Page 152 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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394 Elemental Analysis, Organic Compounds

ELEMENTAL ANALYSIS for organic compounds is a II. DETERMINATION OF CARBON,
branch of quantitative chemical analysis. It deals with the HYDROGEN, AND NITROGEN
principles and methods for determining the percentages
of certain chemical elements present in a pure organic A. Determination of Carbon and Hydrogen
compound or in an organic mixture.
With few exceptions, organic compounds always contain
both carbon and hydrogen. When an organic compound is
decomposedbyheatingathightemperatures(combustion)
I. INTRODUCTION
in the presence of oxygen, carbon dioxide and water are
produced:
Every organic compound contains the element carbon to-
gether with one or more of the other elements in the pe- [O]
Organic compound −−−−→ CO 2 + H 2 O. (1)
riodic table, in definite proportions. Elemental analysis
serves to measure the proportion of each element present The resultant water vapor and carbon dioxide can be
in the compound. Thus, when a new compound is synthe- collected sequentially in separate receivers (absorption
sized in the laboratory or when an unknown compound tubes). From the weights of carbon dioxide and water ob-
is obtained from nature, the product is isolated and care- tained, the percentages of carbon and hydrogen are calcu-
fully purified. Milligram amounts of the pure sample are lated by the following formulas:
subjected to elemental analysis. The percentage of every
wt of CO 2 at wt of C
element is separately determined if necessary, and the sum %C = × × 100 (2)
wt of sample mol wt of CO 2
of all elements should total 100%. In this way the compo-
sition of the synthetic compound is confirmed, while an and
unknown compound is characterized by the quantitative wt of H 2 O at wt of H
relationship of its constituents and its empirical formula %H = wt of sample × mol wt of H 2 O × 100. (3)
can be deduced.
Thisistheabsolutemethodforthedeterminationofcarbon
Besides being used to determine the elements in pure
and hydrogen. The apparatus and technique were devel-
organic compounds, elemental analysis is performed on
oped in the early part of the nineteenth century. Then at
mixtures of organic matter to determine the content of cer-
the turn of the twentieth century, Pregl (Nobel Prize lau-
tainelementstherein.Forinstance,foodstuffsareanalyzed
reate, 1923) improved the method and demonstrated that
for nitrogen, which gives an indication of the nutritional
carbon, hydrogen, and some other elements in organic
value of the material. Shale is analyzed for carbon and hy-
compounds can be determined accurately by using a few
drogen to estimate its fossil oil content. Coal is analyzed
milligrams of the sample.
for sulfur and nitrogen because these two elements are re-
sponsible for environmental pollution due to the burning
of coal. B. Dumas Method for Nitrogen Determination
The general principle of elemental analysis of organic
If a nitrogen-containing organic compound is decomposed
compounds involves the total breakdown (decomposition)
by oxidation, nitrogen oxides are formed:
oftheorganicmoleculessothattheindividualelementsare
converted into their respective elemental forms or simple oxidizing agent
Organic nitrogen compound −−−−−−→ Nitrogen oxides.
inorganic compounds. So that the objective of quantita-
tive analysis can be accomplished, two conditions must (4)
be met: (1) the decomposition of the sample should be
All nitrogen oxides can be converted to elementary ni-
complete and (2) the element to be determined should be
trogen by passing them through metallic copper at high
transformedintoonespecificproductthatcanbemeasured
temperature:
accurately.
At present, it is possible to determine any element that Cu
Nitrogen oxides → N 2 . (5)
may be present in organic materials. There are a great
variety of techniques for decomposing the organic sample. The nitrogen gas is then purified, collected, and measured.
After decomposition, there may be more than one method
of measuring the product. The latter process is known as
C. Simultaneous Determination of Carbon,
the mode of finish. In the following sections, the elements
Hydrogen, and Nitrogen
most commonly determined in organic compounds are
discussed, and selected methods for decomposition and Currently the common practice in analyzing organic com-
finishing are described. pounds for carbon, hydrogen, and nitrogen is to use the