The lower half of the pH scale measures the strength of acids that are dissolved in water. The strength of acids dissolved in a lubricant can also be measured using the pH scale, but the method needs to be altered to allow for the differences between water and oil. Because the readings take longer to stabilise in oil than they do in water, an initial reading is recorded, giving rise to the name initial-pH, or i-pH.
The i-pH can be measured using ASTM D7946, officially called Standard Test Method for Initial pH (i-pH)-Value of Petroleum Products. This method was first published in 2014 and uses a modern electrode to directly measure the strength of acids dissolved within a lubricant.
The i-pH measurement is simply a measurement of the pH of a lubricant, so the results can usually be interpreted in the same way as pH would be. The lower the pH of the lubricant sample, the stronger the acid(s) present in it. Therefore, measuring the i-pH of a used lubricant sample will give a very clear indication of the extent to which acid contaminants have built up in the lubricant during service.
Because the corrosiveness of an acid is dictated by the acid strength, and because i-pH measures the acid strength, i-pH gives an indication of the corrosion risk posed – the lower the i-pH of the used lubricant, the more corrosive it will be.
The final article in this series will cover advantages of using ASTM D7946 i-pH as part of used oil analysis.
For more information, please contact your Lubrizol representative.