C. Remigius Fresenius.

A system of instruction in qualitative chemical analysis online

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SOLUTIONS BY SESQUICHLORIDE OF IRON : Succinic Acid, Benzole
Acid.

168.

a. SUCCINIC ACID (2 HO, C 8 H 4 6 ).

1. HYDRATE OF SUCCINIC ACID forms colorless and inodorous prisms or
tables of slightly acid taste, which are readily soluble in water, alcohol,
and ether, sparingly soluble in nitric acid, and volatilize when exposed
to the action of heat, leaving only a little charcoal behind. The officinal
acid has an empyreumatic odor, and leaves a somewhat larger carbona-
ceous residue upon volatilization. Succinic acid is not destroyed by
heating with nitric acid, and may therefore be easily obtained in the
pure state by boiling with that acid for half an hour, by which means the
oil of amber, if present, will be destroyed. By sublimation crystalline
needles of silky lustre are obtained. The hydrate loses water in this
process, so that by repeated sublimation anhydrous acid is ultimately
obtained. Heated in the air succinic acid burns with a blue flame, free
from soot.

2. The SUCCINATES are decomposed at a red heat ; those which have
an alkali or alkaline earth for base are converted into carbonates in this
process, the change being attended with separation of charcoal. Most of
the succinates are soluble in water.

3. Sesquickloride of iron produces in solutions of neutral succinates cf
the alkalies a brownish pale red bulky precipitate of SUCCINATE OF SES-
QUIOXIDE OF IRON (Fe 2 O 3 , C g H 4 6 ) ; one-third of the succinic acid is
liberated in this reaction, and retains part of the precipitate in solution
if the fluid is filtered off hot. The precipitate dissolves readily in mineral
acids ; ammonia decomposes it, causing the separation of a less bulky
precipitate of a highly basic succiuate of sesquioxide of iron, and com-
bining with the greater portion of the acid to succinate of ammonia,
which dissolves.

4. Acetate of lead gives with succinic acid a white precipitate of
neutral SUCCINATE OF LEAD (2PbO, C 8 H 4 O 6 ), which is very sparingly
soluble in water, acetic acid, and succinic acid, but dissolves freely in



192 BEN&OIC ACID.

solution of acetate of lead and in nitric acid. Treated with ammonia
the neutral succinate of lead is converted into a basic salt (GPbO,
C,H ( 0,).

5. A mixture of alcohol, ammonia, and solution of chloride of barium
produces in solutions of free succinic acid and of succinates a white pre-
cipitate Of SUCCINATE OF BAKYTA (2 Ba 0, C g H 4 O 6 ).

6. Nitrate of suboxide of mercury and nitrate of silver also precipitate
the succinates ; the precipitates, however, are not possessed of any charac-
teristic properties.

169.

b. BENZOIC ACID (HO, C U H 6 3 ).

1. Pure HYDRATE OF BENZOIC ACID forms inodorous white scales or
needles, or simply a crystalline powder. It fuses when heated, and
afterwards volatilizes completely. The fumes of benzoic acid cause a
peculiar irritating sensation in the throat, and provoke coughing ; when
cautiously cooled, they condense to brilliant needles ; when kindled, they
burn with a luminous sooty flame. The common officinal hydrate of
benzoic acid has the odor of benzoin, and leaves a small carbonaceous
residue upon volatilization. Hydrate of benzoic acid is very sparingly
soluble in cold water, but it dissolves pretty freely in hot water and in
alcohol. Addition of water imparts therefore a milky turbidity to a
saturated solution of benzoic acid in alcohol.

2. Most of the BENZOATES are soluble in water ; only those with weak
bases, e.g., sesquioxide of iron, are insoluble. The soluble benzoates have
a peculiar pungent taste. The addition of a strong acid to concentrated
aqueous solutions of benzoates displaces the benzoic acid, which separates
as hydrate in the form of a dazzling white sparingly soluble powder.
Benzoic acid is expelled in the same way from the insoluble benzoates by
such strong acids as form soluble salts with the bases with which the
benzoic acid is combined.

3. Sesquichloride of iron precipitates solutions of free benzoic acid
incompletely ; solutions of neutral benzoates of the alkalies completely.
The precipitate of BENZOATE OF SESQUIOXIDE OF IRON [2 Fe 2 O s ,
3 (C 14 H 5 O 3 )+ 15 aq.], is bulky, flesh-colored, insoluble in water. It is
decomposed by ammonia in the same manner as succinate of sesquioxide
of iron, from which salt it differs in this, that it dissolves in a little
hydrochloric acid, with separation of the greater portion of the ben-
zoic acid.

4. Acetate of lead fails to precipitate free benzoic acid and benzoate of
ammonia, at least immediately ; but it produces white flocculent preci-
pitates in solutions of benzoates with a fixed alkaline base.

5. A mixture of alcohol, ammonia, and solution of chloride of barium
produces NO precipitate in solutions of free benzoic acid or of the alkaline
benzoates.

170.

Recapitulation and remarks. Succinic and benzoic acids are distin-
guished from all other acids by the facility with which they may be
sublimed, and by their deportment with sesquichloride of iron. They
are distinguished from one another by the different color of their salts
with sesquioxide of iron, and also by their different deportment with
chloride of barium and alcohol; but principally by their different degrees



ACETIC ACID. 193

of solubility, succinic acid being readily soluble in water, whilst benzoio
acid is very difficult of solution. Succinic acid is seldom perfectly pure,
and may therefore often be detected by the odor of oil of amber which
it emits.

The detection of the two acids, when present in the same solution
with other acids, may be effected as follows : precipitate with sesqui-
chloride of iron, warm the washed precipitate with ammonia, filter,
concentrate the solution, divide it into two parts, and mix one part with
hydrochloric acid, the other with chloride of barium and alcohol.

Succinic acid and benzoic acid do not prevent the precipitation of ses-
quioxide of iron, alumina, &c., by alkalies.

THIED GROUP OF THE ORGANIC ACIDS.
ACIDS WHICH ARE NOT PRECIPITATED BY CHLORIDE OF CALCIUM NOR

BY SESQITICHLORIDE OF IRON : Acetic Acid, Formic Acid (Lactic Acid,
Propionic Acid, Butyric Acid).

171.
a. ACETIC ACID (HO,C 4 H 3 3 ).

1. The HYDRATE OF ACETIC ACID forms transparent crystalline scales,
which fuse at 62 '6 F. to a colorless fluid of a peculiar pungent and pene-
trating odor, and exceedingly acid taste. When exposed to the action of
heat it volatilizes completely, forming pungent inflammable vapors, which
burn with a blue flame. It is miscible with water in all proportions ;
it is to such mixtures of the acid with water that the name of acetic
acid is commonly applied. The hydrate of acetic acid is also soluble
in alcohol.

2. The ACETATES undergo decomposition at a red heat; among the
products of this decomposition we generally find hydrate of acetic acid,
and almost invariably acetone (C ? H 6 2 ). The acetates of the alkalies
and alkaline earths are converted into carbonates in this process ; of the
acetates with metallic bases many leave .the metal behind in the pure
state, others in the form of oxide. Most of the residues which the
acetates leave upon ignition are carbonaceous. Nearly all acetates dis-
solve in water and in alcohol ; most of them are readily soluble in water,
a few only are difficult of solution in that menstruum. If acetates are
distilled with dilute sulphuric acid, the free acetic acid is obtained in the
distillate.

3. If sesquichloride of iron is added to acetic acid, and the acid is then
nearly saturated with ammonia, or if a neutral acetate is mixed with
sesquichloride of iron, the fluid acquires a deep dark red color, owing to
the formation of ACETATE OF SESQUIOXIDE OF IRON. By boiling the
fluid becomes colorless if it contains an excess of acetate, the whole of
the sesquioxide of iron precipitating as a basic acetate, in the form of
brown-yellow flakes. Ammonia precipitates from it the whole of the
sesquioxide of iron as hydrate. By addition of hydrochloric acid a fluid
which appears red from the presence of acetate of sesquioxide of iron
turns yellow (difference from sulphocyanide of iron).

4. Neutral acetates (but not free acetic acid of a certain degree of
dilution) give with nitrate of silver white crystalline precipitates of
ACETATE OF SILVER (Ag 0, C 4 B" 3 O 8 ), which are very sparingly soluble

i. o



194) FORMIC ACID.

in cold water. They dissolve more easily in hot water, but separate
again upon cooling, in the form of very fine crystals. Ammonia dis-
solves them readily ; free acetic acid does not increase their solubility
in water.

5. Nitrate ofsuboxide of mercury produces in solutions of acetic acid,
and more readily still in solutions of acetates, white scaly crystalline
precipitates of ACETATE OF SUBOXJDE OF MERCURY (Hg 2 0, C 4 H 3 O 3 ),
which are sparingly soluble in water and acetic acid in the cold, but dis-
solve without difficulty in an excess of the precipitant. The precipitates
dissolve in water upon heating, but separate again upon cooling, in the
form of small crystals ; in this process the salt undergoes partial decom-
position : a portion of the mercury separates in the metallic state, and
imparts a gray color to the precipitate. If the acetate of suboxide of
mercury is boiled with dilute acetic acid, instead of water, the quantity
of the metallic mercury which separates is exceedingly minute.

6. Chloride of mercury produces no precipitate of subchloride of mer-
cury with acetic acid or acetates upon heating.

7. By heating acetates with concentrated sulphuric add HYDRATE OF
ACETIC ACID is evolved, which may be known by its pungent odor.
But if the acetates are heated with a mixture of about equal volumes of
concentrated sulphuric acid and alcohol, ACETIC ETHER (C 4 H 6 O, C 4 H s 3 )
is formed. The odor of this ether is highly characteristic and agreeable ;
it is most distinct upon shaking the mixture when somewhat cooled, and
is much less liable to lead to mistakes than the pungent odor of the
free acetic acid.

8. If acetates are distilled with dilute sulphuric acid, and the distillate
is digested with an excess of oxide of lead, part of the latter dissolves
as basic acetate of lead, which may be readily recognised by its alkaline
reaction.

172.

b. FORMIC ACID (HO,C 2 H0 8 ).

1. The HYDRATE OF FORMIC ACID is a transparent and colorless
slightly fuming liquid of a characteristic and exceedingly penetrating
odor. When cooled to below 32 F., it crystallizes in colorless plates.
It is miscible in all proportions with water and with alcohol. When
exposed to the action of heat, it volatilizes completely ; the vapors are
inflammable and burn with a blue flame.

2. The FORMATES, like the corresponding acetates, leave upon ignition
either carbonates, oxides, or metals behind, the process being attended
with separation of charcoal, and escape of carbide of hydrogen, carbonic
acid, and water. All the compounds of formic acid with bases are
soluble in water ; alcohol also dissolves many of them, but not all.

3. Formic acid shows the same reaction with sesquichloride of iron
as acetic acid.

4. Nitrate of silver fails to precipitate free formic acid, and decom-
poses the alkaline formates only in concentrated solutions. The white
sparingly soluble, crystalline precipitate of FORMATE OF SILVER (AgO,
C H0 3 ) acquires very rapidly a darker tint, owing to the separation of
metallic silver. Complete reduction of the oxide of silver to the me-
tallic state takes place, even in the cold, after the lapse of some time ;
but immediately upon applying heat to the fluid containing the preci-
pitated formate of silver. The same reduction of the oxide of silver to



RECAPITULATION AND REMARKS. 195

the metallic state takes place in a solution of free formic acid, and also
in solutions of formates so dilute that the addition of the nitrate of
silver fails to produce a precipitate in them. But it does not take place
in presence of an excess of ammonia. The rationale of this reduction
is as follows : the formic acid, which may be looked upon as a compound
of carbonic oxide with water, deprives the oxide of silver of its oxygen,
thus causing the formation of carbonic acid, which escapes, and of water,
whilst the reduced silver separates in the metallic state.

5. Nitrate ofsuboocide of mercury gives no precipitate with free formic
acid ; but in concentrated solutions of alkaline formates this reagent
produces a white sparingly soluble precipitate of FORMATE OF SUBOXIDE
OF MERCURY (Hg 2 O, C 2 H0 3 ), which rapidly becomes gray, owing to the
separation of metallic mercury. Complete reduction ensues, even in the
cold, after the lapse of some time, but is immediate upon application of
heat. This reduction is also attended with the formation of carbonic
acid and water, and takes place, the same as with the oxide of silver,
both in solutions of free formic acid and in fluids so highly dilute that
the formate of suboxide of mercury is retained in solution.

6. If formic acid or an alkaline formate is heated with chloride of
mercury to from 140 to 158 F. SUBCHLORIDE OF MERCURY precipitates.
Presence of free hydrochloric acid or of somewhat considerable quan-
tities of alkaline chlorides will prevent the reaction.

7. If formic acid or a formate is heated with concentrated sulphuric
acid the formic acid is resolved into water and carbonic oxide gas, which
latter escapes with effervescence and, if kindled, burns with a blue flame.
The fluid does not turn black in this process. The rationale of the
decomposition of the formic acid is this : the sulphuric acid withdraws
from the formic acid the water or the oxide necessary for its existence,
and thus occasions a transposition of its elements (C 2 H 3 = 2 CO + HO).
Upon heating formates with dilute sulphuric acid in a distilling appa-
ratus free formic acid is obtained in the distillate, and may mostly be
readily detected by its odor. Upon heating a formate with a mixture
of sulphuric acid and alcohol formic ether is evolved, which is charac-
terized by its peculiar arrack-like smell.

8. If dilute formic acid is heated with oxide of lead, the latter dissolves.
On cooling the solution, which, if necessary, is concentrated by evapora-
tion, the FORMATE OF LEAD (PbO, C 2 H0 3 ) separates in brilliant prisms
or needles.

173.

Recapitulation and remarks. Acetic acid and formic acid may be dis-
tilled over with water, and form with sesquioxide of iron soluble neuti-ai
salts which dissolve in water, imparting to the fluid a blood -red color,
and are decomposed upon boiling. These reactions distinguish the two
acids of the third group from the other organic acids. From each other
the two acids are distinguished by the odor of their hydrates and ethyle
compounds, and by their different reactions with salts of silver and salts
of mercury, oxide of lead, and concentrated sulphuric acid. The separa-
tion of acetic acid from formic acid is effected by heating the mixture of
the two acids with an excess of oxide of mercury or oxide of silver.
Formic acid reduces the oxides, and suffers decomposition, being resolved
into carbonic oxide and water ; whilst the acetic acid combines with the
oxides, forming acetates, which remain in solution.

o '2



196 LACTIC ACID.

174.

Rarer Acids of the third group of Organic Acids.
1. LACTIC ACID (2HO,C 12 H 10 10 ).

Lactic acid is developed in animal fluids, vegetable matters that have turned sour,
&c. Pure hydrate of lactic acid is an inodorous syrupy liquid ; it has a sour and
biting taste. When it is slowly heated in a retort to 266 F., water containing a
little hydrated lactic acid distils over, leaving a residue of anhydrous lactic acid
(C 12 H, 10 ), which at a temperature between 482 F. and 572 F. is decomposed into
carbonic oxide, carbonic acid, lactide, and other products. Hydrate of lactic acid
dissolves freely in water, alcohol, and ether. Upon boiling the aqueous solution a little
lactic acid volatilizes along with the aqueous vapour. All the lactates are soluble in
water, the greater part of them, however, only sparingly ; it is the same with regard
to spirit of wine ; but they are all insoluble in ether. The production of some of these
salts and the inspection and examination of their form under the microscope supply the
means for the detection of lactic acid ; lactate of lime and lactate of zinc are the best
suited for this purpose. Lactate of lime may be conveniently prepared from animal or
vegetable liquids by the following method devised by SCHERER : Dilute the liquid, if
necessary, with water, mix with baryta- water, and filter. Distil the filtrate with some
sulphuric acid (to remove volatile acids), digest the residue several days with strong
alcohol, distil the acid solution with a little milk of lime, filter warm from the excess
of lime and the sulphate of lime, conduct carbonic acid into the filtrate, heat once more
to boiling, filter from the precipitated carbonate of lime, evaporate the filtrate, warm
the residue with strong alcohol, filter, and let the neutral filtrate stand several days to
give the lactate of lime time to crystallize. Should the quantity of lactic acid present
be insufficient to allow the formation of crystals, evaporate the fluid to syrupy con-
sistence, mix with strong alcohol, let the mixture stand some time, decant or filter the
alcoholic solution into a vessel that can be closed, and add gradually a small quantity
of ether. The process will cause even minute traces of lactate of lime to separate from
the fluid. Lactate of lime shows under the microscope the form of minute crystalline
needles aggregated in tufts with short stalks, pairs of them always being joined at the
stalked ends, so as to look like brushes united together. Lactate of zinc deposited quickly
from its solution shows under the microscope the form of spherical groups of needles.
The slow evaporation of solution of lactate of zinc gives first crystals resembling
clubs truncated at both ends ; these crystals gradually increase in size ; the two ends
apparently diminish, whilst the middle parts increase in size (FUNKE).

2. PROPIONIC ACID (H 0, C 6 H 6 8 ), and BUTYRIC ACID (H 0, C 8 H 7 3 ).

PROPIONIC ACID is formed under a great variety of circumstances ; it is chiefly found
in fermented liquids. The pure hydrate of the acid crystallizes in minute plates ; it
boils at from 284 F. to 287 '6 F. ; it dissolves readily in water. Propionic acid floats
as an oily stratum on aqueous solution of phosphoric acid and on solution of chloride
of calcium. It has a peculiar smell, which reminds both of butyric and acetic acid.
Upon distilling the aqueous solution, the propionic acid passes over into the distillate.
BUTYRIC ACID is frequently found in animal and vegetable matter, more particularly also
in fermented liquids of various kinds. The pure hydrate of the acid is a colorless,
mobile, corrosive, intensely sour fluid, of a disagreeable odor, a combination of the
smell of rancid butter and acetic acid ; it boils at 320 F. It is miscible in all pro-
portions with water and alcohol. It is separated from the concentrated aqueous solu-
tion by chloride of calcium, concentrated acids, &c., in the form of a fluid oil. The
smell of butyric acid is particularly strong in the aqueous solution of the acid.
Upon distilling the aqueous solution, the acid passes over along with the aqueous
vapors.

Propionic acid and butyric acid are often found associated with formic acid and
acetic acid in fermented liquids, in guano, and in many mineral waters. The de-
tection of the several acids may in such cases be effected as follows : dilute the substance
sufficiently with water, acidify with sulphuric acid, and distil ; saturate the distillate
with baryta-water, evaporate to dryness, and treat the residue repeatedly with boiling
alcohol of 85 per cent. This will leave the formate of baryta and part of the acetate,
the remainder of the acetate, together with the propionate and butyrate, dissolving in
the alcohol. Evaporate the alcoholic solution, dissolve the residue in water, decom-
pose cautiously with sulphate of silver, boil, filter, and let the fluid (which ought
rather to contain a little undecomposed baryta salt than any sulphate of silver) evapo-
rate under the desiccator. Take out separately the crystals which form first, those which



PROPIONIC ACID. 197

form after, and those which form last, 'and examine them to ascertain their nature.
Acetate of silver emits upon solution in concentrated sulphuric acid the odor of acetic
acid, and gives no oily drops ; propionate and butyrate of silver emit the peculiar odor
of the acids, and give oily drops, which, however, with minute quantities are visible
only under the microscope. To distinguish positively between propionic and butyric
acid, it is indispensable to determine the amount of silver in the separated silver salts,
and to fix by this the atomic weight of the acids. If much acetate of baryta has passed
into the solution, with a small quantity only of butyrate and propionate, the baryta is
first completely precipitated with sulphuric acid from the aqueous solution of the
baryta salts soluble in alcohol, half of the acid fluid neutralized with soda, the other
half added, the fluid then distilled, the distillate, which now contains principally pro-
pionic and butyric acids, saturated with baryta, then decomposed with sulphate of
silver, and the remaining part of the process conducted as above.



PART II.



SYSTEMATIC COURSE



QUALITATIVE CHEMICAL ANALYSIS.



PART II.



PRELIMINARY REMARKS

ON THE

COURSE OF QUALITATIVE ANALYSIS IN GENERAL, AND ON THE PLAN OP
THIS PART OF THE PRESENT WORK IN PARTICULAR.

THE knowledge of reagents and of the deportment and reaction of other
bodies with them enables us to ascertain at once whether a simple com-
pound of which the physical properties permit an inference as to it3
nature, is in reality what we suspect it to be. Thus, for instance, a few
simple reactions suffice to show whether a body which appears to be
calcareous spar is really carbonate of lime, and that another which we
hold to be gypsum is actually sulphate of lime. This knowledge usually
suffices also to ascertain whether a certain body is present or not in a
compound ; for instance, whether or not a Avhite powder contains sub-
chloride of mercury. But if our design is to ascertain the chemical
nature of a substance entirely unknown to us if we wish to discover
all the constituents of a mixture or chemical compound if we intend
to prove that, besides certain bodies which we have detected in a mix-
ture or compound, no other substance can possibly be present if
consequently a complete qualitative analysis is our object, the mere
knowledge of the reagents, and of the reactions of other bodies with
them, will not suffice for the attainment of this end ; this requires the
additional knowledge of a systematic and progressive course of analysis,
in other words, the knowledge of the order and succession in which
solvents, and general and special reagents, should be applied, both to
effect the speedy and certain detection of every component element of a
compound or mixture, and to prove with certainty the absence of all
other substances. If we do not possess the knowledge of this syste-
matic course, or if, in the hope of attaining our object more rapidly, we
adhere to no method whatever in our investigations and experiments,
analyzing becomes (at least in the hands of a novice) mere guess-work,
and the results obtained are no longer the fruits of scientific calculation,
but mere matters of accident, which sometimes may prove lueky hits,
and at others total failures.

Every analytical investigation must therefore be based upon a definite
method. But it is not by any means necessary that this method should
be the same in all cases. Practice, reflection, and a due attention to
circumstances will, on the contrary, generally lead to the adoption of
different methods for different cases. However, all analytical methods
agree in this, that the substances present or supposed to be present in a



202 PRELIMINARY EXAMINATION.

compound or mixture are in the first place classed into certain groups,
which are then again subdivided, until the individual detection, of the
various substances present is finally accomplished. The diversity of
analytical methods depends partly on the order and succession in which
reagents are applied, and partly on their selection.

Before we can venture upon inventing methods of our own for indi-
vidual cases, we must first make ourselves thoroughly conversant with



Online LibraryC. Remigius FreseniusA system of instruction in qualitative chemical analysis → online text (page 28 of 51)