Making potential measurements in an oil water mixture can be very difficult. The surface energy of oils is much lower than that of water. When they are mixed, the two liquids will separate into distinct phases rather than dissolving into each other. While the addition of surfactants can overcome this somewhat, their use would defeat the purpose of oil-water separators where these mixes are encountered in industry. The lower surface energy of oil will make it preferentially wet any solid surfaces in contact with an oil-water mix. Since relative wettability is a property of the liquids rather than the solids, there are no materials which will preferably be wet by water rather than oil.
When installing a cathodic protection system in the water zone of oil water separators, oil can coat the membrane of the reference electrode during the initial filling of the vessel. The oil film increases the resistance of the measurement circuit making measurements difficult. A work-around which can be used is to coat the membrane end of the reference electrode with clay prior to installing it. As the vessel is refilled and the oil phase rises up past the reference electrode, it will coat the clay on the end. Once the vessel is filled so the reference electrode is in the water phase, turbulence will remove enough clay so that measurements are possible.
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