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dear forum users,
have been recording TAN of a synthetic oil (kin. viskosity = 320) used in a pitting test.
Total duration of test was about 400 hrs. Oil samples were removed at regular intervals.
The TAN of the new oil was just above 1 KOH mg/g and after 100 hrs dropped to about 0.2 KOH mg/g and stayed like that until the end of the test?
To my knowledge so far an oil increases in TAN as it ages, so i am not sure what exactly to make of these results. Am also not too experienced in this area so i would be very glad for all help.

thanks in advance
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thanks for the help Bob K.
One more question. I also recorded the FTIR spectra of the oil samples and attempted to determine the change in carboxylic acid. The only peak in the range typical for oxidation products was at ~1744cm-1 (was looking in the region of ~1640cm-1 to ~1750cm-1). From evaluation it seemed that the carboxylic acid content increased as oil aged. (Thereby adding to my previous confusion regarding TAN). Is it possible that what I recorded at 1744cm-1 was not carboxylic acid but an ester with the carboxyl group (-COO-)? Would an aged oil tend to show an increase in ester content?
Last edited by wayfarer
You can get esters, ketones, aldehydes all of which have C=O along with carboxylic acids(acids also have -OH which have peak in the 3500-3200 cm-1 region). But sounds like the change in the 1700 cm-1 region was minimal so the depletion of acidic additives could more than compensate for small increases in carboxylic acids. In the 1640 cm-1 region salts of carboxylic acids and unsaturation also show up both of which can increase with oxidation
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