150

QUANTITATIVE AGRICULTURAL ANALYSIS

nearly all of the vapor without the use of a water condenser, the
long neck of the digestion flask serving for this purpose. It is
convenient to distill from the flask in which digestion is accom-
plished, in which case the capacity of the flask should be 500 cc.
The digestion must be performed under a hood or some other
provision must be made for carrying away the fumes. An
excellent arrangement for this purpose is a lead pipe, 6 inches in
diameter and with holes in the side so that the necks of a number

PIG. 43.—Kjeldahl flask, stand and lead pipe ventilator.

of digestion flasks may be inserted with the flask in an inclined
position. The end of the lead pipe leads to a chimney.

Catalytic Agents.—The addition of oxides or salts of mercury,
copper or iron to the mixture of the organic material and sul-
phuric acid considerably accelerates the reactions that occur
during digestion. The action is of a catalytic nature and depends
upon the capability of the metal of existing in more than one state
of oxidation. The metal is thus alternately reduced by organic
matter and oxidized by sulphuric acid, somewhat as follows:

2HgS04 -> Hg2S04 + S08 + 0. (1)

The nascent oxygen thus formed attacks the organic matter and
mercurous sulphate is immediately reoxidized:

Hg2S04 + 2H2S04 -> 2HgS04 + 2H20 + S02.

(2)