Application Note - pH and Lubricating Oils

pH is an index of the concentration of Hydrogen Ion (H +) in water. Since oil is not an ionizing solvent, it has no free hydrogen ions and therefore, it does not have a pH per se. If the oil contains materials which when mixed with water supply hydrogen ions to the water phase, then these will register when the pH of the water phase is measured.

Due to dissociation pure water has a pH of 7. The hydrogen ion (H +) concentration in pure water is 1E-7 (pH 7) and the hydroxide ion (OH -) concentration is also 1E-7. Each molecule of H 2O that dissociates produces one of each ion, (HOH H + + OH -). The H + is the acid ion and the OH - is the base ion. Since they are present in pure water in equal concentrations, then the water is “neutral” pH 7.

The fraction ionized is about 0.0000001 (=1E-7) at 22°C; i.e., 10,000,000 liters of water supplies 1 gram- ion of hydrogen. [DEF: The pH value is the logarithm of the number of liters of a solution which must be taken in order to contain one gram ion of hydrogen].

Since this is a reciprocal relationship, raising the hydrogen ion concentration lowers the pH value and vice versa.

1/10,000,000 = 1E-7

pH does not tell us how much total acidic hydrogen is present in a combined of un-ionized form. To determine the concentration of acidic hydrogen we refer to the acid number test. If either ion (H +/ OH -) is present in excess of the other, the excess amount can be found by measuring how much of the other ion is required to bring the system back to neutral.

But what is “neutral” in lubrication oil? As discussed, pure water is neutral at pH 7. Equivalent amounts of “strong” acids and “strong” bases mixed together are neutral at pH 7.

This is because strong acids and strong bases release essentially all (over 90%) of their H + and OH - ions respectively when diluted with water.

However, in most lubricating oil systems we are dealing with “weak” acids and bases. Weak acids or bases ionize or release their H + and OH - reluctantly, on the order of 1%, 0.01% or less, at equilibrium.

All of these systems are in dynamic equilibria. Systems at equilibrium with pH 7 are “neutral” in that the concentration of the hydrogen ions (H +) and the hydroxide ions (OH -) are equal. If the equilibrium is shifted either of both ions may be available depending on what other materials may be present.

You could say, “pH is characteristic of a particular oil,” but remember that the pH is measured in and refers to what is in the water phase only. It is generally accepted that new unused turbine oil will have a pH of about 7. Slightly higher or lower pH values may be encountered depending on those materials (additives), which are present.

Acid Numbers and Lubricating Oils

As discussed, used lubricating oils may contain a combination of strong and weak acid formations.

Determining the concentration of strong acids: Titration with a strong base (specifically KOH) will begin at a pH of less than 4.2 and produce a Strong Acid Number (SAN) at an end point of about pH 4.2.

Determining the concentration of weak acids: Titration with a strong base will begin at a pH above 4.2 and produce an Acid Number at an end point of about pH 11.

In the case of the strong acid titration we add only enough base (OH -) to shift the equilibrium up to pH 4.2. In the case of a weak acid titration we add just enough base to shift the equilibrium from some point above 4.2 to a pH of about 11.

Total Acid Numbers (TAN): It follows then, that if both strong and weak acids are present, the Acid Number (commonly referred to as Total Acid Number, TAN) for the system is obtained by titrating to pH 11. The amounts of each type of acid can be obtained by noting the amount of KOH used to reach pH 4.2 for the strong acids and the incremental amount of KOH added between pH 4.2 a pH 11 for the weak acids. These may be recorded as Strong Acid Number and Weak Acid Number respectively with the TAN being the sum of the two.