Page 159 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Elemental Analysis, Organic Compounds 401

are soluble in aqueous solution, which liberates the halide For the determination of iodine, one mode of finish con-
ions, which can be titrated directly. Similarly, some com- sists of titration with standardized mercuric nitrate solu-
pounds can be dissolved in a nonaqueous solvent like tion, with diphenylcarbazone as the indicator:
ethyl alcohol or liquid ammonia; then addition of metallic
2I + Hg(NO ) 2 → HgI + 2NO . (21)
−
−
sodium dislodges the halogen from the organic molecule 3 2 3
and produces sodium halide. On the other hand, certain
Another method suitable for the determination of small
compounds, especially the polyhalogenated ones, require
amounts of iodine in organic materials involves the am-
drastic reactions at high temperatures to destroy the or-
plification technique. The iodide ions obtained after de-
ganic molecule in order to convert the halogens into ionic
composition of the sample are oxidized to iodate by the
forms. The closed-flask combustion technique (see Fig. 7)
addition of bromine in an acetate buffer. Excess bromine is
is commonly used for this purpose. The absorption liquid
removed with formic acid. Then the iodate is determined
for chlorine or bromine contains sodium hydroxide and
by liberation of iodine on addition of iodide in sulfuric
hydrogen peroxide and that for iodine contains hydrazine acid solution, followed by titration of the liberated iodine
sulfate, so that chloride, bromide, and iodide, respectively, with standardized sodium thiosulfate solution with starch
are obtained as the final products. as the indicator. The sequence of reactions can be depicted
Another technique for decomposing organic halogen as follows:
compounds utilizes a metal bomb constructed of nickel.
acetate
Commercial metal bombs are available in two sizes: a buffer
−
−
I + 3Br 2 + 3H 2 O −−−→ HIO 3 + Br + 5HBr,
2.5-mL bomb for decomposing up to 50 mg of organic
material, and a 22-mL bomb that can handle as much as (22)
0.5 g of sample mixed with 15 g of solid reagents. The
Excess Br 2 + HCOOH −−−→ 2HBr + CO 2 , (23)
reagents are sodium peroxide and sucrose or potassium
nitrate. After being locked tightly, the bomb is heated. HIO 3 + 5KI + 5H 2 SO 4 −−−→ 3I 2 + 3H 2 O + 5KHSO 4 ,
Vigorous oxidation reactions take place, resulting in the
(24)
transformation of the chlorine, bromine, and iodine origi- and
nallypresentintheorganicsubstancetochloride,bromide,
and iodate, respectively. 3I 2 + 6Na 2 S 2 O 3 −−−→ 6NaI + 3Na 2 S 4 O 6 . (25)
B. Modes of Finish VI. DETERMINATION OF FLUORINE
After decomposition, the resultant chloride and bromide
Although fluorine belongs to the halogen group in the
can bedetermined gravimetrically by weighing therespec-
periodic table, the methods described above for the deter-
tive silver halides:
mination of chlorine, bromine, and iodine are not suitable
−
Cl + AgNO → AgCl + NO − (19) for the determination of fluorine. For the decomposition
3
3
of organic compounds containing fluorine, the method of
and choice is fusion with metallic sodium or potassium in a
metal bomb. The drastic reducing action converts organi-
−
−
Br + AgNO → AgBr + NO . (20) cally bound fluorine to alkali fluoride. So that all possible
3 3
interfering substances can be removed prior to the mode
This mode of finish is recommended for occasional runs of finish, fluoride is recovered as fluorosilicic acid from
of one or two samples. For a series of determinations or the fusion mixture by steam distillation in perchloric acid
routine analysis, the expedient method is to titrate the so- solution maintained at 135 C:
◦
lution containing chloride or bromide with standardized
−
−
silver nitrate solution and locate the end point potentio- 6F + SiO 2 + 6HClO 4 → H 2 SiF 6 + 6ClO + 2H 2 O.
4
metrically.
(26)
In the Mitsubishi organic halogen measurement system,
chloro and bromo compounds in the environment (air, wa- Figure 10 shows a series of three distillation apparatuses
ter) are adsorbed in columns filled with activated carbon used for routine analysis. Figure 11 describes the construc-
and then heated at 900 C in a horizontal combustion tube tion of the distilling head and demonstrates the technique
◦
in an oxygen atmosphere. The hydrogen chloride and hy- of steam distillation from a solution at a constant temper-
drogen bromide produced are introduced into the auto- ature above 100 C.
◦
matic titration cell and determined coulometrically with As to the finishing mode, the resultant fluoride can
silver ions. be determined by titrimetry with thorium nitrate or by