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32 OCTOBER 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 10 effect on the reference potential. To measure this effect, special elec- trodes were made up in which two elements shared the same electro- lyte. One element was kept perma- nently in the dark by a double baf- f le, while the other was exposed directly to ambient light. The mag- nitude of the light effect on Ag/ AgCl electrodes was usually less than a millivolt, so it could be safely ig nor e d. Cu/Cu S O 4 ele c t r o de s, however, showed a high degree of light sensitivity. At high noon on the summer solstice, a copper ele- ment in direct sunlight measured –52 mV vs a second element kept in total darkness. In open shade, the value was closer to –10 mV, while interior fluorescent lights caused a –2-mV shift. When the element was removed to a dark location and then re-exposed the following day, po- tential shifts of a similar magnitude were aga i n obser ved. However, when the element was left in the sun for several hours, it apparently became desensitized, and subse- q ue nt photo - er ror s s eldom ex- ceeded –10 mV. Housings of commercial porta- ble electrodes are usually transpar- ent red or yellow with a clear stripe to check the liquid level. When the direct illumination was through the red portion, no photo-error was ob- served, but if the illumination was through the clear stripe, then val- ues similar to t hose above were measured. To get consistent read- ings with a portable reference, the photo-error must be eliminated: the simplest way would be to place a strip of tape over the clear window. Permanent electrodes do not have this problem because they are usu- ally installed in dark locations and have opaque housings. Electrolyte Concentration Effects The reference potential will vary with the logarithm of the concentra- tion of active species in the cell elec- trolyte. This will cause variations in potential readings when using dry electrodes in an electrolyte in which the composition changes, such as Ag/AgCl/seawater references in a tidal estuary. The electrolyte of wet and gelled references has a constant composition t hat eliminates t his source of uncertainty. The concen- trat ion coefficient for Cu/CuSO 4 electrodes is about 20 mV/decade in the negative direction (Figure 4). This means that for each order-of- magnitude drop in the concentra- tion of Cu/CuSO 4 in the electrolyte, the reference potential will shift 20 mV in the negative direction. For Ag/AgCl references, the concentra- t ion coefficient is about 43 mV/ decade in the positive direction. Contamination Effects Contamination of the electrolyte will alter the reference potential of a n elect rode because compet i ng chemical reactions, each with its own characteristic potential, are oc- curring simultaneously. The actual potential of the electrode then be- comes a composite of all the poten- tials established by the individual reactions. Very often, the effect is permanent if the compound formed between the element metal and the contaminant is more stable than the compound formed as part of the reference electrode's intended reac- tion. Chlorides are the contaminant most likely to cause a problem with a Cu/CuSO 4 reference. The effect (Figure 5) is to shift the reference potential in the negative direction. Other halides (iodine, bromine) and sulfides will have a similarly dele- terious effect on bot h Cu/CuSO 4 and Ag/AgCl electrodes. Sulfides in the atmosphere will even affect dry Ag/AgCl references in storage if proper precautions are not taken to protect them. Design Life of Permanent (Gel) Cells The useful life of a dry or wet reference electrode can be indefi- nite if it is properly maintained. A gel electrode, on the other hand, has a fixed life expectancy. In both C L A S S I C C A T H O D I C & A N O D I C P R O T E C T I O N FIGURE 3 Effect of temperature on the reference potential of electrodes in a saturated electrolyte. FIGURE 4 Effect of electrolyte concentration on the reference potential of Cu/CuSO 4 and Ag/AgCl reference electrodes.