TN 4 Element Selection Dry Silver/Silver Choride

Dry silver/silver chloride elements consist of 99.99% pure silver coated with silver chloride.  Reference electrodes using this element are constructed so the external electrolyte comes into direct contact with the element.  Dry type elements are most commonly used in clean full strength seawater. Like the gelled Ag/AgCl elements, they are adversely affected by sulfides. The reference potential of dry Ag/AgCl elements immersed in full strength seawater is 70 mV negative to that of a saturated Cu/CuSO4 reference electrode.  As the ambient chloride level decreases, as would be the case when used in brackish water, the reference potential becomes less positive. Dry Ag/AgCl elements are only available in through-wall, immersion and tubesheet mounted reference electrodes.

Dry silver/silver chloride elements for concrete are a variation of our standard dry Ag/AgCl element which has been adapted for encasement in a cement-based grout. The reference potential depends on the pore water chloride level of the concrete structure in which it is embedded. In concrete immersed in seawater, the pore water chloride level equilibrates with that of the surrounding ocean so that the element provides long term stable service.     This element is only available in our Marine Concrete reference electrode.

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TN 3 Element Selection – Gelled Silver/Silver Chloride

Gelled silver/silver chloride elements are most often used in environments with more than 500 ppm chloride or other halides although they can also be used in chloride free environments.  They consist of 99.99% pure silver coated with silver chloride and immersed in a saturated potassium chloride solution. Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance.  Portable Ag/AgCl electrodes which contain a liquid rather than a gelled electrolyte are limited to laboratory use.

Silver/silver chloride elements can be used in portable, immersion or underground units.  Use of these elements in electrolytes with other halides (iodides or bromides) or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift.  The reference potential of Ag/AgCl/sat. KCl elements is 105 mV negative to that of a saturated Cu/CuSO4 reference electrode.  Use of sodium chloride rather than potassium chloride electrolytes can cause a junction potential error.

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TN 2 Element Selection – Copper/Copper Sulfate

Copper/copper sulfate elements are typically used in environments with less than 500 ppm chloride or other halides. They consist of high purity metallic copper immersed in a saturated copper sulfate (CuSO4) solution. Cu/CuSO4 elements can be used in portable, immersion or underground units.  Reference electrodes intended for long term service will contain a gelling agent and do not require any periodic maintenance.  Portable Cu/CuSO4 electrodes which contain a liquid rather than a gelled electrolyte will require periodic cleaning and electrolyte renewal.  Use of Cu/CuSO4 elements in electrolytes with higher halide levels or in electrolytes with any sulfides present will contaminate the element causing its reference potential to drift. Because of their widespread use, Cu/CuSO4 electrodes are the ones upon which many cathodic protection criteria are based.

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TN 1 How to measure tube sheet potentials

The potential across a tubesheet under cathodic  protection can show large variations from one location to another. Different waterboxes of the same apparent design can produce different potential distributions. Areas with excessively electronegative potentials can cause hydrogen damage to titanium or ferritic stainless steel tubes. Other areas may have potentials insufficiently negative to adequately protect the copper alloy tubesheet. These potential gradients cannot be detected by a reference electrode mounted on the side wall of the waterbox.

EDI’s Model TE Tubesheet Mounted Reference Electrode is designed to mount any place on the face of a tubesheet. They are shipped with a double tube plug which is inserted into the end of a condenser tube and tightened. Tube plugs are available for common tube sizes between 5/8 inch and 1-1/4 inch. The electrode’s lead wire terminates in a waterproof connector which plugs into a mating connector on the Model TW wire which has been affixed to the tube sheet face.  This attachment system allows the electrode to be easily removed during scheduled outages.  For additional information, visit EDI website.

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