C. Remigius Fresenius.

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sufficiently pure.


Preparation. This is prepared like the potassium salt, using
71 parts of pure, normal sodium sulphate and 49 parts of concen-
trated pure sulphuric acid.

Uses. Sodium disi^.phate is used like the potassium disul-
phate, but is to be substituted for the latter when, as in fusing
corundum, the analysis is hampered by alum crystallizing out
(L. SMITH, Zeitschr. f. analyt. Chem., iv, 412).


Neutralize a definite quantity of hydrofluoric acid in a plati-
num dish with pure potassium carbonate or potassium hydroxide,
applying heat toward the last ; then add a quantity of hydrofluoric
acid equal to that first taken, and evaporate the whole to dryness.
The preparation is usually made just before required ; if it is to be
preserved, gutta-percha vessels must be used.

142 REAGENTS. [ 64.

Tests. Solution of hydrogen -potassium fluoride must remain
unaffected on adding hydrogen sulphide, ammonia, ammonia and
ammonium sulphide, or ammonium carbonate and sodium phosphate
with addition of ammonia.

'(fses. This preparation is an excellent flux for many minerals
which are usually very refractory, e.g., tinstone, chrome iron
(GIBBS, Zeitschr.f. analyt. Chem., m, 399).


Preparation. Add ammonia in considerable excess to hydro-
fluoric acid or hydrosilicofluoric acid in a platinum dish, gently
heat for some time, filter if necessary, and evaporate the filtrate to
dryness in a platinum dish. Half the ammonia escapes, while hy-
drogen-ammonium fluoride remains behind. If this is to be pre-
served, a gutta-percha vessel must be used.

Tests. Like those of hydrogen-potassium fluoride. In addi-
tion, a sample heated in a platinum dish (in the open or under a
good vapor hood) must leave no fixed residue.

Uses. The preparation may be advantageously used instead of
hydrofluoric acid in the analysis of silicates.


Preparation. See " Qual. Anal." This reagent serves to
convert the acid alkali sulphates into normal salts. It must com-
pletely volatilize when heated in a platinum dish.


Preparation. Neutralize pure ammonium carbonate with pure
nitric acid, warm, and add ammonia to slightly alkaline reaction;
filter, if necessary, and let the filtrate crystallize. Fuse the crys-
tals in a platinum dish, and pour the fused mass upon a piece of
porcelain ; break into pieces whilst still warm, and keep in a well-
stoppered bottle.

Tests. Ammonium nitrate must leave no residue when heated
in a platinum dish.

Uses. Ammonium nitrate serves as an oxidizing agent; for
instance, to convert lead into lead oxide, or to effect the com-
bustion of carbon, in cases where it is desired to avoid the use of
fixed salts.


Preparation and Tests. See " Qual. Anal."

Uses. Ammonium chloride is often used to convert metallic
oxides and acids, e.g., lead oxide, zinc oxide, stannic oxide, arsenic
aoid, antimonic acid, &c., into chlorides (ammonia and water escape
in the process). Many metallic chlorides being volatile, and others
volatilizing in presence of ammonium chloride fumes, they may be
completely removed by igniting them with ammonium chloride in
excess, and thus many compounds, e.g., alkali antimonates, may
be easily and expeditiously analyzed. Ammonium chloride is
also used to convert various salts of other acids into chlorides, e.g.,
small quantities of alkali sulphates.


Preparation. Hydrogen gas is evolved when dilute sulphuric
acid is added to granulated zinc. It may be purified from traces
of foreign gases either by passing first through mercuric-chloride
solution, then through potassa solution, or as recommended by
STENHOUSE, by passing through a tube filled with pieces of char-
coal. If the gas is desired dry, pass through sulphuric acid or a
calcium-chloride tube. If the zinc used is new, the evolution of gas
may be facilitated by adding a drop of platinic- chloride solution.

Tests. Pure hydrogen gas is inodorous. It ought to burn with
a colorless flame, which, when cooled by depressing a porcelain
dish upon it, must deposit nothing on the surface of the dish except
pure water (free from acid reaction).

Uses. Hydrogen gas is frequently used, in quantitative analy-
sis, to reduce oxides, chlorides, sulphides, &c. , to the metallic state ;
also" to protect certain substances, like metallic sulphides, from the
action of atmospheric oxygen during ignition.


Preparation. See "Qual. Anal." Chlorine gas is purified
and dried by transmitting it through a U-tube containing frag-
ments of manganese dioxide, then concentrated sulphuric acid, or a
calcium-chloride tube.

Uses. Chlorine gas serves principally to produce chlorides,
and to separate the volatile from the non-volatile chlorides ; it is
also used to displace and indirectly determine bromine and iodine,
as well as to convert lower chlorine compounds into higher.

144 REAGENTS. [g 60.


Under this head are arranged the most important of those
substances which serve for the preparation and testing of the
fluids required in volumetric analysis and have not been given
under A and B.


The introduction of crystallized oxalic acid as a basis for alkal-
imetry and acidimetry is due to FR. MOHR. It is also employed
to determine the strength of, or to standardize, a solution of potas-
sium permanganate, 1 molecule of potassium permanganate being
required, in the presence of free sulphuric acid, to convert 5 mole-
cules of oxalic acid into carbon dioxide and water (K,Mn 3 O a +
511,0,04 + 3H 9 SO 4 = K,SO 4 + 2MnSO 4 + 8H 2 O + 10CO a ).
"We use in most cases the pure crystallized acid which lias the
formula H,C,O 4 + 2II a O, and of which the molecular weight is
accordingly 126-048.

Preparation. The pure acid is prepared by shaking powdered
commercial oxalic acid with a small quantity of warm water (so that
considerable of the acid remains undissolved), then filtering, and
crystallizing by rapidly cooling (MOHR). Spread the crystals,
after draining, on blotting paper, and set aside to dry in a dust-
free place at the ordinary temperature (not too high), or press
them gently between renewed layers of blotting paper until the
latter take up no more moisture:

Tests. The crystals of oxalic acid must not show the least sign
of efflorescence (to which they are liable even at 20 in a dry
atmosphere) ; they must dissolve in water to a perfectly clear fluid ;
when heated in a platinum dish, they must leave no fixed and
incombustible residue (calcium carbonate, potassium carbonate,
fec.). If the acid obtained by a first crystallization fails to satisfy
these requirements, it must be recrystallized. In this case the
strengtli of the solution must be such that only 10 or 20 per cent
of the dissolved oxalic acid crystallizes out, and this, containing the
impurities, is then removed, after which the mother-liquor is con-
centrated by evaporation. The crop of crystals next obtained is

65.] fcEAGENTS. 145


Preparation. Digest 1 part of litmus of commerce with 6
parts of water on the water-bath for some time, filter, divide the
blue fluid into 2 portions, and saturate in one half the free alkali,
by stirring repeatedly with a glass ;od clipped in very dilute nitric
acid, until the color just appears red ; add the remaining blue half,
together with 1 part of strong spirit of wine, and keep the tincture
which is now ready for use, in a small open bottle, not quite full,
in a place protected from dust. In a stoppered bottle the tincture
would speedily lose color.

Tests. Litmus tincture is tested by coloring with it about 100
cubic centimetres of water distinctly blue, dividing the fluid into
two portions, and adding to the one the least quantity of a dilute
acid, to the other a trace of solution of soda. If the one portion
acquires a distinct red, the other a distinct blue tint, the litmus
tincture is fit for use, as neither acid nor alkali predominates.


Preparation. Mix 8 parts of very finely powdered pure pyro-
lusite, or manganese dioxide, with 7 parts of potassium chlorate,
put the mixture into a shallow cast-iron pot, and add 37 parts of a
solution of potassa of 1/27 specific gravity (the same solution as
is used in organic analysis *) ; evaporate to dry ness, stirring the
mixture during the operation ; put the residue before it has ab-
sorbed moisture, into an iron or Hessian crucible, and expose to a
dull-red heat, with frequent stirring with an iron rod or iron spa-
tula, until no more aqueous vapors escape and the mass is in a faint
glow. Remove the crucible now from the fire, and transfer the
friable mass to an iron pot. Reduce to coarse powder, and transfer
this, in small portions at a time, to an iron vessel containing 100
parts of boiling water ; keep boiling, replacing the evaporating
water, and passing a stream of carbon dioxide through the fluid
( MULDER f). The originally dark-green solution of potassium
manganate soon changes, with separation of hydrated manganese
dioxide, to the deep violet-red of the permanganate. When it is
considered that the conversion is complete, allow to settle, take
out a small quantity of the clear liquid, boil and pass carbon
dioxide through it. If a precipitate forms, the conversion is not
yet complete.

* Or, instead of the solution, use 10 parts of the hydroxide KOH. In this
case fuse the potash and the chlorate together first, and then project the man-
ganese into the crucible.

\ Jahresbericht von KOPP und WILL, 1858, 581.

146 REAGENTS. f 65.

The following method, recommended by STAEDELER,* is more
rapid : Powder the fused mass and macerate it with an equal weight
of cold water in a flask ; then add an equal quantity of water, and
conduct chlorine into the mixture until the latter has lost its green
color and become a pure red. Then dilute with four times its vol-
ume of water and allow to settle. By this method the yield of
potassium permanganate is increased one-half, because no manga-
nese dioxide is precipitated. On the other hand, if the fused
mass contains potassium hydroxide in excess, potassium chlorate
may form, and which will render somewhat difficult the purifica-
tion of the permanganate crystals.

The clear, red solution, obtained by any one of the above
methods, is decanted from the precipitate, the latter washed by
decantation, and the united liquids evaporated over an open fire
to the crystallizing point, and then allowed to cool. The mother-
liquor, on evaporation, yields a new crop of crystals. The last
mother-liquor contains much potassium chloride, hence it can only
be employed for the preparation of manganese dioxide. If the
crystals obtained are not sufficiently pure, they may be readily
purified by recrystallization. They are freed from adhering
mother-liquor by exposure on a block of plaster of Paris. The
solution of potassium permanganate may be filtered, if necessary,
through gun-cotton, asbestos or previously ignited sand.


FeS0 4 .(NH 4 ) a S0 4 + GH a O.

FR. MOHR has proposed to employ this double salt, which is
not liable to efflorescence and oxidation, as an agent to determine
the strength of the permanganate solution.

Preparation. Take two equal portions of dilute sulphuric
acid, and warm the one with a moderate .excess of small iron
nails free from rust, until the evolution of hydrogen gas has alto-
gether or very nearly ceased ; m>ntrali/r the other portion exactly
with ammonium carbonate, and then add to it a fV\v drops of dilute
sulphuric acid. Filter the solution of the ferrous sulphate into that
f tin- ammonium sulphate, evaporate the mixtuiv a little, if neces-
sary, and then allow the salt to crystallize. Let the crystals, which
are hard and of a pale-green color, drain in a funnel, then wash

* Journ. f. prakt. Clwm., cm, 107.

65.] REAGENTS. , 147

them in a little water, dry thoroughly on blotting-paper in the air,
and keef) for use.

The molecular weight of the salt (392-376) is almost exactly

7 times the atomic weight of iron (5 5 -9). The solution of the

salt in water which has been just acidified with sulphuric acid

must not become red on the addition of potassium sulphocyanate.


Preparation. Bring into a large porcelain dish -58 gnus, of
pure crystallized ferrous sulphate (see Fresenius' "Qual. Anal."
Am. ed., p. 73), together with a quantity of sulphuric acid equiva-
lent to 8*3 grins, of sulphuric anhydride (SO 3 ), (see Table, 191).
Heat upon a sand-bath, adding nitric acid from time to time, in
small portions, until the iron has all passed into ferric sulphate, or
until a drop of the solution gives no blue coloration with potassium
ferricyanide. Heat further, and evaporate until the excess of
nitric acid is expelled, then add 14 grins, of ammonium sulphate,*
and, if need be, hot water sufficient to bring the salt into solution ;
filter into a porcelain capsule and set aside, under cover, to crys-

The iron-alum separates in cubo-octahedrons, which may be yel-
lowish, lilac, or colorless. If dark in color, dissolve in warm water,
add a few drops of oil of vitriol, and crystallize again. Rinse the
pale or colorless crystals, after separation from the mother-liquor,
with cold water, wrap up closely in filter paper, and allow them to
dry at the ordinary temperature, f

* If not on hand, this salt may be prepared by saturating sulphuric acid
with ammonium carbonate and evaporating to dryness. 30 grammes of sul-
phuric acid give somewhat more than is required above.

t Examinations of iron-alum thus prepared show that the variations in the
color of the salt, from colorless to rose, are not connected with appreciable
differences of composition.

J. H. Grove, of the Sheffield Laboratory, obtained the following results in the
examination of ammonia-iron-alum crystals, the ferric oxide being estimated by

ignition :

Fe 2 3

( 16-59

1st \ 16-55

( 16-59

2d 16-53

3d 16-57

4th 16-57

5th ' 16-58

Afl, j 16 ' 50

6th I 16-56
7th 16-55

Calculated 16'60

148 REAGENTS. [ 65.

The yield should be about 80 grins. The dry salt should
be pulverized, pressed between folds of paper until freed from
mechanically adhering water, and preserved in a well-stoppered

Uses. Ammonia-iron-alum furnishes the best means of obtain-
ing a definite quantity of iron in a ferric salt for making standard
solutions, being easily obtained pure and inalterable if kept
away from acid vapors. Its purity may be readily controlled by
ascertaining the loss on careful ignition, which should leave a resi-
due of 16'0 per cent, of ferric oxide, corresponding to 11*62 per
cent, of metallic iron.


Preparation. Triturate iodine of commerce with -J- part of its
weight of potassium iodide, dry the mass in a large watch-glass with
ground rim, warm this gently on a sand-bath, or on an iron plate,
and as soon as violet fumes begin to escape, cover it with another
watch-glass of the same size. Continue the application of heat
until all the iodine is sublimed, and keep in a well-closed glass
bottle. The chlorine or bromine, which is often found in iodine
of commerce, combines, in this process, with the potassium, and
remains in the lower w T atch-glass, together with the excess of
potassium iodide.

Tests. Iodine purified by the process just now described, must
leave no fixed residue when heated on a watch-glass. But, even
supposing it should leave a trace on the glass, it would be of no
great consequence, as the small portion intended for use has to be
resublimed immediately before weighing.

Uses. Pure iodine is used to determine the amount of iodine
contained in the solution of iodine in potassium iodide, employed
in many volumetric processes.


Small quantities of this article may be procured cheaper in
commerce than prepared in the laboratory. For the preparation of
potassium iodide intended for analytical purposes I recommend
BAUP'S method, improved by FREDERKING, because the product
obtained by this process is free from iodic acid.

Tests. Put a sample of the salt in dilute sulphuric acid. If
the iodide is pure, it will dissolve \vithout coloring the fluid ; but
if it contain potassium iodate, the fluid will acquire a brown tint,

65.] REAGENTS. . 149

from the presence of free iodine, the sulphuric acid setting free
iodic and liydriodic acids which react on each other (HIO 3 -f- 5HI 6
= 3II,O + 61) with liberation of iodine which remains in solution.
Mix the solution of another sample with silver nitrate, as long as
a precipitate continues to form ; add solution of ammonia in excess,
shake the mixture, filter, and supersaturate the filtrate with nitric
acid. The formation of a white, curdy precipitate indicates the
presence of chloride in the potassium iodide. Presence of potassium
sulphate is detected by means of solution of barium chloride,
with addition of some hydrochloric acid.

Uses. Potassium iodide is used as a solvent for iodine in the
preparation of standard solutions of iodine ; it is employed also to
absorb free chlorine. In the latter case every atom of chlorine lib-
erates an atom of iodine, which is retained in solution by the agency
of the excess of potassium iodide. The potassium iodide intended
for these uses must be free from potassium iodate and carbonate;
the presence of trifling traces of potassium chloride or potassium
sulphate is of no consequence.

If a potassium -iodide solution of accurately known strength
is to be made, the salt must be dried before weighing. This may
be accomplished by exposing it in powdered form to a tempera-
ture of 180 until its weight is constant. Exposure to a tempera-
ture much above 200 is to be avoided, as then potassium iodate
is likely to be formed, and this would render the iodide impure.
(PETTERSSON, Zeitschr.f. analyt. Chem., ix, 362).


Preparation. The sulphurous acid gas evolved by the action
of sulphuric acid on copper turnings (see u Qual. Anal.") is
washed and then passed into water until the latter is saturated.
The solution is best preserved in small, well- stoppered bottles.

This concentrated solution serves for preparing the diluted
solution of sulphurous acid used in BUNSEN'S method of estimat-
ing iodine.

9. ARSENOUS OXIDE (As 2 O 3 ).

The arsenous oxide sold in the shops in large pieces, externally
opaque, but often still vitreous within, is generally quite pure.
The purity of the article is tested by moderately heating it in a
glass tube, open at both ends, through which a feeble current of
air is transmitted. Pure arsenous oxide must completely volatilize
in this process; no residue must be left in the tube upon the
expulsion of the sublimate from it. If a non-volatile residue is left

150 fcfcAGENTS. [ 65.

which, when heated in a current of hydrogen gas, turns black, the
arsenous oxide contains antimony trioxide, and is unfit for use in
analytical processes. Dissolve about 10 grms. of the arsenous
oxide to be tested in soda, and add 1 to 2 drops lead-acetate solu-
tion. If a brownish color develops, the arsenous oxide contains
arsenous sulphide and cannot be used. Arsenous oxide dissolves
in a solution of sodium carbonate, forming sodium arsenite, which
is used to determine hypochlorous acid, free chlorine, iodine, &c.

Perfectly pure rock-salt is best suited for analytical purposes.
It must dissolve in water to a clear fluid ; ammonium oxalate,
sodium phosphate, and barium chloride must not render the solu-
tion cloudy. Pure sodium chloride may be prepared also by
MARGUERITTE'S process, viz., conduct into a concentrated solution
of common salt hydrochloric gas to saturation, collect the small
crystals of sodium chloride which separate on a funnel, let them
thoroughly drain, wash with hydrochloric acid, and dry the sodium
chloride finally in a porcelain dish, until the hydrochloric acid
adhering to it has completely evaporated. The mother-liquor
which contains the small quantities of calcium sulphate, magne-
sium chloride, &c., originally present in the salt, is used instead
of a corresponding quantity of water, when next preparing hydro-
chloric acid.

Uses. Sodium chloride serves as a volumetric precipitating
agent in the determination of silver, and also to determine the
strength of solutions of silver intended for the estimation of chlo-
rine. We usually fuse it before weighing. The operation must
be conducted with caution, and must not be continued longer than
necessary ; for if the gas-flame acts on the salt, hydrochloric acid
escapes, while sodium carbonate is formed.

The silver obtained by the proper reduction of the pure chlo-
ride of the metal alone can be called chemically pure. The silver
precipitated by copper is never absolutely pure, but contains gener-
ally about pjj^ of copper.

Chemically pure silver is only used in small quantity for stand-
ardizing the NaCl solution employed for the determination of
silver. The solution of silver required for the estimation of
chlorine need not be made with absolutely pure silver, as the
strength of this solution had always best be determined ,//'/, / the
preparation, by im-ans of pure sodium chloride.

66.] REAGENTS. 151



Preparation. Stir pure* copper scales (wliicli should first be
ignite*d in a muffle) with pure nitric acid in a porcelain dish to a
thick paste ; after the effervescence has ceased, heat gently on the
sand-bath until the mass is perfectly dry. Transfer the green basic
salt produced to a Hessian crucible, and heat to a moderate redness,
until no more fumes of hypoiiitric acid escape ; this may be known
by the smell, or by introducing a small portion of the mass into a
test tube, closing the latter with the finger, heating to redness, and
then looking through the tube lengthways. The uniform decom-
position of the salt in the crucible may be promoted by stirring
the mass from time to time with a hot glass rod. When the cruci-
ble has cooled a little, reduce the mass, which now consists of pure
cupric oxide, to a tolerably fine powder, by triturating it in a brass
or porcelain mortar ; pass through a metal sieve, and keep in a
well-stoppered bottle for use. It is always advisable to leave a
small portion of the oxide in the crucible, and to expose this again
to an intense red heat. This agglutinated portion is not pounded,
but simply broken into small fragments.

Another method is to dissolve pure copper in pure nitric acid,
evaporate to dryness in a porcelain dish, ignite the copper nitrate
thus obtained in a Hessian crucible until no fumes arise on stirring
the top of the mass with a rod. A portion in the bottom of the
crucible will be sintered if a proper heat has been applied, while
the upper part will be pulverulent. Treat the sintered portion as
above, and reserve each separately. This method gives a reliable

Tests. Pure cupric oxide is a compact, heavy, deep-black pow-
der, gritty to the touch ; upon exposure to a red heat it must evolve
no hyponitric acid fumes, nor carbon dioxide ; the latter would
indicate presence of fragments of charcoal, or particles of dust. It
must contain nothing soluble in water. That portion of the oxide
which has been exposed to an intense red heat should be hard,
and of a grayish-black color.

* If the scales contain lime, digest them with water, containing a little nitric
acid, for a long time, wash, and then proceed as above.

V)2 KEAGENTS. [ 66.

. Uses. Cupric oxide serves to oxidize the carbon and hydrogen
of organic substances, yielding up its oxygen wholly or in part,
according to circumstances. That portion of the oxide which has
been heated to the most intense redness is particularly useful in the
analysis of volatile fluids.

N.B. The cupric oxide, after use, may be regenerated by oxi-
dation with nitric acid, and subsequent ignition. Should it have
become mixed with alkali salts in the course of the analytical pro-
cess, it is first digested with very dilute cold nitric acid, and washed
afterwards with water. To purify cupric oxide containing chlo-
ride, E. EELENMEYER recommends to ignite it in a tube, first in a
stream of moist air, and finally, when the escaping gas ceases to

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 13 of 69)