C. Remigius Fresenius.

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a slow stream of carbon dioxide is transmitted through the solu-
tion-flask ; in some cases it is sufficient to expel the air, by simply
first putting a little hydrogen-sodium carbonate into the flask, con-
taining an excess of acid, before introducing the substance.

In selecting vessels in which solutions are to be effected, some
care must be exercised that the substance of the vessels be as little
liable to be attacked as possible by the solvent. As a general
rule, glass vessels are but slightly attacked by acids, but are
strongly acted on by alkalies (see 41).

39.
J. SOLUTION, PRECEDED BY FLUXING.

Substances insoluble in water, acids, or aqueous alkalies
usually require decomposition by fluxing, to prepare them for
analysis. Substances of this kind are often met with in the min-
eral kingdom; most silicates, the sulphates of the alkali-earth
metals, chrome ironstone, &c., belong to this class.

The object and general features of theproceas of fluxing have
already been treated of in the qualitative purl of the present work.



40, 41.] SOLUTION. EVAPORATION. 81

The special methods of conducting this important operation will
be described hereafter under "The analysis of silicates," and in
the proper places ; as a satisfactory description of the process, with
its various modifications, cannot well be given without entering
more minutely into the particular circumstances of the several
special cases.

Decomposition by fluxing often requires a higher temperature
than is attainable with a spirit-lamp with double draught, or with
a common gas-lamp. In such cases, the glass-blower's lamp, fed
with gas, is used with advantage.*



40.
4. CONVERSION OF DISSOLVED SUBSTANCES INTO WEIOHABLE FORMS.

The conversion of a substance in a state of solution into a form
adapted for weighing may be effected either by evaporation or by
precipitation. The former of these operations is applicable only
in cases where the substance, the weight of which we are desirous
to ascertain, either exists already in the solution in the form suit-
able for the determination of its weight, or may be converted into
such form by evaporation in conjunction with some reagent. The
solution must, moreover, contain the substance unmixed, or, at
least, mixed only with such bodies as are expelled by evaporation
or at a red heat. Thus, for instance, the amount of sodium
sulphate present in an aqueous solution of that substance may be
ascertained by simple evaporation ; whilst the potassium carbonate
contained in a solution would better be converted into potassium
chloride, by evaporating with solution of ammonium chloride.

Precipitation may always be resorted to, whenever the substance
in solution admits of being converted into a combination which is
insoluble in the menstruum present, provided that the precipitate
is fit for determination, which can never be the case unless it can
be washed and is of constant composition.

41.
a. EVAPORATION.

In processes of evaporation for pharmaceutical or technico-
chemical purposes the principal object to be considered is saving

* Excellent lamps of this kind are made by DESAGA, of Heidelberg.



OPERATIONS.



41.



of time and fuel; but in evaporating processes in quantitative
analytical researches this is merely a subordinate point, and the
analyst has to direct his principal care and attention to the means
of guarding against loss or contamination of the substance operated
upon.

The simplest case of evaporation is when we have to concentrate
a dear fluid, without carrying the process to dry ness. To effect
this object, the fluid is poured into a basin, which, should not be
filled to more than two-thirds. Heat is then applied by placing
the basin either on a water-bath, sand-bath, common stove, or
heated iron plate, or over the flame of a gas- or spirit-lamp, care
being taken always to guard against actual ebullition, as this in-
variably and unavoidably leads to loss from small drops of fluid
spirting out. Evaporation over a gas- or spirit-lamp, when con-
ducted with proper care, is an expeditious and cleanly process.
BUNSEN'S gas-lamp, Fig. 46, may be used most advantageously
in operations of this kind ; a little wire-gauze cap, loosely fitted
upon the tube of the lamp, is a material improvement. By means




Fig. 46. Fig. 47.

of this simple arrangement it is easy to produce even the smallest
flame, without the least apprehension of the flame striking back.
The lamp recently introduced by the MASTE Brothers, of
Iserlohn, and illustrated in Fig. 47, affords excellent service both
for evaporation and ignition, In it the burner is very similar to



41.]



EVAPORATION.



83



that of the BERZELITJS alcohol lamp, and affords a very small as well
as a very powerful large flame, and it has given me excellent
results during long-continued use. Five different sizes are made.

The gas furnace
shown in Fig. 48 is
also excellently adapt-
ed for evaporations
carried en in evapo-
rating-didies. In this
furnace the mixture
of gas and air issues
from many small ori-
fices, and the construc-
tion enables the flames
to be made so small
that the contents of
the dish can be quietly
evaporated without
ebullition.*




Fig. 48.




- 49<



If the evaporation is to be effected on the water-bath, and the
operator happens to possess a BEINDORF, or other similarly con-
structed, steam apparatus, the evaporating-
dish may be placed simply into an opening
corresponding in size. Otherwise recourse
must be had to the water-bath illustrated by
Fig. 49.

It is made of strong sheet copper, and
when used is half filled with water, which is kept boiling over a
gas-, spirit-, or oil-lamp. The breadth from a to 1) should be from
12 to 18 cm. Various flat rings of the same outside diameter as
the top of the bath, and adapted to receive dishes and crucibles of
different sizes, are essential adjuncts to the bath. These rings
when required are simply laid on the bath.

It is very inconvenient to have the* water in the bath com-
pletely evaporate unnoticed, because frequently residues become
heated to a higher degree than is desirable, or concentrated solu-
tions spirt, etc. To avoid such inconveniences, I make use of a
water-bath with constant level, as shown in Fig. 50. This ap-
paratus consists of a zinc vessel, a 1) c d, 10 cm. high and 12 cm.
* My furnaces are made by KILIAN, of Wiesbaden.



84



OPERATIONS.



[41.



in diameter, and connected with the water-bath, </, by means of
the short rubber tube, e, and the copper tube, /. A sheet-zinc




Fig. 50.

bottle, kilcl, the cylindrical part of which is 17 cm. high and
the neck 3 cm. in diameter, is filled with water, and, inverted,
placed in the vessel, a led. The orifice of the bottle at the neck is
15 mm. wide, and, in the inverted position, is closed by a valve, m.
On inserting the bottle into the vessel, the wire carrying the
valve strikes the bottom of the vessel and opens the valve.
The level of the water in g is readily regulated by raising or low-
ering the pillar support, 0, and remains constant so long as any
water remains in the bottle. The tube/* is bent downward in the
water-bath and reaches nearly to the bottom.

A simple arrangement for extinguishing the flame when all the
water in the water-bath has evaporated has been described by
K. HEUSS*; the construction of BUNSEN'S constant water-bath
has been detailed by W. II. "WAiiL.f

If the operator can conduct his processes of evaporation in a
room set apart for the purpose, where he may easily guard against
any occurrence tending to suspend dust in the air, he will find it
no very difficult task to keep the evaporating fluid clean ; in this



* Zeitechr.f. analyt. Ohern., ix, 336.



t Ibid., x, 88.





41.] "*" EVAPOKATION. 85

case it is best to leave the dishes uncovered.* But in a large
laboratory, frequented by many people, or in a room exposed to
draughts of air, or in which coal fires are burning, the greatest
caution is required to protect the evaporating fluid from contami-
nation by dust or ashes.

For this purpose the evaporating dish is either covered with a
sheet of filtering-paper turned down over the edges, or a glass rod
twisted into a triangular shape, Fig. 51, is laid
upon it, and a sheet of filtering-paper spread
over it, which is kept in position by a glass rod
laid across, the latter again being kept from
rolling down by the slightly turned up ends,
a and &, of the triangle.

The best way, however, is the following: Take two small
thin wooden hoops, Fig. 52, one of which fits loosely in the other ;
spread a sheet of blotting-paper over the smaller
one, and push the other over it. This forms a
cover admirably adapted to the purpose ; and
whilst in no way interfering with the operation,
it completely protects the evaporating fluid
from dust, and may be readily taken off ; the paper cannot dip
into the fluid ; the cover lasts a long time, and may, moreover, at
any time be easily renewed.

It must be borne in mind, however, that the common filtering-
paper contains always certain substances soluble in acids, such as
lime, ferric oxide, &c., which, were covers of the kind just
described used over evaporating dishes containing a fluid evolving
acid vapors, would infallibly dissolve in these vapors, and the solu-
tion dripping down into the evaporating fluid, would speedily con-
taminate it. Care must be taken, therefore, in such cases, to use
only such filtering-paper as has been freed by washing from sub-
stances soluble in acids.

Evaporation for the purpose of concentration may be effected
also in fiasks; these are only half filled, and placed in a slanting

* In my own laboratory separate closets are set apart for evaporations in
quantitative analyses. It is best to have the floor and roof of sandstone, and
the walls of brick lined with glazed tiles or finished with plaster of Paris. At
the top of the back wall is a horizontal channel of suitable width, and leading
into a Russian chimney. No fire must be made under this chimney, but it is
very desirable to place this chimney close to another chimney kept constantly
warm (by the fire used for the steam apparatus, for instance). The front wall
of the evaporating chamber may be of sandstone pillars 18 decimeters high, be-
tween which are fitted wooden frames wherein balanced windows may slide up
and down.



86



OPERATIONS.



[41.



position. The process may be conducted on the sand-bath, or over
a gas- or spirit-lamp, or even, and with equal propriety, over a char-
coal fire. In cases where the operation is conducted over a lamp
or a charcoal fire, it is the safest way to place the flasks on wire
gauze. Gentle ebullition of the fluid can do no harm here, since
the slanting position of the flask guards effectively against risk of
Joss from the spirting of the liquid. Still better than in flasks, the
object may be attained by evaporating in tubulated retorts with
open tubulure and neck directed obliquely upwards. The latter
acts as a chimney, and the constant change of air thus effected is
extremely favorable to evaporation.

The evaporation of fluids containing a precipitate is best con-
ducted on the water-bath ; since on the sand-bath, or over the lamp,
it is next to impossible to guard against loss from bumping. This




Fig. 53.

bumping is occasioned by slight explosions of steam, arising from
the sediment impeding the uniform diffusion of the heat. Still
there remains another, though less safe way, viz., to conduct the
evaporation in a crucible placed in a slanting position, as illus-
trated in fig. 53. In tin's process, the flame is made to play upon
the crucible above the level of the fluid.

M In r> 'i il>iid has to l>e evaporated f<> (It-i/ness, as is so often
the case, the operation >houl<l always, if possible, be terminated on
the water-bath. In cafiee where the nature of the dissolved sub-
stance precludes the application of the water-hath, the object in
view mav often l,c most ivadily attained by heating the contents



41.] EVAPORATION. 87

of the dish from the top, which is effected by placing the dish in a
proper position in a drying closet, the tipper plate of which is heated
by a flame (that of the water- or sand-bath) passing over it. If the
substance is in a covered platinum dish or crucible, place the gas-
lanip in such a position that the flame may act on the cover from
above.

In cases where the heat has to be applied from the bottom, a
method must be chosen which admits of an equal diffusion and
ready regulation of the heat.

An air-bath is well adapted for this purpose, i.e., a dish of iron
plate, in which the porcelain or platinum dish is to be placed on a
wire triangle, so that the two vessels may be at all points \ to J
inch distant from each other. The copper apparatus, fig. 49 may
also serve as an air-bath, although I must not omit to mention that
this mode of application will in the end seriously injure it. If the
operation has to be conducted over a lamp, the dish should be
placed high above the flame ; best on wire gauze, since this will
greatly contribute to an equal diffusion of the heat. The use of
the sand-bath is objectionable here, because with that apparatus we
cannot reduce the heat so speedily as may be desirable. An iron
plate heated by gas may perhaps be used with advantage. But no
matter which method be employed, this rule applies equally to all of
them ; that the operator must watch the process, from the moment
that the residue begins to thicken, in order to prevent spirting, by
reducing the heat, and breaking the pellicles which form on the
surface, with a glass rod, or a platinum wire or spatula.

Saline solutions that have a tendency, upon their evaporation, to
creep up {he sides of the vessel, and may thus finally pass over the
brim of the latter, thereby involving the risk of a loss of substance,
should be heated from the top, in the way just indicated ; since by
that means the sides of the vessel will get heated sufficiently to
cause the instantaneous evaporation of the ascending liquid, pre-
venting thus its overflowing the brim. The inconvenience just
alluded to may, however, be obviated also, in most cases, by cover-
ing the brim, and the uppermost part of the inner side of the ve&
sel, with a very thin coat of tallow, thus diminishing the adhesion
between the fluid and the vessel.

In the case of liquids evolving gas-lubUes upon evaporating,
particular caution is required to guard against loss from spirting.
The safest way is to heat such liquids in an obliquely-placed
flask, or in a beaker covered with a large watch-glass; the latter jy



88 OPERATIONS. [ 41.

removed as soon as the evolution of gas-bubbles has ceased, and the
fluid that may have spirted up against it is carefully rinsed into
the glass, by means of a washing-bottle. If the evaporation has to
be conducted in a dish, a rather capacious one should be selected,
and a very moderate degree of heat applied at first, and until the
evolution of gas has nearly ceased.

If a fluid has to be evaporated with exclusion of air, the best
way is to place the dish under the bell of an air-pump, over a ves-
sel with sulphuric acid, and to exhaust; or a tubulated retort may
be used through whose tubulure hydrogen or carbon dioxide is
passed by the acid of a tube not quite reaching to the surface of
the fluid.

The material of the evaporating vessels may exercise a much
greater influence on the results of an analysis than is generally
believed. Many rather startling phenomena that are observed in
analytical processes may arise simply from a contamination of the
evaporated liquid by the material of the vessel ; great errors may
also spring from the same source.*

The importance of this point has induced me to subject it to
a searching investigation (see Appendix, Analytical Experiments,
1-4) ; more recently A. EMMERLING has also exhaustively inves-
tigated the subject, and fully confirms the results, which I will
here briefly intimate.

Distilled water kept boiling for some length of time in glass
(flasks of Bohemian glass) dissolves very appreciable traces of that
material. This is owing to the formation of soluble silicates ; the
particles dissolved consist chiefly of potassa, or soda and lime, in
combination with silicic acid. A much larger proportion of the
glass is dissolved by water containing caustic or carbonated alkali ;
boiling solution of ammonium chloride also strongly attacks glass
vessels. Boiling dilute acids, with the exception, of course, of
hydrofluoric and hydrofluosilicilic acids, exercise a less powerful
solvent action on glass than pure water. Porcelain (Berlin dishes)
is much less affected by water than glass ; alkaline liquids also
exercise a less powerful solvent action on porcelain than on glass ;
the quantity dissolved is, however, still notable. Solution of
ammonium chloride acts on porcelain as strongly as on glass;
dilute acids, though exercising no very powerful solvent action on
porcelain, yet attack that material more strongly than glass. It
results from these data, that in analyses pretending to a high
* Compare A. SOUCHAY, Zeitschr.f. analyt. Chem., iv, 60.



42.] EVAPORATION. 89

degree of accuracy, platinum or platinum-iridium or silver dishes
should, always be preferred. The former may be used in all cases
where no free chlorine, bromine, or iodine is present in the fluid,
or can be formed during evaporation. Fluids containing caustic
alkalies may safely be evaporated in platinum, but not to the point
of fusion of the residue. Silver vessels should never be used to
evaporate acid fluids nor liquids containing alkaline sulphides ;
but they are admirably suited for solutions of alkali hydroxides
and carbonates, as well as of most normal salts. If the use of
porcelain or glass vessels for evaporating large volumes of fluid
cannot be avoided, then porcelain dishes are to be preferred ; with
alkaline fluids glass vessels are totally inadmissible, at least in
accurate analyses.

42.

We come now to weighing the residues remaining upon the
evaporation of fluids. We allude here simply to such as are
soluble in water; those which are separated by filtration will b3
treated of afterwards. Residues are generally weighed in the
same vessel in which the evaporation has been completed, for
which purpose platinum dishes, from 4 to 8 cm. in diameter, pro-
vided with light covers, or large platinum cruci-
bles, are best adapted, since they are lighter than
porcelain vessels of the same capacity.

However, in most cases, the amount of liquid
to be evaporated is too large for so small a vessel,
and its evaporation in portions would occupy too
much time. The best way, in cases of this kind,
is to concentrate the liquid first in a larger vessel,
and to terminate the operation afterwards in the
smaller weighing vessel. In transferring the fluid from the larger
to the smaller vessel, the lip of the former is slightly greased, and
the liquid made to run down a glass rod (Fig. 54).

Finally the large vessel is carefully rinsed with a washing-
bottle, until a drop of the last rinsing leaves no longer a residue
upon evaporation on a platinum knife. When the fluid has thus
been transferred to the weighing-vessel, the evaporation is com-
pleted on the water-bath and the residuary substance finally ignited,
provided, of course, it will admit of this process. For this pur-




90 OPERATIONS. [ 42.

pose the disli is covered with a lid of thin platinum (or a thin glass
plate), and then placed high over the flame of a lamp, and heated
very gently until all the water which may still adhere to the sub-
stance is expelled ; the dish is now exposed to a stronger, and finally
to a red heat. (Where a glass plate is used, this must, of course, be
removed before igniting.) In this case it is also well to make the
flame play obliquely on the cover from above, so as to run as
little risk as possible of loss by spirting. After cooling in a desic-
cator, the covered dish is weighed with its contents. When oper-
ating upon substances which decrepitate, such as sodium chloride,
for instance, it is advisable to expose them after their removal
from the water-bath, and previously to the application of a naked
flame to a temperature somewhat above 100, either in the air-
bath, or on a sand-bath, or on a common stove.

If the residue does not admit of ignition, as is the case, for
instance, with organic substances, ammonium salts, &c., it is dried
at a temperature suited to its nature. In many cases, the tempera-
ture of the water-bath is sufficiently high for this purpose, for the
drying of ammonium chloride, for instance ; in others, the air or
oil-bath must be resorted to. (See 29 and 30.) Under any cir-
cumstances, the desiccation must be continued until the substance
ceases to suffer the slightest diminution in weight, after renewed
exposure to heat for half an hour. The dish should invariably be
covered during the process of weighing.

Since saline residues obtained on evaporation are frequently
prone to attract moisture after drying or ignition, the first weigh-
ing, which always requires some time, may give results which are
too high. To avoid this, the dish is reheated after the first
weighing, then allowed to cool in the exsiccator ; the weight ob-
tained in the first weighing is then placed on one scale-pan and
the dish placed on the other, when the second weighing is ac-
complished with as little loss of time as possible.

If, as will frequently happen, we have to deal with a fluid con-
taining a small quantity of a potassium or sodium salt, the weight
of which we want to ascertain, in presence of a comparatively large
amount of an ammonium salt, which has been mixed with it in the
course of the analytical process, I prefer the following method :
The saline mass is thoroughly dried, in a large dish, on the water-
bath, or, towards the end of the process, at a temperature some-
what exceeding 100. The dry mass is then, with the aid of a



43.] EVAPORATION. 91

platinum spatula, transferred to a small glass dish, which is put
aside for a time in a desiccator. The last traces of the salt left
adhering to the sides and bottom of the large dish are rinsed off
with a little water into the small dish, or the large crucible, in
which it is intended to weigh the salt ; the water is then evaporated,
and the dry contents of the glass dish are added to the residue :
the ammonium salts are now expelled by ignition, and the residu-
ary fixed salts finally weighed. Should some traces of the saline
mass adhere to the smaller glass dish, they ought to be removed
and transferred to the weighing vessel, with the aid of a little
pounded ammonium chloride, or some other ammonium salt, as the
moistening again with water would involve an almost certain loss
of substance.

43
&. PRECIPITATION.

Precipitation is resorted to in quantitative analysis far more
frequently than evaporation, since it serves not merely to convert
substances intp forms adapted for weighing, but also, and more
especially, to separate them from one another. The principal in-
tention in precipitation, for the purpose of quantitative estimations,
is to convert the substance in solution into a form in which it is
insoluble in the menstruum present. The result will, therefore,
cceteris parilms, be the more accurate, the more the precipitated
body deserves the epithet insoluble, and in cases where precipi-
tates are of the same degree of solubility, that one will suffer the
least loss which comes in contact w r ith the smallest amount of
solvent.

Hence it follows, first, that in all cases where other circum-
stances do not interfere, it is preferable to precipitate substances
in their most insoluble form ; thus, for instance, barium had better
be precipitated as sulphate than as carbonate ; secondly, that when
we have to deal with precipitates that are not quite insoluble in
the menstruum present, we must endeavor to remove that men-
struum, as far as practicable, by evaporation ; thus a dilute solution
of strontium should be concentrated, before proceeding to precipi-
tate the strontium with sulphuric acid ; and, thirdly, that when we
have to deal with precipitates slightly soluble in the liquid present,
but insoluble in another menstruum, into which the former may



92 OPERATIONS. [ 43.

be converted by the addition of some substance or other, we ought
to endeavor to bring about this modification of the menstruum.
Thus, for instance, alcohol may be added to water, to induce com-



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