IKA C 7000 Grundausstattung Set 2 - Manual (Page 25)

IKA  -WERKE C 7000 Ver. 07 09.07
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Almost all substances that are tested contain some sulphur and nitrogen. Under the
conditions employed for calorimetric measurements, sulphur and nitrogen burn to
SO
2
, SO
3
and NO
x
. In combination with water from combustion and moisture from
the sample, sulphuric acid and nitric acid are formed and heat of solution is devel-
oped. The influence of the heat of solution must be taken into account when calcu-
lating the standard calorific value.
To obtain a defined end condition, and to quantitatively record all acids, ca. 5 ml of
distilled water or other suitable absorption liquid are added to the decomposition
vessel. The gases resulting from combustion form acids with this absorption liquid
and any water resulting from combustion. After combustion, the decomposition ves-
sel is rinsed thoroughly with distilled water, to collect any condensate that has set-
tled on the walls of the vessel. The acid content of this solution can be determined
using suitable equipment.
You can obtain details of suitable equipment for the purpose from IKA  or from your
authorised dealer.
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To determine the gross calorific value correctly, it is of fundamental importance that
the sample has burnt completely. After a test, the crucible and all solid residues
must be examined for signs of incomplete combustion.
Normally, solids can be burnt directly in powder form. Substances that burn very fast
(e.g. benzoic acid) must not be burnt in a loose form. Such substances tend to ex-
plosive combustion, and complete combustion cannot then be guaranteed. In addi-
tion, the decomposition vessel could be damaged. These substances must be
pressed to form tablets using a special press (C 21 pelleting press, see Accesso-
ries) before testing.
Substances with low flammability (substances with a high mineral content or a low
calorific value) can often only be burnt completely with the aid of combustion cap-
sules or combustion bags (C 10/C 12, see Accessories). The use of liquids to aid
combustion, for example paraffin, is also possible.
Most liquids can be weighed directly in the crucible, but highly volatile substances
should be filled into combustion capsules (gelatine capsules or acetobutyrate cap-
sules, see Accessories) and burnt together with the capsules.
All combustion and ignition aids (e.g. cotton threads) must be burnt completely. If
anything remains unburnt, the test must be repeated.
Halogen-containing substances can cause corrosion of the decomposition vessel.
For such cases, decomposition vessel C 7012 should be used.
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