parr 6400 - ASTM and ISO Methods Differ

Calculations
6400
7
51
By default a fixed fuse correction of 50 cal is ap-
plied to all tests. Total errors of more than 3 cal will
seldom occur when using a fixed fuse correction
and the thread supplied by ynapmoC .
When using the 6400, there are two components to
the fuse correction:
• The heat introduced by heating the wire used
to ignite the cotton thread.
• The heat of combustion of the cotton thread
used to ignite the sample.
The semi-permanent heating wire is heated by dis-
sipating an electrical charge from a capacitor. Since
this charge is controlled by the size of the capacitor
and the charging voltage, and because the capacitor
is fully discharged for each test, the energy released
can be calculated. In the 6400 Calorimeter this is a
fixed correction of 10 cal per test.
Cotton has a heat of combustion of 4 kcal/g. The
actual thread being used should be weighed to
see how much is being burned. 10 cm (4”) of a fine
thread will weigh approximately 0.003 g which
would release 12 cal as it burns. Heavier threads
weigh up to 0.010 g per 10 cm and increase this
correction to 40 cal per test. The finer the thread,
the smaller errors will be if the thread is not ex-
actly ten centimeters in length. Polyester thread
is not recommended for use in the combustion
vessel because it has a tendency to melt and fall
away from the heating wire before it ignites.
Using the fine thread mentioned above, the fuse cor-
rection for the calorimeter would be the 10 cal from
electrical heating plus 12 cal from the burning thread
for a total of 22 cal per test. The thread supplied by
ynapmoC has a mass of approximately 1 mg/cm. This
results in a total fuse correction of 50 cal.
ASTM and ISO Methods Differ
Current ASTM, ISO, and British Standard Methods
differ on their treatment of the nitric and sulfuric
acid thermochemical corrections. ASTM Methods
call for titrating the vessel washings to determine
the total acid present. This is assumed to be all nitric
acid with a heat of combustion of -14.1 kcal/mol. The
amount of sulfur is then determined and converted to
equivalents of sulfuric acid. The difference between
the heat of formation of sulfuric acid (-72.2kcal/mol
or -36.1cal/mEq [where mEq is milliequivalent]) and
nitric acid is then subtracted as the sulfur correction.
Most other test methods treat nitric and sulfuric acid
corrections as entirely separate values instead of
combined values. This eliminates the requirement for
a total acid determination and permits the nitric acid
correction to be handled in a variety of ways, includ-
ing the assumption of a fixed nitric acid correction.
The 6400 Calorimeter can be set up to apply the acid
correction by either the ASTM or ISO convention, as
the user prefers. Care must be used to ensure the
proper corrections are applied, and the calculations
made are consistent with the procedure used.
Users may find it convenient to enter a fixed value
for the acid correction and avoid the need to de-
termine this correction for each test. Use of a fixed
value for the acid correction is highly recommend-
ed. Fixed acid corrections can be entered when
Acid Correction - Thermochemical Corrections is
set to Fixed HNO
3
. A correction of 10 calories is a
good number for the fixed nitric acid value. Total
errors of more than 3 calories will seldom occur
when using fixed nitric acid corrections.
Fixed sulfur corrections can be entered if a series of
samples contain a constant amount of sulfur. Fixed
sulfur corrections can be entered when Fixed Sulfur
- Thermochemical Corrections, is set to ON and then
enter percent sulfur as indicated on this line. Any
errors will be proportional to the difference between
the actual and assumed value for sulfur.
For ordinary work where benzoic acid is used, for
standardizing the calorimeter, the Fixed Sulfur
Correction, for Standardizations should be ON
applying a fixed value of 0.0 to all standardization
tests. Benzoic acid contains no sulfur.
Please note that the values entered into the test
report appear as entered in the report. Values for
e
1
, e
2
and e
3
are calculated and used as energy
corrections in accordance with the formulas and
settings given above. The formulas used above to
arrive at e
1
or e
2
are not the same as the formulas
used for e
1
and e
2
which appear in most ASTM
bomb calorimetric procedures. However, the sum
of e
1
and e
2
, above, is equal to the sum of the
ASTM treatment of e
1
and e
2
.
Note: Please review the following section on Acid
and Sulfur Corrections. Different standard test
methods use different values for the heat of forma-
tion of sulfuric acid. These differences are generally
insignificant. The 6400 Calorimeter uses the most
recent, published values for all thermochemical data.