C. Remigius Fresenius.

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redden litmus paper, in dry air. By these operations any oxides
of nitrogen that may have remained are also removed.


Preparation. Precipitate a clear filtered solution of lead ace-
tate, slightly acidulated with acetic acid, with a small excess oi
potassium dichrornate ; wash the precipitate by decantation, and at
last on a linen strainer ; dry, put in a Hessian crucible, and heat to
bright redness until the mass is fairly in fusion. Pour out upon a
stone slab or iron plate, break, pulverize, pass through a fine
metallic sieve, and keep the tolerably fine powder for use.

Tests. Lead chromate is a heavy powder, of a dirty yellowish-
brown color. It must evolve no carbon dioxide upon the applica-
tion of a red heat ; the evolution of carbon dioxide would indicate
contamination with organic matter, dust, &c. It must contain
nothing soluble in water.

Uses. Lead chromate serves, the same as cupric oxide, for
the combustion of organic substances. It is converted, in the pro-
cess of combustion, into chromic oxide and basic lead chromate.
It suffers the same decomposition, with evolution of oxygen, when
heated by itself above its point of fusion. The property of lead
chromate to fuse at a red heat renders it preferable to cupric oxide
as an oxidizing agent, in cases where we have to act upon difficultly
combustible substances.

N.B. Lead chromate may be used a second time. For this
purpose it is fused again (being first roasted, if necessary), and
then powdered. After having been twice used it is powdered,
naoistened with nitric acid, dried, and fused. In this way the

66.] REAGENTS. 153

lead chromate may be used over and over again indefinitely


Preparation. Triturate 100 grammes of potassium chlorate
with 5 grammes of finely pulverized manganese binoxide, and
introduce the mixture into a plain retort, which must not be more
than half full ; expose the retort over a charcoal fire or a gas-lamp,
at first to a gentle, and then to a gradually increased heat. As
soon as the salt begins to fuse, shake the retort a little, that the
contents may be uniformly heated. The evolution of oxygen
speedily commences, and proceeds rapidly at a relatively low tem-
perature, provided the above proportions be adhered to. As soon
as the air is expelled from the retort, connect the glass tube fixed
in the neck of the retort by means of a tight-fitting cork, with an
india-rubber tube inserted in the lower orifice of the gasometer ;
the glass tube must be sufficiently wide, and there must be sufficient
space left around the india-rubber to permit the free efflux of dis-
placed water. Continue the application .of heat to the retort till
the evolution of gas has ceased. 100 grammes of potassium
chlorate give about 27 litres of oxygen.

The oxygen produced by this process is moist, and may con-
tain traces of carbon dioxide, and also of chlorine. These impuri-
ties must be removed and the oxygen thoroughly dried, before it
can be used in organic analysis. The gas is therefore passed from
the gasometer first through a solution of potassa of 1*27 sp. gr.,
then through U tubes containing granulated soda lime, and finally,
according to circumstances, through II tubes containing calcium
chloride or pumice-stone moistened with sulphuric acid.

Tests. A chip of wood which has been kindled and blown out
so as to leave a spark at the extremity must immediately burst into
flame in oxygen gas. The gas must not render lime-water or a
solution of silver nitrate turbid when transmitted through these


Preparation. Take solution of soda (NaOH), ascertain its
specific gravity, weigh out a certain quantity, calculate the weight of
sodium hydroxide present, add twice this latter weight of the best
quick-lime, allow the lime to slake, and then evaporate to dryness

* Annalen d. CJiem. u. PJiarm., cvi, 127.

154 REAGENTS. [ 66.

in an iron vessel. Heat the residue in an iron or Hessian crucible ;
keep for some time at a low red heat. Break up while still warm
in an iron mortar, and pass the whole through a sieve with meshes
about 3 mm. wide. Reject the finest portion (removing it with
a fine sieve having 2 mm. meshes) and keep the granulated prod-
uct in a well-closed bottle.

Tests. Soda-lime must not effervesce much when treated
with dilute hydrochloric acid ; nor should it, more particularly,
erolve ammonia when mixed with pure sugar and ignited.

Use. Granulated soda-lime prepared as above described forms
an excellent absorbent for carbon dioxide. It was formerly also
used for nitrogen determination instead of the following :

Preparation. Equal weights of sal-soda in clean (washed)

large crystals and of good white promptly slaking quick-lime are
separately so far pulverized as to pass through holes of T y inch,
then well mixed together, placed in an iron pot which should not
be more than half filled, and gently heated, at first without stir-
ring. The lime soon begins to combine with the crystal water of
the sodium carbonate, the whole mass heats strongly, swells up, and
in a short time yields a fine powder, which may then be stirred to
effect intimate mixture and to drive off the excess of water so that
the mass is not perceptibly moist and yet short of the point at
which it rises in dust on handling. "When cold it is secured in
well-closed bottles or fruit jars, and is ready for use.


Metallic copper serves, in the analysis of nitrogenous substances,
to effect the reduction of the nitric oxide gas that may form in
the course of the analytical process.

It is used either in the form of turnings, or copper scales
reduced by hydrogen; or of small rolls made of fine copper wire
ionize. A length of from 7 to 10 centimetres is given to the
spirals or rolls, and just sufficient thickness to admit of their being
inserted into the combustion tube. To have it perfectly free from
dust, oxide, &c., it is first heated to redness in the open air, in a
crucible, until the surface is oxidized ; it is then put into a glass >r
porcelain tuhe, through which mi uninterrupted current of dry
hydrogen gas is transmitted; :md when all atmospheric air has
been expelled from the evolution apparatus and the tube, the
* S. W. Johnson. Report of the Conn. Agr. J:\JH. Station, 1878, p. Ill,

66.] EEAGENTS. 155

latter is- in its whole length lieated to redness. The operator should
make sure that the atmospheric air has been thoroughly expelled,
before he proceeds to apply heat to the tube ; neglect of this pre-
caution may lead to an explosion.

a. Solution of Potassa.

Solution of potassa is prepared from the carbonate, with the
aid of milk of lime, in the way described in the "Qualitative
Analysis," for the preparation of solution of soda. The propor-
tions are 1 part of potassium carbonate to 12 parts of water, and
-f part of lime, slaked to paste with three times the quantity of
warm water.

The decanted clear solution is evaporated, in an iron vessel,
over a strong fire, until it has a specific gravity of 1'27 ; it is then,
whilst still warm, poured into a bottle, which is well closed, and
allowed to stand at rest until all solid particles have subsided. The
clear solution is finally drawn off from the deposit, and kept for

b. Fused Potassa (common).

The commercial potassa in sticks (impure KOH usually com-
bined with more or less H 2 O) will answer the purpose. If you
wish to prepare it, evaporate solution of potassa (a) in a silver ves-
sel, over a strong fire, until the residuary hydroxide flows like
oil, and white fumes begin to ' rise from the surface. Pour the
fused mass out on a clean iron plate, and break it up into small
pieces. Keep in a well-stoppered bottle for use.

c. Potassa (purified with alcohol), see " Qual. Anal.," p. 43.

Uses. Solution of potassa serves for the absorption, and at
the same time for the estimation of carbon dioxide. In many
cases, a tube filled with fragments of fused potassa is used, in
addition to the apparatus filled with solution of potassa. Potassa
purified witli alcohol, which is perfectly free from potassium sul-
phate, is employed for the determination of sulphur in organic


a. Pure Calcium Chloride.

Preparation. Dissolve marble in commercial hydrochloric
acid diluted with four or five times its volume of water. (The
Waste solution resulting from the preparation of carbon dioxide

156 BEAGENTS. [ 66

may be used.) Add to this solution with stirring lime, slaked
with sufficient water to give it the consistency of thin paste until
it gives an alkaline reaction and a pellicle of calcium carbonate
forms on the surface on standing exposed to the air. Iron, man-
ganese, and especially magnesium are usually present in such a
solution, and are precipitated by the calcium hydroxide the iron,
however, not completely. After a few hours, filter and pass hydro-
gen sulphide through the alkaline solution until a filtered portion
is no longer blackened by this reagent. Let the solution stand for
twelve hours, then filter from the iron sulphide. Add next hydro-
chlorio acid to strongly acid reaction to convert the calcium sul-
phide and calcium oxychloride which may be present into chloride.
Concentrate in a porcelain dish. If sulphur separates, after a short
time filter again, and continue the evaporation to dryness with
addition of a little more hydrochloric acid toward the end of the
process. Finally expose the residue to a tolerably strong heat
about (200) on the sand-bath, until it is changed throughout to a
white porous perfectly opaque mass, which point can be ascertained
by breaking up a piece detached from the top. The product is
CaCl,-(- 2H 2 O. Reduce while still hot to granules of the proper
size (-J- to -^ of an inch) by means of suitable sieves and a mortar
previously warmed, and keep in well-closed bottles.

I. Crude fused Calcium Chloride.

Preparation. Neutralize the alkaline solution obtained in a
(without separating the little iron present with H 2 S) exactly with
hydrochloric acid, and evaporate to dryness in an iron pan ; fuse
the residue in an iron or Hessian crucible, pour out the fused mass,
and break into pieces. Preserve it in well-stoppered bottles.

Uses. The crude fused calcium chloride serves to dry moist
gases ; the pure chloride is used in elementary organic analysis for
the absorption and estimation of water formed by the hydrogen
contained in the analyzed substance. A solution of the pure cal-
cium chloride should not show an alkaline reaction. A calcium
chloride tube filled with it should not gain weight when a very
slow current of perfectly dry carbon dioxide is passed through it
an hour.


Potassium dichromate of commerce is purified by repeated
recrystallization, until barium chloride produces, in the solution of


a sample of it in water, a precipitate which completely dissolves in
hydrochloric acid. Potassium dichromate thus perfectly free from
sulphuric acid is required more particularly for the oxidation of
organic substances with a view to the estimation of the sulphur
contained in them. Where the salt is intended for other purposes,
e.g., to determine the carbon of organic bodies, by heating them
with potassium dichromate and sulphuric acid, one recrystallization
is sufficient.





THE quantitative analysis of a compound substance require.-,
as the first and most indispensable condition, a correct and accurate
knowledge of the composition and properties of the new combina-
tions into which it is intended to convert its several individual
constituents, for the purpose of separating them from one another,
and determining their several weights. Regarding the properties
of the new compounds, we have to inquire more particularly, in
the first place, how they behave with solvents; secondly, what is
their deportment in the air ; and, thirdly, what is their behavior on
ignition? It may be laid down as a general rule, that compounds
are the better adapted for quantitative determination the more
insoluble they are, and the less alteration they undergo upon
exposure to air or to a high temperature.

The composition of a substance is expressed either in per cents
or in stoichiometrical formulas ; the latter enable the composition
of the more frequently recurring compounds to be readily re-
membered. In this section the chemical formula is stated in the
first column; the second gives the equivalents (O= 16), while the
third gives the percentage composition.

A compound is the better adapted for quantitative determina-
tion, with respect to its composition, the less it contains relatively
of the substance to be determined, since every operative error,
loss, or inaccuracy is distributed over a larger mass on weighing,
and the error will, hence, be so much the less for the substance to
be determined. Thus platinum-ammonium chloride is better
adapted for the estimation of nitrogen than is ammonium chloride
because the former contains only 6*295 per cent, of nitrogen,
whereas the latter contains 26-24 per cent.


67.] FORMULAE. 159

Suppose we analyze a nitrogenous compound, and, with abso-
lutely accurate manipulation, obtain 1 grin, of platinum-ammo-
nium chloride from 0'3 grm. of the compound. 100 parts of this
platinum salt contain 6*295 parts of nitrogen, hence 1 grm. will
contain 0*06295 parts. Since this is afforded by 0*3 grm. of the
substance, it follows that 100 parts of the latter will contain
20-983 parts of nitrogen.

Let us now make a second analysis, and convert the nitrogen
into ammonium chloride. Working with equal accuracy we ob-
tain from 0*3 grm. of the substance 0-2399 grm. ammonium
chloride, corresponding to '06295 grm. nitrogen, or 20*983 per
cent. Assuming, now, that in both analyses a loss of 10 milli-
grammes had occurred, we would have obtained in the first opera-
tion only 0*99 parts of platinum-ammonium chloride instead of
100, corresponding to 0*06232 nitrogen, or 20*77 per cent. The
loss would, hence, have been 0*213 per cent. In the case of
ammonium chloride, however, we would have obtained 0*2299
parts instead of 0*2399, corresponding to 0*0603 nitrogen, or
20*1 per cent a loss of 0*873 per cent. We thus see that a
similar error would occasion in one case a loss in nitrogen of 0*213
per cent, while in the other the loss would be 0*873 per cent.

Having thus touched generally upon the requirements a com-
pound must possess in order to be adapted for quantitative ex-
amination, we \vill proceed to enumerate those compounds best
adapted and which are as a rule employed. Of course the de-
scription of the external form and appearance relates more par-
ticularly to the condition in which they are obtained in our analy-
sis. In enumerating tho properties of substances reference will
be had exclusively to those which bear directly upon the object
immediately in view.

[The percentage compositions of these compounds are stated in
connection with their description. For this purpose the symbols
of the constituent elements of the compounds in many cases
(viz. : when they are oxygen salts) are grouped in a manner
different from that used to express their chemical constitution.
This grouping constitutes a kind of formulae differing from either
the empirical or rational in ordinary use in modern text-books of
chemistry, but identical with that formerly in general use (the old
system). These formula are based upon the fact that in all
oxygen salts, whether normal, acid, basic, ortho-, meta-, or pyixr

160 FORMS. [ 67.

salts, there is just enough oxygen to form with the radicals present,
both basic and acid, their corresponding oxides or anhydrides, and
with hydrogen, if present, water. They represent oxides (and
water) jointly equivalent in weight to the radicals, hydrogen, and
remaining oxygen, which rational formulae represent as existing in
oxygen salts.


Potassium sulphate, SO, < QJT- = K a O,S0 8 .
Hydrogen potassium sulphate,

Potassium disulphate,



Ammonium magnesium phosphate,
/ .OKH. \

2 V \ o > M /

Magnesium pyrophosphate,

/ PO < 3 > M g

O / g =2MgO,P,0,.

\ PO < o > Mg

Most analytical chemists prefer to present the results of analyses
of oxygen salts in percentages of oxides (or anhydrides) and water
on account of the simplicity of computations required. Accord-
ingly, in the following section, the percentage composition of
oxygen salts is given in this manner, accompanied by correspond-
ing formulae and molecular weights. These formulae are in every
case preceded by rational formulae constructed in accordance with
the theory of the constitution of oxygen salts which is now
generally accepted.]

68.] BASES OF GROUP I. 161





The combinations best suited for the weighing of potassium


a. Potassium sulphate crystallizes usually in small, hard,
straight, four-sided prisms, or in double six-sided pyramids ; in
the analytical process it is obtained as a white crystalline mass.
It dissolves with some difficulty in water (1 part requiring 10 parts
of water of 12), it is almost absolutely insoluble in pure alcohol,
but slightly more soluble in alcohol containing sulphuric acid
(Expt. No. 6). It does not affect vegetable colors ; it is unalter-
able in the air. The crystals decrepitate strongly when heated,
yielding at the same time a little water, which they hold mechani-
cally confined. The decrepitation of crystals that have been kept
long drying is less marked. At a good red heat the salt fuses
without volatilizing or decomposing. At a white heat a little of
the salt volatilizes and also some sulphuric acid, so that the residue
possesses an alkaline reaction (AL. MITSCHERLICH,* BoussiNGAULTf),
When exposed to a red heat, in conjunction with ammonium
chloride, potassium sulphate is partly, and, upon repeated applica-
tion of the process, wholly converted, with effervescence^ intp
potassium chloride (H. ROSE).


S0 > < OK ^ SO, ! 1 1 80-07


The acid potassium sulphate (KHSQ 4 ), which is produced when
^he normal salt is evaporated to dryness with free sulphuric acid,
is readily soluble in water, and fusible even at a moderate heat.

f Journ.f. prakt. Chem., LXXXIII, 486. f Zeitechr.f. anal Chem., vii, 244.

162 FORMS. [ 68.

At a red heat it loses sulphuric acid, and is converted into normal
potassium sulphate, but not readily the complete conversion of
the acid into the normal salt requiring the long-continued applica-
tion of an intense red heat. However, when heated in an atmos-
phere of ammonium carbonate which may be readily procured by
repeatedly throwing into the faint red-hot crucible containing the
acid sulphate small lumps of pure ammonium carbonate, and
putting on the lid the acid salt changes readily and quickly to
the normal sulphate. The transformation may be considered
complete as soon as the salt, which was so readily fusible before, is
perfectly solid at a faint red heat.

1). Potassium nitrate crystallizes ordinarily in the form of
long, striated prisms. In analysis it is obtained as a white saline
mass. .It is readily soluble in water, almost insoluble in absolute
alcohol, and but sparingly soluble in alcohol. It does not affect
vegetable colors, and is unchangeable in air. On being heated it
melts far below a red heat, without decomposition or loss of
weight. Strongly heated it evolves oxygen, and becomes con-
verted into potassium nitrite; intensely heated (to redness) oxy-
gen and nitrogen are evolved, the residue being then caustic
potassa. On being ignited with ammonium chloride, or in a
current of dry hydrochloric-acid gas, it is readily and completely
converted into potassium chloride. Repeatedly evaporated with
oxalic acid in excess (4 to 6 times), it is completely converted into
potassium chloride.


55-11 54-48

v n

> NO, . . . 46-04 45-52

101-15 100-00

o. Potassium chloride crystallizes usually in cubes, often
lengthened to columns; rarely in octahedra. In analysis we
obtain it either in the former shape, or as a crystalline mass. It is
readily soluble in water, but much less so in dilute hydrochloric
acid ; in absolute alcohol it is nearly insoluble, and but slightly
soluble in common alcohol. It does not affect vegetable colors,
and is unalterable in tin- air. When heated, "it decrepitates, unless
it has been kept long drying, with expulsion of a little water

68.] BASES OF GROUP I. 163

mechanically confined in it. At a moderate red heat, it fuses
unaltered and without diminution of weight ; when exposed to a
higher temperature, it volatilizes in white fumes ; this volatilization
proceeds the more slowly the more effectually the access of air is
prevented (Expt. No. T). When repeatedly evaporated with
solution of oxalic acid in excess, it is converted into potassium
oxalate. When evaporated with excess of nitric acid, it is con-
verted readily and completely into nitrate. On ignition with
ammonium oxalate, potassium carbonate and potassium cyanide
are formed in noticeable quantities.


K . . . . 39-11 52-455

Cl 35-45 47-54:5

74-56 100-000

d. Potassium platinic chloride presents either small reddish-
yellow octahedra, or a lemon-colored powder. It is difficultly
soluble in cold, more readily in hot water; nearly insoluble in
absolute alcohol, and but sparingly soluble in common alcohol
one part requiring for its solution, respectively, 12083 parts of
absolute alcohol, 3775 parts of alcohol of 70 per cent, and
1053 parts of alcohol of 55 per cent. (Expt. No. 8, a.) Presence
of free hydrochloric acid sensibly increases the solubility (Expt.
No. 8, I). In caustic potassa it dissolves completely to a yellow
fluid. It is unalterable in the air, and at 100. On exposure to
an intense red heat, four atoms of chlorine escape, metallic plati-
num and potassium chloride being left ; but even after long-con-
tinued fusion, there remains always a little potassium platinic
chloride which resists decomposition. Complete decomposition is
easily effected, by igniting the double salt in a current of hydrogen
gas, or with some oxalic acid.

According to ANDREWS, potassium platinic chloride, even
though dried at a temperature considerably exceeding 100, retains
still -0055 of its weight of water.

164 FORMS. [ 69.


(KC1), . . . 14912 3069 K, . . . 78-22 16-10

PtCl . . . 336-70 69-31 Pt . . . 194-90 40-12

- 01.. . .21270 .4378

485-82 100-00

485-82 100-00

e. Potassium silicofluoride is obtained on mixing a solution of
a potassium salt with hydrofluosilicic acid in the form of a trans-
lucent iridescent precipitate, which increases and completely
separates, when an equal volume of strong alcohol is added to the
fluid* After being filtered off, washed with weak alcohol and dried,
it is a soft white powder. It is difficultly soluble in cold water, far
more readily in boiling water-, not at all or in merest traces soluble
in a mixture of water and strong alcohol in equal parts, but it is
decidedly more soluble in the presence of any considerable quan-
tity of free acid, especially hydrochloric or sulphuric acid. When
potassa is added to the boiling aqueous solution of the salt the
following change takes place : (KF),SiF 4 + 4KOH = 6KF +
Si(OH) 4 , the solution turning from acid to neutral (principle of
STOLE A' s volumetric method of estimating potassium). As soon as
it is ignited the salt fuses, gives off silicon fluoride and leaves
potassium fluoride.


Sodium is usually weighed as SODIUM SULPHATE, SODIUM CHLO-
RIDE, or SODIUM CARBONATE. It is separated from potassium in the
form of SODIUM PLATINIC CHLORIDE, from other bodies occasionally
in the form of sodium silicofluoride.

a. Anhydrous normal sodium sulphate is a white powder or a
white very friable mass. It dissolves readily in water ; but is
sparingly soluble in absolute alcohol ; presence of free sulphuric
acid slightly increases its solubility in that menstruum ; it is some-
what more readily soluble in common alcohol (Expt. No. 9). It
does not affect vegetable colors ; upon exposure to moist air, it

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 14 of 69)