is measure of a lubricant's reserve
alkalinity. It is measured in milligrams of potassium hydroxide per gram
(mg KOH/g). TBN determines how effective the control of acids formed
will be during the combustion process. The higher the TBN, the more
effective it is in suspending wear-causing contaminants and reducing the
corrosive effects of acids over an extended period of time. The TBN
value can be deduced by a couple of different methods, eg: by color
indicator titration, by potentiometric titration.
When the TBN is measured at 2mg KOH/g or less the
lubricant is considered inadequate for engine protection, and is at risk for
allowing corrosion to take place. Higher sulfur fuel will decrease the TBN
faster due to the increased formation of sulfuric acid. Generally
range from 6-80mg KOH/g in modern lubricants, 7-10mg for general automotive
use and 10-15 for Diesel operations. Marine grade lubricants generally
will run from 15-50mgKOH/g, but can be as high as 70 or 80mg KOH/g.
Color Indicating Titration: An appropriate
pH color indicator eg: Phenolphthalein, is used. Titrant is added to the
sample by means of a burette. The volume of titant used to cause a permanent
color change in the sample is recorded and used to calculate the TAN value.
Potentiometric Titration
ASTM-D-2896: The sample is normally dissolved in toluene and
propanol with a little water and titrated with alcoholic potassium hydroxide
(if sample is acidic). A glass electrode and reference electrode is immersed
in the sample and connected to a voltmeter/potentiometer. The meter reading
(in millivolts) is plotted against the volume of titrant. The end point is
taken at the distinct inflection of the resulting curve corresponding to the
basic buffer solution.
Also see
neutralization number.
Application
Notes:
