The total acid number (TAN) is a measurement of acidity that is determined by the amount of potassium hydroxide 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 the potential of corrosion problems. It is usually the naphthenic acids in the crude oil that causes corrosion problems. This type of corrosion is referred to as naphthenic acid corrosion (NAC).
Potentiometric titration: 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 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.
pH is an index of the concentration of Hydrogen Ion (H +) in water.
Since oil is not an ionizing solvent, it has no free hydrogen ions and
therefore, it does not have a pH per se. If the oil contains materials
which when mixed with water supply hydrogen ions to the water phase,
then these will register when the pH of the water phase is measured.
Due to dissociation pure water has a pH of 7. The hydrogen ion (H
+) concentration in pure water is 1E-7 (pH 7) and the hydroxide ion (OH
-) concentration is also 1E-7. Each molecule of H 2O that dissociates
produces one of each ion, (HOH H + + OH -). The H + is the acid ion and
the OH - is the base ion. Since they are present in pure water in equal
concentrations, then the water is “neutral†pH 7.
The fraction ionized is about 0.0000001 (=1E-7) at 22°C; i.e.,
10,000,000 liters of water supplies 1 gram- ion of hydrogen. [DEF: The
pH value is the logarithm of the number of liters of a solution which
must be taken in order to contain one gram ion of hydrogen].
Since this is a reciprocal relationship, raising the hydrogen ion concentration lowers the pH value and vice versa.
pH does not tell us how much total acidic hydrogen is present in a
combined of un-ionized form. To determine the concentration of acidic
hydrogen we refer to the acid number test. If either ion (H +/ OH -) is
present in excess of the other, the excess amount can be found by
measuring how much of the other ion is required to bring the system back
to neutral.
But what is “neutral†in lubrication oil? As discussed, pure
water is neutral at pH 7. Equivalent amounts of “strong†acids and
“strong†bases mixed together are neutral at pH 7.
This is because strong acids and strong bases release essentially
all (over 90%) of their H + and OH - ions respectively when diluted with
water.
However, in most lubricating oil systems we are dealing with
“weak†acids and bases. Weak acids or bases ionize or release their H
+ and OH - reluctantly, on the order of 1%, 0.01% or less, at
equilibrium.
All of these systems are in dynamic equilibria. Systems at
equilibrium with pH 7 are “neutral†in that the concentration of the
hydrogen ions (H +) and the hydroxide ions (OH -) are equal. If the
equilibrium is shifted either of both ions may be available depending on
what other materials may be present.
You could say, “pH is characteristic of a particular oil,†but
remember that the pH is measured in and refers to what is in the water
phase only. It is generally accepted that new unused turbine oil will
have a pH of about 7. Slightly higher or lower pH values may be
encountered depending on those materials (additives), which are present.
As discussed, used lubricating oils may contain a combination of strong and weak acid formations.
Titration
with a strong base (specifically KOH) will begin at a pH of less than
4.2 and produce a Strong Acid Number (SAN) at an end point of about pH
4.2.
Titration with a strong base will begin at a pH above 4.2 and produce an Acid Number at an end point of about pH 11.
In the case of the strong acid titration we add only enough base
(OH -) to shift the equilibrium up to pH 4.2. In the case of a weak acid
titration we add just enough base to shift the equilibrium from some
point above 4.2 to a pH of about 11.
It follows then, that if both
strong and weak acids are present, the Acid Number (commonly referred to
as Total Acid Number, TAN) for the system is obtained by titrating to
pH 11. The amounts of each type of acid can be obtained by noting the
amount of KOH used to reach pH 4.2 for the strong acids and the
incremental amount of KOH added between pH 4.2 a pH 11 for the weak
acids. These may be recorded as Strong Acid Number and Weak Acid Number
respectively with the TAN being the sum of the two.
Additive depletion, contamination and oxidation are common pathways of
lubricant degradation. The acid number (AN) test is one of the methods
available in the oil analysis field used to estimate the amount of
additive depletion, acidic contamination and oxidation. AN does not
directly measure the rate of oxidation, it merely measures the
by-product of oxidation. It is also beneficial to trend AN to determine
the rate of depletion of certain additives. The purpose of this article
is to attempt to answer the following questions:
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What are the main objectives of measuring AN?
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What nomenclature is being used in industry? (strong acid number (SAN), total acid number (TAN), etc.)?
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What standardized methods are currently used in the industry?
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What modified tests exist and why?
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What are the advantages and disadvantages of each test (reproducibility, repeatability, etc.)?
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What are the dos and don'ts of comparing results?
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How is AN trended and what are the common trends?
Once these questions are answered, a better understanding of how to use AN results will be achieved.
Figure 1. Correlating Changes in RUL to Oxidation Life Cycle8
Objectives of Measuring AN
AN is the measure of acid concentration in a nonaqueous solution. It is
determined by the amount of potassium hydroxide (KOH) base required to
neutralize the acid in one gram of an oil sample. The standard unit of
measure is mg KOH/g. AN does not represent the absolute acid
concentration of the oil sample. The AN measurement detects both weak
organic acids and strong inorganic acids. A change in the acid
concentration of an oil can originate from multiple sources. Acidic
contaminants, wrong oil, alkaline-reserve depletion and oxidation
by-products can cause an increase in acid concentration. Table 1 lists
common acids that can be detected.
Understanding the extent of additive depletion is key in determining
the RUL of an oil. Some additives are weakly acidic and can elevate the
oil's initial AN. As the lubricant ages these additives deplete, thereby
reducing the acidity created by the additives. The common antiwear
additive, zinc dialkyl dithiophosphate (ZDDP), produces certain AN
trends during lubricant aging. Concurrently, the oil is possibly being
contaminated with acidic constituents, increasing the acid content in
the oil. The combined effects of additive depletion, acidic
contamination and other acidic-affecting events create a challenge in
determining what the AN represents. Figure 1 shows the underlying
components that affect the AN during lubricant aging. It can be seen
that during an induction period the antioxidant additives are depleting;
once these additives are depleted, the base oil begins to oxidize if
the stressing conditions are sufficiently high. By trending the AN, this
increase can be detected.
Table 1. AN May Detect These Corrosive Oils
Nomenclature Used in Industry Total Acid Number vs. Acid Number
Currently in North America, the term total acid number (TAN) is being
replaced with acid number (AN). This change is based on the fact that AN
tests do not detect the total acid concentration of the lubricant. The
acid concentration of the lubricant contains both strong and weak
components. Strong acidic components are referred to as SAN. The weak
components and the strong components are typically combined as AN. Even
though AN is comprised of both acidic components, it does not represent
all acidic components in the lubricant. For instance, the AN and base
number (BN) tests are not affected by extremely weak acids and bases
that have a dissociation constant of less than 10-9. This is the reason
that TAN is being replaced by AN.
pH vs. AN
The pH and AN test methods measure different aspects of the oil's
acidic or alkaline character. The pH test method measures the apparent
pH of the oil. The apparent pH is a representation of how corrosive the
oil may be, but it does not indicate the concentration of acidic or
alkaline constituents. The pH test method is useful in applications
where corrosive oil could cause considerable damage. It is also valuable
in lubricant systems with a high potential for the formation or the
contamination of strong acids.
The AN and BN test methods respectively measure the concentration of
acidic and alkaline constituents. Both acidic and alkaline constituents
can exist in oil at the same time. In fact, some additives are
amphoteric, meaning they can behave as either a base or an acid. In some
oils, it is important to monitor both the AN and BN to determine the
reactions in the oil. AN and BN do not indicate the strength of the
acidic or alkaline constituents in the lubricant, which reduces their
ability to indicate the oil's corrosiveness. AN has a better ability
than pH to detect and monitor weak acids, which do not readily
dissociate in water. This prevents the pH test method from obtaining a
good indication of how the weak acid concentration is changing in the
lubricant.
Standardized Methods
Table 2 lists the current ASTM standard test methods for determining
AN. Each test has been designed for specific purposes, with ASTM D664
and ASTM D974 being the two most commonly used tests. ASTM D1534 and
ASTM D3339 are similar versions of D974, used for special cases. AN
tests can be broken up into two titration categories: potentiometric or
colorimetric. The potentiometric method uses a potentiometer to detect
the acidic constituents and coverts it to an electronic read out. The
output is plotted and analyzed to determine the inflection of the test
method. The colorimetric method uses paranaphthol-benzene, which
responds to a change in the pH indicator that has been added to the
solution. Once the acidic constituents have been neutralized by the KOH,
the sample will change from orange to blue-green, indicating the end
point.
Table 2. Common ASTM AN Test Methods
ASTM AN Tests
ASTM D664 measures acidic constituents by using a potentiometer
to determine an end point. This method can be used to measure both AN
and SAN. To prepare the sample a mixture of toluene, isopropyl alcohol
and water is dissolved into a sample. Potassium hydroxide is then
titrated into the solution using a burette. The potentiometer output is
monitored while the KOH is titrated into the solution. If the inflection
is indistinguishable, the buffer potential will be considered the AN.
The inflection point is commonly used on new oil; however, for used oils
the inflection may become indistinguishable requiring the use of the
buffer potential as the end point.
ASTM D974 is the measure of acidic constituents using a color
change to indicate the inflection. The sample is dissolved into a
solution of toluene, p-naphtholbenzne, and isopropyl alcohol containing
water. The solution is titrated with KOH while the color is monitored.
This test is used on new oils and oils that are not excessively dark.
ASTM D1534 is similar to ASTM D974 in that they both use a color
change to indicate the end point. ASTM D1534 is designed for electric
insulating oils (transformer oils), where the viscosity will not exceed
24 cSt at 40°C. The standard range of applications is for oils with an
AN between 0.05 mg KOH/g and 0.50 mg KOH/g, which is applicable to the
transformer oils.
ASTM D3339 is also similar to ASTM D974, but is designed for use
on smaller oil samples. ASTM D974 and D664 roughly use a 20 g sample;
ASTM D3339 uses a 2.0 g sample, as shown in Table 2.
Table 3. D974 Repeatability from ASTM Standard
Modified Tests
AN tests are typically conducted to obtain an accurate indication of
additive depletion and possible contamination of ingressed acids. The
standard ASTM methods are time consuming, have relatively poor
reproducibility and utilize hazardous materials. In an effort to control
the source of these issues, many modified versions of the AN test are
currently being used. Each test is specific to its application. For
example, a lab may automate the test to reduce labor and increase
throughput.
For Used Oil Analysis Labs
Laboratories modify tests to improve throughput while decreasing the
use of hazardous materials and their cost. Throughput, or speed, is
important to larger laboratories because it is necessary to find the
fastest test that does not sacrifice quality. Cost also plays a major
role. A standard test slate provided by a lab may also include particle
count, viscosity at 40°C, etc. The cost of this standard test slate
needs to be affordable for the end user; therefore, each individual test
performed may need to be streamlined to ensure both quality and economy
are achieved.
For Field Testing
Field test kits are often used as a first-line AN test. They typically
contain premeasured reagents that allow for convenient field testing.
Some of the field kits use a pass/fail test, which involves adding a
preset amount of KOH to the solution. This indicates whether the AN has
reached a specific point. Field tests can also report actual results.
For example, one such kit uses a volume-sampling syringe to ensure that
the oil samples are the same size. A disposable burette is used to
titrate the KOH. Because the oil sample is a specific size, the burette
has been scaled to indicate the AN. Once the color has changed, the user
only can read the acid number from the burette.
Table 4. D974 Reproducibility from ASTM Standard
Advantages and Disadvantages Repeatability
ASTM defines repeatability as "the difference between successive test
results obtained by the same operator with the same apparatus under
constant operating conditions on identical test material". Based on this
definition, using D664, data was found to be within +/- 7 percent of
the mean 95 percent of the time for fresh oils using the inflection
point method or +/- 12 percent of the mean for used oils with the buffer
end point method. ASTM D974 has the repeatability as stated in Table 3.
For example, a sample that has a 0.15 AN could vary from 0.10 AN to
0.20 AN for ASTM D974 and could vary 0.17 to 0.13 AN for ASTM D664.
Repeatability can be obtained on a modified test. A good lab should be
able to tell how reliable its modified version is. This confirms that
comparing results from one single lab or test procedure is best.
Reproducibility
ASTM's definition of reproducibility is "the difference between two
single independent results obtained by different operators working in
different laboratories on identical test material." Ninety-five percent
of the time, the reproducibility of ASTM D664 is +/- 20 percent of the
mean for fresh oils using the inflection point method or +/- 44 percent
of the mean for used oil using the buffer end point method. For example,
if a mean AN was 0.10 you could expect results from 0.14 to 0.06 95
percent of the time. The reproducibility of ASTM D974 is shown in Table
4. Consider that you received an oil analysis report from multiple labs
on the same oil. It has a mean AN of 0.05, and the results could vary
from 0.09 to 0.01.
It is hard if not impossible to compare results between labs when
modified AN tests are used. Quality labs will likely have a correlation
to the ASTM standard; unfortunately, this would also incorporate more
error. It is best practice to compare only results from the same test
for trending purposes.
According to ASTM, "the AN obtained by this standard (D664) may or may
not be numerically the same as that obtained in accordance with test
methods D974 and D3339." However, the magnitude of the results should be
the same. By trending results from one specific test method, additive
depletion and contamination can be detected.
Figure 2. Variations in AN Trends by Oil Type11
Dos and Don'ts of Comparing Results
Comparing results between samples can become complicated if proper
control is not used. There are many aspects which may and normally will
affect the results from an AN test. As stated previously, there are
multiple test methods used. Some of the methods are within ASTM
standards and some are modified. The average AN result from a laboratory
will likely be from a modified test method.
Dos
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Compare results to historical results on the lubricant (trending).
-
Verify which lab has analyzed the lubricant and the test used.
-
Consistently use the same lab and test method for a specific lubricant.
-
Ensure a representative sample is provided to the lab.
Don'ts
-
Don't switch back and forth between methods.
-
Don't switch back and forth between labs. Don't delay oil analysis;
instead, provide the sample to the lab as soon as possible.
-
Don't compare results between different methods.
Common Trends of AN Trending
In the world of AN tests, there is a current state of disillusionment.
Each laboratory provides results from its own modified test methods,
which forces the end user to rely on precision over accuracy. First, the
user must be wary of comparing results between labs. In an ideal
environment, both accuracy and precision would be provided. In a
next-to-ideal world, only accuracy would be provided. Simple mathematics
could be used to determine the exact value, but in the real world of AN
tests, the precision of each individual labs is what can be counted on.
The results are not on the true mark, but relative to each other they
are good. Comparing results from different labs would result in values
all over the board. By focusing on the precision of one lab or test
procedure, a trend emerges. Trending can enable the end user to properly
evaluate his/her lubricant with greater confidence.
General Trends
Trending results is the best way to work around the accuracy
discrepancies that come from using AN results in machine condition
monitoring. By using results from one specific test or lab, the ability
to trend is good. Figure 2 illustrates the common trends found in
lubricants. Linear trends are for some ester-based synthetics and oils
going through oxidation. It represents the linear oxidation of the base
oil. The parabolic curves may characterize rust and oxidized (R&O)
oils. The AN remains constant during the additive depletion induction
phase. Once the R&O additives have depleted, the base oil will begin
to oxidize. The switching trend is representative of EP oils, where
some of the additives are acidic. As additives deplete and react, the AN
varies. These effects make it hard to trend EP oils unless the normal
switching pathway is known in advance.
AN is an important tool in the oil analysis industry when used
properly. Understanding how the AN is calculated and what variances
exist will help in interpreting the results. SAN is usually not tested,
but it may be useful to an oil analysis program if protection from
corrosion is important or if there is a possibility of contamination
from an inorganic acid. The two commonly used ASTM test methods both
exhibit issues that create the need for modified tests. With the
modified tests currently being used in industry, it is important to
remember why they are in place and the implications when comparing
results. Being able to properly trend results will enable end users to
adequately evaluate their oil condition.
References
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ASTM D664: Standard Test Method for Acid Number of Petroleum Products
by Potentiometric Titration. American Society of Testing and Materials
International, West Conshohocken, Pa.
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ASTM D974: Standard Test Method for Acid Number and Base Number by
Color-Indicator Titration. ASTM Intl., West Conshohocken, Pa.
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ASTM D1534: Standard Test Method for Approximate Acidity in
Electrical Insulating Liquids by Color-Indicator Titration. ASTM Intl.,
West Conshohocken, Pa.
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ASTM D3339: Standard Test Method for Acid Number of Petroleum
Products by Semi-Micro Color Indicator Titration. ASTM Intl., West
Conshohocken, Pa.
-
Finch, Stephen. "Evaluation of New Field Test Methods for Base Number and Acid Number in Lubricating Fluids." Dexsil.
-
Smart, Clifford L. "Get Smart with Improved TAN Titrations." Practicing Oil Analysis magazine. October 2000.
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"Interview Helps Clarify Questions Surrounding AN/BN Test Methods in Used Oil Samples." Practicing Oil Analysis magazine. May 2003.
-
Kauffman, R.E. "Rapid Determination of Remaining Useful Lubricant Life." Handbook of Lubrication and Tribology, Volume III. E. Richard Booser, Editor. CRC Press, Boca Raton, Fla. 1994.
-
Snook, Willet A. "Used Engine Oil Analysis." Lubrication, Volume 54, Number 9, 1968.
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Ball, Peter G. "New pH Test Offers Benefits over TAN/TBN." Practicing Oil Analysis magazine. September 1998.
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Oil Analysis Level I Course Manual, Noria Corporation. 2006.
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