FEEDS

151

Of the three metals named, mercury serves well because its
salts are colorless and they do not obscure the end point of the
oxidation. It is necessary in this case to precipitate the mercury
by the addition of potassium sulphide, before distillation,
in order to prevent the formation of mercurammonium com-
pounds which later are not readily decomposed by sodium
hydroxide. Copper sulphate as a catalyst is often preferred
because it serves as an indicator when sodium hydroxide is
added, a deep blue solution being formed when the solution
becomes basic.

Prevention of Bumping.—During the distillation of ammonia,
after the addition of excess of sodium hydroxide, there is usually
a tendency toward bumping. In order to prevent this, granular
zinc or pumice stone may be used. An excellent substitute is a
small amount (about 0.5 gm) of crushed porcelain from which
the dust has been removed by sifting.

Blank.—Sulphuric acid nearly always contains a small amount
of ammonium sulphate. Distilled water which has been exposed
to laboratory air also may contain a small quantity of ammonium
hydroxide. In order to make the proper correction for the
ammonia that will be derived from the reagents a "blank77
determination must be made, omitting the sample of feed but
carrying out the operations exactly as in the real determination.
In this case cane sugar is added to reduce possible traces of
nitrates existing in the reagents, as they would be reduced by the
organic matter of the feed.

Determination of Organic Nitrogen (of Crude Protein): Kjeldahl Method.—
Prepare the following reagents:
(a) Hydrochloric or Sulphuric Acid Solution, Fifth-normal.—Standardize
against pure sodium, carbonate as directed on page 58, making the necessary
changes in weight of carbonate to account for the different normality of the
acid here used. The standardization of these acids by weighing silver
chloride or barium sulphate (the official methods) is not to be recommended
because chlorides and sulphates, respectively, are nearly always to bo
found in the acids. These would give high values for the acid content, so
determined.
(6) Sodium Hydroxide or Potassium Hydroxide Solution, Fifth^normal—
Standardize by titration against the acid (a), using methyl red as indicator.
(c) Sulphuric Acid.—The concentrated acid of the laboratory, specific
gravity 1.84, as nearly as possible free from nitrates and ammonium salts.