C. Remigius Fresenius.

Quantitative chemical analysis online

. (page 12 of 69)
Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 12 of 69)
Font size
QR-code for this ebook

into the receiver, to 4 parts of sodium chloride in the retort. The
greatest care must be taken to keep the receiver cool, and to change
it as soon as the tube through which the gas is conducted into it
begins to get hot, since it is now no longer hydrochloric acid gas
which passes over, but an aqueous solution of the gas, in form of

* For BETTENDORFF'S process for the preparation of arsenic-free hydro-
chloric acid, and based upon the precipitation of arsenic by stannous chloride,
see Zeitschr.f. analyt. Chem., ix, 107.

130 REAGENTS. [ 58.

vapor, which would simply weaken the fuming acid, if it were
allowed to mix with it.

Tests. The fuming acid must, for many purposes, be perfectly
free from chlorine and sulphurous acid. For the mode of testing
for these impurities, see " Qual Anal." loc. cit. Test for sulphuric
acid as under Nitric Acid, above.

Uses. Fuming hydrochloric acid has a much more energetic
action than the dilute acid ; it is, therefore, used instead of the
latter in cases where a more rapid and energetic action is desirable.


This is employed for the decomposition of silicates and borates,
sometimes in the gaseous form, sometimes in the condition of
aqueous solution. In the first case, the substance to be decomposed
is introduced into the leaden box, in which the hydrofluoric gas is
being generated ; in the latter case, we must first prepare the aque-
ous acid. The raw material employed is fluor spar or kryolite
(LUBOLDT*). Both are first finely powdered, and then treated with
concentrated sulphuric acid. To 1 part kryolite, 2 parts sulphuric
acid are used; to 1 part fluor spar, 2 parts sulphuric acid are
used. If the latter is employed, allow the mixture to stand in
a dry place for several days, stirring every now and then, so that
the silicic acid (which is generally contained in fluor spar) may
first escape in the form of fluosilicic gas. Convenient distil-
latory apparatus have been described by LUBOLDT (loc. cit.) and by
H. BRiEGLEB.f The latter commends itself especially on account
of its relatively small cost. It consists' of a leaden retort, with a
movable leaden top, which can be luted on. The receiver belong-
ing to it is a box of lead, with a tubulure at the side, into which
the neck of the retort just enters. The cover of the receiver is
r;ii>< d conical, and is provided at the top with an exit tube of lead.
In the receiver a platinum dish containing water is placed, all
joints are luted, and the retort is carefully heated in a sand-bath.
The aqueous hydrofluoric acid found at the end of the operation in
the platinum dish is perfectly pure. The small quantity of impure
hydrofluoric acid which collects on the bottom of the receiver is
thrown away. The hydrofluoric acid must entirely volatilize when
heated in a platinum dish on a water-bath. The pure acid gives no

* Jour, fur prakt. Chem., LXXVI, 330.
t Annal. d. Ohem. u. Pharm., cxi, 380.

59.] REAGENTS. 131

precipitate when neutralized with potash, while potassium silico-
fluoride'separates if the acid contains hydrofluosilicic acid. The
acid is best preserved in gutta-percha bottles, as recommended by
STADELER. The acid is now obtainable in the market in gutta-
percha bottles. It should at once be tested ; this must never be
neglected, as I have often found the acid to be impure. The
greatest caution must be observed in preparing this acid, since,
whether in the fluid or gaseous condition, it is one of the most
injurious substances.

3. CHLORINE AND CHLORINE- WATER (see " Qual. Anal.").

4. NITRO-HYDROCHLORIC ACID (see " Qual. Anal.").

5. HYDROFLUOSILICIC ACID (see "Qual. Anal.").

This should be kept in gutta-percha bottles, as when long
kept in glass it attacks the latter and takes up some of its constitu-

c. Sulphur Acids.
1. HYDROSULPHURIC ACID (see " Qual. Anal.").

a. Oxygen Bases and Metals.

a. Alkali Bases.

SODA (see " Qual. Anal.").

All the four sorts of the caustic alkalies mentioned in the quali-
tative part are required in quantitative analysis, viz., common solu-
tion of soda, potassa purified with alcohol, solution of potassa pre-
pared with baryta, and absolutely pure soda. Pure solution of
potassa may be obtained also by heating to redness for half an hour
in a copper crucible, a mixture of 1 part of potassium nitrate, and
2 or 3 parts of thin sheet copper cut into small pieces, treating the
mass with water, allowing the oxide of copper to subside in a tall
vessel, and removing the supernatant clear fluid by means of a
syphon (WOHLER).

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

132 REAGENTS. [ 60.


fi. Alkali-earth Hases.

1. BARIUM HYDROXIDE, OR BARYTA (see " Qual. Anal.").


Finely divided calcium hydroxide mixed with water (milk of
lime), is used more particularly to effect the separation of magne-
sium, etc., from the alkali metals. Milk of lime intended to be
used for that purpose must, of course, be perfectly free from alka-
lies. To insure this the purest lime (calcined white marble)
should be used, and slaked lime should be thoroughly washed by
repeated boiling with fresh quantities of distilled water. This
operation is best conducted in a silver dish. When cold, the
milk of lime so prepared is kept in a w ell -stoppered bottle.

y. Heavy Metals, and their Oxides.

1. ZINC.

Zinc has of late been much used as a reagent in quantitative
analysis. It serves more especially to effect the reduction of ferric
to ferrous salts, and also the precipitation of copper from solutions
of its salts. Zinc intended to be used for the former purpose must
be free from iron, for the latter free from lead, copper, and other
metals which remain undissolved upon treating the zinc with dilute
acids. As it is not easy to prepare in quantity zinc that will answer
botli purposes, it is advisable to keep on hand, besides the ordinary
zinc used for preparing hydrogen, the two following kinds also:

a. Zinc, free from Iron. The distillation of zinc in the labo-
ratory is a tedious and costly operation, hence as a rule the raw
product obtained by distillation from the ore is used in the prepara-
tion of the iron-free zinc. This product contains, at least in many
cases, only such slight traces of iron, that it may be safely used for
the reduction of ferrous salts in solution. Ordinary commercial
zinc contains much more iron, from having been fused in iron

b. Zinc, free from Lead, Copper, etc. To procure zinc which
leaves no residue upon solution in dilute sulphuric acid, there is
commonly no other resource but to re-distil the commercial article.

This is effected in a retort made of the material of Hessian or

60.] EEAGEKTS. 133

black-lead crucibles. The operation is conducted in a wind-furnace
with good draught. The neck of the retort must hang down as
perpendicularly as possible. Over this is placed a small clay drain-
pipe, the lower end of which dips into water contained in a tub or
large stone- ware dish. The joints are all stopped with clay. Under
the neck is placed a basin or small tub, filled with water. The
distillation begins as soon as the retort is at a bright red heat. As
the neck of the retort is very liable to become choked up with zinc
or oxide of zinc, it is necessary to keep it constantly free by means
of a pipe-stem. The zinc obtained by this re -distillation is nearly
or quite free from lead, but still contains notable traces of iron
(from the wire). If the presence of iron is to be totally avoided, a
clay pipe-stem or stick of wood must be used instead of the iron

Tests. The following is the simplest way of testing the purity
of zinc : Dissolve the metal in dilute sulphuric acid in a small flask
provided with a gas-evolution tube, place the outer limb of the tube
under water, and when the solution is completed, let the water
entirely or partly recede into the flask; after cooling, add to the
fluid, drop by drop, a sufficiently dilute solution of potassium per-
manganate. If a drop of that solution imparts the same red tint
to the zinc solution as to an equal volume of water, the zinc may be
considered free from iron. I prefer this way of testing the purity
of zinc to other methods, as it affords, at the same time, an approx-
imate, or, if the zinc has been weighed and the permanganate solu-
tion (which, in that case, must be considerably diluted) measured,
an accurate arid precise knowledge of the quantity of iron present.
If lead or copper is present, the metal remains undissolved upon
solution of the zinc.

Preparation. Commercial copper, with the exception of the
Japanese, which is not always obtainable, is seldom sufficiently pure
for analytical purposes. Hence the pure metal should be prepared
by the chemist, either by the galvanoplastic process, or by the
method of FUCHS, in which copper-sulphate solution is precipitated
by well-cleaned iron, the precipitate of copper boiled with hydro-
chloric acid to remove iron, then washed, dried, and fused, and the
regulus so obtained rolled out into thin sheets.

Tests. Pure copper must be completely soluble in nitric acid,

134 KEAGENTS. [ 60.

and the solution must afford no precipitate with excess of ammonia,
even on long standing (iron, lead, etc.) ; nor should it be rendered
turbid by hydrochloric acid (silver). After precipitation with
hydrogen sulphide, the filtrate should leave no residue on evapo-

Uses. Copper serves occasionally in indirect analysis; for in-
stance, in estimating the copper content of a liquid, for estimating
iron according to FUCHS, etc. Since the development and use of
volumetric analysis, however, it is but seldom used in quantitative


Precipitate pure lead nitrate or acetate with ammonium car-
bonate, wash the precipitate, dry, and ignite gently to complete

Lead oxide is often used to fix an acid, so that it is not expelled
even by a read heat.


Preparation. Add a solution of mercuric chloride to a hot,
moderately dilute caustic-soda solution, taking care that the soda
be always in excess. The yellow precipitate is thoroughly washed
by decantation, then mixed with water, and preserved in this con-
dition in a bottle.

Test. Mercuric oxide must leave no residue on ignition in a
platinum crucible.

Uses. This reagent serves in quantitative analysis for decom-
posing magnesium chloride in the process of separating magnesia
from alkalies.

J. Sulphur Bases.

1. AMMONIUM SULPHIDE (see " Qual. Anal.").

We require both the colorless monosulphide, and the yellow

2. SODIUM SULPHIDE (see "Qual. Anal.").

61.] BEAGENTS. 135


a. Salts of the Alkalies.

1. POTASSIUM SULPHATE (see " Qnal. Anal.").


Preparation. Dilute phosphoric acid (sp. gr. 1*13), prepared
from phosphorus, is mixed with an equal quantity of water, and
pure ammonia water added until the liquid has a strongly alkaline
reaction, when it is set aside for some time, then filtered if neces-
sary, and kept for use.

Tests. Ammonium phosphate must be free from arsenic,
nitric, and sulphuric acids, but more particularly from potassa or
soda. To test it for these two last, add lead-acetate solution so
long as a precipitate still forms, then filter, precipitate the lead
excess with hydrogen sulphide, filter again, evaporate the filtrate
to dryness, and ignite the residue. If an alkaline residue is left,
potassa or soda was present.

In most cases sodium phosphate (see " Qual. Anal.") may be
used instead of ammonium phosphate.

3. AMMONIUM OXALATE (see "Qual Anal.").

4. SODIUM ACETATE (see "Qual. Anal.").


Preparation. Saturate succinic acid, which has been purified
by dissolving in nitric acid and recrystallizing, with dilute ammo-
nia. The reaction of the new compound should be rather slightly
alkaline than acid.

Uses. This reagent serves occasionally to separate ferric iron
from other metals.

6. SODIUM CAKBONATE (see "Qual. Anal.").

This reagent is required both in solution and in pure crystals ;
in the latter form to neutralize an excess of acid in a fluid which
it is desirable not to dilute too much.

7. AMMONIUM CARBONATE (see " Qual. Anal.").

8. SODIUM HYDKOGEN SULPHITE (see " Qual. Anal.").


This salt occurs in commerce. It should be dry, clear, well

136 . REAGENTS. [ 61.

crystallized, and completely and easily soluble in water. The solu-
tion must give with silver nitrate at first a white precipitate, must
not effervesce with acetic acid, and when acidified must give no
precipitate with barium chloride, or, at most, only a slight turbidity.
The acidified solution must, after a short time, become milky from
separation of sulphur.

Uses. Sodium thiosulphate is used for the precipitation of
several metals, as sulphides, particularly in separations, for instance,
of copper from zinc ; it also serves as solvent for several salts (sil-
ver chloride, calcium sulphate, etc.) ; lastly, it is employed in volu-
metric analysis, its use here depending on the reaction 2(Na a S,O t )
+ 21 = 2NaI + Na 2 S 4 6 .

10. POTASSIUM NITRITE (see " Qnal. Anal.").

11. POTASSIUM DICHROMATE (see " Qual. Anal.").

12. AMMONIUM MOLYBDATE (see "Qual. Anal.").

When using the solution of ammonium molybdate in nitric acid
for the estimation of phosphoric acid, the filtrates from the ammo-
nium phospho-molybdate and magnesium-ammonium phosphate
will contain all the molybdic acid. If the filtrates are preserved,
therefore, there will be no loss, and the acid may be recovered as
follows : Evaporate the residue to dryness in the open air or under
a good draught, and heat finally until most of the ammonium
nitrate has been decomposed. Digest the residue with ammonia,
which dissolves the molybdic acid, and filter. To the filtrate add a
kittle magnesia mixture ( 62, 6) in order to precipitate any phos-
phoric acid present. If a precipitate occurs, add sufficient mag-
nesia mixture to assure complete precipitation of all phosphoric
acid. After allowing to stand for some time, filter, acidulate the
filtrate with nitric acid, and then filter off the precipitated molybdic
acid, using suction, and wash it with the smallest quantity of water.
The acid is then available for use again in solution. The filtrate
and washings from the acid will contain but little acid ; they may
be worked up with the next residues treated.

13. AMMONIUM CHLORIDE (see "Qual. Anal.").

14. POTASSIUM CYANIDE (see "Qual. Anal.").
Preparation. Besides the potassium cyanide prepared accord-
ing to LIKKIO, and which contains potassium cyanate and carbonate,
there is required also a pure cyanide for use in certain separations,
e.g., as in AVu'nu:it's method of separating nickel from zinc.

62.] REAGENTS. 137

The pure cyanide is prepared as follows : 2 parts of crystallized
potassiufn ferrocyanide are powdered and then transferred to a
retort, wherein it is heated together with 1 parts concentrated
sulphuric acid and 4 parts water, until the residue begins to bump.
The vapors of hydrocyanic acid are conducted into a cooled
receiver containing a freshly prepared and filtered solution of 1
part caustic potassa (not fused, but evaporated until it just solidi-
fies on cooling) in 3 to 4 parts of not less than 92-per-cent alcohol.
The caustic potassa should be present in slight excess at the close
of the operation. The crystalline mass is filtered by the aid of
suction, then washed with a little alcohol, then dried in a porcelain
dish by the aid of heat, and finally preserved for use in a well-
closed bottle.

. Salts of the Alkali-earth Metals.

1. BARIUM CHLORIDE (see " Qual. Anal.").

The following process gives a very pure barium chloride, free
from calcium and strontium : Transmit through a concentrated
solution of impure barium chloride hydrochloric gas, as long as a
precipitate continues to form. Nearly the whole of the barium
chloride present is by this means separated from the solution, in
form of a crystalline powder. Collect this on a filter, let the
adhering liquid drain off, wash the powder repeatedly with small
quantities of pure hydrochloric acid, until a sample of the wash-
ings, diluted with water, and precipitated with sulphuric acid,
gives a filtrate which, upon evaporation in a platinum dish, leaves
no residue. The hydrochloric mother-liquor serves to dissolve
fresh portions of witherite. I make use of the barium chloride so
obtained, principally for the preparation of perfectly pure barium
carbonate, which is often required in quantitative analyses.


Preparation. Dissolve pure barium carbonate in moderately
dilute acetic acid, filter, and evaporate to crystallization.

Tests. Dilute solution of barium acetate must not be rendered
turbid by solution of silver nitrate. See also " Qual. Anal.," Barium
chloride, the same tests being also used to ascertain the purity of
the acetate.

Uses. Barium acetate is used instead of barium chloride, to
Sect the precipitation of sulphuric acid, in cases where it is desir

138 REAGENTS. [ 62.

able to avoid the introduction of a chloride into the solution, or
to convert the base into an acetate. As the reagent is seldom
required, it is best kept in crystals.

3. BARIUM CARBONATE (see " Qual. Anal.").


Preparation. Strontium chloride is prepared from strontian-
ite or celestine, by the same processes as barium chloride. The
pure crystals obtained are dissolved in alcohol of 96 per cent., the
solution is filtered, and kept for use.

Uses. The alcoholic solution of strontium chloride is used to
effect the conversion of alkali sulphates into chlorides, in cases
where it is desirable to avoid the introduction into the fluid of a
salt insoluble in alcohol.

5. CALCIUM CHLORIDE (see " Qual. Anal.").


Dissolve 1 1 parts crystallized magnesium chloride (MgCl a -j- 6
H 3 O) and 28 parts ammonium chloride in 1 30 parts water, add
70 parts dilute ammonia solution (sp. gr. 0*96). Allow the mix-
ture to stand one or two days and filter. This solution, commonly
called " magnesia mixture," is used to precipitate phosphoric
acid, and also arsenic acid, from aqueous solutions. An excess is
required to effect complete precipitation. Prepared as here de-
scribed, about 10 c. c. should be used in ordinary cases for every
0*1 gramme P a O fc .

A solution containing the same per cent, (approximately) of
magnesium chloride and other constituents may also be prepared
from common calcined magnesia (MgO), provided it is free from
the other alkali-earth metals, as follows: Add to 11 parts magnesia
sufficient hydrochloric acid to effect solution, next add a slight ex-
cess of magnesia and boil to separate traces of iron ; filter, and add
140 parts ammonium chloride and 350 parts dilute ammonia.
Dilute with water until volume equals 1000 c. c. for every 11
grammes of MgO used. Allow the mixture to stand two or three
days, and filter if necessary.

Magnesia mixture may be also made as follows: Dissolve
1 part of crystallized magnesium sulphate and 2 parts pure
ammonium chloride in 8 parts water and add 4 parts ammonia
water. Let stand several days, then filter.

63.] REAGENTS. 139

, c. Salts of the Heavy Metals.


1. FERROUS SULPHATE (see "Qual. Anal.").

2. FERRIC CHLORIDE (see " Qual. Anal.").


I lent finely powdered pitchblende with dilute nitric acid, filter
the fluid from the undissolved portion, and treat the filtrate with
hydrosulplmric acid to remove the lead, copper, and arsenic ; filter
again, evaporate to dryness, extract the residue with water, and fil-
ter the solution from the oxides of iron, cobalt, and manganese.
Urariic nitrate crystallizes from the nitrate; purify this by recrys-
tallization, and then heat the crystals until a small portion of uranic
oxide is reduced. Warm the yellowish-red mass thus obtained
with acetic acid, filter and let the filtrate crystallize. The crystals
are uranic acetate, and the mother-liquor contains the undecom-
posed nitrate ("WEKTIIEIM).

The salt may be more conveniently made from the commer-
cially obtainable sodium uranate (manufactured by the K. K.
Bergoheramt, Joachimsthal). Digest 1 part of this salt in 2 parts
acetic acid (sp. gr. 1-038), then add 25 parts water, heat, filter,
evaporate, and allow to crystallize. The uranic oxide in the last
mother-liquors (containing also sodium acetate) is precipitated by

Uranium being a costly metal, all the residues should be
saved, and worked up as follows: The liquid is poured off from
any sediment of uranium phosphate. All the uranium in it is
then precipitated by adding sodium phosphate. The precipitate
is washed by decantation, mixed with the uranium phosphate
reserved, the whole dissolved in hydrochloric acid, and ferric
chloride added until a sample gives a brownish precipitate with
ammonium carbonate. The mixture is then diluted, and to the
solution, which must contain a sufficient excess of hydrochloric
acid, add a solution of crystallized sodium carbonate in excess.
All the phosphoric acid is thus precipitated as basic ferric phos-
phate; the uranium oxide, however, remains dissolved in the
solution of sodium bicarbonate formed. Filter the mixture, wash,
acidulate the filtrate with hydrochloric acid, warm until the car-
bon dioxide is completely expelled, warm and precipitate the

140 feEAGENTS. [ 64

uranium oxide with ammonia. After washing, dissolve in acetic
acid (E. KEICHARD).*

'Testa. Solution of uranic acetate after acidification with
hydrochloric acid must not be altered by hydrosulphuric acid ;
ammonium carbonate must produce in it a precipitate, soluble in
an excess of the precipitant. A sample of the dilute solution
should acquire a red tint on adding a little sulphuric acid and a
drop of potassium permanganate solution (absence of uranous

Uae-a. Uranic acetate may serve, in many cases, to effect the
separation and determination of phosphoric acid.

4. SILVER NITRATE (see " Qual. Anal.").

5. LEAD ACETATE (see " Qual. Anal.").

6. MERCURIC CHLORIDE (see " Qual. Anal.").

7. STANNOUS CHLORIDE (see " Qual. Anal.").

8. PLATINIC CHLORIDE (see " Qual. Anal.").

It is convenient to know approximately the strength of this
solution. I usually use a solution 10 or 20 c. c. of which contain
1 grm. platinum.

9. SODIUM PALLADIO-CHLORIDE (see " Qual. Anal.").


1. SODIUM CARBONATE, pure anhydrous (see " Qual. Anal.").


3. BARIUM HYDROXIDE OR BARYTA (see " Qual. Anal." and

4. POTASSIUM NITRATE (see " Qual. Anal.").

5. SODIUM NITRATE (see " Qual. Anal.").

6. BORAX (fused).

Preparation. Heat crystallized borax (see " Qual. Anal.) in a
platinum or porcelain dish, until there is no further intumescence ;
reduce the porous mass to powder, and heat this in a platinum cru-
cible until it is fused to a transparent mass. Pour the semi-fluid,

* Zeittchr. f. analyt. Chem., vm, 116.

64.] REAGENTS. 141

viscid mass upon a fragment of porcelain. A better way is to fuse
the borax in a net of platinum gauze, by making tlie gas blowpipe-
flame act upon it. The drops are collected in a platinum dish.
The vitrified borax obtained is kept in a well-stoppered bottle.
But as it is always necessary to heat the vitrified borax previous to
use, to make quite sure that it is perfectly anhydrous, the best way
is to prepare it only when required.

Uses. Vitrified borax is used to effect the expulsion of car-
bonic acid and other volatile acids, at a red heat.


Preparation. Mix 87 parts of normal potassium sulphate (see
" Qual. Anal."), in a platinum crucible, with 49 parts of concen-
trated pure sulphuric acid, and heat to gentle redness until the mass
is in a state of uniform and limpid fusion. Pour the fused salt on
a fragment of porcelain, or into a platinum dish standing in cold
water. After cooling, break the mass into pieces, and keep for

Uses. This reagent serves as a flux for certain native com-
pounds of alumina and chromic oxide. Potassium disulphate is
used also, as we have already had occasion to state, for the cleansing
of platinum crucibles; for this latter purpose, however, the salt
which is obtained in the preparation of nitric acid will be found

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 12 of 69)