II.
~Determination of Organic Carbon in a Limestone.~--Take 1 or 2 grams and
dissolve with a very slight excess of dilute hydrochloric acid,
evaporate to dryness, and determine the carbon in the residue by
combustion with copper oxide.
~Estimation of Carbon in a Sample of Graphite (Black-lead).~--Weigh up 1
or 2 grams in a dish and calcine in the muffle till the carbon is burnt
off. Weigh the residue, and calculate the carbon by difference.
[Illustration: FIG. 76.]
[Illustration: FIG. 77.]
[Illustration: FIG. 78.]
~Determination of Carbon in Iron.~--The carbon exists in two
states--free (graphite) and combined. The following process estimates
the total carbon:--The carbon existing as graphite may be separately
estimated in another portion by the same process, but using hydrochloric
acid to dissolve the iron instead of the copper solution:--Weigh up 2
grams of the iron (or a larger quantity if very poor in carbon), and
attack it with 30 grams of ammonic-cupric chloride[119] dissolved in 100
c.c. of water. Let the reaction proceed for a quarter-of-an-hour, and
then warm until the copper is dissolved. Allow to settle, and filter
through a filtering-tube. This is a piece of combustion tube drawn out
and narrowed at one end, as shown in fig. 76. The narrow part is blocked
with a pea of baked clay, and on this is placed half-an-inch of silica
sand (previously calcined to remove organic matter), then a small plug
of asbestos, and then a quarter-of-an-inch of sand.
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