Parr Instrument 6300 - Manual (Page 74)

B
C
ALCULATIONS
P a r r I n s t r u m e n t C o m p a n y 74
A
CID AND
S
ULFUR
C
ORRECTIONS
(C
ONTINUED
)
Fixed Total: The Acid Correction represents the
total base required to titrate the bomb washings (in
milliliters). This includes both nitric and sulfuric acid.
The correction is a fixed value set by the operator.
The calculation is:
e1 = [((total acid)(acid multiplier)) – (% sulfur)(sample
mass)(sulfur multiplier)](heat of formation of nitric
acid)
Using the default acid and sulfur multipliers as well
as a heat of formation of nitric acid of 14.1 cal/
milliequivalent a 1 gram sample with 25 ml of washings
and 2 % sulfur would result in the following calculation:
e1 = [((25)(.0709)) – (2)(1)(.6238)] 14.1
e1 = [(1.7725) – (1.2476)] 14.1
e1 = [.5249] 14.1
e1 = 7.40109 calories
When the Acid Correction is set to Fixed Total the
value is considered a final value and the operator is not
prompted for an acid value when reporting the results.
Entered Total: The Acid Correction represents the
total base required to titrate the bomb washings (in
milliliters). This includes both nitric and sulfuric
acid. The correction is entered by the operator when
reporting the results.
The calculation is the same as the Fixed Total above.
The value listed on the Acid Correction button is used
for preliminary calculations. When finalizing the report
the operator will be prompted for the acid value.
Calculated HNO3: In ASTM D5865 there are
provisions for calculating the nitric acid contribution.
For test samples that contain no nitrogen, the quantity
of nitric acid formed during the combustion process is a
function of the volume of the bomb, the oxygen filling
pressure, and the quantity of energy released.
For the calculated nitric acid method:
e1 = (nitric acid factor/1000)(Energy Equivalent)
(corrected temperature rise)
Example: For a test run with energy equivalent of
927.4022 and a corrected temperature rise of 6.892
would result:
e1 = (1.58/1000)(927.4022)(6.892)
e1 = 10.0988 calories
The calculated nitric acid method can be applied
to samples containing up to 2% nitrogen without
introducing a significant error in the resulting heat of
combustion value.
Table B-1
Settings for ISO & BSI Methods
Page Line Setting Value
Thermochemical
Corrections
Acid Correction
(STD)
Entered
HNO
3
13
Fixed Sulfur STD Off 7
Acid Correction
(DET)
Entered
HNO
3
13
Fixed Sulfur DET Off 7
Calculations
Factors
Acid Multiplier 0.154
Sulfur Value is
Percent
Off
Sulfur Multiplier 0.1
Use Offset
Correction
On
Offset Value -43.5
Offset Value -43.5
ASTM T
REATMENT FOR
A
CID AND
S
ULFUR
In the ASTM treatment, the correction for acid
formation assumes that all the acid titrated is nitric
acid. Obviously, if sulfur is present in the sample, which
in turn produces sulfuric acid, part of the correction
for the sulfuric acid formed is already included in the
ASTM nitric acid correction (e
1
). This is adjusted by a
separate computation based upon the sulfur content of
the sample. An additional correction of 1.37 kcal must
be applied for each gram of sulfur converted to sulfuric
from sulfur dioxide. This is based upon the heat of
formation of sulfuric acid, from sulfur dioxide, under
bomb conditions, which is -72.2 kcal per mole or -36.1
calories per milliequivalent. But remember, a correction
of 14.1 calories per milliequivalent of sulfuric acid is
already included in the ASTM nitric acid correction
(e
1
). Therefore the additional correction which must be
applied for sulfur will be the difference between 36.1
and 14.1 or 22.0 calories per milliequivalent (44.0 kcal
per mole). For convenience, this is expressed, in the
ASTM e
2
formula, as 13.7 calories (44.0/32.06) for each
percentage point of sulfur per gram of sample.