titrated with base up to this point to yield a
strong acid number. It may also be titrated up to
11 (approximately) to yield a total acid number.
Similarly, a sample whose initial pH reading lies
above 11 (approximately) can be titrated with
acid down to this value to yield a strong base
number, and it can be titrated down to 4
(approximately) to yield a total base number.
strong acid number. It may also be titrated up to
11 (approximately) to yield a total acid number.
Similarly, a sample whose initial pH reading lies
above 11 (approximately) can be titrated with
acid down to this value to yield a strong base
number, and it can be titrated down to 4
(approximately) to yield a total base number.
End Points: Titration end points are not at fixed
pH readings but at inflections that occur in the
curve: reagent versus pH. Whether or not an
end point represents a strictly neutral condition
is of little significance. With test procedure
carefully standardized, the results obtained in
reaching an end point can be compared on an
equal basis with other results obtained in the
same way. A result reported simply as "neut
number," moreover, may be assumed to be a
total acid number. Although it is not provided for
by ASTM procedure, the Initial pH reading may
also be reported.
pH readings but at inflections that occur in the
curve: reagent versus pH. Whether or not an
end point represents a strictly neutral condition
is of little significance. With test procedure
carefully standardized, the results obtained in
reaching an end point can be compared on an
equal basis with other results obtained in the
same way. A result reported simply as "neut
number," moreover, may be assumed to be a
total acid number. Although it is not provided for
by ASTM procedure, the Initial pH reading may
also be reported.
Colorimetric Method: Under the colorimetric
method for determining neut number (ASTM D
974), end point is identified by the change of a
color indicator. This indicator exhibits one color
above a specified pH value, another below. By
this means, a total acid or strong base number
can be determined with a p-naph-tholbenzene
indicator, while a strong acid number can be
determined with a methyl orange indicator.
Obviously, however, this method is not suitable
for the investigation of dark-colored liquids.
method for determining neut number (ASTM D
974), end point is identified by the change of a
color indicator. This indicator exhibits one color
above a specified pH value, another below. By
this means, a total acid or strong base number
can be determined with a p-naph-tholbenzene
indicator, while a strong acid number can be
determined with a methyl orange indicator.
Obviously, however, this method is not suitable
for the investigation of dark-colored liquids.
Reporting the Results: Whatever the method,
all acid numbers are expressed in milligrams of
potassium hydroxide (KOH), a base, required to
"neutralize" a gram of sample. For reasons of
uniformity, base numbers, which are obtained by
titrating with hydrochloric acid (HCI), are
expressed in the same units, the HCI being
converted to the number of KOH units that it
would neutralize.
all acid numbers are expressed in milligrams of
potassium hydroxide (KOH), a base, required to
"neutralize" a gram of sample. For reasons of
uniformity, base numbers, which are obtained by
titrating with hydrochloric acid (HCI), are
expressed in the same units, the HCI being
converted to the number of KOH units that it
would neutralize.
Significance of Test Results: Because acidity
is associated with corrosiveness, there has been
a tendency to attribute undesirable
is associated with corrosiveness, there has been
a tendency to attribute undesirable
properties to an oil that exhibits a high acid
number or a low pH reading. This attitude is
fostered by the fact that deterioration of an oil In
service oxidation, is ordinarily accompanied by
an increase in acid test results. While this
attitude is not in actual disagreement with fact,
its oversimplification may be conducive to
harmful misconceptions.
number or a low pH reading. This attitude is
fostered by the fact that deterioration of an oil In
service oxidation, is ordinarily accompanied by
an increase in acid test results. While this
attitude is not in actual disagreement with fact,
its oversimplification may be conducive to
harmful misconceptions.
In the first place, petroleum oil is not an
aqueous solution, and conventional
interpretations of acidity and alkalinity do not
apply. In the second place, the test results, while
involving certain acid or alkaline implications, do
not distinguish between those that are
undesirable and those that are not. The ASTM
Standards themselves include the statement
that the test "method is not Intended to measure
an absolute acidic or basic property that can be
used to predict performance of an oil under
service conditions. No general relationship
between bearing corrosion and acid or base
number is known."
aqueous solution, and conventional
interpretations of acidity and alkalinity do not
apply. In the second place, the test results, while
involving certain acid or alkaline implications, do
not distinguish between those that are
undesirable and those that are not. The ASTM
Standards themselves include the statement
that the test "method is not Intended to measure
an absolute acidic or basic property that can be
used to predict performance of an oil under
service conditions. No general relationship
between bearing corrosion and acid or base
number is known."
This is not to say, however, that neut number or
pH reading are without significance. They are
applied widely and effectively to turbine oils,
insulating oils, and many other oils in critical
service. With new oils, neutralization test results
provide a useful check on consistency of
product quality. With used oils, they may serve
as a guide to mechanical condition, change In
operating conditions, or product deterioration. A
rise in acid number and/or a drop in base
number or pH reading are generally indicative of
increasing oxidation. They may also be related
to depletion of an additive, many of which are
alkaline.
pH reading are without significance. They are
applied widely and effectively to turbine oils,
insulating oils, and many other oils in critical
service. With new oils, neutralization test results
provide a useful check on consistency of
product quality. With used oils, they may serve
as a guide to mechanical condition, change In
operating conditions, or product deterioration. A
rise in acid number and/or a drop in base
number or pH reading are generally indicative of
increasing oxidation. They may also be related
to depletion of an additive, many of which are
alkaline.
It is impossible, however, to generalize about
the limits to which the neutralization values of
an oil in service may safely be allowed to go.
Each combination of oil, machine, and type of
service follows a pattern of its own. Only
through experience with a particular set of con-
ditions can it be determined at what
neutralization value an oil is no longer suitable
for service.
the limits to which the neutralization values of
an oil in service may safely be allowed to go.
Each combination of oil, machine, and type of
service follows a pattern of its own. Only
through experience with a particular set of con-
ditions can it be determined at what
neutralization value an oil is no longer suitable
for service.
A-11 (FIST 2-4 11/90)