The total acid number (TAN) is a measurement of acidity that is determined by the amount of
potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which expl ...
in milligrams that is needed to neutralize the acids in one gram of oil.
It is an important quality measurement of
crude oil.
The TAN value indicates to the crude
oil refinery
An oil refinery or petroleum refinery is an industrial processes, industrial process Factory, plant where petroleum (crude oil) is transformed and refining, refined into useful products such as gasoline (petrol), diesel fuel, asphalt, asphalt ...
the potential of
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
problems. It is usually the
naphthenic acids in the crude oil that cause corrosion problems. This type of corrosion is referred to as
naphthenic acid corrosion
In organic chemistry, the cycloalkanes (also called naphthenes, but distinct from naphthalene) are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing ...
(NAC).
TAN values may also be useful in other industries where oils are used as lubricants to determine oxidation and the subsequent corrosion risk to machinery.
TAN value can be deduced by various methods, including
* Potentiometric titration: The sample is normally dissolved in
toluene
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
and
propanol
There are two isomers of propanol.
* 1-Propanol, ''n''-propanol, or propan-1-ol : CH3CH2CH2OH, the most common meaning
*2-Propanol, Isopropyl alcohol
Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a ...
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
titration curve corresponding to the basic buffer solution.
* Color indicating titration: An appropriate
pH color indicator e.g.
phenolphthalein, is used. Titrant is added to the sample by means of a burette. The volume of titrant used to cause a permanent color change in the sample is recorded and used to calculate the TAN value.
* Spectroscopic methods: as with many chemical parameters, spectroscopy can be used to make fast, accurate measurements once calibrated by a reference method. Mid and near infrared spectroscopy are most commonly used for this purpose.
Spectroscopic methods are valuable as they can also be used to simultaneously measure a number of other parameters
[ p. 404] and do away with the need for
wet chemistry.
See also
*
Total base number
Total Base Number (TBN) is a measurement of basicity that is expressed in terms of the number of milligrams of potassium hydroxide per gram of oil sample (mg KOH/g). TBN is an important measurement in petroleum products, and the value varies depend ...
References
* – O'Leary describes the theory and practice of measuring Total acid number on-line in beverages.
External links
{{Commons, Oil refinery
Analytical chemistry
Motor oils