Parr Instrument 6400 - Manual (Page 56)

7
Calculations
P a r r I n s t r u m e n t C o m p a n y 54
Thermochemical Calculation Details
Traditionally, standard solutions and procedures
have been established to simplify the calculations
related to the thermochemical corrections. The 6400
Calorimeter has been programmed to permit the
user to use standard solutions and units which are
most convenient, since the microprocessor can
easily apply any conversion factors required.
Acid and Sulfur Corrections
• Total acid is the amount of base required to
titrate the bomb washings (milliliters).
• Nitric acid is that portion of the total acid in the
bomb washings that result when the nitrogen
in the air that is trapped in the bomb is burned
at high pressure. Since this nitric acid does not
result from the sample, and the combustion con-
ditions are reasonably constant from test to test,
the amount of nitric acid formed is also constant.
• Acid multiplier is multiplied by the user entered
acid value to arrive at the number of milliequiva-
lents of acid. This value is normally the concen-
tration (normality) of the base in equivalents per
liter (N).
• Percent sulfur is the concentration of sulfur in
the sample (weight %).
• Molecular weight of sulfur is 32.06.
• Equivalent weight of sulfur in H
2
SO
4
is 16.03
(one half of the molecular weight).
• Heat of formation of nitric acid is 14.1 calories/
milliequivalent.
• Heat of formation of sulfuric acid (from SO
2
) is
36.1 calories/milliequivalent.
• Sample mass is the mass of sample burned in
the bomb (grams).
• Sulfur multiplier is multiplied by the product of
the user entered sulfur value and the sample
mass to arrive at the number of milliequivalents
of sulfuric acid in the bomb washings.
Sulfur Correction:
e2 = (percent sulfur)(sample mass)(sulfur multi-
plier)(heat of formation of H
2
SO
4
).
Acid Correction:
In the 6400 there are a number of settings for the
acid correction.
e1 is the nitric acid portion of the correction.
Fixed HNO
3
: The Acid Correction is a fixed value set
by the operator.
The calculation is:
e1 = (nitric acid value)(acid multiplier)(heat of
formation of nitric acid)
For an 1138 bomb the default nitric acid value is 8
and acid multiplier is .0709. The heat of formation
of nitric acid is 14.1 calories/milliequivalent so the
calculation is:
e1 = (8)(.0709)(14.1) or e1 = 7.9975 (rounds to 8)
When the Acid Correction is set to Fixed HNO3 the
value is considered a final value and the operator is
not prompted for an acid value when reporting the
results.
Entered HNO
3
: The Acid Correction is entered by the
operator when reporting the results.
The calculation is the same as Fixed HNO
3
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.
Fixed Total: The Acid Correction represents the total
base required to titrate the bomb washings (in mil-
liliters). 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 forma-
tion of nitric acid)