C. Remigius Fresenius.

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a manner that the precipitate is enveloped in from four to six folds
of clean paper. The whole is then dropped into the platinum or
porcelain crucible lying obliquely upon a triangle over the lamp,
and pushed down against its sides with the finger. The cover is
then supported against the mouth of the crucible, as shown in Fig.
77, and the ignition commenced by heating the portion of the cru-
cible in contact with the cover. When the flame has the proper
size and position, the filter carbonizes quietly without any appear-
ance of flame or considerable amount of smoke. When the car-
bonization proceeds too slowly, the flame is moved a little toward
the bottom of the crucible. After some time the precipitate ap-

* Ann. d. Chem. u. Pharm., CXLYIII, 285; also Zeitschr. f. analyt. Chem..
viii, 186. In the case of alumina AL. MITSCHERLICH had previously highly rec-
ommended the ignition of the moist precipitate (Zeitschr. f. analyt. Chem., I, 67).

1 1 8 OPERATIONS. [ 53.

pears to be surrounded only by an extremely thin envelope of
carbon, possessing exactly the form (of course diminished in size)
of the original filter ; the flame is then increased, and the crucible
maintained at a bright-red heat until the carbon contained in this
envelope is consumed. The combustion proceeds so quietly that
the resulting ash surrounding the precipitate possesses, even to the
smallest fold, the exact form, of the original filter. If the ash
shows here and there a dark color, it is simply necessary to heat
the crucible over a blast-lamp for a few minutes to effect the com-
plete removal of the trace of carbon. This method of burning a
filter is extremely convenient and accurate ; it is only necessary to
give a little attention at first to the slow carbonization of the paper,
aft-jr which the further progress of the operation may be left to

Gelatinous, finely divided, granular, and crystalline precipi-
tates, such as alumina, calcium oxalate, barium sulphate, silica,
etc., may with equal facility be treated in this manner.

SECOND METHOD. (Ignition of the Precipitate apart from the


This method is resorted to in cases where a reduction of the
precipitate from the action of the carbon of the filter is appro.
hended ; and also where the ignited precipitate is required for
further examination, which the presence of the filter ash might
embarrass. It may be employed also, instead of the first method,
in all cases where the precipitate is easily detached from the filter.
The mode of proceeding is as follows:

The crucible intended to receive the precipitate is placed upon
a sheet of glazed paper ; the perfectly dry filter with the precipitate
is taken out of the funnel, and gently pressed together over the
paper, to detach the precipitate from the filter; the precipitate is
now shaken into the crucible, and the particles still adhering to the
filter are removed from it, so far as practicable, by further pressing
or gentle rubbing together of the folded filter, and are then also
transferred to the crucible. The filter is then cut up, using a clean
pair of scissors, into 8 or 10 pieces, over a sheet of gLi/ed paper,


and one by one placed by means of the tongs on the crucible lid
heated to redness. After all are burned, the lid is ignited until
the last trace of carbon is consumed. If the crucible lid be large,
and the filter small, the cutting up of the filter may be neg-
lected; it need only be folded, and then ignited. The lid is then
placed on a crucible and covered with a beaker. Finally the cru-
cible with the precipitate is ignited (another lid being placed on
it, if necessary), then the lid containing the filter-ash is put on
towards the end of the ignition, and the whole then allowed to cool
a little, when it is placed in a desiccator, and finally weighed when
perfectly cold.

Where the precipitates are not absolutely insoluble in water, as,
for instance, ammonium-magnesium phosphate, in which case the
filter is impregnated with a solution, however dilute, of the salt,
complete ignition frequently requires considerable time. It may,
however, be hastened by pressing the blackened filter against the
red-hot lid by means of a smooth platinum wire or knife. A cer-
tain amount of patience is, however, always required in this opera-

In the case of precipitates which, when reduced, yield no sub-
stances that combine with platinum, the following method of in-
cineration, devised by BUNSEN, may also be used : The filter, freed
so far as possible from the precipitate, is spread open upon the
sheet of glazed paper, and then folded in form of a little square box,
enclosed on all sides by the parts turned up ; any minute particles
of the precipitate that may have dropped on the glazed paper are
brushed into this little box, with the aid of a small feather ; the
box is closed again, rolled up, and one end of a long platinum wire
spirally wound round it. The crucible being placed on or above
a porcelain plate, the little roll is lighted, and, during its com-
bustion, held over the crucible, so that the falling articles of the
precipitate or filter-ash may drop into it, or, at least, into the por-
celain plate. In this way, and by occasionally holding the little
roll again in or against the flame, the incineration of the filter is
readily and safely effected. .When the operation is terminated, a
slight tap will suffice to drop the ash and the remaining particles of
the precipitate into the crucible, which is then covered, and the
ignition completed as in 52. Where it is intended to keep the

120 OPERATIONS. [ 53, a.

ash separate from the precipitate, it is made to drop into the lid
of the crucible, in which case it is better to ignite the crucible with
the principal portion of the precipitate first. Were this method of
incinerating adopted with such precipitates as silver chloride, lead
carbonate, etc., it would entail a certain amount of loss, because
through reduction there would be formed a small quantity of me-
tallic silver or lead, which would form an alloy with the platinum
wire. No matter which method of incineration is resorted to, the
operation must always be conducted in a spot entirely protected
from draughts.

Certain precipitates suffer some essential modification in their
properties, in their solubility, for instance, from ignition. In cases
where a portion of a substance of the kind is required, after the
weighing, for some other purpose with which the effects of a red
heat would interfere, the two operations of drying and igniting
may be combined in the following way : The precipitate is col-
lected on a filter dried at 100; it is then also dried, at 100, and
weighed ( 50). A portion of the dry precipitate is put into a
tared crucible, and its exact weight ascertained ; it is then exposed
to a red heat, allowed to cool in the usual way, and weighed again ;
the diminution of weight which it has undergone is calculated on
the whole amount of the precipitate.

53, a.

A method of filtering, washing, and igniting precipitates with
out the use of paper filters, which in many cases possesses great
advantages, has been devised by F. A. GOOCH,
and is described as follows.* First. White,
silky, anhydrous asbestos is scraped to a fine
short down with an ordinary knife-blade, boiled
with hydrochloric acid to remove traces of iron
or other soluble matter, washed by decantation, and set aside
for use.

* Proceedings of Am. Ac^d. Arts and Sciences, }878, p. 342.

Fig. 79.

Fig. 80.


Secondly. A platinum crucible of ordinary size, preferably of
the broad low pattern (Fig. 78), is chosen, and the bottom (Fig. 79)

perforated with fine holes (the more

numerous and the finer the better)

by means of a steel point ; or better

still, the bottom may be made of fine

platinum gauze. Next, a Bunsen fun-
nel of the proper size is selected, and over the
top a short piece of rubber tubing * is stretched
and drawn down until the portion above the
funnel arranges itself at right angles to the
stem. Within the opening in the rubber, the
perforated crucible is fitted as shown in Fig. 80,
and the funnel is connected with the receiver
of a Bunsen pump or other exhausting appa-
/atus in the ordinary way.

To make the asbestos filter, the pressure of the pump is applied,
and a little of the asbestos prepared as described, and suspended in
water, is poured into the crucible. The rubber and the crucible
are held together by the exhaustion of the vacuum pump with suf-
ficient force to make an air-tight joint ; the water is drawn through
and the asbestos is deposited almost instantly in a close compact
layer on the perforated bottom ; more asbestos (if necessary) in sus-
pension as before being poured upon the first until the layer
becomes sufficiently thick for the purpose for which it is intended.
Finally a little distilled water is drawn through the apparatus to
wash away any filaments which might cling to the under side, and
the filter is ready for use ; the whole process occupying less time
than is required to fold and fit an ordinary paper filter to a funnel.
To prepare the filter for the weighing of a precipitate, the
crucible with the felt of asbestos undisturbed is removed from
the funnel and ignited. In case the precipitate to be subse-
quently collected must be heated to high temperature for a long
time, it is better to enclose the perforated crucible with its felt

* If suitable rubber tubing is not at hand for fitting the crucible to the
funnel, a piece of strong glass tube, preferably tapering slightly, may be used
in place of a funnel. The diameter of the tube should exceed that of the cru-
cible. One end is drawn down to size of a common funnel stem; the crucible
is then fitted to the large end by means of a short section of large rubber tub-
ing, or a bored rubber stopper.


122 OPERATIONS. [ 54.

within another crucible ; because in such cases asbestos felt is apt
to curl at the edges, and without such precaution some of the
precipitate might drop through the perforations and be lost. For
drying at low temperatures, however, and even for ordinary igni-
tion, a second crucible is unnecessary ; but, during the ignition of
an easily reducible substance care must be taken to avoid contact
of unburnt gas with the perforated bottom.

To perform the filtration, the crucible is replaced in the funnel,
the pressure applied, and the process conducted precisely as in
ordinary filtration by the Bunsen pump. It is necessary to observe
that the vacuum pump be started before pouring the liquid upon
the filter. The final drying or ignition, as the case may be, of the
precipitate and filter is made without difficulty, or need of extra

For turbid liquids, or gelatinous precipitates, instead of the
perforated crucible a platinum cone may be used, the upper part
being made of foil, the lower part of gauze. This process is recom-
mended not only for such precipitates as have heretofore usually
been collected upon weighed paper filters, but also for many other
precipitates which are usually ignited, but whose proper ignition
is more or less interfered with by the presence of carbon.


The principle of volumetric analysis has been explained already
in the " Introduction," where we have seen how the quantity of
iron present in a fluid as a ferrous salt may be determined by means
of a solution of potassium permanganate, the value of which has
been previously ascertained by observing the quantity required to
convert a known amount of iron from a ferrous to a ferric salt.

In order to make the matter as clear as possible a few more
examples are here given.

Let us assume that we have prepared a sodium-chloride solution,
100 c. c. of which will precipitate exactly 1 grm. of silver, in the
form of chloride, from a silver-nitrate solution ; we are now in a
position to determine the silver content of unknown silver com-
pounds. For instance, on carefully dissolving 1 grm. of a silver-
cupper alloy in nitric acid, and adding the sodium-chloride solution


as above described just until all the silver is precipitated i.e.,
until another drcfc causes no further precipitate, the quantity of
silver present in the alloy is calculated very simply from the volume
of the sodium-chloride solution used up. Were, for instance,
80 c. c. of the solution used, it would indicate that the alloy had a
silver content of 80 per cent, since, as 100 c. c. of solution pre-
cepitate 1 grm. (or 100 per cent.) of pure silver, each c. c. of
solution will correspond to 1 per cent, of silver.

Again, as is well known, iodine and hydrogen sulphide im-
mediately react when brought together, sulphur and hydriodic acid
resulting (I + H 2 S = HI + 2S). Hydriodic acid has no action
on starch -paste, whereas the slightest trace of iodine colors it blue.
Now, if we prepare a solution of iodine in potassium -iodide solu-
tion, so that every 100 c. c. will contain 0-3721 grm. of iodine, we
can, with such a solution, decompose exactly O'l grm. of hydrogen
sulphide, since

34-086 : 126-85 : : 0-1 : 0-3721.

Let us suppose now that we have a fluid of unknown hydrogen -
sulphide content; we add to it a small quantity of starch-paste,
and then, drop by drop, the iodine solution made as above. The
blue color formed will not be permanent, but will disappear so long
as any iodine and hydrogen sulphide are present to react ; but when
all the latter has been decomposed the liquid suddenly becomes blue
from the formation of starch iodide. The quantity of hydrogen
sulphide may hence be readily calculated from the volume of iodine
solution used up, since 100 c. c. of the latter correspond to 0*1 grm.
hydrogen sulphide. Thus, had we used up 50 c. c. of solution, it
would indicate 0*5 grm. of hydrogen sulphide.

Solutions of accurately known composition or strength, used for
the purposes of volumetric analysis, are called standard (or titrat-
ing*} solutions. They may be prepared in two ways, viz., (a) by
dissolving a weighed quantity of a substance in a definite volume of
fluid ; or (&) by first preparing a suitably concentrated solution of
the reagent required, and then determining its exact strength by a
series of experiments made with it upon weighed quantities of the
body for the determination of which it is intended to be used.

* From the French titre, content (of gold or silver in coin).

124 OPERATIONS. [ 54.

In the preparation of standard solutions by method a, the weight
of the reagent taken for 1000 c. c. may, if desired, be a weight
exactly equivalent to 1 -008 gramme of hydrogen (see 192, c, 6).
In the case of standard solutions prepared by method &, this may
also be easily done, by diluting to the required degree the still some-
what too concentrated solution, after having accurately determined
its strength ; however, as a rule, this latter process is only resorted
to in technical analyses, where it is desirable to avoid all calculation.
1 luids which contain the eq. number of grammes of a substance in
one litre are called normal solutions ; those which contain T ^ of
this quantity, decinormal solutions.

The determination or titration of a standard solution intended
to be used for volumetric analysis is obviously a most important
operation, since any error in this will, of course, necessarily falsify
every analysis made with it. In scientific an 1 accurate researches
it is, therefore, always advisable, whenever practicable, to examine
the standard solution no matter whether prepared by method a
or by method J, with subsequent dilution to the required degree
by experimenting with it upon accurately weighed quantities of the
body for the determination of which it is to be used.

In the previous remarks I have made no difference between
fluids of known composition and those of known power; and this
has hitherto been usual. But by accepting the two expressions
as synonymous, we take for granted that a fluid exercises a chemi-
cal action exactly corresponding to the amount of dissolved sub-
stance it contains that, for instance, a solution of sodium chloride
containing 1 raol. Na Cl will precipitate exactly 1 at. silver. This
presumption, however, is very often not absolutely correct, as will
be shown with reference to this very example, 115, J, 5. In such
cases, of course, it is not merely advisable, but even absolutely
necessary, to determine the strength of the fluid by experiment,
although the amount of the reagent it contains may be exactly
known, for the power of the fluid can be inferred from its com-
position only approximately and not with perfect exactness. If a
standard solution keeps unaltered, this is a great advantage, as it
dispenses with the necessity of determining its strength before
every fresh analysis.

That particular change in the fluid operated upon by means of
a standard solution which marks the completion of the intended


decomposition, is termed the FINAL REACTION. This consists either
in a change of color, as is the case when a solution of potassium per-
manganate acts upon an acidified solution of ferrous salt, or a solu-
tion of iodine upon a solution of hydrogen sulphide mixed with
starch paste; or in the cessation of the formation of a precipitate
upon further addition of the standard solution, as is the case when
a standard solution of sodium chloride is used to precipitate silver
from its solution in nitric acid ; or in incipient precipitation, as is
the case when a standard solution of silver is added to a solution of
hydrocyanic acid mixed w^ith an alkali ; or in a change in the action
of the examined fluid upon a particular reagent, as is the case
when a solution of sodium arsenite is added, drop by drop, to a
solution of chlorinated lime, until the mixture no longer imparts a
blue tint to paper moistened with potassium iodide and starch-
paste, &c.

The more sensitive a final reaction is, and the more readily, posi-
tively, and rapidly it manifests itself, the better is it calculated to
perve as the basis of a volumetric method. In cases where it is an
object of great importance- to ascertain with the greatest practica-
ble precision the exact moment when the reaction is completed, the
analyst may sometimes- prepare, besides the actual standard solu-
tion, another, ten times more dilute, and use the latter to finish the
process, carried nearly to completion with the former.

But a good final reaction is not of itself sufficient to afford a safe
basis for a good volumetric method ; this requires, as the first and
most indispensable condition, that the particular decomposition
which constitutes the leading point of the analytical process should
at least under certain known circumstances remain unalterably
the same. Wherever this is not the case where the action varies
with the greater or less degree of concentration of the fluid, or
according as there may be a little more or less free acid present ; or
according to the greater or less rapidity of action of the standard
solution ; or where a precipitate formed in the course of the process
has not the same composition throughout the operation the basis
of the volumetric method is fallacious, and the method itself,
therefore, of no value.

When volumetric analysis first began to find favor, many
chemists based new volumetric methods upon final end-reactions,
without carefully studying the decompositions involved ; the re-

126 OPERATIONS. [ 54.

suit was a great number of volumetric methods, many of which
were useless. These have, however, been subjected to a sifting
process, particularly by FB. MOHR; * and in the special part of
the present work I have separated the really good methods from
the unserviceable.

* Lefarbuch der Titrirmethode, 3d edit.



FOR general information respecting reagents, I refer the stu
dent to my volume on " Qualitative Analysis."

The instructions given here will be confined to the preparation,
testing, and most important uses of those chemical substances which
subserve principally and more exclusively the purposes of quanti-
tative analysis. Those reagents which are employed in qualitative
investigations, having been treated of already in the volume on the
qualitative branch of the analytical science, will therefore be simply
mentioned here by name.

The reagents used in quantitative analysis are properly arranged
under the following heads :

A. Reagents for gravimetric analysis in the wet way.

B. Reagents for gravimetric analysis in the dry way.

C. Reagents for volumetric analysis.
I). Reagents used in organic analysis.

The mode of preparing the fluids used in volumetric analysis,
will be found where we shall have occasion to speak of their appli-


1. DISTILLED WATER (see " QuaL Anal.").

Water intended for quantitative investigations must be perfectly
pure. Water distilled from glass vessels leaves a residue upon
evaporation in a platinum vessel (see experiment No. 5), and is
therefore inapplicable for many purposes ; as, for instance, for the
determination of the exact degree of solubility of sparingly soluble


128 KEAGEXTS. [ 57

substances. For certain uses it is necessary to free the water by
ebullition from atmospheric air and carbonic acid.

2. ALCOHOL (see " Qual. Anal.").

a. Absolute alcohol. 1. Common alcohol of various degrees of


The application of ether as a solvent is very limited. It is
more frequently used mixed with alcohol, in order to diminish the
solvent, power of the latter for certain substances, e.g., ammonium
platinic chloride. The ordinary ether of the shops will answer the

4. CARBON DISULPHIDE (see " Qual. Anal.").

This should be purified, if necessary, by shaking with metallic
mercury (whereby the disagreeable odor of the commercial article
is removed), and then rectifying over the water-bath. In con-
ducting this latter operation the use of all rubber tubing must be
avoided. Carbon disulphide serves for removing free iodine from
aqueous solutions, and for freeing sulphides of metals from ad-
mixed sulphur.

a. Oxygen Acids.


a. Concentrated sulphuric acid (commercial).
Z>. Concentrated pure sulphuric acid.
c. Dilute sulphuric acid.

See "Qual. Anal."


a. Pure nitric acid of 1*2 sp. gr. (see " Qual. Anal.").

J. Red fuming nitric acid (concentrated nitric acid containing
some hyponitric acid).

J ' reparation. Mix 1000 grin, of pure potassium nitrate with
15 grm. starch in lumps, phu-e the mixture in a capacious tubu-
lated retort, and add 500 grm. sulphuric acid and 500 grm.
fuming sulphuric acid. The retort is then placed on a wire

58.] REAGENTS. 129

gauze over a gas-oven, or in a sand-bath. The distillation will
begin without the application of heat. If the potassium nitrate
is not perfectly free from metallic chlorides, the first portion of
the distillate should be collected separately and set aside. When
the distillation slackens gentle heat is applied, taking care not to
push the distillation too rapidly. The process is complete when,
by the application of moderate heat, no more acid distills over.
As it is impossible to prevent a portion of the hyponitric acid from
escaping, the process should be conducted in the open air, or
under a good vapor hood.

Tests. Red fuming nitric acid must be in a state of the greatest
possible concentration, and perfectly free from sulphuric acid. In
order to detect minute traces of the latter, evaporate a few c. c. of
the specimen in a porcelain dish nearly to dry ness, dilute the resi-
due with water, add some barium chloride, and observe whether a
precipitate forms on standing.

Uses. A powerful oxidizing agent and solvent ; it serves more
especially to convert sulphur and metallic sulphides into sulphuric
acid and sulphates respectively.

3. ACETIC ACID (see "Qual. Anal.").

4. TAETARIC ACID (see " Qual. Anal.").

b. Hydrogen Acids and Hologens.


a. Pure hydrochloric acid of 1*12 sp. gr. (see "Qual. Anal.").*
J. Pure fuming hydrochloric acid of about 1*18 sp. gr.
Preparation. As in " Qual. Anal." 26, with this modifica-
tion, however, that only 3 or 4 parts of water, instead of 6, are put

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 11 of 69)