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30 OCTOBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 10 C L A S S I C W hen measuring corrosion potential, a voltmeter is used to measure the po- tential (voltage) between the struc- t u re a nd a referenc e elec t rode placed in the same electrolyte. The entire circuit, structure plus refer- ence, can be thought of as a single cell. Because a reference electrode comprises one half of this cell, it is commonly referred to as a "half- cell." Measured potentials are actu- ally the difference in potential be- tween that of the reference and that of the structure. It is frequently as- sumed t hat t he potent ial estab- lished by a reference electrode is absolutely invariant; therefore, any observed potential changes must be occurring on the structure. This is not always the case. The observed changes could also be caused by a change in the reference while the structure remains steady. To ensure accurate potential measurements, it is important to understand those factors that affect the potential of a reference electrode. C A T H O D I C & A N O D I C P R O T E C T I O N Reference Electrode Classifications Element Types In its most basic form, a refer- ence electrode is simply a piece of metal immersed in a solution of one of its salts. 1 In a thermodynamically stable reference, a known reversible chemical reaction occurs between the metal element and its environ- ment. At equilibrium, the reaction rate in both directions is equal and t he potential established follows the Nernst equation. 2 In theory, the only factors that affect the potential of a reference electrode are temper- ature, which has a linear effect, and solution concentration, which has a logarithmic effect. There are two kinds of reference electrode elements in common use. The first is a metal in a solution con- taining dissolved ions of that metal. An example of this is the copper/ copper sulfate (Cu/CuSO 4 ) elec- trode. The second is a metal coated with a salt of that metal and im- mersed in a solution of that salt. Bot h silver/silver c h lor ide (Ag/ AgCl) and calomel (mercury/mer- cury chloride) are electrodes of the second kind; their reference poten- tial is established by the chloride ion concentration. Electrolyte Types Reference electrodes can also be classified by the form of the sur- rounding electrolyte. In dry elec- trodes, the element is directly im- mersed in the same electrolyte as the structure. The element is dry until it is placed in service; hence, the name. The silver/silver chloride/seawater reference is a well-known dry refer- ence. 3 Dry electrodes tend to be quite rugged and inexpensive. A disad- vantage is that the reference poten- tial they establish can change if the chemistry of the electrolyte changes. Also, the element of a dry electrode will be ruined if the electrolyte or the surrounding atmosphere during storage contains a contaminant for that type of electrode. The contaminat ion problem is greatly minimized by using a wet electrode, in which the element is im- mersed i n a n elect rolyte w it h a known salt concentration. This elec- trolyte should be renewed periodi- cally. In use, t he electrolyte will slowly leak into the environment through a porous plug. If a reverse- flow condition occurs, the element may become contaminated. Com- This article is published as originally appeared in MP, Vol. 33, No. 11 (1994). Factors Affecting the Accuracy of Reference Electrodes Frank J. Ansuini Email: frank@edi-cp.com James R. Dimond Email: jim@dimondale.com This article discusses several factors that affect the reference potential established by silver/silver chloride and copper/copper sulfate reference electrodes. Data is presented on the effects of temperature, light, contaminants, and solute concentration.