Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
| Potential-pH Diagrams |
 |
|
| Intelligent Tools |
 |
|
 |
|
 |
|
| Corrosion Calculator |
 |
|
| Corrosion Economics Estimator |
 |
|
|
David C. Silverman |
|
Reference Electrode Thermodynamics Reference electrodes that function through equilibrium with a sparingly soluble salt lying between the metal and the environment are the most practical for normal laboratory (and, in many instances, field operations. This section pertains to those types of Class 2 reference electrodes. Among those electrodes, the calomel (mercury-mercurous chloride) and silver-silver chloride electrodes are the most popular for laboratory use. The following discussion outlines the physical chemistry behind the potential for these types of electrodes.For this discussion, M is the metal, Xn- is the anion when in the salt. For simplicity in the equations, the metal is assumed to be in the +1 state when attached to the anion in the sparingly soluble salt. This valence state is found in many Class 2 reference electrodes. With a change in coefficients, the equations can be made applicable to higher metal valence states. The salt formula is MpnXp. In this case, p would be an integer which is determined by the number of metal atoms in the formula for the salt complex. The value is usually one (1) but, as in the case of mercurous chloride (calomel), could be 2 or higher. Thus, if n=2, the salt is M2N. If n=1, the salt could be MpNp where p would be 1 for AgCl and 2 for Hg2Cl2. To be a true reference electrode, the electrode reactions must be reversible in a thermodynamic sense. All of the electrodes discussed in this tutorial fulfill this criterion. The potential of this electrode is determined by the reaction 
(1)The potential of reaction (1) is given by 
(2)In equation (2), E is the half cell potential of equation (1) under the conditions in the system, Eo is the potential of that reaction in the standard state (usually assumed to be unit activity, atmospheric pressure and 298°C, R is the natural gas constant (e.g. 1.98 cal/mol-K), T is the temperature in Kelvin, a is the activity of the dissolved species, and F is the Faraday constant. This electrode is in direct contact with a sparingly soluble salt, not the solution. The solubility reaction for the salt is 
(3)The above equilibrium is governed by the solubility equilibrium expressed through the solubility constant Ksp as 
(4)where, once again p is usually 1. Placing equation (4) into equation (2) results in the reference electrode half cell voltage 
(5)or 
(6)where 
(7)In the above equations, the solubility product is assumed to be written as if the salt is MX (MpXp written as pMX), hence the division by p in all equations for potential. The superscript "o" signifies standard state. Equation (6) is the equation normally written for the reference electrode. The potential depends directly on the activity of the salt anion. Its concentration is usually much higher than that of the metal ion (e.g. the sparingly soluble salt AgCl in the presence of saturated KCl). A soluble positive counter-ion is also present (e.g K+). The activity of the anion (and the related potential) depends indirectly on the counter-ion and its concentration in solution. Examples of the potentials are given elsewhere in this tutorial where some of the individual reference electrodes are discussed. If the concentration of the counter-ion is constant, then the electrode has a constant half-cell potential. It can function as a reference electrode. This criterion is the one that makes the Class 2 reference electrodes "reference electrodes". Two methods exist for maintaining a constant activity. The first is to have the electrode immersed in a "filling solution" of constant activity (concentration). This filling solution is isolated from the outside environment. A small amount of restricted flow is necessary for potential to be measured (electrical continuity). This leakage can lead to contamination. It can also create a liquid junction potential. The second method is for the electrode to be directly immersed in the environment, that outside environment having a constant anion activity (concentration), and the salt being extremely stable from a thermodynamic standpoint. The silver/silver sulfide reference electrode is an example of this latter type. This latter electrode can sometimes function by direct immersion and yet maintain a constant, reproducible voltage. Previous Page: Reference Electrode Classification |
David C. Silverman, Ph.D. - Primary Consultant |