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David C. Silverman |
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Silver/Silver Chloride (Ag/AgCl) Reference Electrode The silver/silver chloride reference electrode has replaced the calomel electrode as the reference electrode of choice by corrosion practitioners. Some of the reasons are that it is easily prepared even for special applications, readily available commercially, very stable, and very robust relative to contamination. It has been used to temperatures slightly above 100°C routinely and reports have appeared in the literature that the electrode has been used to temperatures as high as 300°C for more limited duration.The single junction silver chloride reference electrode is illustrated in this figure ) .
The electrode is not drawn to scale as many sizes exist. The electrode also
can exist as a double junction reference electrode in which a contained outer
solution surrounds the electrode. The outer solution does not have to be
part of the reference electrode itself as shown in this figure
) .
The potential of the reference electrode is related to the concentration of
chloride in the filling solution by
(9)or 
(10)where 
(11)The half cell potential shown in equation (9) or (10) is constant as long as the chloride ion activity is constant. Potassium chloride filling solutions of 3.5M, 4M, and saturation have been reported. The concentrations are kept high to minimize the effect of contamination. The electrode has been used as a direct immersion reference electrode. According to equation (10), for this electrode to function as a true reference electrode in direct immersion applications, the chloride activity (concentration) must be constant. Intefering Ions Silver chloride is one of a number of sparingly soluble salts of silver. Interfering ions can have a large effect on the reference electrode potential if they can react with the silver wire or replace the silver chloride covering the wire. The corrosion potential and any electrochemical measurements that depend on the value of the corrosion potential can be adversely affected. This table shows the relative values at room temperature along with the enthalpy and heat capacity from which the solubility constants can be extrapolated to elevated temperatures. The thermodynamics is for the reaction AgnX↔nAg++Xn-
The data in the above table indicate that a number of silver salts are less soluble than silver chloride at room temperature. These salts are of bromide, iodide, chromate, and sulfide. They would be thermodynamically favored over AgCl and if present in the filling solution might interfere with the potential of the electrode. Silver oxide would be stable under more basic conditions implying that strong base is a contaminant. Oxide formation has been noted when the electrodes are immersed in basic solutions at elevated temperatures. The upper pH limit for immersion is at pH values of 12 to 13 with possible temperature dependence. Sulfide ion interference has been found with both experimental and commercial electrodes. In fact silver sulfide has been used as a direct immersion reference electrode in media containing sulfide ions. Silver sulfate is most likely not a contaminant. Ammonia which can form a number adducts with silver is an additional chemical contaminant and immersion in solutions containing ammonia or ammonia containing buffers is not suggested. Lastly, ultraviolet light can cause a breakdown of silver chloride to metallic silver. Potential vs Temperature The standard potential, (Eo(Ag/AgCl) in equations (9)-(11) has been reported as a function of temperature. This figure shows the values between 0°C and about 100°C ) .
The equation for the line is 
where T is in °C. This figure shows the same information extended to about
275°C) .
The regression equation is
where T is the temperature in °C. (The plot would most likely have been
a straight line had the abscissa been the reciprocal of the absolute
temperature.) These
equations were derived from measurements in which the activity of chloride
was extrapolated to infinite dilution. Knowing these values as a function
of temperature is important because the silver chloride reference electrode
has been used for direct immersion especially at elevated temperatures. If
the chloride concentration is known, the actual potential of the electrode
can be estimated from these figures and the activity of chloride using equation
(10). That value can be compared to the measured value to determine if the
reference electrode has become contaminated by the environment. This figure
shows how the solubility product and chloride concentration change as a function
of temperature
) .
The data at high temperature are somewhat approximate because extrapolation
of ionic free energies to such elevated temperatures is at best an estimate.
The figure shows that the solubility of AgCl increases by almost 3 orders of
magnitude across the temperature range of 25°C to 300°C. Solubility
may become an issue for direct immersion applications at elevated temperatures
if the chloride concentration is not high enough to force the formation of the
salt.
If the electrode is a commercial electrode isolated from the environment by a porous plug, the filling solution is often saturated KCl. In that case, the half cell potential for the electrode is approximately 0.199V vs. the standard hydrogen electrode at 25°C. The temperature coefficient for this electrode has been estimated to be -1.0x10-3, similar to the coefficient of -7.6x10-4 in the above equation. Most of the commercially available electrodes either have a saturated KCl solution or a highly concentrated KCl solution (e.g. 3.5M or 4M). Salt build-up at the plug may become a problem. Use of sodium chloride can help to overcome the issue but at a cost of a larger junction potential. In addition, contamination by silver oxide formation can also become an issue under somewhat basic conditions even with such a high salt concentration. Previous Page: Calomel (Hg/Hg2Cl2) Next Page: Isothermal and Thermal Liquid Junction Potentials - Theory |
David C. Silverman, Ph.D. - Primary Consultant |