3. _Sodium hydroxid solution_, the same as is used in the kjeldahl
method:
4. _Potassium sulfid solution_, made by dissolving 355 grams of
potassium sulfid (K₂S), or sodium sulfid solution, made by dissolving
250 grams of sodium sulfid (Na₂S) in a liter of water.
As apparatus, are necessary oxidation flasks holding about 200
cubic centimeters, and distillation flasks holding about 750 cubic
centimeters, both of bohemian glass.
_Manipulation._—Weigh one gram of substance, moisten it with water,
dry, and introduce into an oxidation flask. Cover with fifteen cubic
centimeters of phenolsulfuric acid and, after cooling, thoroughly mix
by gently shaking. After five minutes add from two to three grams of
zinc in small proportions, keeping the flask cool, then twenty cubic
centimeters of sulfuric acid, and finally two drops of mercury. Boil
the mixture till the fluid is colorless. Cool and dilute. Wash into
a distillation flask and add an excess of sodium hydroxid solution
and twenty-five cubic centimeters of the sodium (or potassium) sulfid
solution. Distil and titrate as in the kjeldahl method.
=192. The Halle-Jodlbaur Method.=—At the Halle station it is
the uniform practice to mix the nitrate with gypsum before the
combustion.[163] In the case of Chile phosphates ten grams are rubbed
with an equal amount of gypsum, and two grams of the mixture, equal
to one gram of the nitrate, taken for the determination. In the case
of saltpeter mixtures which contain over eight per cent of nitrogen,
one gram of the mixture with gypsum is taken, of guanos one and a half
grams, and of lower forms of nitrates or mixtures thereof, from three
to five grams.
The sample, as prepared above, is treated with thirty cubic centimeters
of a mixture of phenolsulfuric acid and phosphoric acid anhydrid.
The mixture is prepared by dissolving sixty-six grams of phenol and
250 grams of phosphoric anhydrid in strong sulfuric acid, and, after
cooling, mixing the two solutions and making the volume up to 1,650
cubic centimeters with pure sulfuric acid. The mixture contains, in
thirty cubic centimeters, one and two-tenths grams of phenol and four
grams of phosphoric anhydrid. In the use of phenolsulfuric acid the
the presence of phosphoric anhydrid is indispensable in keeping the
sulfuric acid water-free and in absorbing the water produced by the
combustion.
The phenolsulfuric acid used contains only enough phenol to reduce half
a gram of saltpeter.
The sample and acid mixture having been put in the combustion flask the
latter is shaken, at intervals, for an hour, and the contents cooled.
The conversion of the nitrates into nitro-phenol compounds is finished
in this time, and the next step consists in reducing these bodies to
the amido-phenol group. This is accomplished in the cold by nascent
hydrogen produced by the addition of zinc dust to the mixture. From one
to three grams of the dust are to be used in proportion to the quantity
of nitrates originally present.
The flask should be placed in a cooling mixture and the zinc dust added
in small portions to prevent a too violent evolution of hydrogen.
After the reduction is ended the flask is allowed to stand for two
hours, after which the combustion, distillation, and titration are
accomplished in the usual way. On cooling, after the end of the
combustion, the contents of the flask become solid. They may be brought
again into liquid state by shaking and gentle warming.
=193. The Official Kjeldahl Method for Nitric Nitrogen.=—As has already
been stated, the presence of certain organic compounds, rich in
hydrocarbons, permits the reduction of nitric nitrogen to ammonia by
combustion with sulfuric acid. Benzol, phenol, and salicylic acid have
all been used for this purpose. The official chemists have adopted for
their method the salicylic acid process first proposed by Scovell.[164]
Besides the reagents and apparatus given under the Kjeldahl method
there will be needed:
(1) _Zinc dust_: This should be an impalpable powder; granulated zinc
or zinc filings will not answer.
(2) _Sodium thiosulfate_:
(3) _Commercial salicylic acid_:
It is found most convenient to prepare a solution of 33.3 grams of
salicylic acid in one liter of the strongest sulfuric acid, and keep
it for use rather than to mix it for each combustion. We prefer the
thiosulfate process first mentioned below. In the zinc dust method
there has been noticed a tendency for the distillation flask to break
just at the end of the process.
_The Manipulation._—Place from seven-tenths to three and five-tenths
grams of the substance to be analyzed in a kjeldahl digesting flask,
add sixty cubic centimeters of sulfuric acid containing one gram of
salicylic acid, and shake until thoroughly mixed, then add five grams
of crystallized sodium thiosulfate; or add to the substance thirty
cubic centimeters of sulfuric acid containing two grams of salicylic
acid, then add gradually two grams of zinc dust, shaking the contents
of the flask at the same time. Finally place the flask on the stand
for holding the digestion flasks, where it is heated over a low flame
until all danger from frothing has passed. The heat is then raised
until the acid boils briskly and the boiling continued until white
fumes no longer pour out of the flask. This requires about five or ten
minutes. Add now approximately seven-tenths gram of mercuric oxid or
its equivalent in metallic mercury, and continue the boiling until the
liquid in the flask is colorless or nearly so. In case the contents of
the flask are likely to become solid before this point is reached add
ten cubic centimeters more of sulfuric acid. Complete the oxidation
with a little potassium permanganate in the usual way, and proceed with
the distillation as described in the kjeldahl method. The reagents
should be tested by blank experiments.
=194. The Gunning Moist Combustion Process.=—The modification proposed
by Gunning was based upon the observation that in the ordinary kjeldahl
process the excess of sulfur trioxid in the beginning of the operation
soon escapes or unites with water in a form not easily decomposed.[165]
During the progress of the combustion the acid diminishes in strength
until it is below the concentration represented by the formula H₂SO₄,
and in this diluted condition the oxidation takes place more slowly.
Gunning proposes to avoid this difficulty by mixing potassium sulfate
with the sulfuric acid. This salt forms with the sulfuric acid, acid
salts which, by heating, lose water easier than acid and as is well
known, they not only act as decomposing and oxidizing media as well as
sulfuric acid, but even in a higher degree, resembling the action of
sulfuric acid at high temperatures and under pressure.
By heating this mixture of sulfuric acid and potassium sulfate with
organic matters in an open vessel, not only the water originally
present, but that which is formed during the oxidation is driven off
without loss of acid. For this reason instead of the oxidizing mixture
becoming weaker, the acid becomes stronger, the boiling-point rises and
this, combined with the fluidity of the mass favors the decomposition
and oxidation of the organic matter in a constantly increasing ratio.
The original mixture used by Gunning had the following composition;
_viz._, one part of potassium sulfate and two parts of strong sulfuric
acid. The substances are united by heat and, on cooling, are in a
semi-solid state, melting, however, easily on the application of heat
and assuming a condition to be easily poured from vessel to vessel.
The quantity of the sample taken should vary in proportion to its
nitrogenous content from half a gram to a gram. The combustion takes
place in flasks entirely similar to those used in the ordinary kjeldahl
process. In the case of liquids, they should be previously evaporated
to dryness before the addition of the oxidizing mixture. At the
beginning of the combustion there is a violent foaming attended with
evolution of some acid and much water, and afterwards of stronger acid.
This loss of acid should not be allowed to go far enough to produce
too great concentration of the material in the flask. One of the best
ways to avoid it is to place a funnel in the flask covered with a
watch-glass which will permit of the condensation and return of the
escaping acid. As soon as the foaming ceases, the flame should be so
regulated as to permit of the volatilized acid being condensed upon the
sides of the flask. In the end a colorless mass is obtained in which no
metallic oxids are present, and this mass can at once be diluted with
water, treated with alkali, and distilled. According to the nature of
the substance from half an hour to an hour and a half are required for
the complete combustion.
_Modifications of the Gunning Method._—As in the case of the kjeldahl
method, numerous minor modifications of the gunning method have
been made, the most important of which relate to its application to
substances containing nitrates. In general the same processes are
employed in this case as with the kjeldahl method. One of the best
modifications consists in the use of the mixture of salicylic and
sulfuric acids followed by the addition of sodium thiosulfate or of
potassium sulfate or sulfid. These modifications will be given in
detail under the official methods.
=195. Reactions of the Gunning Process.=—The various reactions which
take place during the combustion according to the gunning method have
been tabulated by Van Slyke.[166]
The first reaction to take place is the union of sulfuric acid and
potassium sulfate to form potassium acid sulfate in accordance with the
following equation:
(1) K₂SO₄ + H₂SO₄ = 2KHSO₄.
When heated, the potassium acid sulfate decomposes, forming potassium
disulfate and water, thus:
(2) 2KHSO₄ = K₂S₂O₇ + H₂O.
The potassium disulfate at a higher temperature decomposes, forming
normal potassium sulfate and sulfur trioxid, thus:
(3) K₂S₂O₇ = K₂SO₄ + SO₃.
At a sufficiently high temperature the two preceding reactions may take
place in one, thus: