Parr Instrument 1356 - Manual (Page 10)

10
nitrogen that was present as part of the air trapped in the bomb is burned to nitric oxide
which combines with water vapor to form nitric acid. All of this heat is artificial since it
is not a result of the sample burning.
Sulfur correction. In the oxygen rich atmosphere within the bomb, sulfur in the sample is
oxidized to sulfur trioxide, which combines with water vapor to form sulfuric acid. This
liberates additional heat over the normal combustion process, which converts sulfur to
sulfur dioxide. The sulfur correction removes this excess heat from the calculations.
Fuse wire correction. The wire used for a fuse to ignite the sample is partially 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 the wire that is actually burned. It
is normally assumed that the heat generated by the electrical resistance will be the same
when standardizing the bomb and when testing an unknown sample, and can therefore be
ignored. Significant variances can, however, occur in the amounts of fuse wire actually
burned in each test. So this energy is subtracted to account for the heat of combustion of
the metal.
N. Thermochemical Calculation Details
Traditionally, standard solutions and procedures have been established to simplify the
calculations and thermochemical calculations.
ACID and SULFUR Corrections. In certain ASTM methods, the amount of sodium
carbonate used to titrate the bomb washings is equated with e1.
Users may find it convenient to enter a fixed value for the acid correction and avoid the
need to determine this correction for each test. To enter Fixed Acid Corrections,go to
Menu page 5 and turn on line 1 Fixed Fuse for Standardization or line 4 for
Determination. Total errors of more than 3 calories will seldom occur when using Fixed
Acid Corrections.
Fixed Sulfur Corrections can be entered if a series of samples contain a constant amount
of sulfur. For Standizations, line 3 is always ON. For determination runs to enter Fixed
Sulfur Corrections, turn on Line 6 and enter the estimated value.
O. Spiking Samples
It is sometimes necessary to add a spiking material to samples, which are very small,
have a low heat of combustion, or have a high moisture content to add sufficient heat to
drive the combustion to completion. Benzoic acid is an excellent material for spiking for
all of the same reasons that it is a good standard material. White oil is also an excellent
material; particularly for liquid samples. The 1356 Calorimeter can automatically
compensate for the addition of spiking materials to these samples.
When Use Spiking is turned on, on Menu page 2.3 and line 1, the heat of combustion of
the spiking material must be entered on Menu page 2.3, line 2.
During data entry when using a spike, the calorimeter will prompt for both the sample
weight and the spike weight.