C. Remigius Fresenius.

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= 2NaI -|- Na 3 S 4 O 6 . The salt under examination is dissolved in a
large amount of water, starch-paste added, and then the neutral
solution is titrated with iodine. That this method can give correct
results only in cases where no other substances acting upon iodine
are present, need hardly be mentioned. Thiosulphuric acid may,
like sulphurous acid, be converted into sulphuric acid by means
of chlorine or bromine water, and then determined.

4. IODIC ACID.

lodic acid may be determined by the following easy method :
Distil the free acid or iodate with an excess of pure fuming hydro-
chloric acid, in the apparatus described in 130, e, ft (chromic acid),
receive the disengaged chlorine in solution of potassium iodide, and
determine the separated iodine as directed in 1 30, I, 0, ft. The
decomposition of iodic acid by hydrochloric acid is represented by
the equation, IIIO 3 + 5HC1 = f 01 + 401 + 3H S O. Since the 4
at. 01 set free 4 at. I, the amount of iodic acid or iodic anhydride
can be calculated from the weight of the latter ; 1014-8 iodine cor-
respond to 333-7 iodic anhydride (I,O B ) (BUNSEN*). The following
method also yields good results. Mix the solution with dilute sul-
phuric acid, add potassium iodide in excels, ;md determine the
amount of liberated iodine, after 146. One sixth of the iodine
thus formed is derived from the iodic acid (HIO.+ 5HI = 3II,O

See RAMMELSBERG.f

* Annnl. d. Chem. u. Phann., i.xxxvi, Csr>.

\Pogg. Annal , cxxxv, 493; Zcitschr.f. unalyt. Chem., VIII, 456.



131.] NITROUS ACID. 433

5. NITROUS ACID.

4

The nitrous acid in nitrites which are free from nitrates may
be estimated by converting the nitrogen into ammonia and deter-
mining the latter, or by determining the oxidizing action on ferrous
salt. This method is conducted exactly as described under nitric
acid ( 149). When nitric acid is also present, nitrous acid may be
determined very satisfactorily with a solution of pure potassium
permanganate, provided the fluid be sufficiently diluted to prevent
the nitrous acid, which is liberated by the addition of a stronger
acid, being decomposed by water with formation of nitric acid
and nitric oxide. For 1 part of nitrous anhydride at least 5000
parts of water should be present. The decomposition is "repre-
sented by the following equation : 5HNO 2 + K,Mn 2 O 8 + 3H 2 SO 4
= 5HNO 3 + K 2 SO 4 + 2MnSO 4 + 3H 2 O. If the permanganate
be standardized with iron, 4 at. iron correspond to 1 mol. N 2 O 9 ,
since both of these require 2 at. oxygen. Nitrites are dissolved in
very slightly acidulated water, the permanganate is added till the
oxidation of the nitrous acid is nearly completed, the solution is
then made strongly acid, and finally permanganate is added to light-
red coloration.

To determine nitrogen tetroxide N 2 O 4 in red fuming nitric acid,
transfer a few c.c. to about 500 c.c. cold pure distilled water with
stirring, and determine the nitrous acid produced. . 1 mol. nitrous
anhydride found corresponds to 2 mol. nitrogen tetroxide, for the
latter when mixed with such a large quantity of water as is indi-
cated above is decomposed in accordance with the following equa-
tion : _N 2 4 + H 2 = HN0 3 + HN0 2 (Sio. FELDHAUS*).

Nitrous acid and nitrogen tetroxide in presence of nitric acid
may also be estimated by the reduction of chromic acid. An
excess of standard potassium dichromate is added, and the unde-
composed residue of chromic acid is estimated with standard solu-
tion of ferrous salt (FR. Monsf).

As regards the estimation of nitrous acid with lead dioxide,
comp. FELDHAUS, loo. cit. p. 431, also LANG and J. LOWENTHAL.

Regarding the estimation of nitrous acid in water, see 205.

* Zeitschr. f. analyt. Chem., i, 426.

\Lehrbuch der Titrirmethode, 3. Aufl., 236.

$ Zeitschr. /. analyt. Chem., I, 485. lb., HI, 176.



434 DETEKMISTATION. [ 132.

Second Division of the First Group of the Acids.
SULPHURIC ACID ; (Hydrofluosilicic Acid).

132.
SULPHURIC ACID.

I. DETERMINATION.

Sulphuric acid is usually determined in the gravimetric way as
larium sulphate. The acid may, however, be estimated also by
the acidimetric method (2 15), and by certain volumetric methods,
based upon the insolubility of the barium sulphate (and lead sul-
phate).

1. Gravimetric Method.

The exact estimation of sulphuric acid as barium sulphate is by
no means so simple and easy as it was formerly supposed to be, but
requires, on the contrary, great care and attention. This arises
from three causes ( 71, a): First, the barium sulphate is found
to be far more soluble than was imagined in solutions of free acids
and of many salts ; secondly, it is extremely liable to carry down
with it foreign salts, which are of themselves soluble in water;
thirdly, when the precipitate has once separated in an impure
state, it is often very difficult to purify it completely.

The solution should contain but little free hydrochloric acid,
and no nitric or chloric acid. If either of the two last are present,
evaporate repeatedly, on the water-bath with pure hydrochloric
acid. Dilute considerably, heat nearly to boiling, add barium chlo-
ride in moderate excess, and allow to settle for a long time at a
gentle heat. Decant the clear fluid through a filter, treat the pre-
cipitate with boiling water, allow to settle, decant again, and so on,
till the washings are free from chlorine. Finally transfer the pre-
cipitate to the filter, dry and treat according to 53, using only a
moderate red heat.

After the precipitate has been weighed it is well to warm it for
some time with dilute hydrochloric acid on the water-bath. Then
pour off the hydrochloric acid through a small filter, wash the pre-
cipitate by decantation with boiling water without removing it to
the filter, evaporate the filtrate and washings nearly to dry ness in
a platinum or porcelain dish, add water, collect the minute amount



132.] SULPHURIC ACID. 435

of barium sulphate here left undissolved upon the small filter,
wash, dry, incinerate, add the ash to the bulk of the precipitate,
ignite again, and weigh. If the precipitate has lost weight, this
shows that it at first contained foreign salts.

This method of purification sometimes fails when the precipi-
tate contains ferric oxide or platinum (GLAUS*), and it invariably
fails when the solution contained any notable quantity of nitric
acid.f In such cases there is only one resource, namely, to fuse
with about four parts of sodium carbonate, warm with water, filter,
wash with boiling water, acidify the filtrate slightly with hydro-
chloric acid, and determine the sulphuric acid again.

The results are thoroughly satisfactory if these directions are
attended to ; if not, the result may be two or three per cent, too
high or too low.

2. Volumetric Methods.

a. After GAEL MOHK.;); We require a normal solution of
barium chloride, containing 122-166 grm. of the pure crystallized
salt in 1 litre, and also normal nitric or hydrochloric acid and
normal soda ( 215). Add to the fluid to be examined for
sulphuric acid which, should it contain much free acid, is previ-
ously to be nearly neutralized with pure sodium carbonate a meas-
ured quantity of barium chloride solution, best a round number
of cubic centimetres, in more than sufficient proportion to precipi-
tate the sulphuric acid, but not in too great excess. Digest the
mixture for some time in a warm place, then precipitate, without
previous filtration, the excess of barium chloride with ammonium
carbonate and a little ammonia, filter off the barium sulphate and
carbonate, wash until the water running of? acts no longer upon
red litmus paper, and then determine the barium carbonate by the
alkalimetric method given in 223. Deduct the c.c. of normal
acid used from the c.c. of barium chloride, and the remainder will
be the c.c, of barium chloride corresponding to the sulphuric acid
present. The results of this method are quite satisfactory, if the
solution does not contain too much free acid; but in presence of a
large excess of free acid, the action of the salt of ammonia will
retain barium carbonate in solution, which, of course, will make

* Jahresber. von KOPP und WILL, 1861, 323, note.
tCompare my paper in Zeitschr. f. analyt. Chein., ix, 52.
\ Ann. der Chem. u. Pharm., xc, 165.



436 DETERMINATION. [ 132

the amount of sulphuric acid appear higher than is really the
case. It need hardly be mentioned that this method is altogether
inapplicable in presence of phosphoric acid, oxalic acid, or any
other acid precipitating barium salt from neutral solutions, and that
no basic radicals except the alkalies may be present.

b. After AD. CLEMM.* In order to render C. MOHR'S method
more expeditious, and hence better adapted for the use of manu-
facturers, CLEMM lias modified it. In it, also, the absence is
required of all other acids which yield insoluble barium salts ; all
bases except the alkalies must also be absent. In addition to the
standard solutions mentioned under #, there is also required a
normal solution of pure sodium carbonate (53 -45 grm. anhydrous
salt contained in 1 litre). Add a little litmus tincture to the
solution contained in a measuring flask, and if necessary exactly
neutralize with carbonate-free NaOII solution or hydrochloric acid.
Add now a measured excess of barium-chloride solution to pre-
cipitate all the sulphuric acid present, and then add a volume of
normal sodium -carbonate solution equal to that of the barium-
chloride solution used, fill with water to the mark, shake, filter,
and in an aliquot portion of the filtrate (about one half) determine
the sodium carbonate according to 220. The acid required to
neutralize the residual sodium carbonate is, of course, the equiva-
lent of the sulphuric acid present: K a SO 4 + 2BaCl a = BaSO 4 +
2KC1 + Bad, ; and BaSO 4 + KC1 + BaCl, + 2Na a CO, = BaSO 4
+ KC1 + BaCO, + 2JSTaCl + Na 3 CO 8 . In dilute solution the slight
excess of sodium carbonate has no action on the barium sulphate,
hence no error will arise on this score. The results are suffi-
ciently accurate for technical purposes.

c. After E. BoHLio.f This method, which is also adapted
for technical purposes, depends upon the fact that the sulphates
of alkalies are completely decomposed by precipitated barium car-
bonate in the presence of an excess of carbonic acid and at 100,
barium sulphate and an alkali bicarbonate being formed; the
heating prevents the solution of any notable quantity of barium
carbonate because of the presence of free carbonic acid. The
alkali which has combined with the carbonic acid corresponds



Zeitschr f. nnalyt. Chem. ix, 122.
lft., ix, 310.



132.] SULPHURIC ACID. 437

to the sulphuric acid originally present as sulphate. Regarding
the details see the original paper.

d. After R. WILDENSTEIN (first method*). The principle of
this method depends upon precipitating the sulphuric with barium
chloride and estimating the excess of barium chloride, using
potassium chromate. If the solution is neutral, the chromate is
added directly ; if acid, after previous addition of ammonia free
from carbonate in slight excess. There are required: 1. Barium-
chloride solution, 1 c. c. of which should correspond to 0'02 grin,
sulphuric anhydride, SO S , and prepared by dissolving 61 '03
grm. of pure crystallized barium chloride, BaCl a -f- 2H,O, to
make one litre. 2. Potassium-chromate solution, of which
2 c. c. should precipitate 1 c. c. of the barium-chloride solu-
tion. It is prepared by dissolving 18 '3853 grm. potassium bi-
chromate in some water, adding ammonia until the reddish-
yellow color has given place to a pale-yellow, and then diluting
to measure 1 litre. The solution should be neutral.

The two solutions must first be tested, to see whether they
correspond properly. For this purpose dilute 10 c. c. of the
barium-chloride solution with about 50 c. c. of water, boil, and
add 20 '4 c. c. of the potassium-chromate solution. The precipi-
tate rapidly subsides, and the supernatant liquid must be yellow-
ish. On now adding barium-chloride solution by drops, exactly
0'2 c. c. of the solution must be required to effect complete
decomposition in all, therefore, 10 '2 c. c. To carry out the
sulphuric-acid determination, dissolve the substance in about
50 c. c. of water, heat to boiling in a 200 c. c. flask, and run in
barium-chloride solution until perfectly certain that all the sul-
phuric acid has been precipitated, yet avoiding too great an excess.
Boil then .for one-half to one minute, and if acid neutralize
with ammonia free from carbonate, then add to the hot liquid,
whether turbid or not, potassium-chromate solution in quantities
of 0'5 c. c. The liquid rapidly clears up on gently agitating, so
that the appearance of the yellow color, when the chromate begins
to be present in excess, may be readily observed. When this point
arrives, add barium-chloride solution slowly drop by drop until

* Zeitschr. f. analyt. Chem., i, 323.



438 DETERMINATION. [ 132.

complete discolorization is just effected, for which purpose a few
drops, and at most 0*4 c. c., are required. Half the number of
c. c. of potassium-chromate solution used is deducted from the
entire number of c. c. of barium-chloride solution used, and from
the difference calculate the sulphuric acid. Results good.

In applying this method to the sulphates of magnesium, zinc,
or cadmium, dissolve the sulphate in ammonia with the addition
of ammonium chloride, heat with a little calcium chloride in
order to remove any carbonate that may be present, then add
barium chloride, and finally the potassium chromate (FLEISCHER *).
e. After R. WILDENSTEIN (second method f).
Of all the methods for .the volumetric estimation of sulphuric
acid, the simplest, and that which is capable of the most general
application, is to drop into the solution containing excess of hydro-
chloric acid, standard barium- chloride solution, till
the exact point is reached when no more precipita-
tion takes place. This point is difficult to hit, and
hence the method has only found a very limited use.
WILDENSTEIN has given this method a practical
form, which renders it possible to complete an
analysis in about half an hour, and at the same time
to obtain satisfactory results. He employs the
apparatus, Fig. 90. A is a bottle of white glass,
the bottom of which has been removed ; it holds
900 to 950 c. c. B is a strong funnel-tube with
bell-shaped funnel, and bent as shown, provided below with a piece
of india-rubber tubing, a screw compression-cock, and a small piece
of tubing not drawn out. The length from c to d is about 7^-8,
from d to e about 12, cm. The opening of the funnel- tube /*,
which should have a diameter of 2*5 to 3 cm., is covered as fol-
lows : Take a piece of fine new calico or muslin free from sul-
phuric acid and about 6 cm. square, lay on it two pieces of
Swedish paper of the same size and then another piece of stuff
like the first, now bind these all together over the opening/*, care-
fully and without injuring the paper, by means of a strong linen

* Journ f. prakt. Chem., N. F. v, 318. Here also a modification is given by
which the excess of ammonium chromate maybe detected in colored liquids, but
unfortunately the process is far less simple.

\Zeitschr.f. analyt. Chem., i, 432.




132.] SULPHURIC ACID. 439

thread which has been drawn a few times over wax, and cut it off
even all round. We have now a small siphon -filter, which
enables us to filter off a portion of fluid contained in A, and
turbid from barium sulphate, clear and with comparative rapidity.

On gradually adding barium chloride to the dilute acid solution
of a sulphate a point occurs which may be compared with the neutral
point in precipitating silver with sodium chloride (see 115, 5, I) ;
i.e., there is a certain moment w r hen a portion filtered off will give
a turbidity both with sulphuric acid and barium chloride after the
lapse of a few minutes. On this account we must either proceed
on the principle recommended for the estimation of silver, i.e., dis-
regarding the quantity of barium chloride in the solution, to stand-
ardize it by adding it to a known amount of sulphate, till a pre-
cipitate ceases to be formed ; or else we must and WILDENSTEIN
recommends this latter course consider as the end-point of the
reaction the point at which barium chloride ceases to produce a
distinctly visible precipitation in the clear filtrate after a lapse of
two minutes.

The barium chloride solution is prepared so that 1 c.c. corre-
sponds to 0-02 sulphuric anhydride by making a solution contain-
ing the requisite calculated and carefully weighed amount of the
pure salt per litre. A solution of sulphuric acid containing - 02
grm. SO 3 per c.c. may also be required. The process is as follows :

First prepare the solution of the sulphate to be analyzed (using
about 3 or 4 grm.), then fill A with hot water, open the cock with
the screw or by the aid of a glass rod, and wait till the syphon B
is quite full of water. If the water runs down the tube c e with-
out filling it entirely, close and open the cock a few times, and this
inconvenience will be removed. (It is not allowable to suck at <?,
or to fill the syphon with the wash-bottle at e> as either proceeding
would inevitably lead to injuring the filter.) Now close the cock
and pour out the hot water, replace it by 400 c.c. of boiling water,
add the ready-prepared solution of the sulphate, and a small quan-
tity of hydrochloric acid, if necessary, and run in the barium chlo-
ride solution, at first in rather large portions, at last in ^ c.c.
Before each fresh addition of barium chloride open the cock and
allow rather more liquid to flow into a beaker than corresponds to
the contents of the syphon. This quantity should be previously
ascertained, and a mark indicating it made on the beaker. Now



440 DETERMINATION. [ 132.

close the cock and pour the filtrate without loss back into A. (As
the beaker is used over and over again for the same purpose, it
need not be rinsed out.) Now run some of the fluid into a test-
tube, so as to one third fill it, add to the clear fluid 2 drops of
barium chloride from the burette and shake. If a precipitate or
turbidity is produced, return the portion to the main quantity. The
experiment is finished when the last portion tested shows after the
lapse of exactly two minutes no distinctly visible turbidity. The
drops of barium chloride used for the last testing are of course not
reckoned. The slight error involved from the fact that the small
quantity of fluid in the syphon is finally unacted on, is too small
to be noticed. During the experiment the filter must not be
injured by the stirring. In case the end reaction has been over-
stepped, add 1 c.c. of dilute sulphuric acid (equivalent to the barium
chloride) to J., and endeavor to hit it again. Here 1 c.c. will
have to be subtracted from the c.c. of barium chloride used.

The results obtained by WILDENSTEIN are of sufficient accuracy
for technical purposes. Some experiments made in my own labo-
ratory were also quite satisfactory.

/. The methods of LEVOL,* PAPPENHEIM^ SCHWARZ,^: etc.,
depending on the precipitation of the sulphuric acid with stand-
ard lead solution, are only of limited application, because chlo-
rides, hydrochloric acid, and ammonium salts cause disturbances
in the reactions.

II. SEPARATION OF SULPHURIC ACID FROM THE B^sic
RADICALS.

a. In Sulphates which are soluble in Water or Hydrochloric
Avid.

The solution should be free from nitric acid. Precipitate the
sulphuric acid according to I. by barium chloride (or barium ace-
tate). The filtrate contains the excess of barium chloride, together
with the chlorides of the metals present ; separate barium from the
latter by methods given in the fifth section. The fluid obtained by
treating the ignited barium sulphate with hydrochloric acid, evap-

* Bulletin de id Societe d'Encourag., Avril, 1853; Journ. f. prakt. Chem.,
LX, 384.

f MOIIR'S Lehrbuch der Titrirmelhode, 3. Aufl., 411.
\Zeitschr.f. analyt. CJwm., n, 392.



132.] SULPHUKIC ACID. 441

orating and filtering from the small amount of barium sulphate,
must t>e added to the first solution before separating barium
from it.

If the barium sulphate, after treatment with hydrochloric
acid, still contains foreign bases, dissolve the barium sulphate in
sulphuric acid with heat, pour the solution carefully into cold
water, and filter off the precipitated barium sulphate. The foreign
bases will remain in solution.

1). In Sulphates which are insoluble or difficultly soluble in
Water or in Hydrochloric Acid.'

a. From barium, strontium and calcium : Fuse the finely pul-
verized substance in a platinum crucible, with 5 parts of mixed
sodium and potassium carbonates. Put the crucible, with its con-
tents, into a beaker, or into a platinum or porcelain dish, pour
water over it, and apply heat until the alkali sulphates and carbon-
ates are completely dissolved ; filter the hot solution from the resid-
uary alkali-earth carbonates, wash the latter thoroughly with water,
to which a little ammonia and ammonium carbonate has been added,
and determine according to 101 to 103. If the precipitates have
been well washed, it is perfectly admissible to ignite and weigh at
once. Precipitate the sulphuric acid from the filtrate, as in I.,
after acidifying with hydrochloric acid. Finely pulverized calcium
and strontium sulphates may be completely decomposed also by
boiling with a solution of potassium carbonate.*

This process will also answer for barium sulphate, but is far
more difficult.^ and effective only on repeatedly boiling the pre-
cipitate with excess of alkali- carbonate solution, after decanting
the liquid (H. KOSE f). ROSE f).

/3. From lead : The simplest way of effecting the decomposi-
tion of lead sulphate consists in digesting it, at the common tem-
perature, with a solution of hydrogen-sodium or hydrogen-potas-
sium carbonate, filtering, washing the precipitate, determining the
sulphuric acid in the nitrate as in I., dissolving the precipitate,
which contains alkali, in nitric or acetic acid, and determining the
lead in the solution, by one of the methods given in 162.

Presence of strontium and calcium necessitates no alteration in
this method ; but if barium also is present, and it is accordingly

* Sodium carbonate does riot answer so well.
\Journ.f.prakt. Chem., LXIV, 382; LXV, 316.



442 DETERMINATION. [ 133.

necessary to ignite* the mixture with alkali carbonates, a small
portion of lead always remains in solution in the alkaline fluid ; this
must be precipitated by passing through it carbon dioxide before
filtering.

y. from mercury in mercurous sulphate : Mercurous sulphate
is best dissolved by warming with dilute hydrochloric acid with
addition of potassium chlorate or bromine, and the solution is
treated according to a. If the salt is boiled with solution of potas-
sium carbonate, the mercurous carbonate first formed is decom-
posed, and the residue contains . metallic mercury and mercuric
oxide ; a small part of the latter passes into the filtrate.

III. ESTIMATION OF FREE SULPHURIC ACID IN THE
PRESENCE OF SULPHATES.

"We have occasionally to estimate the free acid in presence of
sulphates, as, for instance, in vinegar, w\ne, etc. According to A.
GiRARDf the following is the only direct method which can be
relied on : Evaporate on the water-bath to dryness and exhaust the
residue with absolute alcohol ; determine the combined acid in the
residue, and the free acid in the alcoholic extract, after mixing with
water and evaporating off the alcohol. It has been said that the
object may be obtained by the use of barium carbonate, which is
supposed to throw down the free acid only, hut this is erroneous,
since alkali sulphates in aqueous solution are partially decomposed
at the ordinary temperature by barium carbonate. In some cases
the amount of free sulphuric acid present may be calculated after
having determined the total amount of basic and acid radicals
present. When no other free acid is present, free sulphuric acid
may be determined by the acidimetric process.

Supplement to the Second Division.
133.

IIvDROFLUOSILICIC AdD.

If you have hydrofluosilicic acid in solution, add solution of
potassium chloride, then a volume of strong alcohol equal to the
fluid present, collect the precipitated potassium silicofluoride on a

* This is best done in a porcelain crucible.

\Compt. Rend., LXXXVIII, 515; Zelischr.f. analyt. CJwm., iv, 219.



133.] IIYDROFLUOSILICIC ACID. 443



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