Page 155 - 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 397

FIGURE 4 The complete assembly of the PE 2400 CHN Analyzer. [Courtesy of Perkin-Elmer Corporation.]

H 2 SO 4 sulfate mixture are added, followed by 1 mL of concen-
Organic nitrogen compound −−−−→ (NH 4 )HSO 4 , (6)
trated sulfuric acid. The reaction mixture is boiled gently
NaOH
(NH 4 )HSO 4 −−−−→ NH 3 , (7) for about 10 min until it becomes colorless. On cooling,
the solution is diluted with water and the ammonia is lib-
H 2 O
NH 3 −−−−→ NH 4 OH, (8) erated by using the micro-Kjeldahl distillation apparatus
and shown in Fig. 5. In operation, the ammonium bisulfate so-
lution in the micro-Kjeldahl digestion flask is transferred
NH 4 OH + HCl −−−−→ NH 4 Cl + H 2 O.
through funnel B to the bottom of distilling flask D, fol-
(9)
lowed by 8 mL of 30% sodium hydroxide solution. Funnel
The Kjeldahl method is probably the most frequently B is then closed by putting the Teflon plug in place. Steam
used method in organic elemental analysis. It is performed is then conducted from generator A into flask D, where-
generally on complex mixtures, and it is routinely carried upon ammonia is driven from flask D into condenser C.
out in agricultural stations, food processing plants, and The distillate (ammonium hydroxide solution) is collected
clinical and biochemical laboratories. The equipment and in a 50-mL conical flask containing 5 mL of 2% boric
experimental procedures vary widely, depending on the acid solution. The ammonium hydroxide is titrated with
nature of the organic material. Thus, in some determina- 0.01 N hydrochloric acid [see Eq. (9)], with methyl red–
tions milligram quantities of the sample are used, while bromcresol green as the indicator.
in other cases the sample size may be as large as 5 g. The When the Kjeldahl method is used for nitrogen determi-
reaction vessels employed for the decomposition of the nation, it should be remembered that the nitrogen present
sample range from 10 to 800 mL in capacity. The modes in the organic compound must be the amino type. Other
of finish can be titrimetric, colorimetric, or based on the types such as nitro and nitroso compounds can be reduced
ammonium ion-specific electrode. to amino compounds by suitable treatment prior to con-
A simple procedure for analyzing milligram amounts of centrated sulfuric acid digestion. Complex organic mate-
organic nitrogen compounds can be carried out as follows. rials such as coal and blood samples require the addition
The sample is weighed into a micro-Kjeldahl digestion of catalysts and prolonged heating to achieve complete
flask, which is commercially available or can be home- recovery of nitrogen as ammonium bisulfate.
made from a 150-mm test tube by blowing out its bottom In the mid-1990s, Collins, Chalk, and Kingston devel-
to form a bulb of about 10-mL capacity. Ten milligrams of oped a microwave digestion method which eliminates the
selenium powder and 40 mg of copper sulfate–potassium need for a catalyst and reduces the amount of sulfuric acid