Page 161 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: LLL/LPB P2: FJU Final Pages
Encyclopedia of Physical Science and Technology EN005F220 June 15, 2001 20:44

Elemental Analysis, Organic Compounds 403

experimental procedure should be strictly controlled in is conducted to a tube containing gold foil, which traps the
order to get reproducible results. mercury to produce amalgam. The increase in weight of
Arsenate can be determined by an iodimetric method in the gold tube gives the content of mercury in the sample.
which it is reduced by iodide in acidic solution to produce Amalgamation in a gold trap is also employed in envi-
iodine: ronmental analysis of organic mercury compounds such
as methyl mercury. The mercury is commonly determined
−
+
AsO 3− + 2H + 2I → I 2 + AsO 3− + H 2 O. (29)
4 3 by cold vapor atomic absorption spectroscopy.
The iodine liberated is then titrated with a standardized When an organic mercury compound is decomposed by
sodium thiosulfate solution [see Eq. (25)]. digestion in a mixture containing nitric acid, sulfuric acid,
If the organic sample contains a low percentage of ar- and potassium permanganate, the resultant mercuric ions
senic, arsenate in the digestion mixture is preferably re- are preferably determined by titration with standardized
duced by means of sodium borohydride to liberate arsine, potassium thiocyanate solution:
which is measured by atomic absorption spectrometry.
H 2 SO 4 ,HNO 3 ,KMnO 4
Organic mercury compound −−−−−−−−−−−−→ Hg 2+
(33)
VIII. DETERMINATION OF
METALLIC ELEMENTS and
+
Hg 2+ + 2KCNS −−−−−−−−−−−−→ Hg(CNS) 2 + 2K .
A. Determination by Ashing
(34)
When a metallic element is a constituent of an organic
compound, it will leave a solid residue on complete de- Ferric ammonium sulfate is used as the indicator, and the
composition of the sample by strong heating. This pro- end point is located spectrophotometrically.
vides a simple technique, known as ashing, for determin-
ing metallic elements, except for mercury and osmium,
which form volatile products. The organic substance can IX. OTHER METHODS FOR
be heated in a crucible or microboat: decomposition by DETERMINING ATOMIC RATIOS
heat is aided by moistening the sample with nitric or sul-
furic acid. After heating, noble metals such as platinum
In the methods described in the previous sections, a known
leave the pure metal:
weight of an organic compound is taken for analysis. From
HNO 3 the yield of the product related to a certain element, the
Organic platinum compound −−−−→ Pt. (30)
weight percentage of that element in the sample is deter-
Other metallic elements are converted to oxides, for mined. when two or more elements have been determined,
example, their atomic ratio in the organic compound can be deduced
on the basis of their atomic weights. Besides the above
HNO 3
Organic copper compound −−−−→ CuO, (31) methods, there are other analytical techniques for deter-
mining atomic ratios without weighing the samples. Some
or are recovered in the form of sulfates, for instance,
are discussed below.
H 2 SO 4 When an organic compound containing hydrogen, oxy-
Organic sodium compound −−−−→ Na 2 SO 4 . (32)
gen, and nitrogen is pyrolyzed in a large excess of sulfur in
If the organic sample contains a single metallic element, the absence of air, carbon is converted to carbon dioxide,
the residue is weighed and its metal content can be cal- carbonyl sulfide, and carbon disulfide; hydrogen forms
culated. If two or more metallic elements are present in hydrogen sulfide; oxygen produces carbon dioxide and
the original substance, the residue is brought into solution carbonyl sulfide; and nitrogen is liberated as nitrogen gas:
and subjected to inorganic analysis.
S
CHON compound → CO 2 + COS + CS 2 + H 2 S + N 2 .
B. Determination of Mercury (35)
Because of its volatility, mercury in organic compounds Using these reactions, Hara et al. constructed an appara-
cannot be determined by the ashing technique described tus for determining the atomic ratios of carbon, hydrogen,
above. Instead, the organic sample is placed in a porcelain oxygen, and nitrogen in an organic compound. A quartz
microboat and heated in a combustion tube packed with ampoule of 5.5-mm inside diameter, 6.5-mm outside di-
granulated calcium oxide. The mercury vapor that forms ameter, and 50-mm length is prepared. Purified sulfur is