Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

54 THE DUMAS METHOD
bustion for five to ten minutes longer, moving the burner and the roll
as required. After that remove the burner and, five minutes later
switch off the current in the furnace. Avoid heating the combustion
tube with the burner alone for long periods since such treatment
causes the glass to soften and infallibly to blow out. When heating
has been stopped increase the rate of passage of gas to two bubbles
per second.
As soon as micro-bubbles only are being produced in the nitrometer
close the stop-cock on the connecting tube, detach the rubber
connecting tube from the combustion tube, close the latter with a
cap made from rubber tubing, and leave to cool whilst maintaining
the internal carbon dioxide pressure. Remove the nitrometer to a
somewhat cooler room (room in which the barometer is kept preferably)
after raising the bulb so that the surfaces of the two columns
of liquid are at the same level.
After ten minutes take the reading by holding the bulb behind
the graduated tube with the meniscus in the bulb accurately in the
same plane as the meniscus in the tube. Read the scale division
which is in the same horizontal plane as the lower edge of the
meniscus. Also note the temperature (thermometer in the part of
the nitrometer above the stop-cock) and the barometric pressure.
Calculation.—The percentage of nitrogen is given by the following
formula :
in which v is the volume of nitrogen read, a = ^=0-003663,
2i to
b is the barometric height, S is the correction of the barometric
height to 0°, e is the vapour pressure of potassium hydroxide at t 0 , 1
and s is the amount of substance taken.
Limits of error of the determination : 0-3 per cent high, 0-1 per
cent low.
1 The values of the expression between brackets are given for various values of
b— 8 — e and t in the table on pp. 424, 425. With sufficient accuracy S may be replaced
by Q and e by •=. For example : readings, 6 = 738 mm., t = 20° ; look up in
the table the value for p =738- 2-5- 4 = 731-5. When calculating logarithmically
use Kuster"s tables.