_Apparatus_: The apparatus necessary consists of oxidation flasks of
about 200 cubic centimeters capacity and distillation flasks of about
500 cubic centimeters capacity, both of bohemian glass. Copper may be
used for the distillation flasks.[152]
_The Process_: A gram of the sample to be analyzed is placed in
an oxidation flask together with twenty cubic centimeters of
phosphosulfuric acid and a drop of mercury, about 600 milligrams,
and heated till the fluid becomes colorless. After cooling, dilute
and wash the contents of the flask into a distillation flask. The
resulting volume should be about 300 cubic centimeters. Add 100 cubic
centimeters of the alkaline sodium sulfid solution and some pieces of
ignited pumice stone or granulated zinc. Distill the ammonia, receiving
the distillate in a flask containing a known volume of the standard
sulfuric acid. Titrate with tenth-normal potash, using litmus or
rosolic acid as indicator.
=183. The Kjeldahl Method as Practiced at the Halle Station.=—The
method at present in vogue in the German stations of conducting the
moist combustion process is well illustrated by the method of procedure
followed at Halle.[29] From seven-tenths to one and five-tenths grams
of the sample are taken for analysis according to its richness in
nitrogen. Because of the fact that so small a quantity of the sample is
taken it is of the highest importance that it be perfectly homogeneous
throughout its entire mass. Otherwise, grave errors may arise. From
the sample, as sent to the laboratory, the analyst should take a
subsample and this should be rubbed to a fine powder and the part used
for analysis carefully taken therefrom. If the sample be moist it may
be rubbed up with an equal weight of gypsum, in which case a double
quantity is taken for the determination. Substances like bone-meal,
which do not keep well mixed, especially when occasionally shaken,
should be intimately mixed before each weighing. The sample taken for
analysis is placed in a glass flask of about 150 cubic centimeters
capacity. The flasks should be made of a special glass to withstand
the tension of the combustion. Those made by Kavalier at Sazava, in
Bohemia, have proved to be the most lasting. A globule of mercury
weighing a little less than one gram is placed in the flask and also
twenty cubic centimeters of pure sulfuric acid of 1.845 specific
gravity. The mercury is conveniently measured by an apparatus suggested
by Wrampelmayer. It consists of an iron tube holding mercury, and is
conveniently filled, from time to time, from a supply vessel placed
in a higher position and joined by means of a heavy glass tube and
rubber tube connections. The lower end of the iron tube is provided
with a movable iron stopper having a pocket just large enough to hold
a globule of mercury, weighing a little less than a gram. On turning
the stopper the pocket is brought opposite a discharge orifice and the
measured globule of mercury is discharged. With substances which tend
to produce a strong foaming a little paraffin is used. The flasks after
they are charged are placed on circular digesting ovens under a hood as
shown in figure 11.
[Illustration: FIGURE 11. MOIST COMBUSTION APPARATUS OF THE HALLE
AGRICULTURAL LABORATORY.]
At first the tripodal support of the flasks is so turned as to bring
them between the lamps and in this way a too rapid reaction is at
first avoided. After half an hour the tripods are so turned as to
bring each flask directly over the lamp, the flame of which is allowed
to impinge directly against the glass. The flame is so regulated
that after the evolution of the sulfur dioxid has nearly ceased the
contents of the flask are brought into gentle ebullition. The boiling
is continued until the contents of the flask are colorless, usually
about two hours. As a rule such substances as cottonseed-meal and
dried blood will take a longer time for complete combustion than other
fertilizing materials. During the combustion the flasks are closed with
an oblong loose-fitting unground glass stopper. When the oxidation is
finished the contents of the flasks are allowed to cool, the stoppers
are removed, and enough water is added to fill the flasks about
three-quarters full. The flasks are gently shaken, and the possibility
of breaking, from the heat developed, must not be overlooked. To avoid
confusion the flasks are all numbered before beginning the work, and
the numbers noted by the analyst in connection with the samples. The
contents of each one are next poured into the distillation flask and
the digestion vessels are washed with 100 cubic centimeters of water in
three portions, and the wash-water added to the liquid. Sometimes in
washing out the digestion flask yellow basic mercury compounds separate
on its walls, but this does not, in any way, influence the accuracy
of the results. The distillation flasks should have about 600 cubic
centimeters capacity. To avoid the transfer the digestion may take
place in the distillation flask in which case the latter must be made
of special glass as indicated.
To the liquid thus transferred are added seventy-five cubic centimeters
of soda-lye containing one and one-half times as much potassium sulfid
as is necessary to combine with the mercuric sulfate present. The lye
is of such a strength that sixty cubic centimeters are sufficient to
neutralize the acid present. It has a specific gravity of 1.375 and
contains thirty-three grams of potassium sulfid in a liter.
In order to avoid the bumping which may take place during the
distillation, some granulated zinc should be added.
The distillation flask is closed with a rubber stopper carrying a
bulb-tube which ends above in a glass tube about three-quarters of a
meter long, bent at an acute angle, and passing obliquely downward on
a convenient support. This tube is connected by a rubber, with the end
tube bent nearly at a right angle and dipping into the standardized
acid in the erlenmeyer receiver. The general arrangement of the
distilling apparatus is shown in figure 12. Since the contents of
the vessel are warmed by mixing with the soda-lye, the flame can be
turned on at full head at once at the commencement of the operation.
In about a quarter of an hour the liquid in the receiver will be
at the boiling-point, and the boiling should be continued for five
minutes more, making twenty minutes in all for the completion of the
distillation. By this boiling the contents of the receiver are not
charged with carbon dioxid, as might happen if a condenser were used.
The receiver contains twenty cubic centimeters of a standardized
sulfuric acid solution and about fifty cubic centimeters of water.
[Illustration: FIGURE 12. DISTILLATION APPARATUS OF HALLE
AGRICULTURAL LABORATORY.]
The acid used should contain 38.1 grams of sulfuric acid of 1.845
specific gravity in a liter; and it should be set by titration with
chemically pure sodium carbonate. For this purpose seven-tenths gram of
sodium carbonate is heated in a platinum crucible for two hours over a
small flame, weighed, and placed in an erlenmeyer together with twenty
cubic centimeters of the sulfuric acid, care being taken to avoid loss
from the vigorous evolution of carbon dioxid. After boiling for ten
minutes all the carbon dioxid is removed from solution. After cooling,
the excess of acid is determined by titration with a standardized
barium hydroxid solution, using rosolic acid as indicator.
The solution of barium hydroxid is made as follows: Digest, with warm
water, 260 grams of caustic baryta, Ba(OH)₂, until it is nearly all
dissolved, filter, and make up to a volume of ten liters and keep in
a flask free of carbon dioxid. A solution of barium hydroxid is to be
preferred to the corresponding sodium compound for titration. If traces
of carbonate be formed in the two liquids, the sodium salt will remain
in solution while the barium compound will settle at the bottom of the
flask.
_The Indicator._—The indicator used to determine the end of the
reaction is made by dissolving one gram of rosolic acid in fifty cubic
centimeters of alcohol. From one to two drops are enough for each
titration. The color reaction is less definite as the quantity of
ammonia in the liquid increases. When the titration solutions have been
prepared as above described it is found to require about ninety of the
barium hydroxid to neutralize twenty cubic centimeters of the sulfuric
acid.
By direct titration with sodium carbonate it is ascertained how many
grams of nitrogen the twenty cubic centimeters of sulfuric acid
represent.
_Example._—Suppose the weight of the dried sodium carbonate prepared as
above directed is 0.6989 gram.
½Na₂CO₃, ½N₂
Then 0.6989 : 53 = x : 14
Whence x = 0.184615 gram of nitrogen.
Suppose further that twenty cubic centimeters of sulfuric acid solution
require ninety-four cubic centimeters of barium hydroxid for complete
saturation and after treatment with the above amount of sodium
carbonate, ten and a half cubic centimeters of the barium solution to
neutralize the remaining acid.