C. Remigius Fresenius.

Quantitative chemical analysis online

. (page 33 of 69)
Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 33 of 69)
Font size
QR-code for this ebook


f Annal. de Chem et Pliarm,, LXXXV, 307.



368 DETEKMlXATlON. [ 118.

salt; and it should be of proper dilution also. According to
LIEBIO, 10 c. c. should contain not more than about 0*2 grin, of
mercuric oxide ; hence, if a preliminary experiment shows it to
be too strong, the solution must be diluted. The solution should,
further, be free from metals, and should not contain too much
free acid ; only so much should be present as to aft'ord a non-acid
solution on adding the quantity of sodium-phosphate solution
required in the experiment (say 3 or 4 c. c.). A solution which is
too acid is treated with sodium carbonate until basic salt precipi-
tates, and this is then redissolved by adding a drop or two of
nitric acid.

y. 2 J erformance of the Analytical Process: This may be
carried out in two ways. It is best to employ both, since the first
yields results a little too high, the second a little too low, hence
with both the errors compensate each other. Method 1 : To
10 c. c. of the mercury solution in a beaker add 3 or 4 c. c. of a satu-
rated sodium- phosphate solution, and then, before the precipitate
has time to become crystalline, add sodium-chloride solution, the
last portions being added very cautiously, until the precipitate has
entirely disappeared.

Suppose 20-5 c. c. of sodium-chloride solution have been re-
quired to effect this. We then measure off (Method 2) 20 '5 c. c. of
the same sodium-chloride solution, add to it 3 or 4 c. c. of sodium-
phosphate solution, and run in from a burette sufficient of the same
mercury solution to just afford a permanent precipitate. If 10*25
c. c. of mercury solution have been required to effect this, then
20*5 -f- 20'5 = 41 c.c. sodium-chloride solution have been required
for 10 -|- 10-25 = 20*25 c. c. of mercuric-oxide solution, from
which it follows that, as 1 c. c. of sodium-chloride solution is the
equivalent of 0*0108 grin, of mercuric oxide, 41 c. c. of the solu-
tion will correspond to 0*4428 grin, mercuric oxide the quantity
contained in 20*25 of the mercury solution.

LIEBIG has shown by numerous experiments that this method
gives very close results, e.g., 0*1878 grm. instead of 0-1870;
0-174 grm. instead of 0'1748; 0*1668 grm. instead of 0*1664,
etc. The method is, however, susceptible of only very limited
application. For this reason I omit giving FR. Mora's modifi-
cation of the method,* which consists in replacing the sodium

phosphate by potassium ferricyanide,

-../.. >,

* Lehrbuch der Titrirmethode, 3. Aufl. . 396.



118.] MERCURY IN MERCURIC COMPOUNDS. 369

c. Regarding PERSONNE'S * method, which depends on adding
mercuric-chloride solution to standard potassium-iodide solution
until incipient, permanent precipitation, see Zeitschr. f. analyt.
Chem., n, 381.

d. After J. J. SOBERER.! Mercuric nitrate or chloride may be
directly determined with sodium thiosulphate. The reactions are
as follows : 2H 2 O + 3Hg(NO,) a + 2Na 2 S 2 O 3 = (HgS) 2 Hg(NO,) a +
2Na a S0 4 + 4HNO,; or, 2H 2 O + 3HgCl a + 2Na 2 S 2 O 8 = (HgS),- Hg
Cl 2 -f- 2Na 2 SO 4 -J-4HC1. Tlie process is conducted as follows in
the case of mercuric nitrate : Mix the highly dilute solution with
a little free nitric acid in a tall glass, and add drop by drop solution
of sodium thiosulphate 12'4 grrn. in a litre. Each drop produces
an intense yellow cloud, which on shaking quickly subsides in the
form of a heavy flocculent precipitate (HgS) a Hg(NO s ) a . In order-
to distinguish clearly the exact end of the reaction, SCHERER
recommends to transfer the fluid towards the end to a measuring
flask, to take out -J or -J of the clear fluid and to finish with this.
The portion of thiosulphate last used is multiplied by 3 or 2, as
the case may be, and added to the quantity first used. 1 c.c. of
the solution corresponds to 0*015 mercury, or 0*0162 mercuric
oxide. The relation is not changed even when the fluid contains
another acid (sulphuric, phosphoric).

In the case of mercuric chloride, the highly dilute solution is
mixed with a little hydrochloric acid and warmed, nearly to boil-
ing, before beginning to add the sodium thiosulphate. At first a
white turbidity is formed, then the precipitate separates in thick
flocks. When the solution begins to appear transparent, the pre-
cipitant is added more slowly. In order to hit the end of the
reaction exactly, small portions must be filtered off towards the
close. The precipitate must be completely white ; if too much
thiosulphate has been added, it is gray or blackish, and the experi-
ment must be repeated. SCHERER obtained very accurate results.
Of course no other metals must be present that exert a decompos-
ing action on sodium thiosulphate.



*Journ. dePharm. et de Chem., XLIII, 477.
f Lehrbuch der Chemie, I, 513.



370 DETERMINATION. [ 119.

119.

5. COPPER.

a. Solution

Many cupric salts dissolve in water. Metallic copper is best
dissolved in nitric acid. Cnpric oxide, and those cupric silts \vhich
are insoluble in water, may be dissolved in nitric, hydrochloric, <>r
sulphuric acid. Cupric sulphide is treated with fuming nitric acid,
or it is heated with moderately dilute nitric acid, until the separated
sulphur exhibits a pure yellow tint ; addition of a little hydro-
chloric acid or potassium chlorate greatly promotes the action of
the dilute acid.

1). Determination.

Copper may be weighed in the form of cupric oxide, or in the
metallic state, or as cuprous sulphide ( 85). Into the form of
cupric oxide it is converted by precipitation, or ignition, sometimes
with previous precipitation as sulphide. The determination as
cuprous sulphide is preceded usually by precipitation either as
cupric sulphide or as cuprous sulphocyanate. Copper may be deter-
mined also by various volumetric and indirect methods.

We may convert into

1. CUPRIC OXIDE :

a. By Precipitation as hydrated cupric oxide and subsequent
ignition : All cupric salts soluble in water, and also those insoluble
salts, the acids of which may be removed upon solution in nitric
acid, provided no non-volatile organic substances be present.

1. By Precipitation, preceded by Ignition of the compound:
Such of the salts enumerated under a as contain a non-volatile
organic substance, thus more particularly cupric salts of non- vola-
tile organic acids.

c. By Ignition : Cupric salts of oxygen acids that are readily
volatile or decomposable at a high temperature (cupric carbonate,
cupric nitrate).

2. METALLIC COPPER: Copper in all solutions free from other
metals precipitable by zinc or the galvanic current, also the oxides
of copper.

3. CUPROUS SULPHIDE : Copper in all cas.es in which no other
metals are present that are precipitable by hydrogen sulphide or
potassium sulphocyanate.



119.] COPPER. 371

Of the several methods of effecting the estimation of copper,
No. 3 is* particularly to be recommended for use in laboratories ;
method 2 is also very convenient, and well adapted for assaying.
Of the volumetric methods, one is suited for technical purposes,
the other for the estimation of small quantities of copper. For
technical purposes there are, besides, also several colorimetric
methods, proposed by HEINE, VON HUBERT, JACQUELAIN, A. MUL-
LER, and others, which are, all of them,- based upon the comparison
of an ammoniacal solution of copper, of unknown strength, with
others of known strength.*

LEVOL'S indirect method of estimating copper, which is based
upon the diminution of weight suffered by a strip of copper when
digested in a close-stoppered flask with ammoniacal solution of
copper till decolonization is effected, takes too much time, and is
apt to give false results (PHILLIPS, f ERDMANN^:). The latter remark
applies also to the indirect method proposed by RUNGE, which con-
sists in boiling the solution of copper, free from nitric acid and
ferric salts, in presence of some free hydrochloric acid, in a flask,
with a weighed strip of copper, and, after decolorization of the
fluid, determining the loss of weight suffered by the copper.

1. Determination as Cupric Oxide.

a. J3y direct Precipitation as Oxide.

Heat the rather dilute neutral or add solution in a platinum or
porcelain dish, to incipient ebullition, add a somewhat dilute solu-
tion of pure soda or potassa until the formation of a precipitate
ceases, and keep the mixture a few minutes Ipnger at a tempera-
ture near boiling. Allow to subside, filter, wash by decantation
twice or thrice, boiling up each time, then collect it on the filter,
wash thoroughly with hot water, dry, and ignite in a porcelain or
platinum crucible, as directed 53. Do not use the blow-pipe.
After ignition, and having added the ash of the filter, let the
crucible cool in the desiccator, and weigh. The action of reducing
gases must be carefully guarded against in the process of ignition.

It will sometimes happen, though mostly from want of proper
attention to the directions here given, that particles of the precipi-

* This subject hardly comes within the scope of the present work. I there-
fore refer to AL. MULLER, das Complemcntarcolorimeter, Chemnitz, 1854; Bo-
DEMANN'S Probirkunst von KERL, 222; also to DEHMS, Zeitschr.f. analyt. Chem.,
m, 218, and GUSTAV BISCHOF, jim., ib., vi, 459.
f Annal. d. Chem. u. Pharm., LXXXI, 208. Journ. /. prakt. Chem., LXXV, 211.



372 DETERMINATION. [ 119.

tate adhere so tenaciously to the dish as to be mechanically irremov-
able. In a case of this kind, after washing the dish thoroughly,
dissolve the adhering particles with a few drops of nitric acid, and
evaporate the solution over the principal mass of the precipitated
oxide, before you proceed to ignite the latter. Should the solution
be rather copious, it must iirst be concentrated by evaporation,
until only very little of it is left. For the properties of the pre-
cipitate, see 85.

With proper attention to the directions here given, the results
obtained by this method are quite accurate, otherwise they may be
either too high or too low. Thus, if the solution be not sufficiently
dilute, the precipitant will fail to throw down the whole of the
copper ; or if the precipitate be not thoroughly washed with hot
water, it will retain a portion of the alkali ; or if the ignited pre-
cipitate be allowed to stand exposed to the air before it is weighed,
an increase of weight will, be the result ; and so, on the other hand,
a diminution of weight, if the oxide be ignited with the filter or
under the influence of reducing gases, as thereby cuprous oxide
would be formed. Should a portion of the oxide have suffered
reduction, it must be reoxidized by moistening with nitric acid,
evaporating cautiously to dryness, and exposing the residue to a
gentle heat, increasing this gradually to a high degree of intensity.

Let it be an invariable rule to test the filtrate for copper with
hydrogen sulphide water. If, notwithstanding the strictest compli-
ance with the directions here given, the addition of this reagent
produces a precipitate, or imparts a brown tint to the fluid, this is
to be attributed to the presence of organic matter ; in that case,
concentrate the filtrate and wash- water by evaporation, acidify,
precipitate with hydrogen sulphide water, filter, incinerate the
filter, heat with nitric acid, dilute, filter, concentrate, precipitate
with soda, and add the oxide obtained to the main quantity.

Never neglect to test the cupric oxide after weighing for alkali
or alkali salt by boiling it with water. If either is present, the
oxide must be exhausted with hot water, and then reignited and
reweighed. Finally, dissolve the oxide in hydrochloric acid to
detect and if necessary to estimate any silicic acid it may contain.

In default of sufficiently pure potash or soda, the carbonate
may be used, but the solution must not contain more than 1 grin.
copper in the litre; the alkali carbonate must only be added
slightly in excess, and the mixture must be boiled for half an hour.



119.] COPPED. 373

The bluish-green precipitate will then turn dark brown and gran-
ular, and may b<3 easily washed (GIBBS*).

From ammoniacal solutions, also, copper may be precipitated
by soda or potassa. In the main, the process is conducted as above.
After precipitation the mixture is heated, until the supernatant
fluid has become perfectly colorless ; the fluid is then filtered off
with the greatest possible expedition. If allowed to cool with the
precipitate in it, a small portion of the latter would redissolve.

b. By Precipitation as Oxide, preceded by Ignition of the
Substance.

Heat the substance in a porcelain crucible, until the organic
matter present is totally destroyed ; dissolve the residue in dilute
nitric acid, filter if necessary, and treat the clear solution as
directed in a.

c. By Ignition.

The salt is put into a platinum or porcelain crucible, and
exposed to a very gentle heat, which is gradually increased to
intense redness ; the residue is then weighed. As cupric nitrate
spirts strongly when ignited, it is always advisable to put it into a
small covered platinum crucible, and to place the latter in a large
one, also covered. With proper care, the results are accurate.
Cupric salts of organic acids may also be converted into cupric oxide
by simple ignition. To this end, the residue first obtained, which
contains cuprous oxide, is completely oxidized by ignition with
mercuric oxide (which leaves no residue on ignition), or, with less
advantage, by repeated moistening with nitric acid, and ignition.
A loss of substance is generally incurred by the use of nitric acid
from the difficulty of avoiding spirting.

2. Determination as Metallic Copper,
a. By Precipitation with Zinc or Cadmium,.^
Introduce the solution of copper, after having, if required, first
freed it from nitric acid, by evaporation with hydrochloric acid or

* Zeitschr. f. analyt. Chem., vn, 258.

f The method of precipitating copper by iron or zinc and weighing it in the
metallic form was proposed long ago; see PFAFF'S Ilandbuch der analytischen
Chernie, Altona, 1822, ir, 269; where the reasons are given for preferring zinc as
a precipitant, and hydrogen sulphide is recommended as a test for ascertaining
whether the precipitation is complete. I mention this with reference to
F. MOHR'S paper in the Annal. d. Chem. u. Pharm., xcvi, 215, and BODE-
MANN'S Probirkunst von KERL, 220.



374 DETERMINATION. [ H9.

sulphuric acid, into a weighed platinum (Lsh , dilute, if necessary
with some water, throw in a piece of zinc (soluble in hydrochloric
acid without residue), and add, if necessary, hydrochloric acid in
sufficient quantity to produce a moderate evolution of hydrogen.
If, on the other hand, this evolution should be too brisk, owing to
too large excess of acid, add a little water. Cover the dish with a
watch-glass, which is afterwards rinsed into the dish with the aid
of a washing-bottle. The separation of the copper begins imme-
diately; a large proportion of it is deposited on the platinum in
form of a solid coating; another portion separates, more particu-
larly from concentrated solutions, in the form of red spongy masses.
Application of heat, though it promotes the reaction, is not abso-
lutely necessary ; but there must always be sufficient free acid
present to keep up the evolution of hydrogen. After the lapse of
about an hour or two, the whole of the copper has separated. To
make sure of this, test a small portion of the supernatant fluid
with hydrogen sulphide water ; if this fails to impart a brown tint
to it, you may safely assume that the precipitation of the copper is
complete. Ascertain now, also, whether the zinc is entirely dis-
solved, by feeling about for any hard lumps with a glass rod, and
observing whether renewed evolution of hydrogen will take place
upon addition of some hydrochloric acid. If the results are satis-
factory in this respect also, press the copper together with the glass
rod, decant the clear fluid, which is an easy operation, pour, with-
out loss of time, boiling water into the dish, decant again, and
repeat this operation until the washings are quite free from hydro-
chloric acid. Decant the water now as far as practicable, rinse the
dish with strong alcohol, dry at 100, let it cool, and weigh. If
you have no platinum dish, the precipitation may be effected also
in a porcelain crucible or glass dish; but it will, in that case, take
a longer time, because of the lack of the galvanic action between
the platinum and zinc; and the whole of the copper will be
obtained in loose masses, and not firmly adhering to the sides of the
crucible or dish, as in the case of precipitation in platinum vessels.
The results are very accurate. The direct experiment, No.
69, gave 100 and 100-06, instead of 100. FR. MOHR (loc. cit.)
obtained equally satisfactory results by precipitating in a porce-
lain crucible.*

* STOIIKK (On the alloys of copper and zinc, Cambridge, I860, p. 47) says that
the precipitated copper retains water, but I have not found this to be the case.



119.] COPPER. 375

Zinc being sometimes difficult to obtain of sufficient purity,
cadmium- may be used instead; it dissolves with less violence in
strongly acid copper solutions. It may be used in the form of rod
in which it usually occurs in commerce (CLASSEN*).

1). By Precipitation with the Galvanic Current.

This method makes us independent of pure zinc or cadmium,
and yields the copper in a compact form, readily washed and deter-
mined. It is now largely used in copper works, constant batteries
have been employed for it, and the whole process has been organ-
ized for use on a large scale by LUCKOW, and adopted by the Mans*
feld Ober-Berg-und Hiitten-Direction in Eisleben.f A small elec-
trolytic apparatus without separate battery, for single precipitation s {
has been described by ULLGEEN.^:

c. By Ignition in Hydrogen.

The oxides of copper when ignited in a current of pure hydro
gen are converted into metallic copper, and may thus be coiivei
iently analyzed. Occasionally the cupric oxide obtained by 1, a c.
5, is reduced either at once, or after weighing ; in the latter ca-
the reduction serves as a control.

3. Determination as Cuprous Sulphide.

a. By Precipitation as Cupric Sulphide.

Precipitate the solution which is best moderately acid, btu
should not contain a great excess of nitric acid according to the
quantity of copper present, either by the addition of strong hydro-
gen sulphide water, or by passing the gas. In the absence of nitric
acid it is well to heat nearly to boiling while the gas is passing, as
this makes the precipitate denser, and it is more easily washed.
When the precipitate has fully subsided, and you have made sure
that the supernatant fluid is no longer colored or precipitated by
strong hydrogen sulphide water, filter quickly, wash the precipi-
tate without intermission with water containing hydrogen sulphide,
and dry on the filter with some expedition. Transfer to a weighed
porcelain crucible, add the filter-ash and some pure powdered sul-
phur, and ignite strongly in a stream of hydrogen ( 108, Fig.
83). It is advisable to use a glass blow-pipe. The results are
very accurate (H. ROSE).

* Journ. f. prakt. Chem., xcvi, 259.

\Zeitsclir. f. analyt. Chem., vm, 23 and xi, 1. Compare also G/BBS, ib n
in, 334, and LECOQ DE BOISBAUDAN, ib., vu, 253.

t Ib. , vii, 442. Pogg. Annal. ex, 138.



376 DETERMINATION. [119.

This method, which was recommended by BERZELIUS, and
afterwards by BRUNNER, has only lately received a very practical
form from the apparatus introduced by II. ROSE. I feel great
pleasure in recommending it. In my own laboratory it is in
frequent use.

If the precipitated cupric sulphide is ignited instead in a
current of hydrogen in a covered porcelain crucible, from which
the heat as well as the cover are removed occasionally for a few
seconds, the contents will be converted into a variable mixture of
Cu,S and CuO, which may contain, according to circumstances,
cupric oxide or cuprous sulphide. Since, however, the percent-
age content of cupric oxide and cuprous sulphide in copper is
the same, the copper content in the residue may also be deter-
mined (ULRICI *). This method is simpler than the one detailed
above, but is not quite as accurate.

&. By Precipitation as Cuprous Sulphocyanate, after RIVOT. f
The solution should be as free as possible from nitric acid and
free chlorine, and should contain little or no free acid. Add sul-
phurous or hypophosphorous acid in sufficient quantity, and then
solution of potassium sulphocyanate in the least possible excess.
The copper precipitates as white cuprous sulphocyanate. It is
filtered after standing some time, washed and dried, mixed with
sulphur, ignited in hydrogen in the apparatus mentioned in a, and
this ignition with sulphur is repeated till the weight is constant.
The precipitate may also be collected on a weighed filter, dried at
100, and then weighed. The experiment, No. 71, conducted in
the latter way, gave 99'66 instead of 100. The process yields
satisfactory results, but they are always inclined to be a little too
low, as the cuprous sulphocyanate is not absolutely insoluble. The
loss is larger in the presence of much free acid.

c. Cuprous and cupric oxide, cupric sulphate, and many other
salts of copper (but not chloride, bromide, or iodide) may be directly
converted into cuprous sulphide, by mixing with sulphur and
igniting in hydrogen as in a (II. ROSE, loc. tit.). The results are
thoroughly satisfactory.

* Journ.f. prakt. Chem., cvn, 110.

f Compt. Rend., xxxvm, 868; Journ.f. prakt. Chem., LXU,



119.] COPPER. 377

4. Volumetric Methods.

a. CE HAEN'S METHOD.*

I recommend this method, which was devised in my own
laboratory, f as more especially applicable in cases where small
quantities of copper are to be estimated in an expeditious way.
The method is based upon the fact that, when a cupric salt in
solution is mixed with potassium iodide in excess, cuprous iodide
and free iodine are formed, the latter remaining dissolved in the
solution of potassium iodide : CuSO 4 + 2KI = Cul + K 2 SO 4 + I.
Now, by estimating the iodine by BUNSEN'S method, or with sodium
thiosnlphate ( 146), we learn the quantity of copper, as 1 a*:,
iodine (126-85) corresponds to 1 at. copper (63*6). The following
is the most convenient way of proceeding : Dissolve the compound
of copper in sulphuric acid, best to a neutral solution ; a mouerate
excess of free sulphuric acid, however, does not injuriously affect
the process. Dilute the solution, in a measuring flask, to a defi-
nite volume ; 100 c.c. should contain from 1 to 2 grm. of copper.
Introduce now about 10 c.c. of potassium iodide solution (1 in 10)
into a stoppered bottle, add 10 c.c. of the copper solution, mix,
allow to stand 10 minutes, and then determine the separated
iodine, either with sulphurous acid and iodine ( 146, 1), or with
sodium thiosulphate ( 146, 2). The copper solution must be free
from ferric salts and other bodies which decompose potassium
iodide, also free nitric acid, and free hydrochloric acid; and the
solution must not be allowed to stand too long before titration.
With strict attention to these rules, the results are quite accurate.
DE HAEN obtained, for instance 0*3567 instead of 0*3566 of cupric
sulphate, 99 -89 and 100*1 instead of 100 of metallic copper.
Further experiments (No. 72) have convinced me, however, that,
though the results attainable by this method are satisfactory, they
are not always quite so accurate as would be supposed from the above
figures given by DE HAEN. Acting upon FK. MOHE'S suggestion
I tried to counteract the injurious influence of the presence of

* Annal. d. Chem. u. Pharm., xci. 237.

f BROWN (Quart. Journ. of the Chem. Soc., x, 65), who published this as a
new method in 1857, appears to have been ignorant of its publication in 1854.
Even the slight variation of determining the iodine with sodium hyposulphite
(ScHWAiiz) instead of with sulphurous acid (BUNSEN) was given by MOHR
(Lehrbuch der Titrirmethode, \, 387) in 1855. The same may be said of RUMP-
LER, who in 1868 (Journ. f prakt. Chem., cv, 193) published the method, with
a slight modification, as new.



378 DETEKMINATION. [ 119.

nitric acid, by adding to the solution containing nitric acid, first,
ammonia in excess, then hydrochloric acid to slight excess ; the
result was by no means satisfactory. The reason of this is that a
solution of ammonium nitrate, mixed with some hydrochloric acid,
will, even after a short time, begin to liberate iodine from solution



Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 33 of 69)