Argentum Solutions, Inc.
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David C. Silverman |
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Silver sulfide reference electrodes have found niche applications for examining corrosion in those environments in which the presence of sulfide ions could contaminate the more traditional reference electrodes such as the silver chloride reference electrode. Example environments are certain liquors in the pulp and paper industry that contain sulfide ions. The following table shows the stability constants of silver in the presence of various anions. The thermodynamics is for the reaction AgnX↔ nAg++Xn-.
The stability constant for silver sulfide is many orders of magnitude smaller than that of silver chloride. The lower the stability constant is, the more thermodynamically favorable the solid salt relative to the ion. This stability means that if sulfide ions are present in solution, the electrode comprised of a silver wire surrounded by a layer of silver sulfide would have a response that is almost completely determined by the sulfide ion activity without interference from other ions. If the sulfide ion concentration is constant, as in some process streams, the half cell voltage would be constant as demonstrated by the following for the reaction equations. The voltage variation is a function of the sulfide concentration according to the reaction Ag2S↔2Ag++S2-. 
(20)
or 
(21)
where 
(22)
If the sulfide ion concentration varies, the electrode becomes a selective ion sensing electrode. The electrode can be prepared fairly easily. The preparation recipe may depend somewhat on the ultimate pH of the application. For example, for basic applications, immersing a silver wire in a solution containing sodium hydroxide and sodium sulfide causes a silver sulfide salt to form on the surface. Passing a small current through the cell may or may not be needed. The form of solid silver sulfide changes at 178°C. Unfortunately, little information exists on the effect, if any, on electrode response above this temperature. The electrode itself is reasonably well-behaved. The standard potential at near 25°C has been estimated to be between -0.713V and -0.718V relative to the standard hydrogen electrode. The temperature coefficient has been estimated to be between about -0.5x10-4 and -0.8 x10-4 volts/°C. These results indicate that each electrode must be empirically characterized when used. In addition, according to equations (20)-(22), the characterization should be done for the expected sulfide ion concentration in the test environment. Those requirements are a relatively small price to pay for the ability of this electrode to handle extremely aggressive environments containing sulfide ions, especially by direct immersion. As shown in equation (21), the potential of the silver-silver sulfide electrode depends on the activity (concentration) of sulfide in solution. Very often, this activity is much less than 1. The sulfide ion activity is dictated by the amount of HS- in solution. The implication is that unless the silver-silver sulfide electrode is placed in a solution with unit activity of sulfide ion, the potential will be more positive. This point is important because the silver sulfide coated on silver wire or rod tends to be immersed directly in the test environment, not reside within an isolated sulfide environment. This figure )
shows how the potential can vary with sulfide ion activity (concentration) at 25°C.
The reaction is assumed to transfer 2 electrons though there has been debate if
the charge transfer is a fractional number between 1.5 and 2. The important
point is that the potential varies significantly with sulfide concentration.
The concentration itself does not have to be known as long as the electrode
is calibrated in that
solution. Caution should be exercised if the electrode is directly immersed
in the environment for electrochemical corrosion studies to ensure that the
sulfide ion concentration (electrode potential) remains reasonably constant.
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David C. Silverman, Ph.D. - Primary Consultant |