Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

DETERMINATION OF HALOGEN, SULPHUR, ETC. 69
of a Bunsen burner, allow to cool (on the copper block), and weigh out
the substance for the next analysis.
Immediately after the weight of the absorption vessels has been
finally determined, do not forget to place on the pan the number of
centigramme weights equivalent to the increase in weight and, after
moving the rider to the appropriate notch at the beginning of the scale,
determine the new tare.
Calculation.—The percentages of carbon and hydrogen can be
calculated by means of the following formulae :
0/ p_C02 found 300
'° ~ substance 11 '
H2O found
'° substance
Use Kiister's logarithm tables. The limits of error are : for
carbon ± 0-3 per cent, for hydrogen -(-0-2 and - 0-1 per cent. Good
analyses give about 0-1 per cent C too little and 0-1 per cent H too
much.
III. DETERMINATION OF HALOGEN, SULPHUR, AND
OTHER ELEMENTS
If a substance contains yet other elements besides carbon,
hydrogen, oxygen, and nitrogen, the former are either determined
by oxidising the substance in a sealed tube by heating with red
fuming nitric acid (Carius) or by burning on a platinum catalyst
in a current of oxygen (Dennstedt). Halogens are determined as
silver halide, iodine preferably as iodic acid, sulphur as barium
sulphate, phosphorus as magnesium pyrophosphate.
1. DETERMINATION OF HALOGEN BY THE CAEIUS METHOD
For the determination the following apparatus and chemicals
are required :
Hard glass (Jena) tubes for sealing (length 35 cm., internal diameter
2-5 cm., wall thickness 1-2 mm.). 1
Red fuming nitric acid (d 1-5).
Solid silver nitrate.
Small filter tube with Jena glass fritted filter plate (13 f. G 2).
Halogen-free alcohol and dilute nitric acid (1:100).
Preparing the Carius Tube.—First clean the tube with bichro-
1 The tubes can be used repeatedly (three or four times).