C. Remigius Fresenius.

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test the aluminium hydroxide for silicic acid (which is often
present). This is readily done by heating with dilute sulphuric
acid, or fusing with potassium- or sodium disulphate ( 75). The
method, if properly executed, gives very accurate results. P>ut
if a considerable excess of ammonia is used, more particularly
in the absence of ammonium salts, and the liquid is filtered with-
out boiling or long standing in a warm place to remove the
ammonia, no trifling loss may be incurred. This loss is the
greater, the more dilute the solution, and the larger the excess of
ammonia. The precipitate cannot well be sufficiently washed on
the filter on account of its gelatinous nature ; on the other hand,
if it be entirely washed by decantation, a very large quantity of
wash- water must be used, hence it is advisable to combine the
two methods, as directed.*

b. By Ignition.

a. Aluminium Salt's of Volatile Oxygen Acids.
Ignite the salt (or the residue of the evaporated solution) in a
platinum crucible, gently at first, then gradually to the very high-
est degree of intensity, until the weight remains constant. For
the properties of the residue, see 75. Its purity must be care-
fully tested. There are no sources of error.

/?. Aluminium Salts of Organic Acids.
The same method as described 104, 3, a (Magnesium).



a. Solution.

Many chromic salts are soluble in water. Chromic hydroxide,
and most of the salts insoluble in water, dissolve in hydrochloric
acid. Ignition renders chromic oxide and many chromium suits
insoluble in acids; this insoluble modification must be prepared for

_ . :

* [When a solution of aluminium hydroxide in potassium or sodium hydrox-
ide is boiled with excess of ammonium chloride, the aluminium separates com-
pletely as a hydratcd oxide with two mol. of water, which may be washed with
comparative case. In certain cases, as where aluminium is separated from ferric
iron by boiling their hydroxides with soda, this fact may be taken advantage of.
, Free. Zeitschriff, iv, 355.]

106.] CHROMIUM. 281

solution in hydrochloric acid, by fusing with 3 or 4 parts of po-
tassa in* a silver crucible. In the process of fusing a small quan-
tity of potassium chromate is formed by the action of air ; this,
however, can be decomposed by heating with hydrochloric acid
with formation of chromic chloride. Addition of alcohol greatly
promotes the reduction to chromic chloride. Instead of this f us
ing with potassa, we frequently prefer to adopt a treatment
whereby the chromium is at once oxidized and converted into an
alkali chromate (see 2). For the solution of chromic iron, see

b. Determination.

Chromium is always, when directly determined, weighed as
chromic oxide. It is brought into this form either by precipitation
as hydroxide and ignition, or by simple ignition. It may, how-
ever, also be estimated by conversion into chromic acid, and deter-
mination as such.

We may convert into


a. By Precipitation. J. By Ignition.

All chromic salts soluble in a. All chromic salts of vola-

water, and also those which, in- tile oxygen acids, provided no
soluble in that menstruum, dis- non-volatile substances be pres-
solve in hydrochloric acid, with ent.

separation of their acid. Pro- /?. Chromic salts of organic

vided always that no organic acids,
substances (such as tartaric acid,
oxalic acid, &c.) which interfere
with the precipitation be present.

2. CHROMIC ACID, or. more correctly speaking, ALKALI CHROMATE.

Chromic oxide and all chromic salts.

The methods of analyzing chromic phosphates, borates, silicates,
and chromic chromate, will be found in Part II. of this Section,
under the heads of the several acids of these compounds.

1. Determination as Chromic Oxide.

a. By Precipitation.

The solution, which must not be too highly concentrated, is
best heated to 100 in a platinum dish. One of porcelain may
also answer, but is not so good, but glass should be avoided,


otherwise considerable error is caused by contamination of the
precipitate with silica, and the results, therefore, will be too high
(A. SOUCHAY *). If porcelain is used, this error is slight. Am-
monia is then added slightly in excess, and the mixture exposed
to a temperature approaching boiling, until the fluid over the pre-
cipitate is perfectly colorless, presenting no longer the least shade
of red ; let the solid particles subside, wash three times by decan-
tation, and lastly on the filter, with hot water, dry thoroughly,
and ignite ( 52). The heat in the latter process must be in-
creased gradually, and the crucible kept covered, otherwise some
loss of substance is likely to arise from spirting upon the incan-
descence of the chromic oxide, which marks the passing of the sol-
uble into the insoluble modification. A suction filter ( 47) is
very convenient for washing the precipitate, which may then be
transferred, still moist, to the crucible in which it is ignited and
weighed. (See p. 117.) For the properties of the precipitate
and residue, see 76. This method, if properly executed, gives
accurate results. Precipitation may also be effected with ammo-
nium sulphide, instead of ammonia. In this case precipitation is
complete in the cold ; and it may be carried out in glass vessels.

J. By Ignition.

of. Chromic Salts of Volatile Oxygen Acids.
The same method as described, 105, Z>, OL (Aluminium).

ft. Chromic Salts of Organic Acids.
The same method as described, 104, 3, a (Magnesium).


(For the estimation of chromic acid, see 130.)
The following methods have been proposed with this view :
a. The solution of the chromic salt is mixed with solution of
potassa or soda in excess, until the chromic hydroxide, which forms
at first, is redissolved. Chldrine gas is then conducted into the
cold fluid until it acquires a yellowish-red tint; it is then mixed
with potassa or soda in excess, and the mixture evaporated to dry-
ness; the residue is ignited in a platinum crucible. The whole of

* Zeitschr. f. anatyt. Chem., iv, 66.

106.] CHROMIUM. 283

the potassium (or sodium) chlorate formed is decomposed by this
process, and the residue consists, therefore, now of an alkali chro-
mate and potassium (or sodium) chloride. (YonL.)

1). Potassium hydroxide is heated in a silver crucible to calm
fusion; the heat is then somewhat moderated, and the perfectly
dry chromic compound projected into the crucible. When the
substance is thoroughly moistened with the potassa, small lumps of
fused potassium chlorate are added. A lively effervescence ensues,
from the escape of oxygen ; at the same time the mass acquires a
more and more yellow color, and finally becomes clear and trans-
parent. Loss of substance must be carefully guarded against (H.

c. Dissolve chromic hydroxide in solution of potassa or soda,
add lead dioxide in sufficient excess, and warm. The yellow fluid
produced contains all the chromium as lead chromate in alkaline
solution. Filter from the excess of lead dioxide, add to the filtrate
acetic acid to acid reaction, and determine the weight of the pre-
cipitated lead chromate (G. CHANCEL*).

d. Mix finely comminuted chromic hydroxide with some
potassium chlorate in a porcelain dish, add nitric acid (sp. gr.
1*367), cover the dish with a funnel of somewhat smaller diam-
eter, heat on the water-bath, and add from time to time a frag-
ment of potassium chlorate until all the chromic hydroxide is dis-
solved and converted into potassium chromate. Even with a
hydroxide which has been strongly ignited, the operation does not
last longer than 30 to 60 minutes. The chromic acid is most con-
veniently determined in the solution by precipitation as barium
chromate (STOKER f ; PEARSON J).

[e. Render the solution of chromic salt nearly neutral by a
solution of sodium carbonate, add sodium acetate in excess, heat
and add chlorine water, or pass in chlorine gas, keeping the solu-
tion nearly neutral by occasional addition of sodium carbonate.
The oxidation proceeds readily. Boil off excess of chlorine, when
the chromic acid may be precipitated as lead chromate or barium
chromate (W. GIBBS ).]

* Comp. rend., XLIII, 927. -\Zeitsc7ir.f. analyt. Chem., ix, 71.

% Ibid., ix, 108. {Am. Journ. Sci. 2 Ser., xxxix, 58.]



Supplement to the Third Group.

Titanium is always weighed as titanic oxide (Ti0 2 ), i.e., the
oxide or anhydride corresponding to titanic acid (Ti(OII) 4 ).
Titanic acid is precipitated with an alkali or by boiling its dilute
acid solution. In precipitating acid solutions of titanic acid ammo-
nia is employed; take care to add the precipitating agent only
in slight excess, let the precipitate formed, which resembles alu-
minium hydroxide, deposit, wash, first by decantation, then com-
pletely on the filter, dry, and ignite ( 52). If the solution con-
tained sulphuric acid, put some ammonium carbonate into the
crucible, after the first ignition, to secure the removal of every
remaining trace of that acid. Lose no time in weighing the ignited
titanic oxide, as it is slightly hygroscopic. Occasionally it is more
convenient to precipitate titanic acid from its acid solutions by
nearly neutralizing with ammonia, adding sodium acetate and boil-
ing. The precipitate thus obtained is easily filtered and waslu-d.
If we have titanic acid dissolved in sulphuric acid, as for instance
occurs when we fuse it with potassium disulphate and treat tie-
in ass with cold water, we may, by largely diluting, and long boil-
ing, with renewal of the evaporating water, fully precipitate the
titanic acid. If much free acid is present it must be nearly neu-
tralized with ammonia before boiling. Boiling is best effected in
a platinum dish. After filtration, the free acid in the filtrate is
still further neutralized, and the liquid boiled again for some time,
to see that no titanic acid is precipitated. Testing the last filtrate
with ammonia affords the certainty that precipitation is complete.
In the process of igniting the dried precipitate, some ammonium
carbonate is added. From dilute hydrochloric-acid solutions of
titanic acid, the latter separates completely only upon evaporating
the fluid to dryness ; and if the precipitate in that case were
washed witli pure water, the filtrate would be milky ; acid must,
therefore, be added to the water.

Titanic acid precipitated in the cold, washed with cold water,
and dried without elevation of temperature, is completely soluble
in hydrochloric acid ; otherwise it dissolves only incompletely in
that acid. The metaiitanic acid thrown down from dilute acid

lu'7 .j jiTAJSiiUM. 285

solutions by boiling, is not soluble in dilute acids. Titanic oxide
resulting from ignition of titanic or metatitanic acid does not dis-
solve even in concentrated hydrochloric acid, but it does dissolve
by long heating with tolerably concentrated sulphuric acid. The
easiest way of effecting its solution is to fuse it for some time
with potassium disulphate, and treat the fused mass with a large
quantity of cold water. Upon fusing with sodium carbonate,
sodium titanate is formed, which, when treated with water, leaves
acid sodium titanate, which is soluble in hydrochloric acid. Ti-
tanic oxide (TiO 2 ) consists of 60*07 per cent, of titanium and
39-93 per cent, of oxygen. By fusing titanic oxide with three
times its quantity of potassium hydrogen fluoride, potassium titan
ium fluoride is formed, which readily dissolves in very dilute
hydrochloric acid (of sp. gr. 1'015) in the heat. On fusing, a
very low heat must be applied at first, till the excess of hydro-
fluoric acid has escaped, then the heat is quickly raised till the
mass melts and the titanic oxide is just dissolved (MARIGNAC *) .
Or heating with hydrofluoric and sulphuric acids practically no
titanium fluoride escapes, but by heating with hydrofluoric acid
some loss does occur (RiLEY f ).

Titanium may be estimated volumetrically by first converting
it into titanous oxide, Ti 2 O 3 , and then oxidizing this to titanic
oxide by means of potassium permanganate (compare 112, 2)
(PisANi ). Solutions in sulphuric acid are to be avoided; but
the ordinary solution in hydrochloric acid, or the solution of
titanium-potassium fluoride in dilute hydrochloric acid, is used.
The reduction is effected with zinc under exclusion of air, and
with or without the application of heat. In case of the hydro-
chloric-acid solutions it is accompanied by a violet color ; in solu-
tions of titanium-potassium fluoride, with a greenish color. After
the reduction is effected the zinc is removed, and solution of
potassium permanganate added until the liquid begins to remain
red. The weak point in this method lies in the difficulty of ac-
curately determining the moment when the reduction is complete.
MARIGNAC has fully described the conditions by the observance
of which he almost invariably obtained good results.

* Zeitschr.f. analyt. Chem., vn, 112. jib., n, 71.

JJ6., iv, 419. %2b., vii, 113.





1. ZINC.

a. Solution.

Many of the zinc salts are soluble in water. Metallic zinc,
zinc oxide, and the salts which are insoluble in water, dissolve in
hydrochloric acid. For effecting the solution of precipitated zinc
sulphide, hydrochloric acid is also best. To dissolve zinc blende,
however, it is best to first subject the finely powdered mineral to
the action of hot, concentrated hydrochloric acid, and then effect
complete solution by adding some nitric acid, potassium chlorate,
or a little of some solution of bromine in hydrochloric acid.

b. Determination.

Zinc is weighed either as oxide or as 8ii1j>Jti<l<' ( 77). The
conversion of zinc salts into the oxide is effected either by precipi-
tation as basic zinc carbonate or sulphide, or by direct ignitjofc.
Besides these gravimetric methods, several volumetric methods are
in use.

We may convert into


a. By Precipitation as Zinc 1>. By Precipitation as Zinc
Carbonate. Sulphide.

All zinc salts which are solu- All compounds of zinc \\\\\\-

ble in water, and all zinc salts of out exception,
organic volatile acids; also those
salts of zinc which, insoluble in
water, dissolve in hydrochloric
acid, with separation of their

c. By direct Ignition.

Zinc salts of volatile inorganic oxygen acids.

108.] ZINC. 287


All compounds of zinc without exception.

The method 1, c, is to be recommended only, as regards the
more frequently occurring compounds of zinc, for the carbonate
and the nitrate. The methods 1, b, or 2, are usually only resorted
to in cases where 1, 0, is inadmissible. They serve more especially
to separate zinc from other basic radicals. Zinc salts of organic
acids cannot be converted into the oxide by ignition, since this
process would cause the reduction and volatilization of a small por-
tion of the metal. If the acids are volatile, the zinc may be deter-
mined at once, according to method 1, a: if, on the contrary, the
acids are non- volatile, the zinc is best precipitated as sulphide. For
the analysis of zinc chromate, phosphate, borate, and silicate, look
to the several acids. The volumetric methods are chiefly employed
for technical purposes ; see Special Part.

1. Determination as Zinc Oxide.

a. By Precipitation as Zinc Carbonate.

Heat the moderately dilute solution nearly to boiling in a capa-
cious vessel, a glass vessel is poorly adapted for this purpose,
porcelain is better, and platinum best ; add, drop by drop, sodium
carbonate till the fluid shows a strong alkaline reaction ; boil a few
minutes; allow to subside, decant through a filter, and boil the
precipitate three times with water, decanting each time; they,
transfer the precipitate to the filter, wash completely with hot
water, dry, and ignite as directed 53, taking care to have the filter
as clean as practicable, before proceeding to incinerate it. In
order to prevent the reduction of the zinc oxide and volatilization
of zinc, it is advisable to carefully saturate with ammonium
nitrate the filter after removing as much of the precipitate
from the latter as possible, and then to incinerate it. Should
the solution contain ammonium salts, the ebullition must be con-
tinued until, upon a fresh addition of sodium carbonate, the escap-
ing vapor no longer imparts a brown tint to turmeric paper. If
the quantity of ammonium salts present is considerable, the fluid
must be evaporated boiling to dryness. It is, therefore, in such
cases more convenient to precipitate the zinc as sulphide (see 5).

The presence of a great excess of acid in the solution of zinc
must be as much as possible guarded against, that the effervescence
from the escaping carbonic acid gas may not be too impetuous. The


filtrate must always be tested with ammonium sulphide (with addi-
tion of ammonium chloride) to ascertain whether the whole of the
zinc has been precipitated. This should be done in a flask filled to
the neck and then closed. A slight precipitate will indeed invari-
ably form upon the application of this test; but, if the process lias
been properly conducted, this is so insignificant that it may be al-
together disregarded, being limited to some exceedingly slight and
imponderable flakes, which moreover make their appearance only
after many hours' standing. If the precipitate is more considerable,
however, it must be treated as directed in Z>, and the weight of the
zinc oxide obtained added to that resulting from the first process.
For the properties of the precipitate and residue, see 77. This
method yields pretty accurate results, though they are in most
cases a little too low, as the precipitation is never absolutely com-
plete, and as particles of the precipitate w r ill always and unavoid-
ably adhere to the filter, which exposes them to the chance of
reduction and volatilization during the process of ignition. On
the other hand, the results are sometimes too high ; this is owing
to defective washing, as may be seen from the alkaline reaction
which the residue manifests in such cases. It is advisable also to
ascertain whether the residue will dissolve 'in hydrochloric acid
without leaving silica; this latter precaution is indispensable in
cases where the precipitation has been effected in a glass vessel.

[It is often better, especially in presence of ammonium salts, to
heat the dry zinc salt with excess of sodium carbonate in a. plati-
num dish cautiously to near redness, then treat with hot water and
wash as directed.]

1. By Precipitation as Zinc Sulphide.

Mix the solution, contained in a not too large flask and suffi-
ciently diluted, with ammonium chloride, then add ammonia, till
the reaction is just alkaline, and then colorless or slightly yellow
ammonium sulphide in moderate excess. If the flask is not now
quite full up to the neck, make it so with water, c'ork, allow to
stand 12 to 2-t hours in a warm place, wash the precipitate, if con-
siderable, tir.M by (leoantation, then on the filter with water con-
taining ammonium sulphide and also less and less ammonium chlo-
ride (filially none). If tiie zinc sulphide is to be weighed ;.-
such, it is best to replace the ammonium chloride by ammonium
nitrate. In decanting do not pour the fluid through the filter,

109.] ZINC. 280

but at? once into a flask. After thrice decanting, filter the
fluid that was poured off, and then transfer the precipitate to the
filter, finishing the washing as directed. The funnel is kept cov-
ered with a glass plate. If the zinc is not to be determined accord-
ing to 2, then put the moist filter with the precipitate in a beaker,
and pour over it moderately dilute hydrochloric acid slightly in
excess. Put the glass now in a warm place, until the solution
smells no longer of hydrogen sulphide ; dilute the fluid with a little
water, filter, wash the original filter with hot water, and proceed
with the solution of zinc chloride obtained as directed in a.

The following method also effects a practically complete pre-
cipitation of zinc from acid solution. Add sodium carbonate, at
last drop by drop till a lasting precipitate forms, dissolve the latter
by a drop of hydrochloric acid, pass hydrogen sulphide till the
precipitate ceases to increase perceptibly, add sodium acetate, and
again pass the gas. After washing with water containing hydro-
gen sulphide (which when the zinc sulphide had been thrown
down by hydrogen sulphide from acetic acid solution, is easily
done), treat as above directed.

From a solution of zinc acetate the metal may be precipitated
completely, or nearly so, with hydrogen sulphide gas, even in pres-
ence of an excess of acetic acid, provided always no other free
acid be present (Expt. No. 66). The precipitated zinc sulphide is
washed with water impregnated with hydrogen sulphide, and, for
the rest, treated exactly like the zinc sulphide obtained by precipi-
tation with ammonium sulphide.

Small quantities of zinc sulphide may also be converted directly
into the oxide, by heating in an open platinum crucible, to gentle
redness at first, then, after some time, to most intense redness.
For the properties of zinc sulphide see 77, c,

c. By direct Ignition.

The salt is exposed, in a covered platinum crucible, first to a
gentle heat, finally to a most intense heat, until the weight of the
residue remains constant. The action of reducing gases is to be

2. Determination as Zinc Sulphide.

The precipitated zinc sulphide, obtained as in 1, &, may be
ignited in hydrogen and weighed. H. ROSE,* who recom-

. Anal., ex, 138,




mended the process, employs the apparatus represented by
Fig. 83.

a contains concentrated sulphuric acid, &, calcium chloride.
The porcelain crucible has a perforated porcelain or platinum
cover, into the opening of which fits the porcelain or platinum
tube, d. The latter is provided with an annular projection which
rests on the cover, the tube itself extends some distance into the
crucible. When the zinc sulphide has dried in the filter, it is
transferred to the weighed porcelain crucible, the filter ashes added,
powdered sulphur is sprinkled over the contents of the crucible,
the cover is placed on, and hydrogen is passed in a moderate
stream, a gentle heat is applied at first, which is afterwards raised
for five minutes to intense redness ; finally the crucible is allowed
to cool with continued transmission of the gas, and the zinc sul-
phide is weighed.

Fig. 83.

Instead of the hydrogen apparatus shown, which may not be
at the disposal of the operator, any apparatus that allows the cur-
rent of gas to be regulated may be used. An evolution-appara-
tus in which the current is not under control, is not suitable.

Instead of the porcelain tube and perforated cover, a com-
mon tobacco-pipe may be employed, the bowl of the latter being

100.] MANGANESE. 291

inverted over and fitting exactly within a porcelain crucible.
[Hydrogen sulphide may be advantageously substituted for

OESTEN'S experiments, which were adduced by ROSE in sup-
port of the accuracy of this method, were highly satisfactory.

Zinc sulphate, carbonate, and oxide may be converted into
sulphide in the manner just described. They must, however, be
mixed with an excess of powdered sulphur, otherwise you will
lose some zinc from the reducing action of the hydrogen on the
zinc oxide. Zinc sulphate is best ignited first with excess of air
and before mixing with sulphur, and then igniting in a current of
hydrogen. (H. ROSE.)

The properties of the hydrated and anhydrous zinc sulphide are
given 77 ; the results are accurate. Loss occurs only when the
ignition is performed over the gas blowpipe (which is quite unnec-
essary), and continued longer than five minutes. Compare 77, c.



a. Solution.

Many manganous salts are soluble in water. The manganous
salts which are insoluble in that menstruum, dissolve in hydrochloric
acid, which dissolves also all oxides of manganese. The solution
of the higher oxides is attended with evolution of chlorine equiva-
lent to the amount of oxygen which the oxide under examination
contains, more than manganous oxide (MnO) and the fluid, after
application of heat, is found to contain manganous chloride.

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