And 0.184615: 83.5 = x: 94.
Whence x = 0.207830 gram of nitrogen corresponding to twenty cubic
centimeters of the sulfuric acid used.
Then 0.20783 ÷ 94 = 0.002211 gram of nitrogen corresponding to one
cubic centimeter of the barium hydroxid solution.
If then in the analysis of a fertilizer it is found that 60.5 cubic
centimeters are required to neutralize the excess of sulfuric acid
after distillation the percentage of nitrogen in the sample is found as
follows:
60.5 × 0.002211 = 0.13377.
0.20783 - 0.13377 = 0.07406.
0.07406 × 100 = 7.406 = per cent nitrogen in sample when one gram is
taken for the combustion.
=184. The Official Kjeldahl Method. Not Applicable in the Presence of
Nitrates.=[153]—_Reagents._—(1) _Acids._—(_a_) _Standard hydrochloric
acid_ the absolute strength of which has been determined by
precipitating with silver nitrate, and weighing the silver chlorid as
follows:
To any convenient quantity of the acid to be standardized, add solution
of silver nitrate in slight excess, and two cubic centimeters of pure
nitric acid, of specific gravity 1.2. Heat to boiling-point, and
keep at this temperature for some minutes without allowing violent
ebullition, constantly stirring until the precipitate assumes the
granular form. Allow to cool somewhat, and then pass the fluid
through the asbestos. Wash the precipitate by decantation, with 200
cubic centimeters of very hot water, to which have been added eight
cubic centimeters of nitric acid and two cubic centimeters of dilute
solution of silver nitrate containing one gram of the salt in 100 cubic
centimeters of water. The washing by decantation is performed by adding
the hot mixture in small quantities at a time, and beating up the
precipitate well with a thin glass rod after each addition. The pump is
kept in action all the time, but to keep out dust during the washing
the cover is only removed from the crucible when the fluid is to be
added.
Put the vessels containing the precipitate aside, return the washings
once through the asbestos so as to obtain them quite clear, remove
them from the receiver, and set aside to recover the excess of silver.
Rinse the receiver and complete the washing of the precipitate with
about 200 cubic centimeters of cold water. Half of this is used to
wash by decantation and the remainder to transfer the precipitate to
the crucible with the aid of a trimmed feather. Finish washing in the
crucible, the lumps of silver chlorid being broken down with a glass
rod. Remove the second filtrate from the receiver and pass about
twenty cubic centimeters of ninety-eight per cent alcohol through the
precipitate. Dry at from 140° to 150°. Exposure for half an hour is
found more than sufficient, at this temperature, to dry the precipitate
thoroughly.
(_b_) _Standard sulfuric acid_, the absolute strength of which has
been determined by precipitation with barium chlorid and weighing the
resulting barium sulfate:
For ordinary work half normal acid is recommended, _i. e._, containing
18.2285 grams of hydrochloric or 24.5185 grams sulfuric acid to
the liter; for work in determining very small amounts of nitrogen,
one-tenth normal acid is recommended. In titrating mineral acids
against ammonia solutions, use cochineal as indicator.
(_c_) _Sulfuric acid_, specific gravity 1.84, free of nitrates and
also of ammonium sulfate, which is sometimes added in the process of
manufacture to destroy nitrogen oxids:
(2) _Standard alkali_, the strength of which, relative to the acid, has
been accurately determined. One-tenth normal ammonia solution, _i. e._,
containing 1.7051 grams of ammonia to the liter, is recommended for
accurate work:
(3) _Metallic mercury_ or mercuric oxid, prepared in the wet way: That
prepared from mercuric nitrate can not be safely used.
(4) _Potassium permanganate_ finely pulverized:
(5) _Granulated zinc_, pumice stone, or one-half gram of zinc dust is
to be added to the contents of the flasks in distillation, when found
necessary, in order to prevent bumping:
(6) _Potassium sulfid._—A solution of forty grams of commercial
potassium sulfid in one liter of water:
(7) _Soda._—A saturated solution of sodium hydroxid free of nitrates:
(8) _Indicator._—Solution of cochineal prepared as follows: Tincture
of cochineal is prepared by digesting for a day or two, at ordinary
temperatures, and frequently agitating, three grams of pulverized
cochineal in a mixture of fifty cubic centimeters of strong alcohol
with 200 cubic centimeters of distilled water. The solution is decanted
or filtered through Swedish paper.
_Apparatus._—(1) _Kjeldahl digestion flasks_ of hard, moderately thick,
well-annealed glass: These flasks are about twenty-two centimeters
long, with a round, pear-shaped bottom, having a maximum diameter of
six centimeters and tapering out gradually in a long neck, which is two
centimeters in diameter at the narrowest part, and flared a little at
the edge. The total capacity is from 225 to 250 cubic centimeters.
(2) _Distillation flasks_ of ordinary shape, of 550 cubic centimeters
capacity, or preferably flasks of well-annealed glass, of the same
capacity, of pear-shaped bottom, for both digestion and distillation,
fitted with a rubber stopper and a bulb-tube above to prevent the
possibility of sodium hydroxid being carried over mechanically during
distillation: The bulbs are about three centimeters in diameter, the
tubes being of the same diameter as the condenser and cut off obliquely
at the lower end. This is adjusted to the tube of the condenser by a
rubber tube.
_Manipulation._—(1) _The Digestion._—From seven-tenths to three and
five-tenths grams of the substance to be analyzed, according to its
proportion of nitrogen, are brought into a digestion flask with
approximately seven-tenths gram of mercuric oxid or its equivalent
in metallic mercury and twenty cubic centimeters of sulfuric acid.
The flask is placed in an inclined position, and heated below the
boiling-point of the acid for from five to fifteen minutes or until
frothing has ceased. If the mixture froth badly, a small piece of
paraffin may be added to prevent it. The heat is then raised until the
acid boils briskly. No further attention is required until the contents
of the flask have become a clear liquid, which is colorless or at least
has only a very pale straw color. The flask is then removed from the
frame, held upright, and while still hot, potassium permanganate is
dropped in carefully and in small quantities at a time until, after
shaking, the liquid remains of a green or purple color.
(2) _The distillation._—After cooling, the contents of the flask are
transferred to the distilling flask with about 200 cubic centimeters of
water, a few pieces of granulated zinc, pumice stone, or one-half gram
of zinc dust when found necessary to keep the contents of the flask
from bumping, and twenty-five cubic centimeters of potassium sulfid
solution are added, shaking the flask to mix its contents. Next add
fifty cubic centimeters of the soda solution, or sufficient to make the
reaction strongly alkaline, pouring it down the sides of the flask so
that it does not mix at once with the acid solution. Connect the flask
with the condenser, mix the contents by shaking, and distil until all
ammonia has passed over into the standard acid. The first 150 cubic
centimeters of the distillate will generally contain all the ammonia.
This operation usually requires from forty minutes to one hour and a
half. The distillate is then titrated with standard alkali.
The use of mercuric oxid in this operation greatly shortens the time
necessary for digestion, which is rarely over an hour and a half in
case of substances most difficult to oxidize, and is more commonly
less than an hour. In most cases the use of potassium permanganate is
quite unnecessary, but it is believed that in exceptional cases it is
required for complete oxidation, and in view of the uncertainty it is
always used. The potassium sulfid removes all the mercury from the
solution, and so prevents the formation of mercurammonium compounds
which are not completely decomposed by soda solution. The addition
of zinc gives rise to an evolution of hydrogen and prevents violent
bumping. Previous to use, the reagents should be tested by a blank
experiment with sugar, which will partially reduce any nitrates that
are present, which might otherwise escape notice.
[Illustration: FIGURE 13. DISTILLING APPARATUS.]
=185. The Distillation Apparatus in Use in the Laboratory of the
Department of Agriculture.=—In this laboratory the distilling apparatus
is arranged as shown in Figure 13. The flasks are the same as are
used in the digestion. They are connected to the block tin condensers
by the bulb and rubber tubes, shown hanging on the projecting ends
of the block tin condensers on the right and left of the figure. The
condensers are contained in a trough through which cold water flows
during the distillation. The bulb above the flask carries an emergent
tube which extends to near the center of the bulb and is bent laterally
to avoid any danger of carrying over any alkali that may be projected
into the bulb during boiling. The boiling is continued usually for
nearly an hour or until bumping begins. The table on which the
apparatus is placed is so arranged as to permit of easy access on all
sides. The standard acid is held in erlenmeyers placed on wooden blocks
so that the end of the condenser which is a drawn-out glass tube, dips
beneath the surface of the acid.
=186. Patrick’s Distilling Flask.=—To avoid the expense and annoyance
attending the breaking of the distilling flasks Patrick has proposed to
make them of copper.[154] The size, about half a liter, made for the
evolution of oxygen for experimental purposes, may be used. A little
excess of potassium sulfid is used to make up for any of it which
might be consumed by the copper. About twenty-five cubic centimeters
of this solution are recommended. No zinc or pumice stone is required
to prevent bumping and the distillation may be finished within thirty
minutes, thus securing a saving of time. There will doubtless be a
slight corrosion of the flasks by the sulfid employed but where the
gunning oxidation process is practiced this danger would be avoided.
=187. Modifications of the Kjeldahl Process.=—It would be impracticable
here to give even a summary of the many unimportant changes which
the moist combustion process has undergone since the first papers of
its author were published. These changes may be divided into three
classes; _viz._, 1. Those changes which refer solely to the quantities
of substance taken for analysis, to the composition of the acid
mixture, to the duration of the digestion, to the form and size of
the flasks, both for digestion and distillation, and to the manner of
distillation and of titration. For references to the papers on these
subjects the reader may consult the periodic journals.[155] The most
important of these minor changes are the following: Instead of the
titration by means of separated iodin most chemists have had recourse
to the simpler method of direct titration of the excess of acid by a
set solution of an alkali. Barium, sodium, and potassium hydroxids are
the alkaline solutions most employed. This process permits of a larger
quantity of the sample being taken for combustion and of the use of a
larger quantity of acid in the receiver. It also implies the use of
a larger digestion flask. In fact it is now quite common to make the
digestion in a special glass flask large enough to be used also for the
distillation. This saves one transfer of the material with the possible
danger of loss attending it.
In the distillation it is a common practice, especially in Germany, to
do away with the condensing worm and to carry a long glass tube from
the distilling flask directly into the acid on the receiver. The only
inconvenience in this method is the heating of the contents of the
receiving flask, but this is attended with no danger of loss of ammonia
and the distillate, on account of the high temperature it acquires, is
left free of carbon dioxid. Many of these minor changes have tended to
simplify the process, but without affecting the principle of the method
in the least.
2. In the second place a class of changes may be mentioned in which
there is a marked difference in the method of effecting the oxidation
secured by the introduction of a substance, usually a metal, during
the digestion for the purpose of accelerating the oxidation. In
the original process the only aid to oxidation was applied at the
end of the digestion in the use of potassium permanganate. In the
modifications now under consideration a metallic oxid or metal is
applied at the beginning of the digestion. Copper and mercury are the
metals usually employed. A separate paragraph will be given to the
description of this modification known as the process of Wilfarth.
3. The third class of changes is even more radical in its nature,
having for its object the adaptation of the moist combustion method
to oxidized or mineral nitrogen. The chief feature of this class
of changes consists in the introduction of a substance rich in
hydrocarbons, and capable of easily forming nitro compounds, for the
purpose of holding the oxids of nitrogen which are formed during the
combustion and helping finally to reduce them to the form of ammonia.
The chief varieties of this class of changes were proposed by Asboth,
Jodlbaur, and Scovell, and will be fully set forth in separate
paragraphs.
=188. Method of Wilfarth.=—The basis of this modification as already
noted rests on the fact that certain metallic oxids have the power
of carrying oxygen, and thus assisting in a katalytic way in the
combustion of organic matter.[156] The copper and mercury oxids
are best adapted for this purpose and experience has shown that
mercuric oxid, or even metallic mercury gives the best results. The
manipulation is carried out as follows: From one to three grams of
the sample, according to its richness in nitrogen, are heated with
a mixture of twenty cubic centimeters of acid containing two-fifths
fuming and three-fifths ordinary sulfuric acid. To this is added
about seven-tenths gram of mercuric oxid prepared in the wet way from
a mercury salt free of nitrogen. The combustion takes place in the
usual kjeldahl flask. If the boiling be continued until the liquid
is entirely colorless, final oxidation with potassium permanganate
is unnecessary. To save time the combustion may be stopped when a
light amber color is reached, and then the oxidation finished with
permanganate. Before distilling, a sufficient quantity of potassium
sulfid is added to precipitate all the mercury as sulfid and thus
prevent the formation of mercurammonium compounds which would produce
a deficit of ammonia. A convenient strength of the sulfid solution is
obtained by dissolving forty grams of potassium sulfid in one liter of
water. Bumping at the end of the distillation is not usual, especially
if potash-lye be used, but should it occur it may be stopped by the
addition of zinc dust.
Only when a large excess of potassium sulfid is used is there an
evolution of hydrogen sulfid, the presence of which, however, does not
influence the accuracy of the results.
The presence of mercuric sulfid in the solution tends to prevent
bumping during the distillation, but it is advisable, nevertheless,
to use a little zinc dust. Other minor modifications consist of
forming the acid mixture with equal volumes of concentrated and
fuming sulfuric acid containing in one liter 100 grams of phosphoric
acid anhydrid,[157] and using metallic mercury instead of mercuric
oxid; or a mixture of half a gram of copper sulfate and one gram of
metallic mercury,[158] or half a gram of copper oxid and a few drops of
platinic chlorid solution containing 0.04 gram of platinum in a cubic
centimeter.[159]
=189. Modification of Asboth.=—In order to adapt the moist combustion
process to nitric nitrogen Asboth proposed the use of benzoic
acid.[160] For half a gram of saltpeter 1.75 grams of benzoic acid
should be used. At the end of the combustion the residual benzoic
acid is oxidized by means of potassium permanganate with a subsequent
reheating. If the nitrogen be present as an oxid or as cyanid, one gram
of sugar is added. The metallic element added is half a gram of copper
oxid. Asboth also recommends that the soda-lye used in the distillation
be mixed with sodium potassium tartrate for the purpose of holding the
copper and manganese oxids in solution and thus preventing bumping. The
alkaline liquor contains in one liter 350 grams of the double tartrate
and 300 grams of sodium hydroxid.
The principle on which the use of benzoic acid rests is found in the
fact that it easily yields nitro-compounds and thus prevents the loss
of the nitrogen oxids, these readily combining with the benzoic acid.
The nitro-compounds can be subsequently converted into ammonia by
treatment with potassium permanganate.
The pyridin and chinolin groups of bodies do not yield all their
nitrogen as ammonia by the above treatment.
The conclusions drawn by Asboth from the analytical data obtained were:
(1) Sugar should be used in the ordinary kjeldahl process in those
cases where the nitrogen in the organic substance is present as oxids
or as cyanogen.
(2) In the case of nitrates good results may be secured with benzoic
acid but permanganate must be added at the end.
(3) The kjeldahl-wilfarth process can be applied with substances
difficultly decomposed, _e. g._, alkaloidal bodies.
=190. Variation of Jodlbaur.=—The benzoic acid method, although a step
forward, is not entirely satisfactory in the treatment of nitrates by
moist combustion. Jodlbaur has proposed to substitute for the benzoic,
phenolsulfuric acid.[161]
From two to five-tenths gram of a nitrate are treated with twenty
cubic centimeters of concentrated sulfuric and two and a half of
phenolsulfuric acid, together with three grams of zinc dust and five
drops of a solution of platinic chlorid of the strength mentioned
above. The phenolsulfuric acid is prepared by dissolving fifty grams of
phenol in 100 cubic centimeters of strong sulfuric acid. The combustion
is continued until the solution is colorless, which may take as much as
five hours. If phosphoric acid anhydrid be used as recommended above,
the time of the combustion may be diminished by one-half, but in such
a case the glass of the combustion flask is strongly attacked and is
quite likely to break.
With substances very rich in nitrates it is advisable to rub them first
with dry gypsum.
The theory of the process rests on the fact that by a careful
admixture of a nitrogenous substance diluted with land plaster, with
phenolsulfuric acid, it is possible to change the nitric acid into
nitro-phenol, and by the reducing action of zinc dust to change the
nitro-product formed into amido-phenol. This afterwards is transformed
into ammonium sulfate by heating with sulfuric acid, by which process,
at the same time, all other nitrogenous compounds present in the
substance, as with Kjeldahl’s method, likewise form ammonium sulfate,
only with the difference that addition of mercury is here absolutely
necessary for the complete transformation of the slowly decomposed
amido-phenol which again brings about the necessity of decomposing
the nitrogenous mercury compounds formed in the solution by potassium
sulfid, which is added after or with the soda-lye.
=191. The Dutch Jodlbaur Method.=—The Royal Experiment Station of
Holland directs that the jodlbaur process be carried out as indicated
below.[162]
The reagents necessary are:
1. _Phenolsulfuric acid_, prepared by dissolving 100 grams of pure
crystallized phenol in pure sulfuric acid (1.84) and making up the
solution to a liter with the same sulfuric acid: