C. Remigius Fresenius.

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the ammonium sulphide can be effected.

c. By Ignition.

Expose the compound, in a covered crucible, to a gentle heat
at iiist. and gradually to the highest degree of intensity ; continue
the operation until the weight of the residuary ferric oxide remains

113.] FERRIC IKON. 325

2. Determination as Anhydrous Ferrous Sulphide.

Theiiydrated ferrous sulphide obtained, as in 1, >, may be very
conveniently determined by conversion into the anhydrous sul-
phide. The process is the same as for zinc ( 108, 2). The heat
to which it is finally exposed in the current of hydrogen must no
strong, as an excess of sulphur is retained with some obstinacy. In
fact, it is advisable after weighing to re-ignite in hydrogen and
weigh a second time. It is of no importance if the hydrated sul-
phide has oxidized on drying.

Ferrous sulphate and ferric hydroxide can be transformed into
sulphide in the same manner, after having been dehydrated by
ignition in a porcelain crucible (H. ROSE*).

The results obtained by OESTEN, and adduced by ROSE, as well
as those obtained in my own laboratory, are exceedingly satisfac-
tory. (Expt. No. 67.)

3. Volumetric Determination.

a. Preceded T)y Reduction of Ferric to Ferrous Iron.

The methods here to be described depend upon the reduction
of ferric to ferrous iron, the quantity of which latter is then esti-
mated. We have hence to occupy ourselves simply with the re-
duction of ferric to ferrous solutions, the other part of the process
having been fully discussed in 112 (Ferrous Iron). This reduc-
tion can be effected by many substances (zinc, stannous chloride,
hydrogen sulphide, sulphurous acid, &c.), but only those can be
used with advantage an excess of which may be added with im-
punity. If an excess must be very carefully avoided, or, being
added, must be carefully removed, the method becomes trouble-
some, and a ready source of inaccuracy is introduced. On these
grounds, zinc, although somewhat slow in action, unquestionably
deserves to be preferred to all other reducers.

Reduction ~by Zinc. Heat the hydrochloric- or sulphuric-acid
solution, which must contain a moderate excess of acid, but be
free from nitric acid,f in a small, long-necked flask placed in a
slanting position ; drop in small pieces of iron-free zinc ( 60) and

* Pogg. AnnaL, ex, 126.

f If nitric acid is present, there forms, under the action of zinc, nitrous acid,
which reduces potassium permanganate. This would hence give rise to erro-
neous results (TEKREIL, Zeitschr.f. analyt. Chem., vi, 116),


conduct a slow current of carbon dioxide through the flask (Fig.
85). Evolution of hydrogen gas begins at once, and the color of
the solution becomes paler in proportion as the ferric sulphate (or
chloride) changes to ferrous sulphate (or chloride). Apply a mod-
erate heat to promote the action, and add also, if necessary, a
little more zinc. As soon as the hot solution is completely de-
colorized (one cannot judge of the perfect reduction of a cold solu-
tion so well, as the color of a ferric salt is deeper when hot), and
all the zinc is dissolved,* allow to cool completely in the stream
of carbon dioxide (to hasten the cooling the flask may be im-
mersed in cold water) ; then dilute the contents with water, pour
off and wash carefully into a beaker, leaving- behind (so far as
possible) any flocks of lead that may have separated from the zinc,
wash repeatedly with water, and in the case of a sulphuric-acid
solution proceed preferably as directed in 112, 2, /?; in the case
of a hydrochloric-acid solution proceed according to p. 319, y. If
the solution contain metals precipitable by zinc, these will separate
and may render filtration necessary. In this case the filtrate
must be again heated with zinc before using the standard solution.
If iron-free zinc cannot be procured, the percentage of iron in
the metal used must be determined and weighed portions of it
employed in the process of reduction ; the known amount of iron
contained in the zinc consumed is then subtracted from the total
amount of iron found.

In the analysis of solid ferric compounds it is advisable to add
some zinc while they are dissolving in hydrochloric acid. Solu-
tion is thereby facilitated (O. L. ERDMANNf). Regarding the re-
duction of ferric chloride by stannous chloride compare with b.

[Reduction by Hydrogen Sulphide. Pass hydrogen sulphide
through the cold ferric solution in a flask. The solution should
occupy about two-thirds of the capacity of the flask, and should
not contain much more than 0*2 grm. iron per 100 c. c., but may
be more dilute when but little iron is present. Continue the
treatment with hydrogen sulphide at least 10 minutes after
the color due to the ferric salt has disappeared, or until the

* If any zinc remains undissolved, the results may be too low, because iron
often deposits on zinc and does not dissolve until the zinc itself dissolves
(A. MITSCHERLICH, ZeiUcliT. f. analyt. Ctom.,ii, 72).

\Journ.f.prakt. Cliem. t LXXVI, 176.

113.] FERKIC IKON. 327

solution appears to be well saturated with that gas. Heat, at first
cautiousty, to boiling. Escape of hydrogen sulphide at this period
indicates that enough of that reagent has been applied. Continue
boiling so rapidly that air cannot enter the flask, the mouth of
which may be partially closed by a loose roll of filter paper, or other
means, until the solution is reduced to one half its first volume.
This will insure the removal of excess of hydrogen sulphide. (The
escaping vapor will cease to blacken paper dipped in an alkaline
lead solution somewhat before this point is reached.) During the
boiling, let the flask be inclined so as to prevent mechanical loss.
When the boiling is discontinued fill the flask immediately with
cold water to within an inch of the mouth, close with a stopper,
and cool in a stream of water. Before reducing the ferric solu-
tion by either of the above processes, it is desirable to remove
hydrochloric acid, if it is present, so that the iron after reduction
can be satisfactorily determined by KMnO 4 . Chlorides can be
decomposed and ITC1 removed by evaporating the solution with
excess of sulphuric acid so long as hydrochloric acid vapors are
given off at a temperature slightly exceeding 100. A liberal
excess of sulphuric acid is advantageous. After cooling add water
and digest till the ferric sulphate is dissolved. This treatment is
simple and safe when nothing is present which is thereby converted
into a compound insoluble in dilute sulphuric acid (silicic acid,
barium, strontium, much calcium, &c.). Such insoluble com-
pounds may persistently retain iron. When, therefore, by evapo-
ration w T ith sulphuric acid and subsequent treatment with water
an insoluble residue remains, it should not be thrown away before
testing it to ascertain whether it retains iron.]

&.' Without Previous Reduction to Ferrous Iron.

These methods all depend on adding a reducing agent to the
solution till the sesquioxide is entirely converted into protoxide,
and then determining, either directly or indirectly, the reducing
agent used. Many methods have been proposed, but I have
found those given under a and ft the best.

a. Reduction by stannous chloride.*

This method, if properly executed, gives very good results;

* The reduction of ferric chloride by stannous chloride has already been
utilized by PENNY and WALLACE in another manner, but I believe I was tht
first to give the method a practical form (Zeitschr.f. analyt. Chem., i, 26).


and, having had many years' experience with it, I can strongly
recommend it. There are required for it :

a. A Ferric-Chloride Solution of known strength. This is
prepared by dissolving 10*04 grm. of perfectly clean, thin, soft
iron wire (corresponding to 10 grin, pure iron) in HC1 in a long-
necked flask placed slantingly, oxidizing with potassium chlorate,
completely driving off the excess of chlorine by protracted, gentle
boiling, and finally making up the volume to 1 litre.

5. A clear' Solution of Stannous Chloride. This should be of
such a strength that one volume may reduce about two volumes
of the ferric-chloride solution.

c. A Solution of Iodine in Potassium Iodide, each c. c. of
which contains O'Ol grin, iodine. The iodine content need not
be exactly known. The operations are as follows:

1. Measure off 2 c. c. of the stannous-chloride solution and
add a little starch solution, 5 c. c. water, and sufficient of the
iodine solution to permanently color the liquid blue. Note the
quantity used. Every c. c. of stannous-chloride solution will
require about 5 c. c. of iodine solution.*

2. Measure off 50 c. c. of the ferric-chloride solution, add a
little hydrochloric acid, and heat the liquid in a small flask, pref-
erably on an iron plate, to boiling. Then run in the stannous-
chloride solution from a burette, first in large quantities, then in
smaller, with suitable intervals between the additions, while the
fluid is kept gently boiling all the time. The yellow color be-
comes lighter in proportion as the reduction progresses. Towards
the end the stannous-chloride solution is added by drops, with
sufficient time for action. The precise moment when reduction
is complete may be readily noted by the yellow liquid becoming
colorless. Now cool the contents of the flask, add some starch
solution, and run in from a burette solution of iodine by drops
until a permanent blue color supervenes. The quantity of iodine
solution used gives, according to the relations determined in 1,
the excess of the etannous-chloride solution added. f On deduct-

* The quantity of iodine solution here used varies somewhat according as
more or less hydrochloric acid is added to the stanuous-chloride solution. The
differences are, however, so trifling as to have no appreciable influence on the
result, because in this method only a very small excess of stannous chloride has
to be determined.

f If the staunous chloride.has been added very cautiously toward the last,
no determinable excess of stannous chloride is frequently left in solution, par-

113.] FERRIC IRON. 329

ing tliis excess from the quantity of stannous-chloride solution
used at first, the corrected quantity which is necessary to reduce
0*5 grm. of iron from a ferric to a ferrous condition is found.

3. The stannous-chloride solution having been thus standard-
ized, it may be used for estimating unknown quantities of iron
by dissolving these in hydrochloric acid, converting any ferrous
chloride into ferrio chloride by one of the methods detailed in
112, 1, a or <?, driving off every trace of chlorine, and finally
adding the stannous-chloride solution to the suitably concentrated
iron solution as described under 2, until the liquid becomes color-
less, the excess of stannous chloride present being then determined
with the iodine solution. The iron content may then be calculated,
from the volume of stannous-chloride solution used, by the rule-
of-three. Assuming that 25 c. c. of stannous-chloride solution cor-
respond to 0'5 grm. iron (i.e., sufficient to reduce 0*5 grm. iron
from a ferric to a ferrous condition), and assuming that we have
used 20 c. c. of the solution to reduce the unknown quantity of
ferric salt, then the latter will have contained "4 grm. of iron,

25: 0-5: : 20: a?; x = 0-4 grm.

This method, as already stated, affords excellent results,* but only
when all the operations are carried out at once, in order that the
titer of the staniious chloride may not be altered by the action of
the air. On this account, also, it is preferable to operate with
rather concentrated stannous-chloride solutions, and consequently
fairly large quantities of iron, rather than with dilute solutions,
on which the air would act more strongly.

To prepare the stannous-chloride solution, heat pure powdered
tin (obtained by melting tin in a porcelain dish and triturating
with a pestle until cold) with hydrochloric acid (sp. gr. 1-12) until
the tin, being in excess, evolves no more hydrogen. Cool the
liquid, pour or filter it off from the undissolved tin and add
3 volumes of hydrochloric acid and 6 volumes of water. The
solution is best preserved in an apparatus which prevents or limits
the action of the air on it.

ticularly when operating with concentrated iron solutions. In other cases,
however, a small excess will be found. In order to render the method perfectly
reliable hence i consider it absolutely indispensable to test for an excess of
Stannous chloride in the manner stated, and to determine it.
* These are given in the Zeitschr.f. analyt. Chem., i, 26.



[ 113.

Formerly * the air entering the vessel containing the solution
was made to pass through tubes filled with phosphorus and potas-
sium pyrogallate in order to deprive it of oxygen ; the apparatus
shown in Fig. 86 is, however, now preferred, and especially in
large laboratories where relatively much of the solution is fre-
quently used.

Fig. 86.

a is a vessel containing the stannous- chloride solution ; f is
a siphon, which is filled by blowing air into , after which the
compression -cock g is closed, and the constant carbonic-acid ap-
paratus c connected with the tube b. The stopper of a is then
loosened and the air in the vessel replaced by carbonic acid, when
the stopper is again tightly inserted. On withdrawing some of
the solution by opening <?, an equal volume of it is replaced by
carbonic acid which passes from c to a, the evolution of gas ceas-
ing as soon as g is closed again and the acid is forced out of the
flask d, which contains marble and is provided with a small open-
ing at the bottom ; the flask is held down by a plaster plate, h.

* Zeitechr.f. analyt. Chem., n, 58.

113.] FERRIC IRON. 331

/?. Reduction "by Potassium Iodide, and the determination
of the liberated Iodine ~by Sodium Thiosulphate. *

The principle of the method is as follows : When an excess of
potassium iodide acts upon ferric chloride at a moderate heat,
there are formed ferrous chloride, potassium chloride, and free
iodine, the last being dissolved by the excess of potassium iodide,
and remaining in solution; (Fe 2 Cl 6 + 2KI = 2FeCl 2 + 2K01
-f- 21). On determining the free iodine by means of a standard
solution of sodium thiosulphate [2Na 2 S 2 O 3 -f- 21 = 2NaI +
NaJS 4 O e (sodium tetrathionate)], the quantity of iron present may
be calculated, since 1 eq. of iodine, 126-85, liberated corresponds
to 1 eq. of iron, 55*9, present.

The method requires: 1, a solution of sodium thiosulphate
(about 12 grm. of the crystallized salt in the litre) ; 2, potassium
iodide, which must be free from iodate ( 65, 5) ; 3, a ferric-chlo-
ride solution of known iron strength, and free from ferrous
chloride and free chlorine, see 113, &, <*, the solution there
described, containing O'l grm. iron in 10 c. c., being well adapted
for the purpose ; 4, several flasks with well-fitting ground-glass
stoppers and of 100 to 150 c. c. capacity; 5, thin, freshly pre-
pared starch paste.

The sodium- thiosulphate solution is first standardized as fol-
lows : 10 c. c. of ferric-chloride solution are introduced into each
of two flasks, and diluted caustic-soda solution added to each until
a few flocks of ferric hydroxide begin to precipitate, when suffi-

* This method passed through many phases before it reached the state of
development in which it is at present used. After DUFLOS, and later on,
STRKNG, had employed the method of estimating iron by reducing ferric chlo-
ride with hydriodic acid, C. MOHR in 1858 ( Annal. d. Chem. u. Pharm:, cv,
53) studied the influence of dilution in this reaction. In 1860 FR. MOHR
(Annal. d. Chem. u. Pharm., cxin, 257; described a method in which the po-
tassium iodide was not used in excess, but played the part more of an indica-
tor. The method did not, however, answer the strict requirements demanded
of a good process. In the same year C. D. BRAUN (Journ. /. prakt. Chem.,
LXXXI, 423) employed the method in estimating iron in ferric chloride obtained
by oxidizing ferrous chloride with nitric acid, but he improved the process by
employing an excess of potassium iodide, facilitating the reaction by warming,
and estimating the liberated iodine on cooling. In 1863 FR. MOHR (Zeitschr. /.
analyt. Chem., u, 243) described the method, adopting BRAUN'S improvements,
but standardizing the thiosulphate solution with iodine liberated by potassium
bichromate. In 1864 BRAUN (Zeitschr. analyt. Chem., in, 452) again described
his method in detail.


cient hydrochloric acid (about 0'5 to 1 c. c.) of 1 -1 specific gravity
is added to each to render the solutions again clear. By this
method undue acidity of the liquids is avoided, and the fluids will
no longer be brownish- red, but will have a dark-yellow color.
Three grm. of potassium iodide are now introduced into each of
the flasks, the stoppers tightly inserted and tied down with
moistened parchment paper or with wire or cord, and the flasks
warmed to 50 or 60( best accomplished by suspending the flasks
in the ascending steam of a water-bath). The reduction of the
ferric chloride is completed in from 15 to 20 minutes, when the
solutions will have a brownish-red color. After allowing to cool,
run in from the burette the sodiurn-thiosulphate solution until
the liquid has a wine-yellow color, then add from 0*5 to 1 c. c. of
thin starch paste, and continue again to add the thiosulphate solu-
tion until the blue color of the starch iodide just disappears. The
volume used up corresponds to the iodine liberated by O'l grin,
iron, and hence also to 0-1 grm. iron present as ferric chloride.

The Estimation of the Iron in a solution of unknown strength
is accomplished in the same manner as in standardizing the
sodium -thiosulphate solution. Care must be taken that all the
iron must be present as ferric oxide or chloride, and that the
liquid contains no other substance that will decompose potassium
iodide, e.g., free chlorine or nitric acid. It is also advisable to
employ solutions containing as nearly as possible about O'l grm.
iron, and that not too little nor too much thiosulphate be used.
The free acid must also be reduced in quantity, as detailed above.

If it was found that 18 -4 c. c. of the thiosulphate solution corre-
sponded to 0*1 grm. iron, and if there had been required 24*5 c. c.
of the thiosulphate solution to -combine with the iodine liberated
by the unknown quantity of iron present, this last would then
amount to 0-13315 grm., since 18'4:0'1 : : 24-5:0-13315.

The method gives good results, and is much to be recommended
for determining small quantities of iron.

y. Reduction ly Sodium Thiosulphate in the presence of a
Cupric Salt, after OUDEMANS.*

If an acid solution of ferric chloride is mixed with a little cupric

* Sodium thiosulphate was first employed by SCHERER (Qel. Am. der K.
Bayerischen Akademie, vom 31 Aug. 1859), afterwards by KREMKR and LAN-
DOLT (Zcitschr.f. analyt. Chem., i, 214). The method of OUDEMANS is to be found

113.] FERRIC IRON. 333

sulphate and some potassium sulphocyanate and then sodium
thiosulphate is added, the red color of the ferric sulphocyanate
gets paler and paler, and finally when the ferric salts are reduced
to ferrous, disappears altogether. "Warming is unnecessary. To
hit the point is not easy, so we add a slight excess of sodium thio-
sulphate and then titrate back with standard iodine. The reaction
is as follows: Fe.Cl. + 2Na a S 2 O 3 = 2FeCl 2 -f 2NaCl -f NaJS 4 O 6 ;
it is promoted by the addition of a small quantity of cupric sul-
phate, which is alternately reduced by the thiosulphate and oxi-
dized by the ferric chloride. If a small quantity of cuprous salt
is produced by the excess of thiosulphate this does not matter, as
its action on the iodine solution is the same in extent as the action
of the thiosulphate which produced it. The method is not
accurate unless the fluid remains clear; neither cuprous sul-
phocyanate nor cuprous iodide nor sulphur must be thrown
down. Hence care must be taken to maintain the proper amounts
of the reagents and to dilute the fluid sufficiently.

This metho^d is much like that detailed under /?, in so far as it
is most convenient to standardize the thiosulphate against a ferric-
chloride solution of known strength, and then to use it on solutions
of unknown strength to determine their iron content.

"We require 1. A solution of sodium thiosulphate containing
about 12 grm. (of the crystallized salt) per litre. 2. A solution of
ferric chloride of known strength, prepared by dissolving 10*04
grm. of clean, fine, and soft iron wire (=10 grm. pure iron) in
hydrochloric acid in a slanting long-necked flask, oxidizing the
solution with potassium chlorate, completely removing the excess
of chlorine by protracted gentle boiling, and finally diluting
the solution to 1 litre. 3. A solution of cupric sulphate, 1
in 100. 4. A solution of potassium sulphocyanate, 1 in 100.
5. A solution of iodine in potassium iodide, containing 5 or
6 grm. iodine in the litre (compare 146, 3). 6. Thin starch

Measure off some of the sodium thiosulphate, add starch paste
( 146, 3), and then titrate with iodine solution, in order to de-

in Zeitschr. f. nnalyt. Chem., vi, 129; it was criticised and rejected in MOHR'S
Lehru. d. Titrirmethocte, 3 Aufi. 291. OUDEMANS replied toMoim in Zeitschr. f.
analyt. Chem., ix, 342. and an examination of the method by C. BALLING ap-
peared in the same journal, ix, 99.

334 DETERMINATION-. [ 113.

terraine the relation between the two solutions. Now transfer 10
or 20 c. c. of the ferric chloride to a beaker, add 2 c. c. concen-
trated hydrochloric acid, 100 or 150 c. c. water, 3 c. c. copper
solution, and 1 c. c. potassium- sulphocyanate solution, titrate with
sodium thiosulphate till the fluid just loses its color, add at once
some starch paste, and titrate back with iodine solution till the
blue color appears. Deduct the thiosulphate equivalent to the
iodine solution from the total quantity of thiosulphate used ; the re-
mainder will represent the amount required to reduce the iron
present. In the analysis the conditions should be similar to those
in the standardizing of the thiosulphate.

This method is very rapid, and the results, though not so
accurate as those by methods a arid /?, are quite good enough for
many technical purposes.

Supplement to 112 and 113.

Besides the methods given in 112 and 113, there have
been many others, particularly indirect ones, advocated. Since
these, however, possess no advantages over those described above,
or are capable of only limited application, I will confine myself to
a description of only the most important.

1. FUCK'S method *: Add hydrochloric acid to the solution,
which must contain the iron as a ferric salt, and be free from
nitric acid, and boil in contact with a few strips of metallic copper
until the solution acquires a light-green color. Then estimate the
iron from the loss in weight of the copper (Fe a Cl, -f- Cu = 2FeCl,
+ 2CuCl). The method yields good results only when the most
careful attention is paid in excluding the air. The conditions most
favorable to success have been studied by J. LOWE and KO'NIG,
and are detailed, under the " Analysis of Iron Ores," in the
Special Part.

2. The solution containing the iron as a ferric salt, and free
from the metals of the fifth and sixth groups, as well as other sub-
stances decomposable by hydrogen sulphide, is precipitated by an
excess of a clear solution of hydrogen sulphide, avoiding all heat.
The precipitated sulphur is determined after a few days, and the
quantity of iron calculated therefrom according to the equation

* Jour. /. prakt. Chem., xvn, 160.


Fe 3 O, + H a S = 2FeO + H 3 O + S (H. EOSE). Eesulis accurate.
Compare* also DELFFS.*

3. Add an excess of gold and sodium chloride to tlie solution
containing the iron as a ferrous salt, close the bottle, and deter-
mine the precipitated gold : 6FeCl 2 + 2 AuCl, = 3Fe a Cl. + 2 Au

Supplement to the Fourth Group.


If the compound in which the uranium is to be determined
contains no other fixed substances, it may often be converted into
uranous uranate U(UO 4 ) 2 (called also nranoso-uranic oxide UO 2 -
2UO 8 ) by simple ignition. If sulphuric acid is present, small por-
tions of ammonium carbonate must be thrown into the crucible
towards the end of the operation.

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