"What are the limits of water content in engine oil and engine oil additives? Is it possible to measure it with the Karl Fischer method (ASTM D6304) or could the chemical reagents of the Karl Fischer method and the engine oil additives cause an error to occur?"
Water content as a root cause of machine or lubricant failure is often overlooked. In almost all applications, the amount of water present in the oil should be monitored. Moisture target levels in lubricants must be determined based on the sensitivity of the machine/lubricant to water and how critical the machine is for operation. In some machines, a small amount of water can be highly destructive, while other machines may be quite robust and designed for frequent moisture contamination.
In the case of engines, high levels of water contamination in the oil should not be anticipated. Any water content above 300 parts per million (0.03 percent water) should be monitored closely. When water content reaches 1,500-2,000 parts per million (0.15-0.2 percent water), there is cause for alarm. In normal conditions, water contamination should not be an issue for an engine, since the operation temperature evaporates it.
Moisture levels in lubricants can be tested with several different instruments. Karl Fischer (KF) titration methods are widely used for accurate and precise results. Depending on the lubricant type, the constituents in the oil could cause interference in the accuracy of the test, particularly additives such as detergents, extreme-pressure and anti-wear additives. Even some contaminants and byproducts of oil degradation could cause interference. For engine oils, multiple interferences are possible when using the typical Karl Fischer methods.
The two primary Karl Fischer methods are volumetric and coulometric. Volumetric (ASTM D1744) titrates using an iodine reagent. The volume of the reagent used is then measured. This method is subject to interference from certain additives, such as those containing sulfur. The coulometric method (ASTM D6304) is more reliable and less affected by interferences. When the coulometric method is coupled with the co-distillation procedure prior to titration, the results are more accurate. This co-distillation method involves heating the oil to vaporize water. The vapors are then transferred to be condensed with toluene.
When measuring water content in oil containing a potential interference, comparative analysis should always be performed. This means samples should be compared to a baseline result to provide an indication of how the instrument would report moisture content even when no (or minimal) water is present. The baseline can be determined by running the KF test with a new, dry sample of the same oil. Therefore, if the baseline results indicate 200 parts per million, for example, the cautionary and critical limits should be measured with the addition of the baseline. While this practice can improve the results interpretation process, the root cause of the high water content levels reported should be investigated.