Differences obtained in the estimation of nitrogen in the above
substances are frequently the source of much annoyance. The cause of
these discrepancies is chiefly due to the lack of uniformity in the
material, and from its not being in a sufficiently fine state during
the combustion. The hair which is found in commerce for the
manufacture of fertilizers, is generally mixed with sand and dust.
Wool dust often contains old buttons, pieces of wood, shoe pegs, and
all sorts of things. The flesh fertilizers are composed of light
particles of flesh mixed with the heavier bone dust.
Even after taking all possible precautions to finely comminute these
substances by mechanical means, still only imperfect results are
obtained, for the impurities, that is to say, the sand, can never be
so intimately mixed with the lighter particles that a sample of 0.5 to
0.8 gramme, such as is used in the determination of nitrogen, will
correspond to the correct average contents. In substances such as
dried blood, pulverization is very tedious. A very good method of
overcoming these difficulties, and of obtaining from the most mixed
substances a perfectly homogeneous mass, is that recommended by
Grandeau[1] of decomposing with sulphuric acid--a method which as yet
does not seem to be generally known. From a large quantity of the
substance to be examined, the coarse stones, etc., are removed by
picking or sifting, and the prepared substance, or in cases where the
impurities cannot be separated, the original substance, is treated
with sulphuric acid; after it is decomposed, the acid is neutralized
with calcium carbonate, and the nitrogen is determined in this mass.
[Footnote 1: _Handbook d. Agrict. Chem. Analyst._, p. 18.]
In order to operate rapidly, it is best to use as little sulphuric
acid as possible. If too much sulphuric acid is used, necessarily a
large amount of calcium carbonate is essential to get it into proper
condition for pulverizing. Under such circumstances the percentage of
nitrogen becomes very low, and a slight error will become
correspondingly high.
20 c.c. of concentrated sulphuric acid and 10 c.c. are sufficient for
30 to 40 grammes of material. After the substance and liquid have been
thoroughly stirred in a porcelain dish, they are warmed on a water
bath and continually stirred until the mass forms a homogeneous
liquid. The sirupy liquid thus obtained is then mixed with 80 to 100
grammes of pulverized calcium carbonate (calcspar), dried for fifteen
minutes at 40 to 60° C., and after standing for one to two hours the
dish and its contents are weighed. From the total weight the weight of
the dish is subtracted, which gives the weight of the calcium sulphate
and the calcium carbonate, and the known weight of the wool dust, etc.
This material is then intimately ground, and 2 to 3 grammes of it are
taken for the determination of the nitrogen, which is then calculated
for the original substance.
Although the given quantities of water and sulphuric acid hardly
appear sufficient for such a large quantity of hair or wool, still in
the course of a few minutes to a quarter of an hour, after continual
stirring, there is obtained a liquid which, after the addition of the
calcium carbonate, is readily converted into a pulverized mass.
Frequently a smaller quantity of sulphuric acid will suffice,
especially if the material is moist. The chief merit of this process
is that in a short time a large quantity of material, having a uniform
character, is obtained. Its use is, therefore, recommended for general
employment.
When the coarser stones, etc., are weighed, and the purified portion
decomposed, absolutely correct results are obtained, and in this way
the awkward discrepancies from different analysts may be
avoided.--_Chemiker Zeitung_, v. 7, p. 703.
* * * * *