C. Remigius Fresenius.

Quantitative chemical analysis online

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140.] SILICIC ACID. 509

aqueous or acid solution free from other fixed bodies, simply
evaporate the solution in a platinum dish, ignite and weigh the

Respecting a volumetric estimation of silicic acid (conversion
into and acidimetric determination of potassium silicofluoride, see
97, 5), I must refer to STOLBA.*


a. In all compounds which are decomposed l)y Hydrochloric
or Nitric Acid, on digestion in open vessels.

To this "class belong the silicates soluble in water, as well as
many of the insoluble silicates, as, for instance, nearly all zeolites.
Several minerals not decomposable of themselves by acids become
so by persistent ignition in a state of fine powder (F. MOHB f). If
the ignition is too strong, particles of alkali may be lost.

The substance is very finely powdered, dried at 100, and put
into a platinum or porcelain dish (in the case of silicates whose solu-
tion might be attended with disengagement of chlorine, platinum
cannot be used) ; a little water is then added, and the powdei
mixed to a uniform paste. Moderately concentrated hydrochloric
acid, or if the substance contains lead or silver nitric acid, is now
added, and the mixture digested at a very gentle heat, with con-
stant stirring, until the substance is completely decomposed, in
other terms, until the glass rod, which is rounded at the end,
encounters no more gritty powder, and the stirring proceeds
smoothly without the least grating.

The silicates of this class do not all comport themselves in the
same manner in this process, but show some differences ; thus most
of them form a bulky gelatinous mass, whilst in the case of others
the silicic acid separates as a light pulverulent precipitate ; again,
many of them are decomposed readily and rapidly, whilst others
require protracted digestion.

When the decomposition is effected, the mixture is evaporated
to dryness on the water-bath, and the residue heated, with frequent
stirring, until all the small lumps have crumbled to pieces, and the

* Zeitschr. f. analyt. Cliem , iv, 163. f lb., vii, 293.

\ Very hard silicates cannot be powdered in an agate mortar without taking
up silica ; these must, therefore, be powdered in a steel mortar, sifted, and freed
from particles of steel with the magnet.


whole mass is thoroughly dry, and until no more acid fumes escape.
It is always the safest way to conduct the drying on the water-bath.
Occasionally it is well to moisten the dry mass with water and evap-
orate again. In cases where it appears desirable to accelerate the
desiccation by the application of a stronger heat, an air-bath may
be had recourse to ; which may be constructed in a simple way, by
suspending the dish containing the substance, with the aid of wire,
in a somewhat larger dish of silver or iron, in a manner to leave
everywhere between the two dishes a small space of uniform width.
Direct heating over the lamp is not advisable, as in the most
strongly heated parts the silicic acid is liable to unite again with
the separated bases to compounds which are not decomposed, or
only imperfectly, by hydrochloric acid.

When the mass is cold, it is brought to a state of semi-fluidity
by thoroughly moistening it with hydrochloric acid ; after which
it is allowed to stand for half an hour, then warmed on a water-
bath, diluted with hot water, stirred, allowed to deposit, and the
fluid decanted on to a filter ; the residuary silicic acid is again
stirred with hydrochloric acid, warmed, diluted, and the fluid once
more decanted ; after a third repetition of the same operation, the
precipitate also is transferred to the filter, thoroughly washed with
hot water, well dried, and ignited at last as strongly as possible, as
directed in 52 or 53. For the properties of the residue, see 93, 9.
The results are accurate. The basic metals, which are in the filtrate
as chlorides, are determined by the methods given above. Devia-
tions from the instructions here given are likely to entail loss of
substance ; thus, for instance, if the mass is not thoroughly dried,
a not inconsiderable portion of the silicic acid passes into the solu-
tion, whereas, if the instructions are strictly complied with, only
traces of the acid are dissolved ; in accurate analyses, however, even
such minute traces must not be neglected, but should be separated
from the metals precipitated from the solution. The separation
may, as a rule, be readily effected by dissolving them, after ignition
and weighing, in hydrochloric or sulphuric acid, by long digestion
in the heat, the traces of silicic acid being left undissolved. Some-
times it is better to fuse the metallic oxides with potassium disul-
phate, or to reduce them to the metallic state by ignition in hydro-
gen, and then to treat with hydrochloric acid. Again, if the silicic
acid is not thoroughly dried previous to ignition, tlu> aqueous vapor
disengaged upon the rapiJ application of a strong heat may carry

140.] SILICIC ACID. 511

away particles of the light and loose silica. If a suction appara-
tus has been used, however, and the precipitate has been quite
thoroughly freed from water, the precipitate may be ignited at
once, as described in 52, p. 116. In this case, however, the
incineration of the filter is frequently imperfect

The silicic acid may be tested as follows : This testing must on
no account be omitted if the silica has been separated in a pulveru-
lent and not in a gelatinous form. Heat a portion on a water-bath
with moderately concentrated solution of sodium carbonate for an
hour in a platinum or silver dish; with less advantage in a porce-
lain dish. EGGERTZ* recommends, for Ol grm. silicic acid, 6 c.c.
of a saturated solution of sodium carbonate and 12 c.c. of water.
Pure silica would dissolve. If a residue remains, pour off the clear
fluid and heat again with a small quantity of sodium carbonate. If
a residue still remains, weigh the rest of the impure silica and
treat it according to &, to estimate the amount of impurity.

If you \\&ve,pure hydrofluoric acid, you may also test the silicic
acid in a very easy manner, by treating it with this acid and a few
drops of sulphuric acid in a platinum dish ; upon the evaporation
of the solution, the silicic acid, if pure, will volatilize completely
(as silicon fluoride). If a residue remains, moisten this once
more with hydrofluoric acid, add a few drops of sulphuric acid,
evaporate, and ignite ; the residue consists of the sulphates of the
metals retained by the silicic acid, as well as any titanic acid that.
w r as present (BERZELIUS). Ammonium fluoride may be used
instead of hydrofluoric acid.

1). Compounds which are not decomposed by Hydrochloric or
Nitric Acid on digestion in open vessels.

a. Decomposition by f^isiwi with Alkali Carbonate.

Reduce the substance to an impalpable powder, by trituration
and, if necessary, sifting ( 25) ; transfer to a platinum crucible,
and mix with about 4 times the weight of pure anhydrous sodium
carbonate or sodium and potassium carbonate, with the aid of a
rounded glass rod ; wipe the rod against a small portion of sodium
carbonate on a card, and transfer this also from the card to the
crucible. Cover the latter well, and heat, according to size, over a
gas or spirit-lamp with double draught, or a blast gas-lamp ; or

* Zeitschr.f. analyt. Chem., vii, 502.


insert in a Hessian crucible, compactly filled up with calcined
magnesia, and heat in a charcoal fire.

Apply at first a moderate heat for some time to make the mass
simply agglutinate ; the carbonic acid will, in that case, escape from
the porous mass with ease and unattended with spirting. Increase
the heat afterwards, finally to a very high degree, and terminate
the operation only when the mass appears in a state of calm fusion,
and gives no more bubbles.

The platinum crucible in which the fusion is conducted must
not be too small ; in fact, the mixture should only half fill it. The
larger the crucible, the less risk of loss of substance. As it is of
importance to watch the progress of the operation, the lid must be
easily removable ; a concave cover, simply lying on the top, is there-
fore preferable to an overlapping lid. If the process is conducted
over the spirit or simple gas-lamp, the mixed sodium and potas-
sium carbonates are preferable to sodium carbonate, as they fuse
much more readily than the latter. In heating over a lamp, the
crucible should always be supported on a triangle of platinum wire,
with the opening just sufficiently wide to allow the crucible to
drop into it fully one third, yet to retain it firmly, even with the
wire at an intense red heat. When conducting the process over a
spirit-lamp with double draught, or over a simple gas-lamp, it is
also advisable, towards the end of the operation, when the heat is
to be raised to the highest degree, to put a chimney over the cruci-
ble, with the lower border resting on the ends of the iron triangle
which supports the platinum triangle ; this chimney should be
about 12 or 14 cm. high, and the upper opening measure about 4
cm. in diameter. The little clay chimneys recommended by O. L.
ERDMANN are still more serviceable (Fig. 20, p. 22, k 'Qual. Anal.").
When the fusion is ended, the red-hot crucible is removed with
tongs, and placed on a cold, thick, clean iron plate, on which it
will rapidly cool ; it is then generally easy to detach the fused cake
in one piece.

The cake (or the crucible with its contents) is put into a beaker,
from 10 to 15 times the quantity of water poured over it, and heat
applied for half an hour, then hydrochloric acid is gradually added,
or, under certain circumstances, nitric acid ; the beaker is kept
covered with a glass plate, or, which is much better, with a large
watch-glass or porcelain dish, perfectly clean outside, to prevent
the loss of the drops of fluid which the escaping carbonic acid car-

140.] SILICIC ACID. 513

ries along with it ; the drops thus intercepted by the cover avo
afterwards rinsed into the beaker. The crucible is also rinsed with
water mixed with dilute acid, and the solution obtained added to
the fluid in the beaker.

The solution is promoted by the application of . a gentle heat,
which is continued for some time after this is effected to insure the
complete expulsion of the carbonic acid ; since otherwise some loss
of substance might be incurred, in the subsequent process of evapo-
ration, by spirting caused by the escape of that gas. If in the pro-
cess of treating the fused mass with hydrochloric acid, a saline
powder subsides (sodium or potassium chloride), this is a sign that
more water is required.

If the decomposition of the mineral has succeeded to the full
extent, the hydrochloric acid solution is either perfectly clear, or
light flakes of silicic acid only float in it. But if a heavy powder
subsides, which feels gritty under the glass rod, this consists of
undecomposed mineral. The cause of such imperfect decomposi-
tion is generally to be ascribed to imperfect pulverization. In
such cases the undecomposed portion may be fused once more with
alkali carbonate ; the better way, however, is to repeat the process
with a fresh portion of mineral more finely pulverized.

The hydrochloric or nitric acid solution obtained is poured,
together with the precipitate of silicic acid,' which is usually floating
in it, into a porcelain or, better, into a platinum dish, and treated
as directed in II., a. That the fluid may not be too much diluted,
the beaker should be rinsed only once, or not at all, and the few
remaining drops of solution dried in it ; the trifling residue thus
obtained is treated in the same way as the residue left in the evapo-
rating basin. This is the method most commonly employed to
effect the decomposition of silicates that are undecomposable by
acids ; that it cannot be used to determine alkalies in silicates is

ft. Decomposition ~by means of Hydrofluoric A cid.

aa. By Aqueous Hydrofluoric Acid.

The silicate should be finely pulverized, dried at 100 (in some
cases ignition is advisable*). It is mixed, in a platinum dish, with

* Many minerals are much more readily decomposed by hydrofluoric acid
also if they are previously ignited in a state of fine division (HERMANN, RAM-
MELSBERG, FR. MOHR, Zeitschr. f. analyt. Chem., vn, 291).


rather concentrated, slightly fuming hydrofluoric acid, the acid
being added gradually, and the mixture stirred with a thick plati-
num wire. The mixture, which has the consistence of a thin paste,
is digested some time on a water-bath at a gentle heat, and pure
concentrated sulphuric acid, diluted with an equal quantity of
water, is then added, drop by drop, in more than sufficent quantity
to convert all the basic metals present into sulphates. The mixture
is now evaporated on the water-bath, during which operation sili-
con fluoride gas and hydrofluoric acid gas are continually volatiliz-
ing ; then it is finally exposed to a stronger heat at some height above
the lamp, until the excess of sulphuric acid is almost completely
expelled. The mass, when cold, is thoroughly moistened with con-
centrated hydrochloric acid, and allowed to stand at rest for one
hour ; water is then added, and a gentle heat applied. If the
decomposition has fully succeeded, the whole must dissolve to a
clear fluid. If an undissolved residue is left, the mixture is heated
for some time on the water-bath, then allowed to deposit, the clear
supernatant fluid decanted as far as practicable, the residue dried,
and then treated again with hydrofluoric acid and sulphuric acid,
and, lastly, with hydrochloric acid, which will now effect complete
solution, provided the analyzed substance was very finely pulver-
ized, and free from barium, strontium (and lead). The solution is
added to the first. The basic metals in the solution (which con-
tains them as sulphates, and contains also free hydrochloric acid)
are determined by the methods which will be found -in Section Y.

This method, which is certainly one of the best to effect the
decomposition of silicates, was proposed by BERZELIUS. It has
been but little used hitherto, because we did not know how to pre-
pare hydrofluoric acid, except with the aid of a distilling appa-
ratus of platinum, or, at least, with a platinum head ; nor to keep
it, except in platinum vessels. These difficulties can now be con-
sidered as overcome, comp. 58, 2. Xever omit testing the acid
before using it.

The hydrofluoric acid may also be employed in combination
with hydrochloric acid ; thus 1 grin, of finely elutriated felspar,
mixed with 4<)c.c. water, 7 c.c. hydrochloric acid of 25#and 3Jc.c.
hydrofluoric acid, and heated to near the boiling point, dissolves
completely in three minutes. 4 c.c. sulphuric acid are then added,
the barium sulphate which may separate is filtered off, and the

140.] SILICIC ACID. 515

filtrate ^evaporated till no more hydrofluoric acid escapes (AL.


The execution of the method requires the greatest possible
care, both the liquid and the gaseous hydrofluoric acid being most
injurious substances. The treatment of the silicate with the acid
and the evaporation must be conducted in the open air, otherwise
the windows and all glass apparatus will be attacked. As the
silicic acid is in this method simply inferred from the loss,f a
combination with method a is often resorted to.

l)b. By Gaseous Hydrofluoric, Acid.

Instead of the aqueous solution of hydrofluoric acid, the
gaseous acid may also be used for decomposing silicates. This
method, which was formerly much used, was proposed by BRUN-
NER,J and is as follows: Place 1 or 2 grammes of the very finely
powdered silicate in as thin a layer as possible in a shallow platinum
dish, moisten the powder with diluted sulphuric acid, and place
the dish on a leaden tripod or other support within a leaden box
about 6 inches in diameter and about 6 inches high, and on the
bottom of which there has been just placed a half -inch layer of
powdered fluor-spar mixed into a paste with concentrated sul-
phuric acid. (Take care to avoid the vapors evolved ; the mixing
of the fluor-spar and sulphuric acid should be done with a long
glass rod, or, better still, with a leaden rod). As soon as the plat-
inum dish has been placed in the box, by the aid of a pair of pin-
cers or tongs, put on a tightly-fitting cover, lute the joints airtight
with plaster-of-paris, and set the whole for 6 to 8 days in a warm
place. If it is desired to facilitate the process do not lute air-
tight, but heat the apparatus in the open air over a gas- or
alcohol-lamp ; by this method 1 to 2 grammes of the silicate may
be decomposed in a few hours, provided the silicate has been
spread out in a very thin layer, or else stirred from time to time,
an operation which must be cautiously effected.

* Journ. f. prakt. Chem., LXXXI, 108.

f The silicon escaping in the form of fluoride may sometimes be determined
directly by the method of STORY MASKELYNE ( Zeitschr. f. analyt. Chem., ix,
380), which, however, requires a platinum retort of peculiar construction.

% Pogg. Annal. , XLIV, 134.

An apparatus that may be used in the laboratory has been described by
A. MULLEB (Journ. f. prakt. CJiem., xcv, 51).


If the decomposition has succeeded, the residue in the plati-
num dish will consist of silicofluorides of the metals, and sulphates.
Place the dish now in a larger platinum dish, add sulphuric acid
drop by drop, using a little more than is sufficient to convert the
bases into sulphates, evaporate in an air-bath, nearly but not quite
expel finally the excess of sulphuric acid over the naked name,
and treat the residue with hydrochloric acid and water as detailed
under aa. The decomposition may be considered as complete
only when a perfect solution results (apart from the presence of
a little barium sulphate).

If a platinum tube adapted for the purpose is at hand the de-
composition may also be effected by heating the finely powdered
mineral placed in a platinum boat inserted into the tube while
passing through the latter a current of dry hydrofluric-acid gas.
The platinum tube is bent downwards in front, and the end should
dip into water; the water takes up the volatile fluorides, while
the non-volatile remain in the platinum boat. (SAINT-CLAIRE

cc. By Ammonium Fluoride.

Mix the very finely powdered substance in a platinum dish
with four times its weight of ammonium fluoride, moisten well
with concentrated sulphuric acid, heat on the water-bath till the
evolution of silicon fluoride and hydrofluoric acid slackens, add
more sulphuric acid, heat again, finally somewhat more strongly
till the greater part of the sulphuric acid has escaped, and treat
the residue according to aa (L. v. BABO, J. POTYKA, E.. HOFF-
MANN f). II. ROSE J first warms the silicate gently with seven
times its amount of the fluoride and some water, then heats gradu-
ally to redness till no more fumes escape, and finally treats with
sulphuric acid.

dd. By Hydrogen Potassium Fluoride, etc.

In silicates, which more or less resist the action of hydrofluoric
acid, such as zircon and beryl, the basic nu'tals with the exception
of the alkalies may be determined by fusing with hydrogen
potassium fluoride (MARIGNAC, GIBBS ), or by mixing with
3 parts of sodium fluoride, adding 12 parts of potassium disnlphate

* Compt. Rend., LVIIT, 545. \ Zeitsc?ir.f. analyt. Chem., vi, 366.

. Annal., cvui, 'Jn. Zcil.^hr. f. uimlyt. Chem., m. 399.

140.] SILICIC ACID. 617

to the crucible, and then heating at first very gently, afterwards
more strongly till the mass fuses calmly. The residue is dissolved
in water or hydrochloric acid (CLARKE*).

y. Decomposition ly Fusion with Barium Hydroxide or
Barium Carbonate.

The fusion of silicates with barium carbonate requires a very
high heat, obtainable only with a good blast-lamp, a SEFSTROM fur-
nace, a DEVILLE turpentine lamp, etc. ; even the highest temper-
ature afforded by a wind furnace is insufficient to effect the melt-
ing together of the barium carbonate and silicate, and only when
this occurs is decomposition complete. When this does occur,
however, it is so energetic that even the most refractory fossils
are easily and completely decomposed. From 4 to 6 parts of
barium carbonate are taken for 1 part of the very finely powdered
mineral. The fusion is effected in a platinum crucible, which,
if a SEFSTROM furnace is used, is placed within another crucible
of refractory fire-clay filled with magnesia. The crucible is left
in the furnace for at least half an hour. The greater the quan-
tity of barium carbonate taken the greater is the danger of alka-
lies volatilizing. DEVILLE, in fact, recommends taking only O8
part barium carbonate for 1 part of felspathic mineral.

More readily decomposable minerals may be more easily de-
composed by means of barium hydroxide freed from its water of
crystallization. To 1 part of the mineral, from 4 to 5 parts of
barium hydroxide are taken, the whole intimately mixed and
covered with a layer of barium carbonate. The fusion may be
effected over an ordinary gas- or BERZELIUS alcohol -lamp ; and it
is best to use silver crucibles, as platinum is attacked. The mass
fuses either completely or at least melts together into a mass.
In order to render platinum crucibles also applicable, v. FELLEN-
BERa-RiviER f recommends 'melting 4 or 5 parts of calcium chlo-
ride in the platinum crucible, shaking the crucible around while
cooling, then adding 1 part of barium hydroxide and fusing this
in turn. After cooling, 1 part of the finely powdered silicate is
introduced and heat is applied, gently at first, but strongly later
when no gas appears to be evolved. SMITH recommends fusing

* Zeitschr. /. analyt. Chem , vn, 463. f Ib., ix, 459.

\Journ.f. prakt. Chem., LX, 246.


1 part silicate with 3 to 4 parts barium carbonate and 2 parts
barium chloride.

When the operation is at an end no matter whether barium
carbonate or barium hydroxide has been used allow the crucible
to cool, clean its outside, cover it with 10 to 15 parts of water in
a beaker, allow to macerate for some time, then add hydrochloric
or nitric acid, and proceed as in 5, a. Care must be taken to
avoid adding too much hydrochloric acid at a time, because the
barium chloride is difficultly soluble in it, and hence may retard
or check the solution of the still undissolved portion by forming
over this an insoluble protective coating. In the filtrate from
silicic acid the bases are estimated according to the methods given
under Section Y. The silicic acid is to be tested as to its purity
according to the method described in a, before the operation may
be regarded as having been successful. These methods, which
were formerly much employed in determining the alkalies in sili-
cates, have been more or less superseded by decomposition with
aqueous hydrofluoric acid and with ammonium fluoride, as both
of these are now readily obtainable commercially.

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