C. Remigius Fresenius.

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action of the carbonic acid of the atmosphere upon the excess of
the baryta water, add the washings to the filtrate, and determine
in the fluid the barium as directed in 101. Calculate for each
1 at. barium 2 mol. nitric acid. Lastly, free nitric acid may also
be determined in a simple manner by supersaturating with am-
monia, evaporating in a weighed platinum dish, drying the resi-
due at 110 to 120, and weighing the NH 4 NO 8 (SCHAFFGOTSCH).



DETERMINATION. [ 149.

Of course, the results can be accurate only then when the ammo-
nia employed leaves no residue when evaporated from platinum.

II. Separation of nitric acid from the basic radicals ,
and determination of the acid in nitrates.

The determination of combined nitric acid is an important and
at times difficult problem, the solution of which has engaged the
attention of many chemists of late. I would advise that, after
selecting the method it is intended to use, it be tried repeatedly
on weighed quantities of a pure nitrate in order to become per-
fectly familiar with the method, and to acquire the skill without
which accuracy cannot be expected in the frequently complicated
processes. Of the great number of methods proposed I shall
confine myself to describing only the simplest and best.

a. Methods based on the decomposition of Nitrates in the
Dry Way.

ot. In anhydrous metallic nitrates which leave upon ignition a
metallic oxide of known and definite composition, the nitric acid
may be determined by ignition and calculation from the weight of
the residue.

ft. In the case of nitrates the residue of which on ignition
has no constant composition, or by the ignition of which the cru-
cible is much attacked (alkali and alkali-earth nitrates), fuse the
substance (which must be anhydrous and also free from organic
and other volatile bodies) with a non-volatile flux, and estimate
the nitric acid from the loss. The following have been proposed
as fluxes : Borax glass, by SCHAFFGOTSCH * (3 parts borax glass to
1 part nitrate) ; potassium bichromate, by PERSOZ f (2 parts of
the bichromate to 1 part nitrate), and silicic acid, by REICH. :f
All three give satisfactory results when careful regard is paid to
the peculiarities of the individual fluxes. Silicic acid is the best
flux, as it may be readily procured, and the execution is the most
easy and the most certain to succeed. I shall describe the method
in its application to potassium or sodium nitrate.

Fuse the latter at a low temperature, pour out on to a warm

*Pogg. Annal., LVII, 260.

f Rep. decliim. appliquee, 1861, 253; Zeitschr. f. analyt. Chem., T, 85.
\Berg- und llnttcitmannische Zeitschrift, 1861, No. 21; Zeitschr. f. analyt.
, i, 86. Zeitschr. f. analyt. Chem., i, 181.



149.] NITRIC ACID. 573

porcelain dish, powder, and dry again before weighing. Now
transfer to a platinum crucible 2 to 3 grm. powdered quartz, ignite
well, and weigh after cooling. Add about 0'5 grin, of the salt
prepared as above, mix well, and convince yourself by the balance
that nothing has been lost during mixing. The covered crucible
is then exposed to a low red heat (just visible by day) for half an
hour and weighed, after cooling, with the cover. The loss of
weight represents the quantity of N 2 O 6 . Sulphates or chlorides
are not decomposed at the given temperature ; if a higher heat be
applied, the latter may volatilize. The action of reducing gases
must be avoided. The test analyses, communicated by REICH (loo.
cit.\ as well as those performed in my own laboratory,* gave very
satisfactory results.

1). Method based on the distillation of Nitric Acid.

All nitrates may be decomposed by distillation with moderately
dilute sulphuric acid. The nitric acid passing into the receiver
may then be determined according to I, volumetrically or gravi-
metrically. This process was originally proposed by GLADSTONE f,
but was later carefully studied by II. ROSE and FINKENER J. 1
to 2 grm. of the nitrate should be treated with a cooled mixture
of 1 volume concentrated sulphuric acid and 2 volumes water.
For 1 grm. nitre take 5 c. c. sulphuric acid and 10 c. c. water.
The distillation may be performed either with a thermometer at
160 to 170 in a paraffin- or sand-bath (duration of the distilla-
tion for 1 to 2 grm. nitre, 3 to 4 hours) or in vacuo, with the use
of a water-bath. The latter process is the better. In the former,
the neck of the tubulated retort (which is drawn out and bent
down) is connected with a bulbed U-tube containing a measured
quantity of standard soda or potassa solution ( 215). The dis-
tillation in vacuo may be conducted, without the use of an air-
pump, according to FINKENER, as follows : Transfer the measured
quantity of water and concentrated sulphuric acid to the tubulated
retort, and the necessary quantity of standard potassa or soda solu-
tion, diluted to 30 c. c., to a flask with a narrow neck of about
200 c.c. capacity. Then, by means of an india-rubber tube, con-

* Zeilschr. /. analyt. Chem., I, 184. f Journ. /. prakt. Chem., LXIV, 442.

IZeilschr.f. analyt. Chem., i, 309.

The bulbed U-tube will be found figured in 185.



674 DETERMINATION. [ 149.

nect the flask with the retort air-tight, so that the drawn-out point
of the latter may extend to the body of the flask, and with tub-
ulure open heat the contents of the retort and of the flask to
boiling. "When the air has been expelled from the apparatus by
long boiling, transfer the salt (weighed in a small tube) to the
retort through the tubulure, close the latter immediately, and
at the same time take away the lamp. The retort is then heated
on a water-bath, the flask being kept cool. The quantity of
nitric acid that has passed over is finally ascertained by determin-
ing the still free alkali with standard acid. If it is suspected that
all the nitric acid has not been driven into the receiver by one
distillation, you may by heating the flask and cooling the retort
distil the water back into the latter, and then the distillation
from the retort may be repeated. The distillate thus obtained is
always free from sulphuric acid, hence the results are very exact.
The base remains as sulphate in the retort. In the presence of
chloride add to the contents of the retort a sufficiency of dissolved
silver sulphate, or when much chloride is present moist silver
oxide. The nitric acid is then obtained entirely free from
chlorine.

c. Methods based on the decomposition of Nitrates ~by Alka-
lies and Alkali Earths.

a. Nitrates of metals which are completely precipitated by
alkali hydroxides or carbonates provided basic salts are not pre-
cipitated at the same time may be analyzed by simple boiling
with an excess of standard potassa or soda or their carbonates.
After cooling, dilute to J or -J- litre, mix, allow to settle, draw off
a portion of the supernatant clear fluid, determine the free alkali
remaining in it, and calculate therefrom the amount which has
been converted into nitrate. This method was used by LANGER
and WAWNIEKIEWICZ, * but was, however, already previously known.
HAYES obtained with silver and bismuth nitrates good results ; but
with mercurous nitrate (using sodium carbonate) the results were
not so satisfactory. f If the method is applied to ammonium
nitrate, heat must be applied after adding the alkali, until all the
ammonia is expelled. That the method is applicable only when
no other acid is present need scarcely be mentioned.

* Annal. d. Chem. u. Pharm., cxvri, 230.

f II. ROBE, Zoitechrift f. analyt. Ctiem., i, HOO.



149.] NITRIC ACID. 575

ft. Itt nitrates from which the basic metals are precipitated by
barium or calcium hydroxides or their carbonates (or by barium
sulphydrate, recently precipitated and free from barium thiosul-
phate GLAUS*), the nitric acid may be estimated with great
accuracy if no other acids are present by filtering, after pre-
cipitation has been effected, warm or cold, passing carbonic acid
through the filtrate, if necessary, till all the barium is precipitated,
warming, filtering, and determining the barium in the filtrate by
sulphuric acid. 1 at. of the barium corresponds to 1 mol. nitric
anhydride (N" a OJ. In case of bismuth-nitrate, boil after adding
the barium hydroxide until the separated oxide is perfectly yellow.



y. In many nitrates the bases of which are precipitable by
hydrogen sulphide, the nitric acid may be determined accord-
ing to GIBBS by adding to the salt in solution about its own weight
of some neutral organic salt, e.g., Rochelle salt, and throwing
down the metal by H a S. The filtrate and washings are brought
to a definite bulk, and the free acid is determined in aliquot por-
tions alkalimetrically. \

d. Methods 'based upon the decomposition of Nitric Acid by
Ferrous Chloride.

a. PELODZE T was the first to utilize the action of free nitric
acid on ferrous chloride in the determination of nitric acid. The
decomposition is as follows :

6FeCl a + 2KIS T O 3 + 8HC1 = 3Fe,Cl 6 + 2KC1 + 4H 3 O + N 3 O a .

PELOUZE used a known quantity of ferrous chloride in excess,
and titrated the excess with potassium permanganate, l^ie
method used by him, and given in the foot-note, affords variable
results, sometimes good, sometimes not reliable ; on this point all

* ZeitscJir. /. analyt. Chem., I, 372. f lb -> Vi, 233.

| Amer. Jour. Sd.. XLIV, 209.

f Journ. /. prakt. Chem. XL, 324.

Dissolve 2 grm. piano wire in 80 to 100 c. c. pure concentrated hydrochloric
acid with the aid of heat, in a 150-c. c. flask the cork of which is fitted with a
glass tube. Then add ! 2 grm. of the potassium nitrate, or an equivalent quan.
tity of another nitrate to be analyzed, stopper, and rapidly heat to boiling.
After five or six minutes pour the fluid, which has again cleared, into a larger
flask, dilute with much water, and estimate the ferrous chloride present with
permanganate.



576 DETERMINATION. [ 149.

who used the method agree (compare FR. MOHR,* and ABEL and
BLOXAM f). The numerous experiments made in my own labor-
atory also confirm this. The reasons for the lack of accuracy of
the method are as follows :

a. Action of the air on" the nitric oxide in the flask in the
presence of the aqueous vapor therein, whereby nitric acid is
regenerated ; this is the principal cause of inaccuracy.

b. Incomplete expulsion of the nitric oxide from the liquid,
in consequence of which more permanganate is reduced than cor-
responds to the ferrous chloride present; this is to be appre-
hended only in dilute solutions.

c. Escape of nitric acid before it has acted upon the ferrous
chloride; this is to be apprehended when the liquid is boiled
rapidly after adding the nitrate, and when the excess of ferrous
chloride is comparatively small.

d. Occasionally loss of iron from incautious boiling ; this is to
be apprehended when a part of the ferrous chloride deposits in
solid form on the sides of the vessel above the fluid.

I have succeeded in so modifying the process as to avoid all
these sources of error and to obtain results which, so far as relia-
bility and accuracy are concerned, are perfectly satisfactory. My
process is as follows :

Select a tubulated retort of about 200 c. c. capacity, with a
long neck, and fix it so that the latter is inclined a little upwards.
Introduce into the body of the retort about 1-5 grm. fine piano-
forte wire, accurately weighed, and add about 30 or 40 c. c. pure
fuming hydrochloric acid. Conduct now through the tubulure, by
means of a glass tube reaching only about 2 cm. into the retort,
h\ -.In. -vn -us washed by solution of potassa, or pure carbonic acid,
and connect the neck of the retort with a U-tube containing some
water. Place the body of the retort on a water-bath, and heat
gently until the iron is dissolved. . Let the contents of the retort
cool in the current of hydrogen gas or carbonic acid ; increase the
latter, and drop in, through the neck of the retort, into the body,
a small tnhc containing a weighed portion of the nitrate under
examination, which should not contain more than about 0*2 grm.

* I^hrbuch der Titrirmethode, i, 216.

\Quart. .lonrn. of the Chem. Soc., ix, 97; also Journ. f. prakt. CTiem.,



149.] NITRIC ACID. 577

of N a O*. After restoring the connection between the neck and
the U-tube, heat the contents of the retort in the water-bath for
about a quarter of an hour, then remove the water-bath, heat with
the lamp to boiling, until the fluid, to which the nitric oxide had
imparted a dark tint, shows the color of ferric chloride, and con-
tinue boiling for some minutes longer. Care must be taken to
give the fluid an occasional shake, to prevent the deposition of dry
salt on the sides of the retort. Before discontinuing boiling,
increase the current of hydrogen or carbonic-acid gas, so that no air
may enter through the U-tube when the lamp is removed. Let the
contents cool in the current of gas, dilute copiously with water, and
determine the iron still present as ferrous chloride volumetrically
by potassium dichromate or permanganate 335 -4 of iron con-
verted by the nitric acid from ferrous to ferric chloride correspond
to 108*08 (N 2 O 5 ). My test-analyses of pure potassium nitrate
gave 100-1 100-03 100-03, and 100-05, instead of 100.*
[The iron remaining as ferric chloride may also be determined by
sodium thiosulphate.]

y?. Since we have learned to titrate ferric salts . di-
rectly with accuracy, it is, as a rule, more convenient and
exact not to estimate (as in a) the residual ferrous salt, but to
determine the ferric salt produced, as first pointed out by C. D.

BRAUN.f

I would recommend the following method as the best. J Besides
the requisites for titrating ferric chloride by means of stannous
chloride (p. 327), there is required a solution prepared by dissolving
100 grm. ferrous sulphate as free from ferric salt as possible in 150
to 200 c. c. of hydrochloric acid (sp. gr. 1-1 1-2) by the aid of
heat in a 500-c. c. flask, and finally filling the flask to the mark
with fuming hydrochloric acid, and shaking. As, however, a
solution absolutely free from ferric salt cannot be obtained, esti-
mate according to the method given on page 327 how much stan-
nous chloride solution is required to reduce the ferric chloride
present in 50 c. c. of the solution made as above. It is advisable

* Annal. de CJiem. u. Pharm., cvi, 217.

\Journ.f. prakt. Chem., LXXXI, 421.

\ It is particularly convenient when several analyses are to be made ; when
only one or two estimations are to be made, however, the iron wire may be dis-
solved in hydrochloric acid as in a.



578 DETERMINATION. [ 149.

to heat the solution in an atmosphere of carbon dioxide, and to
titrate it either immediately before or directly after the
analysis.

Place the weighed nitrate (the quantity must be such as to con-
tain not more than 0'2 grin, nitric acid) in a long-necked flask,
provided with a doubly- perforated stopper carrying two glass
tubes. One of these reaches to the body of the flask, while the
other but just enters it. Through the former pass in a current of
carbon dioxide, and when all the air has been displaced introduce
50 c. c. of the hydrochloric-acid solution of ferrous sulphate; con-
tinue to pass in the carbon dioxide for some time longer, then heat,
at first gently for some time, and then gradually to boiling, until
the liquid has lost its blackish color and exhibits the pure color of
ferric chloride, and until the escaping gas, passed into dilute starch
paste containing a little potassium iodide, ceases to exhibit the
blue color of starch iodide. Now remove the stopper from the
flask, rinse off the longer tube if necessary, dilute the residue
with double its volume of water, and estimate the ferric chloride
as on p. 327. The cooling for determining the slight excess of
stannous chloride with iodine is best conducted in a current of car-
bon dioxide. From the stannous-chloride solution used altogether
deduct first the small excess ascertained by the iodine solution,
and then the small quantity corresponding to the ferric salt pre-
sent in 50 c. c. of the acid ferrous-sulphate solution. The re-
mainder gives the iron in the ferric salt produced ; and, when
multiplied by 0'3222-t, gives the nitric acid. This factor is ob-
tained thus : 6 eq. of iron (335 -4) : 1 eq. of N a O 6 (108 08) : : ferric
iron present : x.

It will be seen that it is best, once for all, to multiply the
known quantity of iron in the ferric-chloride solution used for
standardizing the stannous-chloride solution by the above factor,
and to mark the product on the label as the quantity of nitric
acid corresponding to 10 c. c. of ferric-chloride solution. If no
Bt&ndardized ferric-chloride solution is at hand, the stannous-chlo-
ride solution may l.e standardized directly against nitric acid by
adding a weighed quantity of pure potassium nitrate to 50 c. c. of
the acid ferrous-sulphate solution, and then determining the fer-
ric chloride formed according to the method given above. The



149.] NITRIC ACID. 579

results, are perfectly satisfactory if the process is properly carried
out, and the estimations succeed each other immediately.*

y. SCHL(>SING'S method, f which was employed more espe-
cially for estimating nitric acid in tobacco, affords the important
advantage that it may he employed in the presence of organic
matter. Numerous experiments have shown this method to be
thoroughly satisfactory. It is conducted in the apparatus shown
in Fig. 107.




Fig. 107.

The dissolved nitrate is introduced into the flask J., the
drawn-out neck of which is connected by means of a rubber tube,
a, with a narrow glass tube, 5; c is another rubber tube, 15 cm-
long, and connected with J. The solution of the salt, which
must be neutral or alkaline, is boiled down to a small volume,
the aqueous vapor completely expelling all the air from A and
the tubes, c is then immersed in a solution of ferrous chloride
in hydrochloric acid contained in a glass vessel, the lamp removed,
and the receding regulated by compressing the tube c with the
fingers. "When the iron solution is almost entirely absorbed a
little hydrochloric acid is allowed to recede, in separate portions,
three or four times, in order to entirely free the tube from fer-
rous chloride, this being absolutely necessary. Before the air caix
force its way into the tubes c is closed by means of an iron com-
pression-cock, and its end immersed into the mercury in the
trough and brought up under the bell B. The lamp is again

*Zeitschr.f. analyt. Chem., i, 38. HOLLAND gives a method in which the
use of an indifferent gas is unnecessary. Zeitschr. f. analyt. Chem., vin, 452 ;
Ghem. News, xvn, 219.

f Annal. de Chem. 3 ser., XL, 479 ; Journ.f. prakt., CJiem., LXII, 142.




580 DETERMINATION. [ 149.

placed under A, in order to let the reaction proceed, and the
compression -cock immediately replaced by the pressure of the
fingers, this compression being in turn relieved as soon as a press-
ure is felt from within. The reaction is ordinarily at an end in
eight minutes, when c is removed from under B. 13 is a small
bell- jar, made from an adapter, and must have a capacity three

to four times the volume of gas to
be received. In cases where the
evolution of gas is impetuous it is
at times necessary to immerse it in
the trough in order to better con-
Fig. 108. dense the vapors. The upper part
of B is drawn out as shown in Fig. 108, in order to facilitate its
ready insertion into the rubber tube and also the breaking off of
Its point. The bell is first filled with water in order to expel all
the air, and then with mercury ; well-boiled milk-of-lime is then
introduced by means of a curved pipette. The nitric oxide
entering B is thus freed from the slightest trace of acid vapor.
The nitric oxide is now to be transferred- into the flask (7, there
to be reconverted into nitric acid by means of oxygen. The
flask C contains a little water ; it is connected by means of the
rubber tube d with the glass tube <?, the other end of which car-
ries the narrow rubber tube, /, 10 cm. long.

The water in C is now heated to boiling, and all the air ex-
pelled from the flask and tubes by the aqueous vapor ;/ is then
connected with the tip of the bell-jar 7?, which has previously been
slightly scratched with a diamond, and the tip is then broken off.
At first the aqueous vapor condenses in the bell-jar, and expels
at the same time the small quantity of milk-of-lime remaining in
the tip. On now removing the lamp, a return current is soon
established, which drives the . nitric oxide into C. If this pro-
ceeds too rapidly, f is compressed by the fingers. As soon as the
milk-of-lime in the bell-jar has nearly reached the rim of/, the
latter is closed by a compression-cock. 20 to 30 c. c. of pure
hydrogen gas are now introduced into the bell-jar, in order to
insuiv the trunsferral of the last traces of nitric oxide into 6 y , to
b<- :ih.-orle.l likr th,- n ->t. f \& im\v elo>ed with the rompressioii-
cock, its cud removed from the tip of the bell- jar and connected
insU-a-1 with the glass tube h of the oxygen jar D\ the cock, r,



149.] NITRIC ACID. 581

f

of this is now opened, and then the compression-cock, thus allow-
ing oxygen to enter c. As soon as the object of the operation is
effected, r- is closed, h and /separated, and, after waiting fifteen
minutes, the regenerated free nitric acid is estimated by means of
very dilute soda lye ( 215).

The success of the. method depends materially upon the com-
plete expulsion of air from a and c. The test experiments made
by SCHLOSING, as well as the analyses made in my own labora-
tory, yielded highly satisfactory results.* Similarly satisfactory
results were also obtained by R. FRUHLiNGf and H. GROUVEN,
as well as by E. SCHULZE. When the quantity of nitric acid is
small, it is advantageous to use a larger quantity of ferrous
chloride.

It is quite evident that the apparatus described above may be
variously modified, while the principle of its action is still retained.
For instance, SCHLOSING recommended a slightly modified appa-
ratus for the estimation of very small quantities of nitric acid (less
than O'Ol grm). FRUHLING and GROUVEN (loc. cit.\ also devised
various, though unimportant, modifications. The most decided
modification is that proposed by E. REICH ARDT. In this the mer-
cury trough is dispensed with, and the nitric oxide is received in
a vessel filled with soda-lye, after all the air in the apparatus
has been expelled by hydrogen. Since, however, it is difficult to
obtain this gas perfectly free from oxygen, the results obtained
with this modification are apt to be too low.

The modification proposed by F. SCHULZE, and described by
H. WULFERT, [ differs from the other modifications in that the
nitric oxide is first collected in a bell- jar filled with mercury and
provided with a glass cock, then conducted into a measuring-
tube, and its volume measured. If a foreign gas is also present,
its volume is also ascertained by absorbing the nitric oxide with
ferrous-chloride solution. In his experiments, however, WUL-
FERT never found more than 0*33 c. c. unabsorbable gas. The
results obtained were very satisfactory, even with very small quan-
tities o nitric acid, and in the presence of considerable organic
matter.

* Zeitschr. /. analyt. Chem., i, 39. 75., ix, 24.

\Landwirthschqftl. Versuchsstat., ix, 14 and 150. ||76., ix, 400.

j Zeitschr. f. analyt. Chem., YI, 384.



582



DETERMINATION.



[ 149.



<?. SCIIULZE'S Method* modified by

The solution containing the nitrate is concentrated if necessary
to a volume of about 50 c. c. and introduced into the flask A,
which should have a capacity of about 200 c. c. This flask
(Fig. 109) is provided with a rubber stopper, through which pass




Fig. 109.

two bent tubes, a b c and efg. The first is drawn out to a point
(not too small) at , and projects through the stopper about
2 cm. ; the second terminates without diminution of size exactly
at the lower surface of the stopper. These two tubes are .con-
nected by rubber tubes (bound with thread) at c and y with the
glass tubes c <l and g h. A rubber tube is drawn over the lower
end of (j h to protect it from fracture. B is a glass vessel con-
taining 10-per cent, soda solution. A measuring tube graduated
in O'l c. c., of not too great diameter, filled with previously boiled



* Zeitschr. f. analyt. Chem., 1870, 400.

t Anleitung zur Uidersuchung. von Wasser, von W. KUBEL, Zweite AuflagQ
von F. TIEMANN, Bruunch.sweig, Fr. Vieweg u. Solm., 1870, s. 55,



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