C. Remigius Fresenius.

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enclosed water, with the removal of which the operation of drying
in the sense here in view is alone concerned.

In the drying of substances for quantitative analysis, our object
is to remove all moisture, without interfering in the slightest degree
with combined water or any other constituent of the body. To
accomplish this object, it is absolutely requisite that we should
know the properties which the substance under examination mani-
fests in the dry state, and whether it loses water or other constitu-
ents at a red heat, or at 100, or in dried air, or even simply in
contact with the atmosphere. These data will serve to guide us in
the selection of the process of desiccation best suited to each sub-
stance. The dried substance should always at once be transferred
to a well-closed vessel ; glass tubes, sealed at one end, and of suf-
ficiently thick glass to bear the firm insertion of tight-fitting
smooth corks weighing-tubes are usually employed for this
purpose. It is also advisable to cover the corks with tinfoil.

The following classification may accordingly be adopted :

a: Substances which lose water even on simple contact with the
atmosphere ; such as sodium sulphate, crystallized sodium carbon-
ate, etc. Substances of this kind turn dull and opaque when
exposed to the air, and finally crumble wholly or partially to a white
powder. They are more difficult to dry than many other bodies.
The process best adapted for the purpose, is to press the pulverized
salts with some degree of force between thick layers of fine white
blotting-paper, repeating the operation with fresh paper until the
last sheets remain absolutely dry.

It is generally advisable in the course of this operation to repow-
der the salt.

b. S'libstances which do not yield water to the atmosphere (unless
it is perfectly dry), but effloresce in artificially dried air / such as
magnesium sulphate, sodium-potassium tartrate (Kochelle salt), &c.
Salts of this kind are reduced to powder, which, if it be very
moist, is pressed between sheets of blotting-paper, as in a j after
this operation, it must be allowed to remain for some time spread
in a thin layer upon a sheet of blotting-paper, effectually protected
against dust, and shielded from' the direct rays of the sun.



56



OPERATIONS.



[ 27,



27.

c. Substances which undergo no alteration in dried air, but
lose water at 100 ; calcium tartrate, for instance. These are finely
pulverized ; the powder is put in a thin layer into a watch-glass or
shallow dish, and the latter placed inside a chamber in which the
air is kept dry by means of concentrated sulphuric acid or calcium
chloride. This process is usually conducted in one of the follow-
ing apparatus, which are termed desiccators, and which subserve
still another purpose besides that of drying, viz., that of allowing
hot crucibles, dishes, etc., to cool in dry air.





Fig. 26. Fig. 27.

In Fig. 26, a represents a glass plate (ground-glass plates
answer the purpose best), &, a bell jar with ground rim, which is

greased with tallow; c is a glass
basin with concentrated sulphuric
acid; d, a round iron plate, sup-
ported on three feet, with circular
holes of various sizes, for the recep-
tion of the watch-glasses, crucibles,
k etc., containing the substance.

In Fig. 27, a represents a beaker
W with ground and greased rim, and
filled to one-fourth or one-third
with concentrated sulphuric acid ;
1) is a ground-glass plate; c is a

bent wire of lead, which serves to support the watch-glass contain-
ing the substance.

Fig. 28 is a similarly constructed calcium-chloride desiccator.




Fig. 28.



27.]



DESICCATION".



57




Fig. 29 represents a readily portable desiccator, used more par-
ticularly to receive crucibles in course
of cooling, and carry them to the bal-
ance. The apparatus consists of a
strong glass jar ; the lid must be ground
to shut air-tight ; the place on which it
joins is greased with tallow. The outer
diameter of my jars is 105 mm.; the
sides are G mm. thick. The aperture
has a diameter of 80 mm. ; the box up
to the small part is 65 mm. high; the
lid has the same height ; the small part
itself is 15 mm. high, and ground to a
slightly conical shape.
A brass ring, with
rirn, fits exactly into
the aperture ; the rim
must not project be-
yond the glass. The ring bears a triangle of
iron or, better, platinum wire, intended for the
reception of crucibles, &c. The vessel is one-
third filled with calcium chloride.

Fig. 30 represents an exsiccator devised by
A. SCHKOTTER; it affords free egress to the air,
which expands when a
hot crucible is placed
within the exsiccator
and passes through the
small tube, #, escaping
through two small holes
placed at the base of &,
whence it rises through
sulphuric acid contained
in c, and finally escapes
through the bulb d
filled with calcium chlo-
ride. When the appa- Fig. 30.
rattis is cooling, per-
fectly dry air re-enters by the same way. The operation may be




08 OPERATIONS. [ 6.

considered at an end when no more air-bubbles pass through the
sulphuric acid. The small tube, e, serves to catch any sulphuric
acid that might be carried down through a ; it must not close air-
tight, the lower orifice of the apparatus serving as a stopper for
the bell- jar, hence the cork carrying it must be channelled.
f serves as a stand for the bell-jar. This, desiccator possesses the
advantage that the substances placed in it are cooled in dry air
at the ordinary atmospheric pressure, and hence, when >3moved
from the apparatus, have no tendency to absorb air and, with this,
moisture, which can not be said of substances cooled in air slightly
rarefied by heat.

The substance to be dried is exposed to the action of the dry
air, until it ceases to lose weight. Substances which are acted on
by atmospheric oxygen are in a similar manner dried under the
bell- jar of an air-pump. Substances which, although they lose
no water, yet lose ammonia, in dry air, are dried over caustic
lime mixed with a little powdered ammonium chloride, i.e., in an
anhydrous ammoniacal atmosphere.

28.

d. Substances which at 100 completely lose their moisture,
without suffering any other alteration, such as hydrogen potassium
tartrate, sugar, etc. These are dried in the water- bath ; in the case
of slow-drying substances, or where it is wished to expedite the

operation, with the aid of a current of
dry air.

Fig. 31 represents the water-bath
most commonly used. . It is made of
sheet copper. The engraving renders
a detailed description unnecessary. The
inner eliamber, c, is surrounded on
five sides by the outer case or jacket,
Fi &- 31- d e, without communicating with it.

The object of the apertures y and h is to effect change
of air, which purpose they answer sniliciently well. AVhen
it is intended to use the apparatus, the outer case is filled to about
one- half with rain-water, and the aperture a is closed with a perfor-
ated cork, into which a glass tube is fitted ; the aperture I is entirely







28.] DESICCATION. 59

closed. If the apparatus is intended to be heated over charcoal, it
should have a length of about 20 cm. from d to/*; but if over a
gas-, alcohol-, or oil-lamp, it should be only about 13 cm. long. In
the former case, the inner chamber is 17 cm. deep, 14 cm. broad,
and 10 cm. high ; in the latter case, it \i 10 cm. deep, 9 cm. broad,
and 6 cm. high. The temperature in the inner chamber never
quite reaches 100 ; to bring it up to 100, F. ROCHLEDER has sug-
gested to close It with a double-limbed tube, the outer longer limb
of which dips into a cylinder filled with water; d is in that case
closed with a perforated cork bearing a sufficiently tall funnel
tube, which fits air-tight in the cork. The lower end of this tube
reaches down to one inch from the bottom.

In large analytical laboratories water is usually kept boiling all
day long, for the production of distilled water. The boilers used
in my own laboratory have the shape of somewhat oblong square
boxes, about 120 cm. long, 60 cm. broad, and 24 cm. high ; the
front of the boiler has soldered into it, one above the other, two
rows of drying chambers, of the kind shown in Fig. 31. This
gives so many ovens that almost every student may have one for
his special use. Most of these ovens are from 11 to 12 cm. deep
and broad, and 8 cm. high; some of them, however, are 16 cm.
deep and broad, to enable them to receive large-sized dishes. The
substances to be dried are usually put on double watch-glasses,
laid one within the other, which are placed in the oven, and the
door is then closed. In the subsequent process of weighing, the
upper glass, which contains the substance, is covered with the
lower one. The glasses must be quite cold before they are placed
on the scale. In cases
where we have to deal with
hygroscopic substances, the
reabsorption of water upon
cooling is prevented by the
selection of close-fitting

glasses, which are held Fig. 32.

tight together by a clasp

(Fig. 32), and allowed to cool with their contents under a bell-
glass over sulphuric acid (see Fig. 26). These latter instructions
apply equally to the process of drying conducted in other appa-
ratus.

The clasp used for pressing the watch-glasses together and




60



OPERATIONS.



[28.



which in all cases where it is intended to ascertain the loss of
weight which a substance suffers on desiccation, is to be looked upon
as belonging to the glasses, and must accordingly be weighed with
them is constructed of two strips of thin brass plate, about 10 cm.
long and 1 crn. wide, which are laid the one over the other, and
soldered together at the ends, to the extent of 5 to 6 mm. ; or,
they may be made of one piece, as our illustration shows.

The following apparatus serves for drying substances in a cur-
rent of air :

In Fig. 33 the air-current is caused by simply warming the air,
hence the apparatus is very convenient to use. a ~b is a copper or










Fig. 33.

tinned-iron box into which the canal c d is soldered, and communi-
cates with the chimney ef. The latter is surrounded on three
sides by the case g A, which also communicates with a fi, but has no
opening at the top ; i is a round hole leading into the canal, and
which maybe closed with a cork; Z/^is provided with a well-fitting
sliding door running in grooves.

In use, the aperture n, which serves as an outlet for the water,
is closed with a cork, when the outer case is half filled, through the
opening m, with water, which is then heated to boiling. The
watch-glasses containing the substances to be dried are then placed
on the holes in the sliding shelf B (Fig. 33), which is then intro-
duced into the canal at I /-, and the latter closed. The steam sur-
rounding the chimney soon causes an upward current of the
warmed air within it, and this causes cold air to be drawn in at
the opening *', and to pass over the substance to be dried, carrying
vrith it the evaporating moisture.



28.]



DESICCATION.



61



The disadvantage that the substances are always kept at a point
below 100 C. by the current of cold air, is easily remedied by sol-
dering a tube under the bottom of the canal along its entire length
and back again, and conducting the air through it into the canal.
The air is thus heated to 100 C. before it comes into contact with
the substances. This tube is not shown in the illustration, in order
to avoid confusion. It is very practical, also, to omit the opening
m, and instead to cut in the top of the case round holes of different
sizes (provided with suitable covers) for receiving small evaporating
dishes. According to requirements, the apparatus may be made
20 to 30 cm. long, 15 cm. wide, and about 10 cm. high. The
chimney should be 6 cm. wide and 3 cm. high. Should a stronger
current of air be desired than that afforded by the small chimney, a
current of air previously passed through sulphuric acid or over
calcium chloride may be blown through the opening i by means of
a gasometer, rubber bulb, or other suitable contrivance. Or, air
dried by passing through sulphuric acid may be drawn through the
apparatus by means of an air-pump ( 47), or an aspirator (d in
Fig. 34) connected with the small chimney by a tube-bearing cork
inserted into a short tube with which the chimney is in this case
provided. If a higher temperature than that of boiling water is
desired, the (copper) apparatus is filled with oil, and the tempera-
ture taken with a thermometer inserted in a perforated cork fixed
in the opening m.

In Fig. 34 the air-current is produced by a stream of water.

Fig. 34.




Fig. 35.



a represents a flask one-third filled with concentrated sulphuric
acid; c is a glass vessel (commonly called a LIEBIG'S drying-tube),



62 OPERATIONS. [ 29.

and d a tin vessel (the aspirator) provided with a stop-cock at <?,
and arranged in other respects as the cut shows. Fig. 35 repre-
sents a small tin vessel, containing water and covered with a lid ;
two apertures are cut into the border of the latter, to receive
the ascending limbs of c.

The tube c is first weighed with the substance, then placed in
the water-bath (Fig. 35), which is placed over an alcohol- or gas-
lamp; the aspirator d is then filled with water, and c connected
with the flask a by the perforated cork g, and with d by means of
a caoutchouc tube f. If the stop-cock e be now opened so as to
allow the water to drop from d, the air will pass through the tube
, and after being dehydrated by the sulphuric acid, will pass over
the heated substance in c. After the operation has been continued
for some time, it is interrupted for the purpose of weighing the
tube c and its contents, and then resumed again, and continued
until the weight of c (and its contents) remains stationary. The
current of cold air exercising its constant cooling action upon the
substance, the latter never really reaches 100. It is, therefore,
sometimes advisable to substitute for the water in the bath a satu-
rated solution of common salt.

With this substitution, the apparatus will be found to effect its
purpose most expeditiously. It is not adapted, however, for dry-
ing such substances as have a tendency to fuse or agglutinate at
100. It is, moreover, less adapted for determining the moisture
in substances than for simply drying them, because the glass is some-
what attacked by the prolonged action of the boiling water, hence
causing a slight loss in weight of the drying-tube in the course of
the operation. This loss, too, varies with different kinds of glass.



29..

e. Substances which persistently retain moisture at 100, or
become completely dry only after a very long time, but which are
decomposed by a red heat.

The desiccation of such substances is affected by means of air-,
oil-, paraffin , or mercury-baths, or on drying-disks (Fig. 4-2), at a
temperature of 100-120 or even still higher ; and, according to
circumstances, with or without the aid of a current of air, some-
times in a partial vacuum, and sometimes in diluted carbon dioxide.



29.]



DESICCATION.



63



Figs. 36 and 37 represent two air-baths of simple construction.
The latter is adapted for the desiccation of a single substance; the
former is suited for the simultaneous drying of several substances.

In Fig. 36, a ~b is a case of stout sheet copper soldered with
brass, 15 to 20 cm. wide and deep, and of suitable height. In the




Fig. 36.



Fig. 37.



aperture c there is fixed a cork carrying a thermometer, <#, which
extends into the interior of the case, e is a wire stand on which the
watch-glasses containing the substances to be dried are placed.
The case is heated by means of a gas-, alcohol-, or oil-lamp. When
the temperature has reached the point desired, it is easily main-
tained at this point by regulating the flame. * In order to limit as
much as possible the cooling from without, it is advisable to cover
the apparatus with a pasteboard hood having a movable front.

In Fig 37, A -is a box of strong sheet copper, about 11 cm.

* When using gas, BUNSEN'S improved KEMP regulator (made by DESAGA, of
Heidelberg) may be advantageously employed in order to obtain constant tem-
peratures. A modification has been recommended by TH. SCHOREK (Zeitschr. f.
analyt. Chem., ix, 213). SCHEIBLER'S regulator (ibid., vn, 88) is more certain in
action, even also under sudden changes in gas pressure, but, as its action depends
on an electromagnet, its construction is more complicated.



64



OPERATIONS.



[29.



high and 9 cm. in diameter. The box is closed with the loosely-
fitting cover .Z?, which is provided with a narrow rim, and has two
apertures, C and E ; C is intended to receive the thermometer Z>,
which is fitted into it bj a perforated cork; J? affords an exit to the

aqueous vapors, and is, according
to circumstances, either left open
or loosely closed. Within the
box, about half-way up, are fixed
three pins, for the support of a
triangle of moderately stout wire,
upon which the crucible with the
substance is placed. The ther-
mometer bulb should be as near
the crucible as possible, but must
not touch the triangle. Heating
is effected by means of a gas- or
alcohol-lamp. When the appa-
ratus has cooled to the extent that
it may be conveniently grasped,
the cover is removed, and the still
warm crucible taken out, covered,
and allowed to become cold in an
exsiccator, when it is weighed.

The air-bath shown in Fig. 38
serves for drying substances in a
bulb-tube with the simultaneous
employment of a current of dry
air. The apparatus consists of a
sheet-iron box having the follow-
ing dimensions in cm. : a I = 20 ;
ac = 13; ad = 12; ef= 11;
e g = 6. The diameter of tho
njKTture on each side is 16 mm.
The thermometer is thrust so far
down until its bulb is on a level with and touches the side of the
bulb-tube. To this end the opening h is not placed exactly in the
middle line, but 1 cm. behind it. In this apparatus a temperature
of 200 to 260 may be easily attained. To produce the current of
dry air, one of the projecting ends of the bulb-tube is connected



I




Fig. 38.



29.]



DESICCATION.



with a hydraulic air-pump ( 47) or an aspirator, as in Fig. 34 ;
the other end is connected with a calcium -chloride tube or a flask
containing concentrated sulphuric acid (Fig. 34, a) ; the current
should be somewhat rapid at first, slower afterwards. If the tube
with the dried substance is to be weighed, it must be allowed to
cool, with a current of dry air still passing through it.

In the air bath shown in Fig. 39, the drying is promoted by
alternate exhaustion and readmission of air. a is a vessel of stout




Fig. 39.

sheet copper, provided with two apertures, and soldered with brass ;
1) is a glass tube in which the substance is dried ; c is a thermom-
eter ; d is a calcium-chloride tube ; e is a small hand air-pump,
which may, of course, be replaced by a hydraulic or mercurial
air-pump.

In use a is heated to the desired degree ; then I and d are ex-
hausted. After a few minutes the stop-cock/ is opened and air is
allowed to re-enter, first becoming perfectly dry by passing over
the calcium chloride. The exhaustion and readmission of air are
then repeated until not the slightest trace of moisture is visible in
the tube g when the latter is cooled by surrounding it with cotton
saturated with ether.



OPERATIONS.



[30.



30.

As an oil-bath, the copper drying closet figured in Fig. 31 is
employed as a rule, being then filled two-thirds with refined rape-
oil. The temperature is ascertained by means of a thermometer
borne by a perforated cork inserted in the aperture a. The ther-
mometer bulb must reach nearly to the bottom, or must at least be
entirely immersed in the oil. As the oil emits a disagreeable and
most annoying odor when heated, it is preferable to use paraffin
instead. The air-bath shown in Fig. 39 may also serve as an oil-
bath. If it is intended to weigh the substance, after drying, in
the tube, a shorter tube should be selected whidi may be readily
inserted into the tube standing in the oil. '

Some organic substances, when dried at high temperatures,
suffer alteration from the action on them of atmospheric oxygen
(see FK. ROCHLEDER, Jour, fur prcikt. Chemie, LXVI, 208).
When drying such substances, hence, contact with oxygen must be
avoided.

Figs. 40 and 41 show apparatus devised by ROCIILEDER for this
purpose. The former may also be advantageously employed for
drying with an air-current ; in the latter, the drying is effected in
a rarefied gas. B, Fig. 40, is a sheet-copper
"" cylinder 18 cm. high and 9 cm. in diameter,
containing a suitable quantity of oil or paraffin
in which is suspended and suitably fixed an
iron or glass vessel, A, containing mercury.
In the mercury there dip a thermometer, and
the glass tube, (7, containing the substance
to be dried. The dried gas (hydrogen, car-
bonic acid, air, etc.) enters at J, and escapes
at a if necessary, through a weighed cal-
cium-chloride tube. To prevent any possible
air-currents acting on the substance, the end
of ~b is bent upwards. The advantage of hav-
ing mercury in a is that, on removing the
tube 6 7 , it is perfectly clean.
In Fig. 41 the cock // is screwed on to the air-pump at a ;
b is connected by means of rubber tubing with a rubber bag or
bladder filled with carbon dioxide. B is an oil-bath, the tern-




Fig. 40.



31.]



DESICCATION.



67



perature of which is ascertained by a thermometer. In the oil-
bath is suspended a wide-mouthed, stout glass vessel, S, in which




Fig. 41.

iw placed the substance to be dried, and contained in a glass tube,
as wide as practicable, and sealed at one end. On pumping, while
the ccck IT is open and the cock II' is closed, the air in S is rare-
fied; on now closing IT and opening ZP, the vessel becomes filled
with carbon dioxide, previously dried by passing through the
calcium-chloride tube C f . By repeating this procedure the appa-
ratus is entirely filled with dried carbon dioxide. H'~ is then
closed, and the pump operated. The oil-bath is then heated to the
desired temperature, carbon dioxide being admitted from time to
time by opening II "' '. On closing II' and pumping again, the
moisture taken up by the carbon dioxide is removed with the lat-
ter, and is retained in the calcium- chloride tube 0. Within half
an hour the drying is complete.



31.

In technical and agricultural chemical investigations, in which
a number of specimens are to be simultaneously dried at a high
temperature, the drying-disk illustrated in Fig. 42 and devised
by me is recommended.

The apparatus consists of a turned cast-iron plate 21 cm. in
diameter and 37 mm. thick, supported by a tripod, and weighing
about 8 kilos. This weight enables the plate to be uniformly
heated, and permits the desired temperature to be readily main-
tained. At equal distances from the centre of the plate six smooth,



68



OPERATIONS.



[31.



ard

l



cylindrical cavities are turned. Each cavity is fitted with a turned

brass pan 55 mm. in diameter and
18 mm. deep, and fitting rather
loosely, so as to be readily removable
after heating. Every pan is provided
with a small handle pointing toward
the periphery of the plate, and restii
in appropriate grooves made in tl
latter. Each handle, moreover, beai
stamped upon it a number, from 1
to 6, corresponding to a similar num-
ber stamped in the plate behind the
cavities so that each pan has its own
proper cavity. The centre of each
pan is 6.5 cm. distant from the centre
of the plate; and the rims of the
pans are level with the surface of the
plate. Five of the pans are intended
for the samples (ores, parts of plants,
etc.), and the sixth for the thermom-
eter. The sixth cavity bears fitted
into it a brass rim extending 3 cm-
above the surface of the plate; the
pan so heightened is filled with brass
or copper filings, and the thermom-
eter bulb is embedded in these down
to the bottom. Heat is applied under
the centre of the plate.
f. Substances which suffer no alteration at a red heat, such as
>arium sulphate, pearlash, etc., are very readily freed from ini>-
ure. They need simply be heated in a platinum or porcelain
crucible over a gas- or spirit-lamp until the desired end is attaini-d.
The crucible, having first been allowed to cool a little, is put, still
,iot, under a desiccator, and finally weighed when cold.




Fig. 42.



32.] DESICCATION. 69

III. GENERAL PROCEDURE IN QUANTITATIVE ANALYSES,



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