Parr Instrument 6725 - Manual (Page 55)

6725
Semi-micro Calorimeter Instruction Manual
6-7
ASTM, ISO and Other Methods
Current ASTM, ISO, and British Standard Methods differ on their treatment of the nitric
and sulfuric acid thermochemical corrections. ASTM Methods call for titrating the bomb
washings to determine the total acid present. This is assumed to be all nitric acid with a
heat of combustion of -14.1 Kcal per mole. The amount of sulfur is then determined and
converted to equivalents of sulfuric acid. The difference between the heat of formation of
sulfuric acid (-72.2 Kcal per mole or -36.1 calories per milliequivalent) and nitric acid is
then subtracted as the sulfur correction.
Most other test methods treat nitric and sulfuric acid corrections as entirely separate
values instead of combined values. This eliminates the requirement for a total acid
determination and permits the nitric acid correction to be handled in a variety of ways,
including the assumption of a fixed nitric acid correction.
The 6772 Calorimetric Thermometer can be set up to apply the acid correction by
either the ASTM or ISO convention, as the user prefers. Care must be used to
ensure the proper corrections are applied, and the calculations made are consistent
with the procedure used. See Table 2.
ASTM
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.
ISO
Both the ISO 1928 and BSI 1016: Part 5 methods for testing the calorific value of coal
and coke, deal with acid and sulfur corrections in a manner which is somewhat different
than ASTM procedures. The analysis of bomb washings in these methods call for a
titration, first using 0.1N barium hydroxide (V2) followed by filtering, and a second
titration using 0.1N HCL(V1) after 20 ml of a 0.1N sodium carbonate has been added to
the filtrate. Table 2 gives the settings which allow the results of the two titrations, V1 and
V2, to be entered into the controller directly for the calculation of the total acid correction.
V1 should be entered at the prompt for acid and V2 is entered at the prompt for sulfur.
The settings in Table 2 assume that the same procedure is carried out for both
standardization and determination. The offset value is the product of -1, the Heat of
Formation of Nitric Acid, the acid multiplier, and the 20 ml of 0.1 N sodium carbonate
used in the analysis. The formula used to get the total correction in calories is as
follows:
V1 (Acid Multiplier) (Heat of Formation of Nitric Acid) V2 (Sulfur Multiplier) (Heat of Formation of Sulfuric Acid) + offset value