Page 157 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005F220 June 15, 2001 20:44

Elemental Analysis, Organic Compounds 399

FIGURE 7 Closed-flask combustion technique. [Courtesy of Thomas Scientific.]

The end point is a sky-blue color that persists for at least
O 2
Organic sulfur compound → SO 3 (15)
30 sec.
and When the organic compound to be analyzed contains
only carbon, hydrogen, oxygen, and sulfur after closed-
SO 3 + H 2 O → H 2 SO. (16)
flask combustion as described above, the resultant solution
will be dilute sulfuric acid. In this case, it is more conve-
A simple technique for decomposing organic sub- nient to use acidimetry as the mode of finish. For this
stances in a flask filled with pure oxygen is closed-flask purpose, the solution in the Sch¨oniger flask is transferred
combustion (or oxygen-filled flask combustion) with into a 100-mL conical flask, boiled for 2 min to remove
platinum as the catalyst. This method is suitable for residual hydrogen peroxide, and then titrated with 0.01 N
sulfur determination. The appartus is illustrated in Fig. 7. sodium hydroxide, with methyl red as the indicator:
When the sample size is between 3 and 25 mg, the flask
H 2 SO 4 + 2NaOH = Na 2 SO 4 + 2H 2 O. (18)
(Sch¨oniger flask) has a capacity of 500 mL. For the
decomposition of larger quantities of organic materials, In the CHNS/O analyzer, sulfur is determined in the
1000-mL flasks are used. The stopper of the flask is form of SO 2 . The flow diagram of the Leco Analyzer
sealed to platinum gauze. In operation, the sample to for CHNS/O (Fig. 8) is shown in Fig. 9. The organic
be analyzed is placed on a piece of paper cut in the sample is weighed in a tin capsule and dropped into
shape shown at the left in Fig. 7. The wide part of the
paper is then folded and inserted into the platinum gauze.
Meanwhile, 10 mL of distilled water and 0.3 mL of 30%
hydrogen peroxide are added to the flask to serve as
absorption liquid, and the air inside the flask is displaced
with pure oxygen. Next the tip of the paper is ignited
and the stopper is immediately attached to the flask. To
prevent escape of sulfur trioxide, the analyst must tilt the
flask as illustrated at the right in Fig. 7.
After cooling to room temperature, the contents of the
conical flask are quantitatively transferred to a 200-mL
graduated beaker. The pH of the solution is adjusted to
3.0 ± 0.2 by addition of 0.5 N ammonium hydroxide.
Then 50 mL of acetone and 0.3 mL of dimethylsulfonazo-
III indicator are added. While the mixture is stirred vig-
orously, the sulfate ions are titrated with 0.01 M barium
chloride:

FIGURE 8 Leco Analyzer for CHNS/O. [Courtesy of Leco
SO 2− + Ba(ClO 4 ) 2 = BaSO 4 + 2ClO . (17) Corporation.]
−
4 4