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David C. Silverman |
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Reference Electrode Classification Reference electrodes can be grouped into two main classes:
Class 1 electrodes are two phase systems (solid electrode and liquid environment) reversible to the solution components. A good example is the reversible hydrogen electrode in which a platinum electrode is in contact with hydrogen ion and hydrogen gas dissolved in the liquid. (The hydrogen gas is in equilibrium between the vapor and liquid phases.) The electrode has a high hydrogen exchange rate. The resulting electrode response depends completely on the hydrogen ion activity and hydrogen gas fugacity (partial pressure). As long as these variables are constant, the electrode potential is constant. The stringent requirements for this electrode make the standard hydrogen electrode impractical for corrosion studies. But, for the case of a hydrogen ion activity of 1 and a hydrogen gas fugacity of 1, the half cell reaction of this electrode forms the standard potential against which all other potentials are referenced. The voltage for this one case is set at 0.00 Volts. This type of electrode is not considered in this tutorial. Class 2 electrodes are three phase systems reversible to anions in solution. The phases are composed of (1) the metal, (2) a solid, sparingly soluble salt composed of the metal cation and (3) the anion in a solution surrounding the electrode. The solution surrounding the electrode usually (but not always) communicates with the external environment by means of a junction that limits diffusion. This geometry enables the anion concentration (activity) and subsequent half cell potential to remain constant. These important electrodes are discussed in this tutorial. Some of the commonly used Class 2 reference electrodes are as follows: Calomel (mercury-mercurous chloride) (Hg/Hg2Cl2) - The counter ion is chloride, generally in a saturated potassium chloride solution in commercially available electrodes. The electrode is very stable. The maximum temperature for use is about 60°C. Silver/Silver Halide (Ag/AgX) - In this case X can be chloride, bromide, iodide. The most common choice for the halide is chloride. That case is discussed in this tutorial. The filling solution tends to be the potassium salt of the anion X (e.g. KCl when X=chloride). Commercial concentrations of the potassium chloride salt solution tend to be saturated or 4 molar though others are used. This electrode is generally applicable to about 100°C with more limited applicability reported slightly above that temperature. Mercury/mercurous sulfate (Hg/Hg2SO4) - The counter ion is sulfate generally in a concentrated, often saturated, potassium sulfate solution. This reference electrode finds most application where leakage of halide ion (e.g. chloride ion) into the environment can be a problem. The maximum working temperature tends to be well below 100°C. Silver/silver sulfide (Ag/Ag2S) - The counter ion is sulfide ion. This electrode is often immersed directly in the environment because silver sulfide is extremely stable with an extremely small solubility product. It has been used at elevated temperature in sulfide containing environments. Referring to this electrode as a Class 2 reference electrode applies only where the silver sulfide salt is in equilibrium with a constant activity (concentration) of sulfide ion in the environment. Copper/cupric sulfate (Cu/CuSO4•xH2O - The counter ion is sulfate in a saturated copper(II) sulfate solution. The most stable form of copper sulfate has 5 waters of hydration (X = 5) at room temperature but under saturated conditions, the average value of X might be lower. Characterization information on the solid phase is sparse under saturated conditions. The salt is often labeled as copper sulfate with no waters of hydration, a label which may be somewhat simplistic. Mercury/mercuric oxide (Hg/HgO) - The counter ion is actually hydroxide. This reference electrode is used under strongly basic condition, often at elevated temperatures. These six electrodes seem to be the most common Class 2 reference electrodes reported in electrochemical corrosion evaluations. Most are available commercially from a number of manufacturers. Other types of metal/metal oxide reference electrodes have been reported, especially for high temperature or selected environmental applications. Some metal oxide electrodes may not function at equilibrium but at steady state so they do not always fall into either of these two classifications. Previous Page: Introduction and Overview |
David C. Silverman, Ph.D. - Primary Consultant |