Originally Posted By: alexeft
Originally Posted By: Voltmaster
pH is how acidic ( or basic ) something is , theoretically the higher the TBN the lower the pH.
As TBN drops pH increases making the oil solution more acidic.
That's exactly what I was thinking. A new oil should have a high pH, which should become lower as acid cancelling additives are depleted. All this process should be very easily metered with a pH meter or just pH measuring strips.
Unless, of course, I am missing something, hence the initial question.
I doubt that its quite as simple as that, and if it was "As TBN drops pH increases making the oil solution more acidic." would be exactly wrong, since an acid solution has a low pH, it being a negative scale.
IIRC, in general “wet chemistry” (which, as has been pointed out, isn’t exactly what we are discussing here), pH does not initially change much as you add acid to a base, until you get close to the equivalence point, where most of the base is neutralized by the acid. This is even more true where acid is being added to a buffer solution, which I’d think would be analogous to the situation with motor oil.
I should perhaps have made it clearer that using wet indicator paper was my speculative suggestion, and I havn’t seen or heard any industry endorsement of it. My rationale would be that changes in the wet paper might be analogous to what happens in the water-phase of water-contaminated oil (which I’d guess is relevant for machinery corrosion), and I’m assuming that the separate water phase would prevent oil infiltration into the paper (which might interfere with “wet chemistry” colour development and visualization) but I’m also assuming that the diffusion path is short enough for the water in the paper to equilibrate with oil-bourne but water soluable acids/bases in the oil on a practical timescale.
“Official” TBN and TAN methods use isopropyl alcohol as a “bridging solvent”, in an effort to apply traditional “wet chemistry” titration methods to oil analysis, so that’s a different approach. The Machinery Lubrication article I mentioned (but didn’t have time to cite) “Total Acid Number Titration Method” (it covers TBN as well) gives a detailed description of a (modified) titration method, but doesn’t say much about the significance of the results.
http://www.machinerylubrication.com/Read/4/tan-titrations
This article
“Optimizing Drain Intervals Using TBN vs. TAN” http://www.machinerylubrication.com/Read/2170/oil-drain-interval-tan-tbn
shows a graph depicting the relationship between them. It has some puzzling features (that “clear wedge” with no TAN points, for example) and apparently the authors never been told to label his X-axis, (tsk tsk!) but the basic inverse relationship is pretty much what you’d expect.
This article
A Comprehensive Look At the Acid Number Test goes a bit further into the significance of the acid number and additive depletion.
http://www.machinerylubrication.com/Read/1052/acid-number-test
This author has apparently never been told to label his X-axis and he hasn’t been told not to use locally undefined acronyms either.(RUL stands for Remaining Useful Life, apparently, which I suppose his specialist audience would know.) but he does explain some differences between AN and pH.
Beyond that, the picture is rather complex, and perhaps the main take-home message is that accuracy and reproducibility are low, and results from different laboratories can’t be reliably compared.