At the temperature at which these reactions take place, the water
that is set free does not recombine with the sulfur trioxid nor with
the sulfuric acid that is present in excess, but is expelled from
the mixture; hence the mixture becomes more concentrated during the
digestion. The sulfur trioxid set free acts upon the organic matter in
the powerful manner peculiar to it, and the potassium sulfate formed
in the last reaction above unites with another molecule of sulfuric
acid, and the same round of reactions is repeated continuously so long
as there is an excess of free sulfuric acid present in the mixture.
As the liquid becomes more concentrated with the continuation of the
digestion, the boiling-point increases so that the effect is the same
as heating under pressure. The danger of too great concentration and
risk of consequent loss of nitrogen is avoided by using increased
proportions of sulfuric acid.
As compared with the kjeldahl the gunning method presents the following
advantages:
(1) The gunning method requires fewer reagents. As no form of mercury
is used no potassium sulfid is needed, and there is no risk of loss
from the presence of mercurammonium compounds.
(2) The solution to which caustic soda is added is clear, so that in
neutralizing, it is an easy matter to avoid great excess of alkali, and
so, in most cases, to avoid foaming and bumping in distillation.
(3) In the blank determinations less nitrogen is found in the reagents
used in the gunning method. In only one case was more nitrogen reported
in a blank by this method; in all the others the amount averaged
considerably less.
=196. The Official Gunning Method.=—In a digestion flask holding
from 250 to 500 cubic centimeters place from seven-tenths to two and
eight-tenths grams of the substance to be analyzed, according to its
proportion of nitrogen. Then add ten grams of powdered potassium
sulfate and from fifteen to twenty-five cubic centimeters (ordinarily
about twenty cubic centimeters) of concentrated sulfuric acid. Conduct
the digestion as in the kjeldahl process, starting with a temperature
below boiling-point and increasing the heat gradually until frothing
ceases. Digest until colorless or nearly so. Do not add either
potassium permanganate or potassium sulfid. Dilute, neutralize, and
distil as in the kjeldahl method. In neutralizing, it is convenient
to add a few drops of phenolphthalein indicator, by which one can
tell when the acid is completely neutralized, remembering that the
pink color, which indicates an alkaline reaction, is destroyed by
a considerable excess of strong fixed alkali. The distillation and
titration are conducted as in the kjeldahl method. In distilling,
the use of zinc or of any substance to prevent bumping or foaming
is generally unnecessary, if too great an excess of fixed alkali be
avoided. The amount of sulfuric acid recommended by Gunning is two
grams for each gram of potassium sulfate; but Voorhees has found that
this mixture is so viscous as to cause troublesome foaming frequently,
and after cooling it cakes in a hard mass, which may be difficult
to redissolve.[167] To avoid foaming and caking, he has found it an
effective means to increase the amount of sulfuric acid used, taking
instead of two grams to one of potassium sulfate three or four grams
of acid to one of potassium sulfate. It is, therefore, suggested
in carrying out the work, to use from five to twenty-five cubic
centimeters (ordinarily about twenty cubic centimeters) of sulfuric
acid for ten grams of potassium sulfate. In case the potassium sulfate
is not free from nitrogen compounds, one or two recrystallizations will
make it pure.
=197. Gunning Method Adapted to Nitrates.=—The essential features
of this modification are due to Winton and Voorhees.[168] The
modifications of the kjeldahl method, for similar purposes, furnished
the material details for the gunning modified process. Winton reports
good results from digesting for two hours, from half a gram to a gram
of the sample with thirty cubic centimeters of sulfuric containing
two grams of salicylic acid, in a flask of half a liter capacity. Two
grams of zinc dust are then slowly added, with constant shaking, and
the flask heated, at first gently, until, after a few minutes boiling,
dense fumes are no longer emitted. Three grams of potassium sulfate are
next added and the boiling continued until the solution is colorless,
or if iron be present, until a light straw color is produced. On
cooling, when the mixture begins to solidify, water is added with
caution, and afterwards sodium hydroxid, and the ammonia is obtained by
distillation.
In the process, as conducted by Voorhees, about one gram of the sample
is digested with ten grams of potassium sulfate and thirty cubic
centimeters of sulfuric containing one gram of salicylic acid, and
three grains of zinc sulfid. The heat is kept down until frothing
ceases, and then the mass kept in gentle ebullition until clear. The
distillation is accomplished with the usual precautions. The voorhees
process is superior to that recommended by Winton in adding the
potassium sulfate at the beginning of the combustion.
=198. Official Gunning Method Modified to Include the Nitrogen
of Nitrates.=—In a digestion flask holding from 250 to 500 cubic
centimeters, place from seven-tenths to three and five-tenths grams
of the substance to be analyzed, according to the amount of nitrogen
present. Add from thirty to thirty-five cubic centimeters of salicylic
acid mixture, namely, thirty cubic centimeters of sulfuric to one gram
of salicylic acid, shake until thoroughly mixed, and allow to stand
from five to ten minutes, with frequent shaking; then add five grams of
sodium thiosulfate and ten grams of potassium sulfate. Heat very gently
until frothing ceases, then strongly until nearly colorless. Dilute,
neutralize, and distil the same as in the gunning method.