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plete precipitation of ammonium platinic chloride, lead chloride,
calcium sulphate, &c.; thus again, ammonium magnesium phosphate
may be rendered insoluble in an aqueous menstruum by adding
ammonia to the latter, &c.

Precipitation is generally effected in beakers. In cases, how-
ever, where we have to precipitate from fluids in a state of ebulli-
tion, or where the precipitate requires to be kept boiling for some
time with the fluid, flasks or dishes are substituted for beakers,
with due regard always to the material of which they are made
(see Evaporation, 41, at the end).

The separation of precipitates from the fluid in which they are
suspended, is effected either by decantation or filtration, or by
both these processes jointly. But, before proceeding to the sepa-
ration of the precipitate by any of these methods, the operator
must know whether the precipitant has been added in sufficient
quantity, and whether the precipitate is completely formed. To
determine the latter point, an accurate knowledge of .the properties
of the various precipitates must be attained, which we shall en-
deavor to supply in the third section. To decide the former ques-
tion, it is usually sufficient to add to the fluid (after the precipitate
has settled) cautiously a fresh portion of the precipitant, and
to note if a further turbidity ensues. This test, however, is not
infallible, when the precipitate has not the property of forming
immediately; as, for instance, is the case with ammonium phos-
pho-molybdate. When this is apprehended, pour out (or transfer
with a pipette) a small quantity of the clear supernatant fluid into
another vessel, add some of the precipitant, warm if necessary ;
and after some time look and see whether a fresh precipitate has
formed. As a general rule, the precipitated liquid should be
allowed to stand at rest for several hours, before proceeding to the
separation of the precipitate. This rule applies more particularly
to crystalline, pulverulent, and gelatinous precipitates, whilst curdy
and flocculent precipitates, more particularly when the precipitation
was effected at a boiling temperature, may often be filtered off im-
mediately. However, we must observe here, that all general rules,
in this respect, are of limited application.



44.] DECANTATIOtf. 3



a. SEPARATION OF PRECIPITATES BY DEVASTATION.

When a precipitate subsides so completely and speedily in a
fluid that the latter may be decanted off perfectly clear, or drawn
off with a syphon, or removed by means of a pipette, and that
the washing of the precipitate does not require a very long time,
decantation is often resorted to for its separation and washing ;
this is the case, for instance, with silver chloride, metallic mer*
cury, &c.

Decantation will always be found a very expeditious and accu-
rate method of separation, if the process be conducted with due
care ; it is necessary, however, in most cases, to promote the speedy
and complete subsidence of the precipitate ; and it may be laid down
as a general rule, that heating the precipitate with the fluid will
produce the desired effect. Nevertheless, there are instances in
which the simple application of heat will not suffice ; in some cases,
as with silver chloride, for instance, agitation must be resorted to;
in other cases, some reagent or other is to be added hydrochloric
acid, for instance, in the precipitation of mercury, &c. We shall
have occasion, subsequently, in the fourth section, to discuss this
point more fully, when we shall also mention the vessels best
adapted for the application of this process to the various precipitates.

After having washed the precipitate repeatedly with fresh
quantities of the proper fluid, until there is no trace of a dissolved
substance to be detected in the last rinsings, it is placed in a
crucible or dish, if not already in a vessel of that description ; the
fluid still adhering to it is poured off as far as practicable, and the
precipitate is then, according to its nature, either simply dried, or
heated to redness.

A far larger amount of water being required for washing pre-
cipitates by decantation than on filters, the former process caji be
expected to yield accurate results only where the precipitates are
absolutely insoluble. For the same reason, decantation is not ordi-
narily resorted to in cases where we have to determine other con-
stituents in the decanted fluid.

The decanted fluid must be allowed to stand at rest from
twelve to twenty-four hours, to make quite sure that it contains
no particles of the precipitate ; if, after the lapse of this time, no



94 OPERATIONS. [ 45.

precipitate is visible, the fluid may be thrown away; but if a pre-
cipitate has subsided, this had better be estimated by itself, and the
weight added to the main amount ; the precipitate may, in such
cases, be separated from the supernatant fluid by decantation, or
by filtration.

45.
/?. SEPARATION OF PRECIPITATES BY FILTRATION.

This operation is resorted to whenever decantation is imprac-
ticable, and, consequently, in the great majority of cases ; provided
always the precipitate is of a nature to admit of its being com-
pletely freed, by mere washing on the filter, from all foreign
substances. Where this is not the case, more particularly, there-
fore, with gelatinous precipitates, aluminium hydroxide for in-
stance, a combination of decantation and filtration is resorted to
( 48). Filtration is effected either with or without exhaustion of
the liquid ; in the latter case, however, it is greatly accelerated.

45.

aa. OEDIKAEY FILTEATIOIST.
cxof. FILTERING APPARATUS.

Filtration, as a process of quantitative analysis, is almost
exclusively effected by means of paper.

Plain circular filters are most generally employed ; plaited fil-
ters are only occasionally used. Much depends upon the quality
of the paper. Good filtering paper must possess the three follow-
ing properties: 1. It must completely retain the finest precipi-
tates ; 2. It must filter rapidly ; and 3. It must be as free as
possible from any admixture of inorganic bodies, but more espe-
cially from such as are soluble in acid or alkaline fluids.

It is a matter of some difficulty, however, to procure paper
fully answering these conditions. The Swedish filtering p<tj>. /-,
with the water-mark J. II. HUNKTELL, is considered the best, and,
consequently, fetches the highest price; but even this answers only
the first two conditions, being by no means sufficiently pure for
\ TV accurate analyses, since it leaves upon incineration about O3



45.] FILTRATION. 95

per cent, of ash,* and yields to acids perceptible traces of lime, mag-
nesia, and ferric oxide. For exact experiments it is, consequently,
necessary first to extract the paper with dilute hydrochloric acid,
then to wash the acid completely out with water, and finally to
dry the paper. In the case of very fine filtering paper, the best
way to perform this operation is to place the ready-cut filters,
several together, in a funnel, exactly the same way as if intended
for immediate filtration ; they are then moistened with a mixture
of one part of ordinary pure hydrochloric acid with two parts of
water, which is allowed to act on them for about ten minutes ;
after this all traces of the acid are carefully removed by washing
the filters in the funnel repeatedly with warm water. The funnel
being then covered with a piece of paper, turned over the edges,
is put in a warm place until the filters are dry. Compare the
instruction given in the "Qual. Anal.," Am. Ed., p. 8, on the
preparation of washed filters. Filter paper containing lead, and
which is consequently blackened by sulphuretted hydrogen, should
be rejected, f

Heady-cut filters of various sizes should always be kept on hand.
Filters are either cut by circular patterns of pasteboard or tin, or,

still better, by MOHR'S filter-
patterns, fig. 55. This little
apparatus is made of tin-plate,
and consists of two parts. B is
a quadrant fitting in A, whose
straight edges are turned up,
and which is slightly smaller
than B. The sheets of filter-
paper are first cut up into squares, which are folded in quarters,
and placed in A, then B is placed on the top, and the free edge of
the paper is cut off with scissors. Filters cut in this way are per-
fectly circular, and of equal size.

Several pairs of these patterns of various sizes (3, 4, 5, 6, 6*5,
and 8 cm. radius) should be procured. In taking a filter for a
given operation, you should always choose one which, after the
fluid has run through, will not be more than half filled with the
precipitate*

* Plantamour found the ash of Swedish filtering paper to consist of 63 -23
silicic acid, 12-83 lime, 6 '21 magnesia, 2 -94 alumina, and 13-92 ferric oxide, in
100 parts.

f WICKE, Annal. d. Chem. u. Pharm., cxn, 127.




96 OPERATIONS. [ 45.



As to the funnels, they should be inclined at the angle of 60,
and not bulge at the sides. Glass is the most suitable material for
them.





Fig. 56. Fig. 57.

The filter should never protrude beyond the funnel. It should
come up to one or two lines from the edge of the latter.

The filter is firmly pressed into the funnel, to make the paper
fit closely to the side of the latter; it is then moistened with
water ; any extra water is not poured out, but allowed to drop
through.

The stands shown in figs. 56 and 57 complete the apparatus for
filtering.

The stands 'are made of hard wood. The arm holding the
funnel or funnels must slide easily up and down, and be fixable by
the screw. The holes for the funnels must be cut conically, to
keep the funnels steadily in their place.

These stands are very convenient, and may be readily moved
about without interfering with the operation.

/3ft. BULKS TO BE OBSERVED ix THE PROCESS OF FILTRATION.

In the case of curdy, flocculent, gelatinous, or crystalline pre-
cipitates tluTe is no danger of the fluid passing turbid through the
filter. But with fine pulverulent precipitates it is generally necea-
Hiii'ij. and always tulvixablC) to let the precipitate subside, and then
filter the supernatant liquid, before proceeding to place the precipi-



45.] FILTRATION. 97

tate upon the filter. We generally proceed in this way also with
other kiitds of precipitates, especially with those that require to
stand long before they completely separate. Precipitates which
have been thrown down hot, are most properly filtered off before
cooling (provided always there be no objections to this course),
since hot fluids run through the filter more speedily than cold ones.
Some precipitates have a tendency to be carried through the filter
along with the fluid ; this may be prevented in some instances by
modifying the latter. Thus barium sulphate, when filtered from
an aqueous solution, passes rather easily through the filter the
addition of hydrochloric acid or ammonium chloride prevents this
in a great measure.

If the operator finds, during a filtration, that the filter would
be much more than half filled by the precipitate, he would better
use an additional filter, and thus distribute the precipitate over the
two ; for, if the first were too full, the precipitate could not be
properly washed.

The fluid ought never to be poured directly upon the filter,
but always down a glass rod (see Fig. 54), and the lip or rim of the
vessel from which the fluid is poured should always be slightly
greased with tallow.* The stream ought invariably to be directed
towards the sides of the filter, never to the centre, since this
might occasion loss by splashing. In cases where the fluid has to
be filtered off, with the least possible disturbance of the precipitate,
the glass rod must riot be placed, during the intervals, in the
vessel containing the precipitate ; but it may conveniently be put
into a clean glass, which is finally rinsed with the wash-water.

The filtrate is received either in flasks, beakers, or dishes,
according to the various purposes for which it may be intended.
Strict care should be taken that the drops of fluid filtering through
glide down the side of the receiving vessel ; they should never be
allowed to fall into the centre of the filtrate, since this again
might occasion loss by splashing. The best method is that shown
in Fig. 56, viz., to rest the point of the funnel against the upper
part of the inside of the receiving vessel.

If the process of filtration is conducted in a place perfectly
free from dust, there is no necessity to cover the funnel, nor the

* The tallow may be kept under the edge of the work-table at a convenient
point, where it will adhere by a little pressure. The best way of applying the
tallow to the lip of a vessel i.s with the greased finger,



98 OPERATIONS. [ 46.

vessel receiving the filtrate ; however, as this is but rarely the case,
it is generally indispensable to cover both. This is best effected
with round plates of sheet-glass. The plate used for covering the
receiving vessel should have a small U-shaped piece cut out of its
edge, large enough for the funnel-tube to go through. The effect
desired may be produced by cautiously chipping out the glass bit
by bit with the aid of a key. Plates perforated in the centre are
worthless as regards the object in view.

After the fluid and precipitate have been transferred to the
filter, and the vessel which originally contained them has been
rinsed repeatedly with water, it happens generally that small par-
ticles of the precipitate remain adhering to the vessel, which can-
not be removed with the glass rod. From beakers or dishes these
particles may be readily removed by means of a feather prepared
for the purpose by tearing off nearly the whole of the plumules,
leaving only a small piece at the end which should be cut per-
fectly straight. From flasks, minute portions of heavy precipitates
which are not adherent, are readily removed by blowing a jet of
water into the flask, held inverted over the funnel ; this is effected
by means of the washing-bottle shown in Fig. 60, after the tube
J has been properly directed. If the minute adhering particles of
a precipitate cannot be removed by mechanical means, solution in
an appropriate menstruum must be resorted to, followed by re-pre-
cipitation. Bodies for which we possess no solvent, such as barium
sulphate, for instance, must not be precipitated in flasks.

46.
yy. WASHING OF PRECIPITATES.

After having transferred the precipitate completely to the filter,
we have next to perform the operation of washing ; this is effected
by means of one of the well-known washing-bottles, Figs. 58, 59,
and 60.* The doubly perforated stoppers are of vulcanized rubber.

By the arrangement shown in Fig. 60, in which a short piece
of wide glass tubing a is connected by means of pieces of rubber
tubing with the tip 5, the jet of water may be turned in any direc-



* A wash-bottle for odorous liquids has been devised by JACOB, Zeitschr.f.
analyt. Chern., v, 168.



46.]



FILTRATION".



99



tion, and even upwards, Iry simply turning 5. Care must be
taken that "no loss is occasioned by too violent a stream of water.




Fig. 58.



Fig. 59.



Fig. 60.



"Where great caution is required in washing a precipitate, the
arrangement shown in Fig. 61 can be used with good results;
its construction requires no explanation.
The point of a is drawn out and broken off.
On inverting the flask it delivers a fine,
continuous stream of water.

Precipitates requiring washing with
water are washed most expeditiously with
hot water, provided always there be no
special reason against its use. The wash-
ing-bottle shown in Fig. 59 is particularly
well adapted for boiling water. The
wooden handle which is fastened to the
neck of the flask with wire serves to facil-
itate holding it. If this is not desired, the
neck of the bottle may be wound with cord
of suitable thickness.

It is a rule in washing precipitates not to add fresh wash- water
to the filter till the old has quite run through. In applying the
jet of water you have to take care on the one hand that the upper
edge of the filter is properly washed, and on the other hand that




Fig. 61.



100 OPERATIONS. [ 4t.

no canals are formed in the precipitate, through which the fluid
runs off, without coming in contact with the whole of the precipi-
tate. If such canals have formed and cannot he broken up by the
jet, the precipitate must be stirred cautiously with a small platinum
knife or glass rod.

The washing may be considered completed when all soluble
matter that is to be removed has been got rid of. The beginner
who devotes proper attention to the completion of this operation
shuns one of the rocks which he is most likely to encounter.
Whether the precipitate has been completely washed may generally
be ascertained by slowly evaporating a drop of the last washings
upon a platinum knife, and observing if a residue is left. But in
cases where the precipitate is not altogether insoluble in water
(strontium sulphate, for instance), recourse must be had to more
special tests, which we shall have occasion to point out in the
course of the work. The student should never discontinue the
washing of a precipitate because he simply imagines it is finished
he must be certain.

Formerly continuous w r ash-bottles were employed for pro-
tracted washings. They have, however, fallen into disuse because
in their employment canals readily form in the precipitates, a
large quantity cf water is required, and the use of hot water
is excluded. Hence it is now customary to treat precipitates as
described in 48. Those interested in the construction of con-
tinuous wash-bottles will find them described and illustrated in the
Handwdrterbuch dcr Chemie, 2d edit., n, 584586.

47.
55. Filtration ~by Suction.

Filtration being a frequent and tedious operation, many at-
tempts to facilitate the operation by employing suction have been
made for a long time. BUNSEN * has more recently studied the
subject exhaustively. In order to avoid the danger of breaking
the filter, the fear of which has prevented chemists from generally
(.mploying this method, care must be taken that the filter lies close
to the funnel down to the point. Hence funnels should be chosen

Ann. d. Chem. u. Pharm., CXLVJII, 269 ; also Zeitsclir. f. analyt. Chem.,
viii, 174.



47.] FILTRATION. 101

the sides of wliicli are inclined at an angle of 60, and free from
inequalities! of surface. In the funnel should be placed a vary
thin, exactly fitting platinum cone, and in the latter is placed the
filter so that, after being moistened, it will be in contact at all
points, and without any intervening air-bubbles.

The preparation of the platinum cone is thus given by BUNSEN :
A sheet of writing-paper is formed into a filter, and accurately
adjusted to the sides of a carefully selected funnel, when it is fixed
in place in the funnel by a few drops of sealing-wax applied to the
upper margin. The filter is then impregnated with oil, and filled
with plaster of Paris, in which a handle is inserted before the
plaster has set. After a few hours the plaster cast, with its paper
again oiled, is imbedded in a crucible 4 or 5 cm. high and filled
with plaster of Paris. After this has set the plaster cone is re-
moved and freed from its oiled-paper covering. There are thus
obtained a solid cone and a conical hollow which fit each other
perfectly and exactly correspond to the funnel. The platinum
cone is now prepared by cutting a piece of sheet platinum (weigh-
ing about 0-154 grm. per square cm.), of
suitable size, to the shape shown in Pig. 62.
With a scissors a slit, ab^ is cut from the
centre of the piece to a point midway on the
line cd. The foil is next softened by igni-
tion, after which it is laid against the solid
cone with the point of the latter at &, and
abd pressed against the cone and the foil
wrapped around as closely as possible. The platinum cone is now
again ignited, after which it is molded to the plaster cone by hand,
then inserted into the hollow cone in which it is tightly pressed.
The platinum cone, when finished, should let no light pass through
its apex, and even without being soldered is sufficiently firm for all
uses.

The glass funnel, carrying its platinum cone and filter, is now
inserted air-tight into one hole of a doubly perforated rubber stopper,
so that the stem projects from 5 to 8 cm. from the stopper ; the
other perforation carries a short tube bent at right angles, the lower
end of which should not project below the stopper. On now
inserting the stopper in the neck of a bottle, and applying suction
to the tube, the filtration of any liquid in the filter is effected the




102



OPERATIONS.



[47.




47.] FILTKAT10K. 103

more rapidly the greater the difference in pressure between the
external aiu and that within the flask. If it is intended to filter
under a great difference of pressure, an ordinary flask will not
suffice, as it may be shattered by the pressure of the external air.
For such a purpose, therefore, a stout glass flask must be used,
placed conveniently within a tin vessel, <?, Fig. 64, down the inner
conical sides of which three strips of thick cloth or india rubber are
glued. This arrangement is advantageous in affording a firm stand
for flasks of various sizes, and in that any danger from a possible
explosion may be avoided by simply covering it with a cloth. As
an exhauster, any aspirator capable of effecting a difference in
pressure of one quarter of an atmosphere will answer, as a rule.
A very simple arrangement is shown in Fig. 63, in which the
thick-walled rubber tube 5 connects C with A, while a similar tube
connects A and B. On opening the cocks in A and B, water
runs from the former into the latter, creating a partial vacuum in
A, and drawing in air through the filter, thus effecting filtration
under a pressure which is the greater the difference in height
between the water levels in the two bottles. These may have a
capacity of 2 to 4 litres, and they should be similar, so that when
the upper one is emptied it may be replaced by the filled one.

The most convenient of all aspirators, however, is the hydraulic
air-pump, more or less perfect forms of which had been known
for a long time * before BUNSEN brought it to a high degree of
perfection. Fig. 64 represents his apparatus, connected with a
flask, and as now furnished by P. DESAGA, of Heidelberg. The
following is BUNSEN'S description of it:

On opening the pinch-cock , water flows through the supply
pipe w into the enlarged glass tube 0, and passes off through the
leaden pipe d which has a diameter of 8 mm. This pipe leads to
the bottom of a drain 30 to ;40 feet belcw.f The tube 0, fused
into c, reaches to the lower end of c, and its lower aperture is very
small. Its upper continuation is connected at/*, by means of a side



* Compare Zeitschr.f. analyt. Chem., n, 359, and iv, 46.

f If the hydraulic air-pump is in the upper part of the house, it will suffice
to conduct the 1 'ad pipe to the bottom of a cistern placed in the cellar and
provided with a lateral tube inserted a little more than half-way from the bottom.
On connecting this tube with a deep drain, the apparatus performs its functions
Without requiring any attention



104 OPERATIONS. [ 47.

tube, with a mercurial manometer ; at g it is connected with the
vessel A, which is intended to retain the steam given off on wash-
in- with hot water. A side tube in h connects the latter, by




Fig. 64.

means of a rubber tube, with the filtering flask. All the rubber
tubing used should be thick-walled and of 2 to 3 mm. bore. The
entire apparatus should be so screwed to a board, fixed to the wall,
as to avoid any tendency on the part of the glass tubes to break by
reason of the board warping.

On opening </, the water runs down J, and the thirty-foot
column of water in this draws with it the air, which escapes
through the opening in the tube c in the form of a stream of bub-
bles. Even if the water runs at its fastest, however, and even with



47.] FILTRATION. 105

a column 40 feet in length, it is impossible to create any considerable
exhaustion", because of the friction, increasing in rapid progression,
between the water and the lead pipe. A second pinch-cock, &, is
hence applied, which so regulates the flow of water once and for
all as to minimize the friction by reducing the flow, thus enabling
a maximum of exhaustion to be obtained when a is completely
open. Such an apparatus, properly regulated, exhausts in a com-



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