The Silver / Silver Chloride Reference Electrode
Purpose of a Reference Electrode
An Electrochemical Cell (A little background)
The simplest cell consists of two electrodes in an electrolyte solution. The electrodes are usually made of metal, where current is carried by movement of electrons, the electrolyte solution consists of ions in a polar solvent, typically water. The ions are typically those produced by dissolution of an ionic compound such as potassium chloride (KCl). Potassium chloride dissolves in water producing potassium ions (K+) and chloride ions (Cl-).
The potassium ions carry a positive charge, the chloride ions carry a negative charge. (Positive ions are called cations, negative ions are called anions.)
Electric current consists of the net movement of charged particles in a particular direction. In metals the charge carrier is the negative electron. In an electrolyte solution the charge carriers are ions, both negative and positive. In the presence of an electric field (a potential gradient), electrons in a metal will move toward the more positive potential. In the electrolyte, current (movement of charge) is carried by the ions. Cations will move toward the more negative potential, anions toward the more positive potential.
In the absence of a current there is no potential difference across a conducting phase; the entire phase is at the same potential. (In this example each electrode is a phase, the electrolyte solution is another phase. The air above the cell is a phase and the container (a glass beaker) is another phase.)
Measurement of Potential Differences in an Electrochemical Cell
To measure the potential difference between two electrodes in a cell we can connect a voltmeter between the two electrodes. The appropriate term for the point of attachment at the top of each electrode is the terminal. If the two electrodes were identical, platinum for instance, there would be no potential difference between the two terminals. But there would be a difference in potential between an electrode and the liquid phase, the electrolyte solution. Since the potential is the same across each phase (in the absence of current) the potential difference between the phases takes place in a very narrow region, the interface, between the phases. Figure 1 is a diagram of the cell with a voltmeter attached.
If the electrodes are of different materials we may have a significant difference in potential between the terminals. This depends on the specific composition of the electrolyte as well. In this case the difference between the potential of one electrode and the solution would be different than the potential difference between the other electrode and the solution. In both cases there are two interfacial regions at which the potential differences occur. In one case they are the same, but opposite in direction; in the other they differ. In both cases we can read only a single value with the voltmeter. If the two interfacial potential differences are equal but opposite in direction, as they would be in the case of identical electrodes, the voltmeter would register zero volts. So the voltmeter gives us the sum of two potential differences. An important point is that we cannot know from such a measurement what the magnitude or direction of the potential difference is at either of the two interfaces.
Reference Electrodes
If it were possible to set up an electrode-solution combination that would have a predictable and stable potential difference between the electrode and the solution and combine that with another electrode-solution interface this would enable us to calculate the second potential difference. We would know the potential difference between the two terminals and the potential difference between one electrode and the solution. Taking the difference would give us the potential between the other electrode and the solution.
It is possible to set up such a system with a predictable potential difference. This is called a reference electrode and the potential difference is referred to as an electrode potential or half-cell potential. This usually requires two separate solutions and a porous junction between them. The junction should prevent mixing of the two solutions but allow ions to pass through. There may be a small potential difference across the junction, but this can be kept small and constant. In order to minimize the junction potential it is common to use a salt bridge, often a concentrated solution of potassium chloride, connected to each solution chamber by a junction.
Beside being able to establish a reproducible potential difference a reference system must also be able to pass a small amount of current without a significant change in potential. This requires what is called a high exchange current. This will be discussed in more detail later.
Quantifying the Potential
We can now prepare a system with a reproducible potential difference between two phases, but we still cannot quantify this potential difference. This problem is solved by selecting a reference system and arbitrarily assigning the value of zero to the potential difference. The reference selected was the normal hydrogen electrode which consists of a platinum electrode in a solution of 1M HCl over which hydrogen gas at one atmosphere is bubbled. The electrochemical reaction of this system is
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It is assigned a potential of zero volts at all temperatures. A cell can be assembled with a normal hydrogen electrode and the reference we want to measure. The entire value of the voltmeter reading of this cell is attributed to the reference being measured.
Qualifications of a Reference Electrode
Defined Potential
A reference electrode has to be designed so that there is a well defined potential difference between the metal electrode and the solution in contact with it. The term reference electrode refers to a system which includes a metal electrode, the solution in which the electrode is immersed and often other solid chemical components. All these components are enclosed in a chamber with a junction connecting the internal fill solution with the solution in the cell. The reference potential depends on the composition of the fill solution. The fill solution must be protected from dilution and/or contamination through the junction.
In most reference systems an ion is present in both the metal and the fill solution. In the silver / silver chloride reference that ion is silver I (Ag+). The metal consists of a lattice structure of positive silver ions (Ag+) and loosely held negative electrons. In the solution there are silver ions (Ag+) and chloride ions (Cl-). The silver ions can move across the interface between these two phases. It is the distribution of these silver ions between the metal and solution that determines the potential. Thus it is important to stabilize the concentration of silver ions in the solution. This is done by introducing the slightly soluble silver salt, silver chloride. The concentration of silver ion is determined by the solubility product constant of the salt. The silver concentration in the presence of solid silver chloride also depends on the concentration of chloride ion in the solution. An additional source of chloride ion in rather high concentration (1-4M) further stabilizes the silver ion concentration. This will be described in more detail in the section "Theory of the Potential".
High Exchange Current
At the interface between the metal and the solution, silver ions are crossing from one phase to the other in both directions at equal rates. This is called an exchange current. There is no net movement of charge. If a current is passed through the circuit including the reference electrode, the charge transfer between the metal and the electrolyte solution involves a net movement of silver ions across the interface. If electrons are withdrawn from the silver wire, excess silver ions remain and go into solution. Oxidation is taking place at the reference electrode. If electrons are flowing into the reference electrode, silver ions from the solution attach themselves to the silver electrode. This process is called reduction. In both cases the concentration of silver ion near the electrode is altered. This causes a change in the potential difference between the two phases.
If the exchange current between the two phases is rapid, the system is able to accommodate a small net current density (amps per sq cm) with a minimum displacement of the potential. A high exchange current means that there are both oxidation and reduction currents going on at the surface at the same time at a high rate. Thus the passage of a small net current through the system is a minor perturbation and can be tolerated without a significant shift in potential. Thus when there is apt to be a significant current passed through the reference electrode, it is a requirement that the reference have a high exchange current. The silver / silver chloride system has a high exchange current.
For a large current there is a slight displacement of potential. This has its limits. In usage you do not want to pass much current through a reference. If significant current is required at a working electrode you want to use a three electrode system so that an auxiliary electrode carries the current and the reference is used only as a reference for the potential of the working electrode.
Silver / Silver Chloride Electrode
Construction
The electrode is commonly a silver wire or a silver plated platinum wire, perhaps 0.020 inch in diameter. A layer of silver chloride is grown on the wire by electrolysis in one molar hydrochloric acid, keeping the current density below 10 mA/cm2 . This is placed in an electrolyte solution containing chloride ion, either from hydrochloric acid or potassium chloride. Common concentrations of potassium chloride (KCl) are one molar and saturated (4.1M). The wire and solution may be contained in a glass tube with a ceramic porous junction at one end. At the other end the silver wire is attached to a copper wire of the external circuit.
Theory of the Potential
A metal in equilibrium with its ion in solution is called an electrode of the first kind. As an electrode of the first kind the reaction for a silver electrode is
At equilibrium the escaping tendency of the silver ion in
the two phases is equal. The escaping tendency is related to the chemical potential (m). The electrochemical potential (
) of component i in phase a is defined as
where zi is the charge on the particle, F is the Faraday constant, and fa is the inner potential or phase a. A neutral particle is not effected by the potential. In the neutral metal phase there is an equilibrium between the electrochemical potentials of the silver atoms, silver ions, and electrons.
Since the metal atom is neutral the electrochemical potential is the same as the chemical potential.
Of these particles the only one existing in both the metal phase and the solution phase is the silver ion. At equilibrium the electrochemical potential of the silver ion is equal in both phases.
The electrochemical potentials can be expressed in terms of chemical potential and inner potential of the phase. The chemical potential can be expressed in terms of standard chemical potential and an activity term. In the case of the metal phase the activity is one and thus the logarithmic activity term disappears.
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R is the gas constant, 8.314 J K-1 mol-1, F is the Faraday constant, 96485 coul mol-1, T is the temperature in Kelvins, mo is the standard chemical potential in J mol-1, and f is the inner potential in volts. This can be rearranged to give
which says that the difference in inner potential between the silver metal and the solution is proportional to the sum of the difference in standard chemical potential of silver ions in the metal phase and solution phase and the logarithm of the activity of the silver ion in the solution phase. This is a form of the Nernst equation and can be written
If the cell contains a slightly soluble salt of the metal ion it is called an electrode of the second kind. As an electrode of the second kind the concentration of the silver ion in solution is determined by the solubility product of the salt. The solubility product, written in terms of concentration, is defined as
and its value is 1.75 x 10-10. Approximating the activities with concentration, the activity of the silver ion can be expressed as a function of the chloride ion concentration.
If the fill solution contains 1M KCl then the silver ion concentration becomes very small. With this substitution the Nernst equation can be written as
The standard potential of the Ag,Ag+ (EoAg,Ag+) electrode is 0.7996V. The first two terms on the right-hand-side are both constants and can be combined.
The standard potential of AgCl (EoAgCl) is 0.2223V. This is the potential of the electrode if the activity of chloride ion is 1M. Thus the potential is a function of the chloride activity.