Page 156 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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398 Elemental Analysis, Organic Compounds

be achieved by passing the gas through copper oxide at
600 C,
◦
CO + CuO → CO 2 + Cu, (13)
◦
or through anhydroiodic acid at 130 C,
CO + I 2 O 5 → CO 2 + I 2 . (14)

Several finishing modes are applicable. The carbon
dioxide can be retained in an absorption tube packed with
Ascarite (sodium hydroxide mixed with asbestos) and
weighed, or absorbed in a solution and titrated. The iodine
produced in Eq. (14) can also be determined by titration.
Alternatively, the iodine vapor can be led by a stream
of nitrogen into an electrolysis cell, where iodine is re-
duced at controlled potential and the amount of electricity
isrecorded.Figure6shows,fromrighttoleft,thecomplete
equipment for oxygen determination, which comprises the
combustion train, the furnace for the oxidation of carbon
monoxide, and the assembly for electrometric finish.
The PE 2400 CHN Analyzer (Fig. 4) can be modified
to perform automatic determination of oxygen. The com-
bustion tube is filled with platinized carbon reagent. The
samples are pyrolyzed in an inert atmosphere of argon or
helium. The reaction product CO is separated by frontal
FIGURE 5 Simple micro-Kjeldahl distillation apparatus. chromatography and measured by thermal conductivity. In
[Reprinted with permission from Cheronis, N. D., and Ma, the Leco CHN Analyzer, carbon monoxide is converted
T. S. (1964). “Organic Functional Group Analysis.” Wiley, New to carbon dioxide to be measured by infrared absorption.
York Copyright 1964 Wiley.]
Neither CHN method is suitable for the analysis of organic
substances which contain fluorine, phosphorus, silicon, or
considerably. For biological materials, 10 mL of concen- most metallic elements.
trated sulfuric acid per gram sample is recommeneded,
followed by 6–12 mL of 30% hydrogen peroxide. The
digestion time is about 20 min. IV. DETERMINATION OF SULFUR

Sulfur in organic compounds is usually determined in the
III. DETERMINATION OF OXYGEN form of sulfate. When organic material is heated in a large
excess of oxygen, as in the burning of petroleum, its sulfur
When an oxygen-containing compound is pyrolyzed at content is converted to sulfur trioxide, which combines
◦
950 C in the presence of platinized carbon in a nitrogen with water to produce sulfuric acid:
or helium atmosphere, the organic molecule is broken up
and all oxygen is converted to carbon monoxide according
to the following reactions:

Compound containing C, H, O
platinized
carbon
−−−−→ CO + CO 2 + H 2 O, (10)
H 2 O + C −−−−→ CO + H 2 , (11)
and
CO 2 + C −−−−→ 2CO. (12)
Hence measurement of the amount of carbon monoxide
produced will indicate the oxygen content of the organic
substance. In practice, the carbon monoxide is determined FIGURE 6 Equipment for the determination of oxygen in organic
indirectly by transforming it to carbon dioxide. This can compounds. [Courtesy of Prof. Z. Y. Hu.]