to the reference electrode is desired to be zero. Or rather it must be used under zero current. Connecting terminal for reference electrode in potentiostat has extremely high input impedance so that it is impossible for current to flow through the reference electrode. That is one of the important functions of potentiostat.
The most used reference electrode nowadays is silver-silver chloride electrode (SSCE). SSCE is environmentally friendly compared to calomel electrode using mercury and its salt. Its stability, reliability and reproducibility are also sufficient.
The reaction on SSCE is shown as below.
Ag + Cl- ⇔ AgCl + e- (+0.20V)
Forward reaction is oxidation and reverse is reduction. These reactions are in equilibrium.
Chloride ion (Cl-) comes from internal electrolyte (KCl or NaCl). Ag is silver wire itself. AgCl is thin solid film formed on surface of the silver wire.
It is easy to manufacture Ag/AgCl electrode. It is just anodizing cleaned silver wire in water solution containing chloride ion (KCl or HCl etc.). The film formed on surface of the wire is faint pink and turns to dusky grey with time. Thus obtained wire is put into KCl solution confined in a glass tube with a frit in its end for electric contact with test solution as shown in right Figure. That’s all.
Reference electrode detailed
The electrode potential of above reaction is given as follows.
E = E0Ag+ + (RT/F)ln aAg+ eq.(1)
Substitution of the activity of silver ion with the solubility product constant of AgCl , KsAgCl results eq(1) into following equation (2).
E = E0Ag+ + (RT/F) ln KsAgCl - (RT/F) ln aCl- eq.(2)
Using literature values of E0Ag+ and KsAgCl ( E0Ag+＝0.7991 V, and KsAgCl = [Ag+][Cl-] = 1.6×10-10), well-known equation (3) is obtained.
E = 0.222 - (RT/F) ln aCl- eq.(3)
aCl- is the activity of chloride ion in internal solution of the electrode. So, the reference potential of SSCE depends on the activity of chloride ion in addition to temperature. An increase of the activity results in a cathodic shift of the potential. Hence, keeping internal chloride concentration to constant is important to obtain reproducible results.
Reference potential of saturated SSCE(in saturated KCl solution) is 0.197 V at 25°C versus standard hydrogen electrode(SHE). Electric contact of reference electrode with test solution is performed through the frit which permits mutual transfer between the internal solution and the test solution. Hence, dilution of the internal solution and contamination of the test solution provides a possible significant trouble sometimes.
Instead of KCl, sometimes NaCl is chosen as an internal electrolyte. When perchlorate anion is involved in test solution, NaCl is often employed as internal electrolyte because of the low solubility of potassium perchlorate compared with sodium salt. Deposition of potassium perchlorate within the frit may cause serious trouble like large increase of reference electrode impedance.
A main reason of using KCl is that potassium cation and chloride anion have almost identical mobility so that the liquid junction potential at the liquid interface composed of these ions is minimized. While, the liquid junction potential for the case using NaCl as internal electrolyte may become rather considerable. Which one is better choice might depend on a compromise among practical convenience.
Management of reference electrode is important, otherwise resulting in shift of reference potential. Reference electrode should be kept in solution of the same composition of internal solution, while it is not used. Contamination or deposition within the frit due to external substances should be excluded, otherwise resulting in big trouble such as oscillation of potentiostat caused by external noise etc.
Check method of Ag/AgCl reference electrode
For the check, if the reference electrode is properly, you can measure comparing with the new one of the same type.
Since the reference electrode in the cell is the same, the ideal potential difference is 0 Volt, however in a practice a small potential difference will occur. It can be used if the potential difference, between the electrodes, is within 0 ± 20 mV. For the measurement, immerse the reference electrode in 3 M NaCl solution or saturated KCl solution, and then measure the potential difference between both them using a voltmeter.
Reference Potential for difference type of the reference electrode.
NHE(Normal Hydrogen Electrode)
|SCE(Saturated Calomel Electrode)||241mV|
SSCE(Sodium Saturated Calomel Electrode)
Equilibrium reaction shown below takes place in calomel electrode.
Hg + Cl- ⇔ 1/2Hg2Cl2 + e- (1)
The electrode potential is given as following equation.
E = E0Hg22+/Hg + RT/2F ln Ks(Hg2Cl2) - RT/F ln aCl- (2)
Early two terms in right side of equation (2) is replaced to E0Hg2Cl2/Hg which is calculated to be 0.273V from the literature values of redox potential
(E0Hg22+/Hg = 0.7960V) and the solubility product constant (Ks(Hg2Cl2)＝2×10-18).
Replacing again E0Hg2Cl2/Hg to E0, the equation (3) is obtained, which is familiar formula involving the activity of chloride ion.
E = E0 - (RT/F) ln aCl- (3)
Although a small discrepancy in above calculated value exists, E0 is known as 0.268 V for standard potential at 25°C. The electrode potential depends on the concentration of chloride ion as like silver-silver chloride electrode. It is 0.280 V at 1M KCl and 0.241 V at saturated KCl which is so often called as SCE as abbreviation of Saturated Calomel Electrode.
SCE has been used as reference electrode instead of SHE, since early days of electrochemical reserach. However, its use has been put the brakes by concern coming from environmental issues. However, still SCE remains at an important and indispensable position. As a matter of fact, we are able to see many recent reserach papers in which SCE is used. A typical structure of SCE is shown in right figure.
The reference potential of SCE is 0.241 V(at 25°C) , because of increased activity of chloride ion in internal solution of saturated KCl (ca. 4.8 M). In addition to the chloride concentration dependence of potential, the temperature dependence of potential is about -0.5 mV per degree. Except for SCE, the reference potentials are 0.280 V and 0.334 V for 1 M KCl and 0.1 M KCl, respectively. The higher the concentration of KCl is, the larger the temperature dependence of the potential is.
The side arm as salt bridge extended from the body of SCE in above figure is not convenient for daily use because of its fragility. Hence, mercury electrode is placed uppermost and a paste of mercury-mercurous chloride is placed under it and supported by ceramic etc. Such type of SCE electrode is available from BAS Inc. (RE-2BP as shown right Figure).
calomel reference electrode RE-2BP
We often receive the question of "Is SCE usable for experiment in organic solvent?" . Answer is "Yes". However, in the case when you dislike a contamination with water or chloride ion, you are recommended to use a salt bridge, in which both the same organic solvent and electrolyte used in your test solution is contained. Even so, you have to pay attention the fact that a rather large junction potential between SCE and the salt bridge still remains unknown. In addition to that, considering a growing risk of deposition in the frit, it might be better to use a reference electrode for non-aqueous solvent(such as Ag-Ag+ in organic solvent ) or pseudo-reference electrode (such as Pt wire), then refer to the internal standard such as ferrocene.
Usage of pseudo-reference just connecting to the reference terminal of potentiostat is recommendable from the stand point of low impedance.
You may not want to use the typical reference electrode (Ag-AgCl or calomel electrode. For example, you may be discouraged to use it in the cases that your test solution has to be kept away from a contamination with chloride ion or it is strong alkali or strong acid etc. In those cases, you have a choice of mercury-mercuric oxide (Hg-HgO) reference electrode for alkaline solution and mercury-mercurous sulfate reference electrode for neutral or acidic solution.
Mercury-mercuric oxide reference electrode (Hg-HgO electrode)
This is used for strong alkaline solution. It is utilized the equilibrium reaction between mercuric oxide layer formed on mercury and hydroxide anion in alkaline solution as internal solution. Consecutive reactions are shown below.
Hg ⇔ Hg2+ + 2e- (1)
HgO + H2O ⇔ Hg(OH)2 ⇔ Hg2+ + 2OH- (2)
The electrode potential for the reaction (1) is given shown below as equation (3).
E= E0Hg/Hg2+ + (RT/2F) ln aHg2+ (3)
Since the activity of mercuric ion is given by using the solubility product constant as aHg2+ = Ks(Hg(OH)2) /(aOH-)2, equation (3) is replaced to equation (4).
E = E0Hg/Hg2+ + (RT/2F ) ln Ks(Hg(OH)2) - (RT/F ) ln aOH- (4)
As described in the section of silver-silver chloride, the standard potential of the reference electrode utilizing highly insoluble salt depends on significantly its solubility product constant.
Using E0Hg/Hg2+ = 0.8537 and Ks(Hg(OH)2) ≃ 10-25, equation (4) is finally given as below.
E= 0.116 - (RT/F ) ln aOH- (5)
As the internal electrolyte, 0.1 M, 1 M, 3 M and saturated solution of NaOH or KOH are employed. Saturated solution of Ca(OH)2 or Ba(OH)2 are sometimes used because of its low solubility compared with NaOH or KOH. Lower OH- activity of these alkali earth solution is favorable for use of glass container and raises the standard potential a little bit upward. The standard potentials of 1 M KOH, 1 M NaOH, saturated Ba(OH)2 and saturated Ca(OH)2 are 110 mV, 113 mV, 146 mV and 192 mV, respectively.
Mercury-mercurous sulfate electrode
The electrode reaction and the electrode potential are shown as follows.
Hg + 1/2SO42- ⇔ 1/2Hg2SO4 + e- (6)
E = E0Hg/Hg22+ + (RT/2F) ln aHg22+ (7)
The equation (7) is replaced by using the relation between the activity of mercurous ion and the solubility product constant Ks(Hg2SO4),
Ks(Hg2SO4) = [Hg22+][SO42-]) as follows.
E = E0Hg/Hg22+ + (RT/2F ) ln Ks(Hg2SO4) - (RT /2F ) ln aSO42- (8)
Putting E0Hg/Hg22+ = 0.7960V(25°C) and Ks(Hg2SO4) ≃ 7×10-7, into equation (8), the final form is shown as below.
E= 0.6125 - (RT/2F ) ln aSO42- (9)
The reference potential is considerably positive compared to that of calomel electrode reflecting rather large solubility of mercurous sulfate. Saturated K2SO4 or 1 M H2SO4 are used as the internal electrolyte. This reference electrode is an appropriate choice for the case to avoid a contamination of chloride ion.
Line-up of those kinds of electrode are available from ALS Japan as RE-61AP for Hg-HgO electrode and RE-2CP for Hg-Hg2SO4 electrode.