exactly 100 cubic centimeters at 0° and 760 millimeters pressure of
mercury, is calculated by the formula
V = (100(273 + _t_)760)/(273(_b_ − _f_));
where _t_ equals observed temperature, _b_ the barometric pressure less
the correction noted above and _f_ the tension of the vapor of water
under existing conditions. For example:
Let the temperature be 18°
Barometric reading 755
Correction for _t_ 2
Corrected barometer 753
Vapor of water tension 16
Then V = (100(273 + 18)760)/(273(753 − 16)) = 109.9.
This indicates that 109.9 cubic centimeters of air would occupy a volume
of 100 cubic centimeters when subjected to standard conditions.
The tubes A, B, and C are filled with mercury of which about two and a
half kilograms will be required. By means of the leveling tube B, the
stopper in C being opened, the mercury in C is brought exactly to 109.9
cubic centimeters. The stopper in C is then closed, mercury poured into
D, which is then closed with a rubber stopper, carrying a small glass
tube as indicated in the figure.
The leveling tube B serves to regulate the pressure on the gas in A and
this is secured by depressing or elevating it as the case may require.
The tube for reducing the volume to standard conditions of temperature
and pressure, _viz._, 0° and 760 millimeters of mercury, is shown in C.
In its narrow part which has the same internal diameter as A it is
graduated into tenths of a cubic centimeter. The upper end of C is
furnished with a heavy glass neck D surmounted by a glass cup. In the
neck is placed a ground-glass stopper, carrying a groove below, which
corresponds to a similar groove above in the side of the neck whereby
communication can be established at will between the interior of C and
the exterior. The joint is also sealed by pouring mercury into D as is
shown in the figure. When the stopper is well ground and greased the
reduction tube may be raised or lowered as much as may be necessary
without any danger of escape or entrance of gas. To determine the
position of the reduction tube C the reading of the barometer and
thermometer at room temperature is taken. From the reading of the
barometer subtract one millimeter if the temperature be below 12°, two
millimeters at a temperature from 12° to 19°, three from 20° to 25°, and
four above 25°.
When a gas has been introduced into the measuring tube A it is brought
to the volume which it would assume under standard conditions by
adjusting the tube C in such a way as to bring the level of mercury in C
and A to the same point and the level of the mercury in C is exactly at
100 cubic centimeters. The gas in A is then at the volume which it would
occupy under standard conditions and this volume can be directly read.
This adjustment is secured by moving the tubes B and C up or down. If
gases are to be measured wet, a drop of water should be put on the side
of the upper part of C, and, if dry, of sulfuric acid, before the
adjustment for temperature and pressure.
=475. Method of Manipulation.=—By the action of mercury in the presence
of sulfuric acid, the nitrogen in nitrates, nitrites, nitrosulfates,
nitroses, nitrocellulose, nitroglycerol, and the greater number of
explosives, may be obtained and measured as nitric oxid. The nitrogen
compounds are decomposed in the apparatus shown in Fig. 84.
To make an analysis, the apparatus is filled with mercury, through F,
until the two openings in the cock and _i_ are entirely occupied with
that liquid. The cock _h_ is then closed, and the nitrogen compound, in
solution, introduced through _g_, care being taken that no air enters
_g_ when F is depressed and _h_ opened to admit the sample. The funnel
_g_ is washed several times with a few drops of sulfuric acid, which are
successively introduced into G. The total liquid introduced should not
exceed ten to fifteen cubic centimeters, of which the greater part
should be sulfuric acid. The rubber tube connecting G and F is carefully
closed with a clamp and G violently shaken for a few minutes until no
further evolution of nitric oxid takes place. In shaking, the apparatus
should be so held as to prevent the escape of the mercury from the small
tube _i_ by keeping it closed with the finger or drawing over it a
rubber cap.
[Illustration:
FIGURE 84.
LUNGE’S ANALYTIC APPARATUS.
]
After the evolution of the gas has ceased, the tube _e_, Fig. 83, is
brought into contact with _i_, Fig. 84, and the two are joined by a
tight-fitting piece of rubber tubing in such a way as to exclude any
particle of air. The tube F, Fig. 84, is lifted and B and C, Fig. 83,
depressed. On carefully opening the cocks _h_ and _b_ and bringing _i_
and _e_ into union, the gas is passed from G into A. When all the gas
has entered A and the acid mixture from G has reached _b_ the latter is
closed, and also _h_. The apparatus G is disconnected and removed. The
gas in A is then reduced to normal conditions by manipulating the
reduction tube C in the manner already described.
The gas in A is measured dry by reason of having been generated in
presence of rather strong sulfuric acid. Consequently, for this
operation the adjustment of the volume of gas in C should be made in
contact with a drop of strong sulfuric acid. In order to make the
readings, a quantity of material must be taken which will give less than
thirty or from 100 to 140 cubic centimeters of nitric oxid.
The quantities of the different compounds of nitric acid corresponding
to the number of cubic centimeters of nitric oxid, measured under
standard conditions, are shown in the following table:
CORRESPONDING TO
———————————— ———————————— ————————————
Cubic Weight in N₂O₃ in HNO₃ in NaNO₃ in
centimeters milligrams. milligrams. milligrams. milligrams.
of NO.
1 1.343 1.701 2.820 3.805
2 2.682 3.402 5.640 7.610
3 4.029 5.103 8.460 11.415
4 5.372 6.804 11.280 15.220
5 6.715 8.506 14.100 19.025
6 8.058 10.206 16.920 22.830
7 9.401 11.907 19.740 26.635
8 10.744 13.608 22.560 30.440
9 12.087 15.309 25.380 34.245
=476. Utility of the Method.=—Where it is desirable that the nitric oxid
method be used, and at the same time heating be avoided, the
decomposition of a nitrate by means of metallic mercury and sulfuric
acid affords a convenient and accurate procedure. But, as a rule, there
is no objection to the application of the lamp, and in such cases the
mercury method appears to have no advantage over the ferrous chlorid
process. Nevertheless, in the hands of a skilled worker the results are
reliable, and the process is a quicker one, on the whole, than by
distillation with ferrous chlorid and hydrochloric acid. This method,
however, can not be recommended as in any way superior to the reduction
methods to be hereinafter described.
ESTIMATION OF NITRIC ACID BY OXIDATION OF A COLORED SOLUTION.
=477. Method of Boussingault.=—The process for the estimation of nitric
acid by the decoloration of a solution of indigo is due originally to
Boussingault.[305] In this method the extract, obtained by washing
slowly 200 grams of soil until the filtrate amounts to 300 cubic
centimeters, is evaporated until its volume is no greater than two or
three cubic centimeters, and it is transferred to a test-tube, with
washings, and again evaporated in the tube until the volume is not
greater than that last mentioned. A few drops of solution of indigo are
added, and then two cubic centimeters of pure hydrochloric acid; the
whole is then heated. As the color of the indigo disappears more is
added. When the color ceases to fade, the liquid in the test-tube is
concentrated by boiling. If concentration fail to destroy the blue or
green color, another one-half cubic centimeter of hydrochloric acid is
introduced. The reaction is completed when neither concentration nor
fresh addition of hydrochloric acid destroys the excess of indigo
present. The color produced by a small excess of indigo is a bright
sap-green; this tint is the final reaction sought. The small excess of
indigo necessary to produce a green color is deducted in every
experiment.
When more than mere traces of organic matter are present, Boussingault
advises that the nitric acid be first separated by distillation and then
reduced by the indigo solution. For this purpose the concentrated
solution of the nitrate, two or three cubic centimeters, is placed in a
small tubulated retort with two grams of manganese dioxid in fine
powder. The retort is next half filled with fragments of broken glass,
over which is poured one cubic centimeter of concentrated sulfuric acid.
The retort is heated carefully by means of a small flame, which is kept
in motion so as to successively come in contact with all parts of the
bottom of the retort. The distillate is received in a graduated
test-tube which is kept cool. The distillation is continued until the
vapors of sulfuric acid begin to appear. The apparatus is allowed to
cool, the stopper of the retort removed, two cubic centimeters of water
introduced, and the distillation again made until fumes of sulfuric acid
are again seen. The distillation with water is made twice in order to
remove every trace of nitric acid from the retort. The distillate is
neutralized with a solution of potassium hydroxid and concentrated to
two cubic centimeters, and the nitric acid estimated in the manner
already described. The manganese dioxid used should be previously well
washed and the sulfuric must be free of nitric acid.
_Preparation of the Indigo Solution._—Fifty grams of indigo in fine
powder are digested for twenty-four hours, at 40°, in a liter of
distilled water. The water is then poured off and replaced with a fresh
supply. After the second decantation the residue is treated with 750
cubic centimeters of equal parts of water and pure concentrated
hydrochloric acid and boiled for an hour. After cooling, the undissolved
portion is collected on a filter and washed at first with hot, and
afterwards with cold water, until the filtrate is no longer colored and
is free of acid. The dried residue is treated with ether under a
bell-jar, or in a continuous extraction apparatus, until the ether is
only of a faint blue tint. The fifty grams of indigo at first taken will
give about twenty-five grams of the purified article, which, however,
will still leave a little ash on combustion.
_Solution in Sulfuric Acid._—Five grams of the purified indigo are
placed in a flask having a ground-glass stopper, treated with
twenty-five grams of fuming sulfuric acid, and allowed to digest two or
three days at a temperature of from 50° to 60°. From seventy to 200
drops of the solution thus made are placed in 100 cubic centimeters of
water for use in the process.
_Standardization of the Indigo Solution._—The solution as prepared above
is standardized by a solution of one gram of pure potassium nitrate in
1,000 cubic centimeters of distilled water. The oxidation of the indigo
solution is accomplished as described above. For this strength of
standard nitrate solution two cubic centimeters are taken corresponding
to two milligrams of potassium nitrate. The indigo solution for this
strength should have only twenty drops of the sulfuric acid solution of
indigo to 100 cubic centimeters of water. If twenty grams of potassium
nitrate are taken for 1,000 cubic centimeters of the standard solution
then 200 drops of the sulfindigotic acid should be used to 100 cubic
centimeters of water.
=478. Method of Marx.=—As usually practiced, the indigo method is
conducted according to the variation described by Marx.[306] There are
required for the process the following reagents and apparatus:
_a._ A solution of pure potassium nitrate containing 1.8724 grams per
liter. One cubic centimeter of the solution is equivalent to one
milligram of nitric anhydrid (N₂O₅).
_b._ A solution of the best indigo carmine in water which should be
approximately standardized by solution in the manner described
hereafter, and then diluted so that six to eight cubic centimeters equal
one milligram of nitric acid.
_c._ Chemically pure sulfuric acid of specific gravity 1.842, perfectly
free from sulfurous and arsenious acids and nitrogen oxids.
_d._ Several thin flasks of about 200 cubic centimeters capacity.
_e._ A small cylindrical measure holding fifty cubic centimeters and
divided into cubic centimeters.
_f._ A Mohr’s burette divided into tenths of a cubic centimeter.
_g._ A twenty-five cubic centimeter pipette or another burette.
_h._ A five cubic centimeter pipette divided into cubic centimeters or
half cubic centimeters.
_i._ A measuring flask of 250 cubic centimeters capacity.
_Preliminary Trial._—Twenty-five cubic centimeters of the sample are
transferred to a flask; the fifty cubic centimeter measure is filled
with sulfuric acid and the burette with indigo solution. The sulfuric
acid is added to the sample all at once, shaken for a moment, and the
indigo run in as quickly as possible with shaking until a permanent
greenish tint is produced. If the sample do not require more than twenty
cubic centimeters of indigo solution of the above strength, it can be
titrated directly, otherwise it must be diluted with a proper quantity
of pure water, and subjected again to the preliminary trial.
_The Actual Titration._—(1) Twenty-five cubic centimeters of the sample
properly diluted if necessary, are measured and poured into a flask, and
as much indigo as was used in the preliminary trial, is added; a
quantity of sulfuric acid, equal in volume to the liquid in the flask,
is added all at once, the mixture shaken, and indigo solution run in
quickly out of the burette until the liquid remains permanently of a
greenish tint.
(2) The last experiment is repeated as often as may be necessary adding
to the water at first half a cubic centimeter less indigo than the total
quantity used previously, afterwards proceeding as in (1) until the
final test shows too little indigo used.
(3) From the rough titration of the indigo, calculate the amount of
potassium nitrate solution corresponding with the indigo solution used
in (2), multiply the result by ten, transfer this quantity of the
standard nitrate solution to a 250 cubic centimeter flask, fill with
pure water to the mark, and titrate twenty-five cubic centimeters of
this fluid with indigo as in (2). If the quantity of indigo solution
used is nearly the same as that required in (2), its exact value may be
calculated, but if it is not, another nitrate solution may be made up in
the 250 cubic centimeter flask, more closely resembling the sample in
strength, and the titration with the indigo solution must be repeated.
(4) If the water contain any considerable amount of organic matter, it
must first be destroyed by potassium permanganate. In this case, the
estimation of the organic matter and nitric acid may be conveniently
combined.
The use of permanganate in the above case is likely to introduce an
error as has been shown by Warington. The method therefore can not be
recommended in the presence of organic matter.
=479. Method of Warington.=—The modification of the indigo method as
used by Warington, applicable only in absence of organic matter, is the
one chiefly employed in England.[307]
Instead of the ordinary indigo of commerce, indigotin is used. The
normal solution of the coloring matter is made of such a strength as to
be equivalent to a solution of potassium nitrate containing 0.14 gram of
nitrogen per liter. Where large quantities of the coloring matter are to
be used it is advisable to prepare it about four times the strength
given above and then dilute it as required. Four grams of sublimed
indigotin will furnish more than two liters of the color solution.
The solution is prepared as follows:
Four grams of indigotin are digested for a few hours with five times
that weight of Nordhausen sulfuric acid, diluted with water, filtered,
and made up to a volume of two liters. The strength of the indigotin
solution is determined with a solution of potassium nitrate of the
strength mentioned above. The process is performed as follows:
From ten to twenty cubic centimeters of the standard nitrate solution
are placed in a wide-mouthed flask of about 150 cubic centimeters
capacity. A portion of the indigotin solution is next added, such as
will be deemed sufficient for the process, and the whole is well mixed.
Strong sulfuric acid is next measured out from a burette into a
test-tube, in volume equal to the united volumes of the nitrate solution
and indigotin. The whole of the sulfuric acid is then poured as quickly
as possible, into the solution in the flask and rapidly mixed, and the
flask transferred to a calcium chlorid bath, the temperature of which
should be maintained at 140°. It is essential to the success of the
operation that the sulfuric acid should be mixed with the greatest
rapidity. It should be poured in at once and the whole well shaken
without waiting for the test-tube containing the acid, to drain. The
flask should be covered by a watch-glass while it is held in the bath.
As soon as the larger part of the indigotin is oxidized the flask in the
bath should be gently rotated. With very weak solutions of nitrate it
may be necessary sometimes to keep the flask in the bath for five
minutes. When the indigo color is quickly discharged it shows the
presence of nitric acid in considerable excess and a considerably larger
quantity of indigo must be taken in the next experiment. The experiments
are continued until just the quantity of indigo necessary to consume the
nitric acid is taken, the amount of indigo being in very slight excess,
not exceeding one-tenth cubic centimeter of the indigo solution used.
The tint produced by the small excess of indigo remaining is best seen
by filling the flask with water. On substances of approximately known
strength about four experiments are usually necessary to determine the
amount of indigo to be taken, but with unknown substances a larger
number may be necessary.
Usually in determinations of this kind it is directed to use double the
volume of sulfuric acid mentioned above. In this case not only is the
quantity of indigo oxidized much greater than with a smaller portion of
acid, but the prejudicial effect of organic matter is also greater than
when the smaller quantity of acid is employed.
An indigo solution standardized as above is strictly to be used for a
solution of nitrate of the strength employed during the standardization.
The quantity of indigo oxidized in proportion to the nitric acid present
diminishes as the nitrate solution becomes more dilute. Instead of
determining this during each series of experiments it may be estimated
once for all and a table of corrections used.
The following table is based upon experimental determinations:
Strength of Indigo Difference Nitrogen Difference Difference
niter required, between corresponding between the in the
solution cubic amounts of to one cubic nitrogen nitrogen
used. centi- indigo, centimeter of values, values for a
meters. cubic indigo, gram. gram. difference
centi- of one cubic
meters. centi- meter
in the
amount of
indigo,
gram.
⁸⁄₆₄ Normal 10.00 0.000035000
⁷⁄₆₄ „ 8.71 1.29 0.000035161 0.000000161 0.000000125
⁶⁄₆₄ „ 7.43 1.28 0.000035330 0.000000169 0.000000132
⁵⁄₆₄ „ 6.14 1.29 0.000035627 0.000000298 0.000000231
⁴⁄₆₄ „ 4.86 1.28 0.000036008 0.000000381 0.000000298
³⁄₆₄ „ 3.57 1.29 0.000036763 0.000000756 0.000000586
²⁄₆₄ „ 2.29 1.28 0.000038209 0.000001445 0.000001129
¹⁄₆₄ „ 1.00 1.29 0.000043750 0.000005541 0.000004295
The table is used as follows:
Suppose that twenty cubic centimeters of water under examination have
required 5.36 cubic centimeters of indigo solution for the oxidation of
the nitric acid contained therein. By inspection of the table it is seen
that this number is five-tenths cubic centimeter above the nearest
quantity given; _viz._, 4.86 cubic centimeters. From the last column in
the table it is found that the correction for five-tenths cubic
centimeter of indigo solution is 0.000000149 cubic centimeter, being
half that for the one cubic centimeter given in the table. This is to be
subtracted from the unit value in nitrogen given in the first “gram”
column of the table; _viz._, 0.000036008. It is thus seen that the 5.86
cubic centimeters of indigo solution are equivalent to 0.000035859 gram
of nitrogen per cubic centimeter. The water under examination,
therefore, contains nine and six-tenths parts of nitrogen as nitric acid
per million.
Attention must also be paid in standardizing indigo solutions to the
initial temperature of the solutions. A rise in the initial temperature
will be attended by a diminution in the quantity of indigo oxidized.
Experiments with a room temperature of 10° and a room temperature of
20°, being the initial temperatures of the experiments, showed that at
the higher temperature the amount of indigo consumed was about five per
cent less when the strong solutions of nitrate were employed. The indigo
solution should, therefore, be standardized at the same temperature at
which the determinations are made.
If twenty cubic centimeters of the standard nitrate solution employed be
used in setting the indigo solution, this standard will enable the
operator to determine nitric acid up to 17.5 parts of nitrogen per
million in water or soil extracts.
The presence of an abundance of chlorids in the water under examination
tends to diminish the content of nitric acid found, and also tends to
introduce an error, which is sometimes of a plus and sometimes of a
minus quantity, according to the strength of the nitric acid present.
The reaction is shortened in weak solutions by the presence of chlorids,
and the quantity of indigo consumed is consequently increased. The error
introduced by chlorids is usually of an insignificant nature.
On account of the interference of organic matters with the reaction of
indigo it is not of much use in the examination of nitrates washed out
of soils, although in some cases the results may be quite accurate. This
method must, therefore, be considered as applicable, in general, to
waters or soil extracts which contain little or no organic matter.
In analytical work pertaining particularly to agriculture, the use of
the indigo method for determining nitric acid has been largely employed,
both in the analyses of soil extracts and drainage and irrigation
waters. The method, however, can hardly survive as an important one in
such work in competition with more modern and speedy processes of
analysis.
DETERMINATION OF NITRIC NITROGEN BY REDUCTION TO AMMONIA.
=480. Classification of Methods.=—When nitrogen is present in a highly
oxidized state, _e. g._, as nitric acid, it may be quickly and
accurately estimated by reduction to ammonia. This action is effected by
the reducing power of nascent hydrogen, and this substance may be
secured in the active state by the action of an acid or alkali on a
metal, or by means of an electric current. The processes depending on
the use of a finely divided metal in the presence of an acid or alkali
have come into general use within a few years, and are now employed
generally instead of the more elaborate estimations depending on the use
of copper oxid or indigo.
The typical reaction which takes place in all cases is represented by
the following equation:
2HNO₃ + 8H₂ = 2NH₃ + 6H₂O.
The method will be considered under three heads; _viz._, 1. Reduction in
an alkaline solution; 2. Reduction in an acid solution; 3. Reduction by
means of an electric current.