Parr Instrument 1341 - Manual (Page 11)

1341 Plain Jacket Calorimeter
w w w . p a r r i n s t . c o m 11
Discussion of Calculations
Acid Correction
Since combustion in the bomb takes place in an
atmosphere of nearly pure oxygen at high tem-
perature and pressure, several reactions take place
which would not occur in burning the same material
under normal atmospheric conditions. These side
reactions are important because they generate an
appreciable amount of heat which cannot be cred-
ited to the sample, and for which a correction must
be made.
For example, in the normal combustion of coal, all
sulfur is oxidized and liberated as SO
2
but nitrogen
in the material usually is not affected. Likewise, no
change occurs in the nitrogen of the air required
for normal combustion. But, when the same coal is
burned in the oxygen combustion vessel, oxidation
of the sulfur is carried further to form SO
3
which
combines with water vapor to form H
2
SO
4
; and
some of the nitrogen in the bomb is also oxidized
and combined with water vapor to form HNO
3
.
These two side reactions result in acids within the
bomb, and require a correction to account for the
heat liberated in their formation.
In computing the correction for acid formation it
is assumed that all of the acid titrated is nitric acid
(HNO
3
), and that the heat of formation of 0.1N HNO
3
under bomb conditions is -14.1 Kcal per mol. Obvi-
ously, if sulfuric acid is also present, part of the
correction for H
2
SO
4
is included in the nitric acid
correction. The sulfur correction described below
takes care of the difference between the heats of
formation of nitric and sulfuric acids.
Sulfur Correction
A correction of 1.4 Kcal must be applied for each
gram of sulfur converted to sulfuric acid. This is
based upon the heat of formation of 0.17N H
2
SO
4
which is -72.2 Kcal per mol. But a correction of 2 X
14.1 Kcal per mol of sulfur is included in the nitric
acid correction. Therefore the additional correction
which must be applied for sulfur will be 72.2-(2 X
14.1) or 44.0 Kcal per mol, or 1.37 Kcal per gram of
sulfur. For convenience, this is expressed as 13.7
calories for each percentage point of sulfur per gram
of sample.
Fuse Wire Correction
The wire used as a fuse for igniting the sample is
partly consumed in the combustion. Thus the fuse
generates heat both by the resistance it offers to the
electric firing current, and by the heat of combustion
of that portion of the wire which is burned. It can be
assumed that the heat input from the electric firing
current will be the same when standardizing the
calorimeter as when testing an unknown sample,
and this small amount of energy therefore requires
no correction. However, it will be found that the
amount of wire consumed will vary from test to test,
therefore a correction must be made to account for
the heat of combustion of the metal.
The amount of wire taking part in the combustion is
determined by subtracting the length of the recov-
ered unburned portion from the original length of 10
cm. The correction is then computed for the burned
portion by assuming a heat of combustion of 2.3
calories per cm. for Parr 45C10 (No. 34 B & S gage
“Chromel C”) wire, or 2.7 calories per cm for No. 34
B & S gage iron wire.
Radiation Correction
The method recommended for calculating the
correction for heat gain or loss from a plain jacket
calorimeter is that specified by the American Society
for Testing and Materials as published under ASTM
Designations D240 and D3286. It is based upon the
work of Dr. H.C. Dickinson at the National Bureau of
Standards who showed that the amount of heat leak
during a test could be approximated by assuming
that the calorimeter is heated by its surroundings
during the first 63 percent of the temperature
rise at a rate equal to that measured during the
5-minute preperiod. The method then assumes that
the cooling (or heating) rate during the remaining
37 per cent of the rise is the same as the rate
observed during the 5-minute postperiod. For most
experimental work the dividing point between these
two periods is taken as that point in time, b, when
the temperature has reached six-tenths (instead
of 63%) of the total rise. Note that these two time
intervals must be expressed in minutes and decimal
fractions (Example: 1.4 min. and 6.6 min.).