C. Remigius Fresenius.

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not appreciably altered by time. Solid matter in powder such as
graphite, hastens coagulation, alkali salts induce it rapidly. Aque-
ous solutions of silicic acid may, on the contrary, be mixed with
hydrochloric acid, nitric acid, acetic acid, tartaric acid and alcohol
without coagulating. The gelatinous silicic acid produced by
coagulation may contain more or less water, and it appears to be
the more difficultly soluble in water, the less water it contains;
thus a jelly of silicic acid containing 1 per cent, of silica (SiO 2 ) gives
a solution with cold water containing 1 part of silica in about 5000
parts, a jelly of 5 per cent, gives a solution containing 1 part of silica
in about 10000 parts of water. A jelly containing less water is still
less soluble, and when the jelly is dried up to a gummy mass it is
barely soluble at all ; this is also the case with the pulverulent
hydrate of silica obtained in the analysis of silicates by drying a
jelly containing much salts at 100 (GRAHAM*). The hydrated
silica dried at 100 dissolves but very slightly in acids (with the
exception of hydrofluoric acid) ; it dissolves, however, in solutions
of fixed alkalies and alkali carbonates, especially on heating. Aque-
ous ammonia dissolves the jelly in tolerable quantity and the dry
hydrate in very notable quantity (PRiBRAM)f. Regarding the
amount of water in the hydrate dried at given temperatures chem-
ists do not agree.J

On ignition all the hydrates pass into anhydrous silica. As
the vapor escapes small particles of the extremely fine powder
are liable to whirl up. This may be avoided by moistening the
hydrate in the crucible with water, evaporating to dryness on a
water bath, and then applying at first a slight and then a gradu-
ally increased heat.

The silica obtained by igniting the hydrate appears in the
amorphous condition, with a sp. gr. of 2*2 to 2*3. It forms a

* Pogg. AnnaL, cxin, 529. \Zeitschr.f.analyt. CTiem., vi, 119.

$ DOVERI (Annal. de Ghim. et de Phys., xxi, 40; Annal. d. Chcm. u. Pharm.,
LXIV, 256) found in the air-dried hydrate 16-9 to 17'8 water; J. FUCHS
(Annal. d. Cfiem. u. Pharm., LXXXII, 119 to 123), 9 '1 to 9-6; G. LIPPERT, 9-38
to 9-95. DOVERI found in the hydrate dried at 100, 8'3 to 9 -4; J. FUCHS, 6'63
to 6 96; G. LIPPERT, 4-97 to 5-52. H. ROSE (Pogg. Annal., cvm, 1; Journ.fiir
prakt. Chem., LXXXI, 227) found in the hydrate obtained by digesting stilbit
with concentrated hydrochloric acid, and dried at 150, 4'85# water.


white powder insoluble in water, and acids (hydrofluoric excepted),
soluble in solutions of the fixed alkalies and their carbonates,
especially in the heat. Hydrofluoric acid readily dissolves amor-
phous silica; the solution leaves no residue on evaporation in
platinum, if the silica was pure. The amorphous silica, when
heated with ammonium fluoride in a platinum crucible, readily
volatilizes. The ignited amorphous silica, exposed to the air,
eagerly absorbs water, which it will not give up at from 100 to
150 (H. ROSE). The lower the heat during ignition the more
hygroscopic is the residue (SOUCHAY*). Silica fuses at the strong-
est heat ; the mass obtained being vitreous and amorphous. Amor-
phous silica ignited with ammonium chloride, at first loses weight,
and then, when the ignition has rendered it denser, the weight
remains constant.

The amorphous silica must be distinguished from the crystallized
or crystalline variety, which occurs as rock crystal, quartz, sand, &c.
This has a sp. gr. of 2*6 (SCHAFFGOTSCH), and is far more difficultly,
arid in far less amount, dissolved by potash solution or solution of
fixed alkali carbonates ; it is also more slowly attacked by hydro-
fluoric acid, or ammonium fluoride. Crystallized silica is not hygro-
scopic, whether strongly or gently ignited (SOUCHAY). Vegetable
colors are not changed either by silica or its hydrates.


Si 28-4 47-02

O 2 32-0 52-98

60-4 100-00



Hydrochloric acid is almost invariably weighed in the form of
SILVER CHLORIDE for the properties of which see 82.

Hydrobromic acid is always weighed in the form of SILVER


* Zeitschr. /. analyt. Chem., vm, 423.

236 FORMS. [ 94.

Silver bromide, prepared in the wet way, forms a yellowish-
white precipitate. It is wholly insoluble in water and in nitric
acid, tolerably soluble in ammonia, readily soluble in sodium thio-
sulphate and potassium cyanide. Concentrated solutions of potas-
sium, sodium, and ammonium chlorides and bromides dissolve it to
a very perceptible amount, while in very dilute solutions of these
salts it is entirely insoluble. Traces only dissolve in the alkali
nitrates. It dissolves abundantly in a concentrated warm solution
of mercuric nitrate. On digestion with excess of potassium iodide
solution it is completely converted into silver iodide (FIELD). On
ignition in a current of chlorine silver bromide is transformed into
chloride ; on ignition in a current of hydrogen it is converted into
metallic silver. Exposed to the light it gradually turns gray, and
finally black. Under the influence of heat, it fuses to a reddish
liquid, which, upon cooling, solidifies to a yellow, horn-like mass.
Brought into contact with zinc and water, it is decomposed ; a
spongy mass of metallic silver forms, and the solution contains zinc


Ag . . . . 107-92 57-44

Br 79-95 42-56

187-87 100-00


Hydriodic acid is usually determined in the form of SILVER
IODIDE, and occasionally also in that of PALLADIOUS IODIDE. '

a. Silver iodide, produced in the wet way, forms a light-yellow
precipitate, insoluble in water, and in dilute nitric acid, and very
slightly soluble in ammonia. One part dissolves, according to
WALLACE and LAMONT* in 2493 parts of aqueous ammonia sp. gr.
0*89 ; according to MARTINI, in 2510 parts of 0-96 sp. gr. It is copi-
ously taken up by concentrated solution of potassium iodide, but it
is insoluble in very dilute ; it dissolves readily in sodium thiosul-
phatc and in potassium cyanide; traces only are dissolved ly alkali
nitrates. In concentrated warm solution of mercuric nitrate it is
copiously soluble. Hot concentrated nitric and sulphuric acids
cnn vert it, but with some difficulty, into silver nitrate and sulphate
respectively, with expulsion of the iodine. Silver iodide acquires a

* Chem. Gaz., 185$.Jahresbericht, KOPP and WILL, 1859, 070.


black color when exposed to the light, "When heated, it fusea
without decomposition to a reddish fluid, which, upon cooling,
solidifies to a yellow mass, that may be cut with a knife. Under
the influence of excess of chlorine in the heat it is completely con-
verted into silver chloride ; ignition in hydrogen reduces it but
incompletely to the metallic state. When brought into contact
with zinc and water, it is decomposed but incornpletley ; zinc iodide
is formed, and metallic silver separates.


Ag . . . . 107-92 45-97

I 126-85 54-03

234-77 100-00

J. Palladium iodide, produced by mixing an alkali iodide
with palladious chloride, is a deep brownish -black, flocculent pre-
cipitate, insoluble in water and in dilute hydrochloric acid, but
slightly soluble in saline solutions (sodium chloride, magnesium
chloride, calcium chloride, &c.). It is unalterable in the air. Dried
simply in the air it retains one molecule of water = 5'05 per cent.
Dried long in vacua, or at a rather high temperature (70 to 80),
it yields the whole of this water, without the least loss of iodine,
Dried at 100, it loses a trace of iodine ; at from 30C to 400, tin?
whole of the iodine is expelled. It may be washed \&iihhot wa.er
without loss of iodine.


Pd 107-00 29-66

I, 253-70 70-34

360-70 100-00


Hydrocyanic acid, if determined gravimetrically and directly, is
always converted into SILVER CYANIDE for the properties of which
compound see 82.


The forms into which the sulphur in hydrogen sulphide or
metallic sulphides, is converted for the purpose of being weighed,

238 FORMS. [ 95.



For the properties of the sulphides named, see 82, 85, 92 ;
for those of barium sulphate, see 71.



These two acids are never determined directly that is to say,
in compounds containing them, but always in an indirect way ;
generally volumetrically.




IN the preceding Section we have examined the composition
and properties of the various forms and combinations in which
radicals are separated from each other, or in which they are weighed.
We have now to consider the special means and methods of con-
verting them into such forms and combinations.

For the sake of greater clearness and simplicity, we shall, in
the present Section, confine our attention to the various methods
applied to effect the determination of single radicals, deferring to
the next Section the consideration of the means adopted for sepa-
rating them from each other.

We shall here deal with the estimations of substances in the
free state, or compounds consisting of one base and one acid, or
containing one metal and one metalloid.

As in the ' c Qualitative Analysis, ' ' the acids of arsenic will be
treated of among the bases, on account of their behavior to hydro-
gen sulphide ; and those elements that form acids with hydrogen !
will be treated of under their respective hydrogen acids.

In the quantitative analysis of a compound we have to study
first, the most appropriate method of dissolving it ; and, secondly,
the modes of determining the quantity of one or more of its con-

With regard to the latter point, we have to turn our attention,
first, to the performance j and secondly, to the accuracy of the

It happens very rarely in quantitative analyses that the amount
of a substance, as determined by the analytical process, corresponds
exactly with the amount theoretically calculated or actually pres-
ent ; and if it does happen, it is merely by chance.

It is of importance to inquire what is the reason of this fact,
and what are the limits of inaccuracy in the several methods.

The cause of this almost invariably occurring discrepancy
between the quantity present and that actually found, is to be
ascribed either exclusively to the execution, or it lies partly in the
method itself.


240 bETERMINATJOtf. [ 96.

The execution of tlie analytical processes and operations can
never be absolutely accurate, even though the greatest care and
attention be bestowed on the most trilling minutiae. To account
for this, we need only bear in mind that our weights and measures
are never absolutely correct, nor our balances absolutely accurate,
nor our reagents absolutely pure ; and, moreover, that we do not
weigh in vacua; and that, even if we deduce the weight in vacua
from the weight we actually obtain by weighing in the air, the
very volumes on which the calculation is based are but approxi-
mately known; that the hygroscopic state of the air is liable to
vary between the weighing of the empty crucible and of the cru-
cible -f- the substance; that we know the weight of a filter ash
only approximately ; that we can never succeed in completely
keeping off dust, &c.

With regard to the methods, many of them are not entirely
free from certain unavoidable sources of error ; precipitates are
not absolutely insoluble; compounds which require ignition are
not absolutely fixed ; others, which require drying, have a slight
tendency to volatilize; the final reaction in volumetric analyses is
usually produced only by a small excess of the standard fluid,
which is occasionally liable to vary with the degree of dilution, the
temperature, &c.

Strictly speaking, no method can be pronounced quite free
from defect ; it should be borne in mind, for example, that even
barium sulphate is not absolutely insoluble in water. Whenever
we describe any method as free from sources of error, we mean,
that no causes of considerable inaccuracy are inherent in it.

We have, therefore, in our analytical processes, invariably to
contend against certain sources of inaccuracy which it is impossi-
ble to overcome entirely, even though our operations be conducted
with the most scrupulous care and with the utmost attention to
established rules. It will be readily understood that several defects
and sources of error may, in some cases, combine to vitiate the
results ; whereas, in other cases, they may <-<HHJH axate one another,
and thus enable us to attain a higher degree of accuracy. The
comparative accuracy of the results attainable by an analytical
method oscillates between two points these points are called the
limits of error. In the case of methods free from sources of error,
these limits will closely approach each other ; thus, for instance, in


the determination of chlorine, with great care one will always be
able to obtain between 99*9 and 100*1 for the 100 parts of chlorine
actually present-
Less perfect methods will, of course, exhibit far greater dis^
crepancies ; thus, in the estimation of strontium by means of sul-
phuric acid, the most attentive and skilful operator may not be
able to obtain more than 99 (and even less) for the 100 parts
of strontium actually present. I may here incidentally state that
the numbers occasionally given in this manner, in the course of the
present work, to denote the degree of accuracy of certain methods,
refer invariably to the substance estimated (chlorine, nitrogen,
baryta, for instance), and not to the combination in which that
substance may be weighed (silver chloride, ammonium platinic
chloride, barium sulphate, for instance) ; otherwise the accuracy of
various methods would not be comparable.

The occasional attainment of results exactly corresponding with
the numbers calculated does not always justify the assumption, on
the part of the student, that his operations, to have led to such a
result, must have been conducted with the utmost precision and
accuracy. It may sometimes happen, in the course of the analyti-
cal process, that one error serves to compensate another ; thus, for
instance, the analyst may, at the commencement of his^operations,
spill a minute portion of the substance to be analyzed ; whilst, at a
later stage of the process, he may recover the loss by an imperfect
washing of the precipitate. As a general rule, results showing a
trifling deficiency of substance may be looked upon as better proof
of accurate performance of the analytical process than results
exhibiting an excess of substance.

As not the least effective means of guarding against error and
inaccuracies in gravimetric analyses, I would most strongly recom-
mend the analyst, after weighing a precipitate, &c., to compare
its properties (color, solubility, reaction, dec.) with those which it
should possess, and which have been amply described in the pre-
ceding Section.

In my own laboratory, I insist upon all substances that are
weighed in the course of an analysis being kept between watch-
glasses, until the whole affair is concluded. This affords always a
chance of testing them once more for some impurity, the presence
of which may become suspected in the after-course of the process.


First Group.



a. Solution.

Potassa and potassium salts of those inorganic acids which we
have to consider here, are dissolved in water, in which menstruum
they dissolve readily, or at all events, pretty readily.

Potassium salts of organic acids it is most convenient to convert
into potassium carbonate by long-continued, gentle ignition in a
covered crucible. Heated to fusion, the carbon separated acts on
the potassium carbonate ; carbonic oxide escapes, and some potas-
sium hydroxide is formed. On simple carbonization a slight loss
is caused ; on fusing, which must be avoided, a further loss occurs.

5. Determination.

Potassium is weighed either as potassium sulphate, as potas-
sium Moride, or as potassium-platinic chloride (see 68). It
may also be determined volumetrically. For the alkalimetric
estimation of potassa or potassium carbonate, see 219 and 220.
For estimating potassium as potassium hydrogen tartrate, and
which only gives approximate results, a chapter will be given in
the Special Part.

We may convert into


Potassium salts of strong volatile acids; e.g., potassium chlo-
ride, potassium bromide, potassium nitrate, etc., and salts of
organic acids.


Potassium hydroxide and compounds of potassium with weak,
volatile acids not decomposable by nitric acid, e.g., potassium
carbonate (potassium salts with organic acids).


In general, caustic potassa and potassium salts of weak volatile
acids; also, and more particularly, such as are decomposed by

97.] POTASSIUM. 243

nitric acid, e.g., potassium sulphide, potassium sulphate, chromate,
chlorate, and silicate.


Potassium salts of non-volatile acids soluble in alcohol. This
method is particularly important for salts of the non- volatile acids ;
e.g., potassium phosphate, potassium borate; also for separating
potassium from sodium.

The potassium in potassium borate may be determined also as
sulphate ( 136) ; and the potassium in the phosphate, as potas-
sium chloride ( 135).

The form of potassium platinic chloride may also be resorted
to in general, for the estimation of potassium in all potassium salts
of those acids which are soluble in alcohol. This form is, more-
over, of especial importance, as that in which the separation of
potassium from sodium, etc., 'is effected.


Potassium salts of those acids which are soluble in weak alcohol,
except borate.

1. Determination as Potassium Sulphate.

Evaporate the aqueous solution of the potassium sulphate to
dryness, ignite the residue in a platinum crucible or dish, and
weigh ( 42). The residue must be thoroughly dried before you
proceed to ignite it ; the heat applied for the latter purpose must
be moderate at first, and very gradually increased to the requisite
degree ; the crucible or dish must be kept well covered neglect of
these precautionary rules involves always a loss of substance from
decrepitation. If free sulphuric acid is present, we obtain, upon
evaporation, acid potassium sulphate ; in such cases the acid salt is
to be converted into the normal by igniting first alone (here it is
best to place the lamp so that the flame may strike the dish-cover
obliquely from above), then with ammonium carbonate. See 68.

For properties of the residue, see 68. Observe more particu-
larly that the residue must dissolve to a clear fluid, and that the
solution must be neutral. Should traces of platinum remain behind
(the dish not having been previously weighed), these must be care-
fully determined, and their weight subtracted from that of the
ignited residue.


With proper care and attention, tins method giyes accurate

To convert the above-mentioned salts (potassium chloride, &c.)
into potassium sulphate, add to their aqueous solution a quantity
of pure sulphuric acid more than sufficient to form normal sulphate
with the whole of the potassium, evaporate the solution to ury-
ness, ignite the residue, and convert the resulting acid potassium
sulphate into the normal, by treating with ammonium carbonate

( 6)-

As the expulsion of a large quantity of sulphuric acid is a very
disagreeable process, avoid adding too great an excess. Should too
little of the acid have been used, which you may infer from the
non-evolution of sulphuric acid fumes on ignition, "moisten the
residue with dilute sulphuric acid, evaporate, and again ignite. If
you have to deal with a small quantity only of potassium chloride,
&c., proceed at once to treat the dry salt, cautiously, with dilute
sulphuric acid in the platinum crucible ; provided the latter be
capacious enough. In the case of potassium bromide and iodide,
the use of platinum vessels must be avoided.

Potassium salts of organic acids are directly converted into
potassium sulphate by first carbonizing them at the lowest possible
temperature, and after cooling adding some crystals of pure ammo-
nium sulphate and a little water to the mass. The crucible being
covered, the water is evaporated by heating the crucible cover, and
the whole is afterwards heated to dull redness, until the excess of
ammonium sulphate is destroyed. If the carbon is not fully con-
sumed by this operation, add a little ammonium nitrate and repeat
the ignition. The potassium sulphate is then weighed. (KXM-
MERER.*) It is usually advisable to ignite finally in an atmosphere
of ammonium carbonate. The results are accurate.

2. Determination as Potassium Nitrate.

The general method is the same as in 1. The potassium nitrate
must be heated gently to the melting-point, otherwise loss will
arise from evolution of oxygen. For properties of the residue
see 68. The process is easily carried out, and the results are
accurate. In converting potassium carbonate into the nitrate,
consult 38.

[* Fre. ZeiL, vn,

1 97.] POTASSIUM. 245

* 3. Determination as Potassium Chloride.
General method the same as described in 1. The residue of
potassium chloride must, previously to ignition, be treated in the
same way as potassium sulphate, and for the same reason. The
salt must be heated in a well-covered crucible or dish, and only to
dull redness, as the application of a higher degree of heat is likely
to cause some loss by volatilization. No particular regard need be
had to the presence of free acid. For properties of the residue,
see G8. This method, if properly and carefully executed, gives
very accurate results. The potassium chloride may, instead of
being weighed, be determined volumetrically by 141, I). This
method, however, has no advantage in the case of single estima-
tions, but saves time when a series of estimations has to be

In determining potassium in the carbonate it is sometimes
desirable to avoid the effervescence occasioned by treatment with
hydrochloric acid, as, for instance, in the case of the ignited resi-
due of a potassium salt of an organic acid, which is contained in
the crucible. This may be effected by treating the carbonate with
solution of ammonium chloride in excess, evaporating and igniting,
when ammonium carbonate and the excess of ammonium chloride,
will escape, leaving potassium chloride behind.

The methods of converting the potassium compounds specified
above into potassium chloride, will be found in Part II. of this
Section, under the respective heads of the acids which they con-

4. Determination as Potassium- Platinio Chloride.
a. Potassium salts of volatile acids (nitric acid,, acetic acid, <fec.).
Mix the solution with hydrochloric acid, evaporate to dry ness,
dissolve the residue in a little water, add a concentrated solution of
platinic chloride, as neutral as possible, in excess, and evaporate in
a porcelain dish, on the water-bath, nearly to drjness, taking care
not to heat the water-bath quite to boiling. If the platinum con-
tent of the platinum chloride solution is known, the proper quan-
tity of the latter to add is more readily used ( 63, 8). Add
alcohol of about 80 per cent, by volume to the residue and let it
stand for some time, pour the alcoholic solution through a small
filter, and treat. the residue if necessary a few times with small
quantities of alcohol of the same strength, until it appears to be
pure potassium-platinic chloride. Bring this upon the filter and


wash completely by applying repeatedly small quantities of the
same alcohol. Dry next the filter and its contents in the funnel,
for it is necessary that the alcohol should be completely volatilized.
Transfer the contents of the filter carefully to a watch-glass, and
place the filter back into the funnel and dissolve and wash out the
small quantity of adhering potassium-platinic chloride with hot
water. Evaporate the yellow solution thus obtained to dryness in

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