Calculations
52
7
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 vessel washings (mL).
• Nitric acid is that portion of the total acid in the
vessel washings that result when the nitrogen
in the air that is trapped in the vessel is burned
at high pressure. Since this nitric acid does not
result from the sample, and the combustion
conditions 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
milliequivalents of acid. This value is normally
the concentration (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 cal/mEq.
• Heat of formation of sulfuric acid (from SO
2
) is
36.1 cal/mEq.
• Sample mass is the mass of sample burned in
the oxygen combustion vessel (grams).
• Sulfur multiplier is multiplied by the product of
the user entered sulfur value and the sample
mass to arrive at the number of milliequiva-
lents of sulfuric acid in the vessel washings.
Sulfur Correction:
e
2
= (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.
e
1
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:
e
1
= (nitric acid value)(acid multiplier)(heat of
formation of nitric acid)
For an 1138 Oxygen Combustion Vessel the default
nitric acid value is 8 and acid multiplier is .0709.
The heat of formation of nitric acid is 14.1 cal/mEq
so the calculation is:
e
1
= (8)(.0709)(14.1) or e
1
= 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 vessel washings
(in milliliters). This includes both nitric and sulfuric
acid. The correction is a fixed value set by the
operator.
The calculation is:
e
1
= [((total acid)(acid multiplier)) – (% sulfur)
(sample mass)(sulfur multiplier)](heat of
formation of nitric acid)
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