Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

64 CAEEYING OUT THE COMBUSTION
poise) and 10 minutes after wiping the tube determine its approximate
weight by adding centigramme weights until the rider
can be set as near the beginning of the scale as possible. During
the fifteenth minute determine the weight accurately. Directly
thereafter determine the weight of the calcium chloride tube.
When the combustion has been completed weigh the absorption
vessels after exactly similar treatment and exactly the same time.
When the weight has been exactly determined in the fifteenth minute
rapidly put on the number of centigramme weights .corresponding
to the increase in weight, move the rider to the corresponding notch
at the beginning of the scale and again check the swing, which will
now be somewhat different owing to the deviation of the weights
from the reading given by the rider. By proceeding in this way
one can avoid using additional weights, which would first have to
be standardised, and can weigh by moving the rider alone after the
next combustion.
Weigh the substance in a small open platinum boat; weigh hygroscopic
substances in a small weighing bottle.
To clean the boat boil it in a test tube with dilute nitric acid,
attach it to a platinum wire, and ignite it for a short time in the
(non-luminous) Bunsen flame. Then set it to cool for about twenty
seconds on a nickelled copper block.
With a pair of forceps place the empty boat on the pan of the
balance and carefully weigh accurately to 0-01 mg. Set the boat
on a clean piece of paper, put in the substance, brush the outside
clean with a fine hair brush, and determine the increase in weight
with an accuracy of 0-01 mg. For C and H determinations weigh
20-30 mg. After weighing, set the boat again on the copper block
and cover with a small glass dish.
THE COMBUSTION
Switch on the current in the electric furnace while the absorption
apparatus is cooling, at the same time passing air through the tube
at the rate employed during analyses. 1 If the tube has not previ-
1 When heating begins it will be observed that there is a distinct decrease in the
rate of flow of bubbles in the counter although the adjustment of the pressure
regulator remains unaltered. This decrease occurs because the brake-pad prevents
immediate attainment of equilibrium following the rise in pressure caused by the
increase in temperature. Thus every increase of pressure in the tube causes a
diminution of the rate of flow of bubbles and hence greatly facilitates accurate
control of the process of combustion during the analysis.