Ye olde TAN test
Evan Zabawski | TLT From the Editor July 2014
Righting what was never correct.
If we could all endeavor to use the correct, official terms then perhaps we can eradicate the incorrect, misleading terms.
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WE ARE CREATURES OF HABIT. Sometimes a small error is adopted so pervasively that it is difficult to accept the correction later. The correct name for the test to measure acidic constituents in petroleum products is no exception.
That error is analogous to a linguistic error, and both errors appear in the title of this column. The word “ye” is often mistakenly thought to be the Old English equivalent of “the,” but it is only correct as the nominative case of the second-person plural pronoun “you.” Think of the classic inscription on the entrance to Hell in Dante’s Divine Comedy: “Abandon all hope, ye who enter here.”
How ‘the’ became ‘ye’ is simple enough. In Old English (6th to 10th Century), the th sound was represented by the letter Þ, called thorn (or Þorn, as it were). Middle English (12th to 15th Century) began to adopt the digraph th in place of Þ, about the same time William Caxton introduced the printing press to England.
The German and Italian type fonts used in early printing presses did not have a Þ, but a Y looked very similar (compare Þ to Y). This ubiquitous substitution even found its way into the first King James Version of the Bible in 1611, but it was never pronounced with a Y sound. A more modern example is how older courthouse buildings are often inscribed with ‘jvstice’ rather than ‘justice’ since the Latin alphabet does not contain a ‘u’, and ‘v’ represents both consonant and vowel (now you know why ‘w’ is called double-u and not the seemingly more correct double-v). I digress.
ASTM D664 and ASTM D974 are the two common standard methods for measuring acidic constituents in oil and the result from either was referred to as the Total Acid Number or the TAN. When these standards were written, it was thought they were capable of measuring all the acids, both weak and strong, but this was deemed incorrect more than 25 years ago. The extremely weak acids (defined as having a dissociation constant of less than 10
-9) are not measured, and as such the title and scope but not the method of the standards were changed to reflect this fact. The correct term is simply Acid Number or AN.
This holds true for both ASTM D2896 and ASTM D4739, which, conversely, are standard methods used to measure the basic substances in oil. The result from either is correctly termed Base Number or BN and not Total Base Number or TBN. The official names of AN and BN did once include ‘Total’, but they have not for at least a generation.
The use of TAN, rather than AN, is still common in technical materials and presentations. There have been no errors or misrepresentations in the data, but the community is conditioned to use the outdated term. If we could all endeavor to use the correct, official terms then perhaps we can eradicate the incorrect, misleading terms.
While we are at it, I have one more thought about these tests. For those not familiar with either test, AN is a titration using potassium hydroxide (KOH) in a toluene/acetone solution, with the units for reporting as mg KOH/g of sample. BN is a titration using perchloric acid (HClO4) in a glacial acetic acid solution, yet it also reports with the units mg KOH/g of sample, rather than mg HClO4/g of sample. Perhaps it is time for a change in this convention as well? Your thoughts?
Evan Zabawski, CLS, is the senior reliability specialist for Fluid Life in Edmonton, Alberta, Canada. You can reach him at evan.zabawski@fluidlife.com.