C. Remigius Fresenius.

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of freshly precipitated lead carbonate must be first added and
the solution filtered before proceeding to the determination.

c. Fordos and Gelis^s Volumetric Method, f This method
is founded on the reaction of free iodine on potassium cyanide,
described by SERULLAS and WOHLER, and which is as follows:
KCN + 21 = KI + ICN. According to this, 2 eq. of iodine
correspond to 1 eq. of cyanogen or 1 eq. hydrocyanic acid, or
1 eq. of potassium cyanide. The iodine solution is best pre-
pared according to 146. If free hydrocyanic acid is to be deter-
mined, add first some soda solution cautiously until an alkaline
reaction, then add carbonic-acid water to convert any possible
excess of alkali into carbonate (the fluid must not render curcuma
paper brown), and finally sufficient iodine solution to just perma-

* Zeitschr.f. analyt. Chtm. t 11, 180.

\Journ. de Ghim. et de Pharm., xxiu, 48; Journ. f. prakt. Chem., LIX, 255.

147.] CYANOGEN. 551

nently tinge the colorless solution yellowish. "When analyzing
potassium cyanide prepare a solution first of known strength and
use a volume containing about 0*05 grm. of potassium cyanide.
In this case, too, the addition of carbonic-acid water is necessary.
The potassium cyanide must contain no potassium sulphide, as
this vitiates the results. The method on the whole gives good
results. Compare SOTJCHAY (Loo. cit.)\ it is not applicable for
bitter- almond water, however.

II. Separation of Cyanogen from the Metals.

a. In Cyanides of the Alkali Metals.

Mix the substance (if solid, without previous solution in water)
with excess of silver nitrate solution, then add water, finally nitric
acid in slight excess, allow to settle without warming, and deter-
mine the silver cyanide as w in L, a. The basic .metals are deter-
mined in the filtrate after separating the excess of silver.

J. In Cyanides and double Cyanides, which are completely
decomposed by Silver Nitrate and Nitric Acid or Silver Nitrate
and Ammonia.

Digest for some time with a dilute sohition* of silver nitrate,
stirring frequently,* then add nitric acid in moderate excess, and
digest at a gentle heat, till the foreign cyanide is fully dissolved
and the silver cyanide has become pure and quite white. Then
add water and filter. As a precautionary measure it is well to test
the metal obtained by long ignition of the silver cyanide, whether
it is free from those metals which were combined with the cyano-
gen. The filtrate is used for estimating the basic metals, the silver
being first precipitated with hydrochloric acid. This method affords
us an exact analysis of the double cyanides of potassium with
nickel, copper, and zinc (H. ROSE).

"W. WEITH-J- recommends a solution of silver nitrate in ammo-
nia for the decomposition of many cyanogen compounds, such as
potassium ferrocyanide, Prussian blue, and even potassium cobalti-
cyanide. He digests them in sealed tubes at 100 (in the case of
potassium cobalticyanide, 150) for 4 or 5 hours. Warm the con-
tents of the tube gently in a dish, until the crystals of ammonio-
cyanide of silver are dissolved, filter off the separated metallic

* Double cyanide of nickel and potassium yields by this process a mixture of
silver cyanide with nickel cyanide. Like double cyanides are similarly decom-
posed, f Zeitschr. /. analyt. Chem., ix, 379,

552 DETEfcMIKATIOK. [ 147.

oxide, wash it with ammonia, dilute, and precipitate the silver
cyanide by acidifying with nitric acid. In the filtrate separate the
silver from the alkalies, &c. In respect to the undissolved oxides
it should be noted that metallic silver is always mixed with the
ferric oxide.

c. In Mercuric Cyanide.

Precipitate the aqueous solution with hydrogen sulphide ; the
mercuric sulphide may be filtered without difficulty if a little
ammonia or hydrochloric acid be added ; it is determined accord-
ing to 118, 3. If the compound is in the solid condition, the
cyanogen may be determined in another portion by ignition with
cupric oxide, the nitrogen and carbonic acid being collected and
separated (comp. Organic Analysis).

H. HOSE and FINKENER* have, after much trouble, succeeded
in finding out a method for determining cyanogen with precision
also in solutions of mercuric cyanide. Mix the solution of the mer-
curic cyanide with zinc nitrate dissolved in ammonia. To 1 part
of mercuric salt you may add about 2 parts of the zinc salt. Add
to the clear solution hydrogen sulphide water gradually till it pro-
duces a perfectly white* precipitate of zinc sulphide. The precipi-
tate, which is a mixture of the mercuric and zinc sulphides, settles
well. After a quarter of an hour filter it off and wash with very
dilute ammonia. The filtrate contains zinc cyanide dissolved in
ammonia, together with ammonium nitrate. It does not smell of
hydrocyanic acid, and consequently no escape of the latter takes
place. Mix it with silver nitrate and then add dilute sulphuric acid
in excess. The silver cyanide is next washed a little by decantation,
then to free it from any zinc cyanide simultaneously precipitated.
heated with a solution of silver nitrate, finally filtered off.
washed, and determined after 147. I., a. The precipitated sul-
phides may be dissolved in aqua regia, and the mercury precipitated
as mercurous chloride according to 118, 2. The test-analyses com-
municated by ROSE yielded excellent results.

d. In compounds decomposable by Mercuric Oxide in f/n IT, /

Many simple cyanides, and also double cyan ides both of the
character of the double cyanide of nickel ami putassimn, and of
the ferro- or ferricyanides (not, however, cobalticyanides) may, as

* ZeitscJir. f. analyt. Chem., i, 288-

147] CYANOGEN. 553

is well known, be completely decomposed by boiling with excess
of mercuric oxide and water, all cyanogen being obtained as mer-
curic cyanide and the metals passing into oxides.

H. ROSE (loc. cit.) has shown that Prussian blue, potassium
ferro- and ferricyanide, more particularly, may be readily analyzed
in this manner.

Boil a few minutes with water and excess of mercuric oxide till
complete decomposition is effected, add in order to render the
ferric hydroxide and mercuric oxide removable by filtration nitric
acid in small portions, till the alkaline reaction has nearly disap-
peared, filter, wash with hot water, dry the precipitate, ignite
very gradually raising the heat under a hood (with a good
draught), and weigh the ferric oxide remaining. In the filtrate
the cyanogen is determined according to c, and any potassium that
may be present is determined in the filtrate from the silver cya-

e. Determination of Metals contained in Cyanides with decom-
position and volatilization of the 'Cyanogen.

Of the various means for completely decomposing compounds
of cyanogen, especially also the double cyanides, according to II.
ROSE (loc. cit.) three particularly are worthy of recommendation
viz., concentrated sulphuric acid, mercuric sulphate, and ammo-
nium chloride. The nitrates seemed decidedly less suitable on
account of their too violent action.

pounds, simple or double, are completely decomposed and con-
verted into sulphates or oxides, as the case may be, if treated in a
powdered condition in a platinum dish or a capacious platinum
crucible with a mixture of about 3 parts concentrated sulphu-
ric acid and 1 part water, and heated till almost all the sulphuric
acid had been expelled. The residual mass is then free from cyan-
ogen. It is dissolved in water, if necessary with addition of
hydrochloric acid, and the metals determined by the usual methods.
This way is not adapted for mercuric cyanide, as a little of the
metal would escape with the fumes of the sulphuric acid.

sulphates, those suitable to our present purpose are the normal and
the basic (Turpeth mineral). The substance is mixed with 6 parts
of the latter, heated in a platinum crucible gradually, and finally
maintained for a long time at a red-heat, till all the mercury has


volatilized, and the weight of the crucible remains constant. If
alkalies are present, a little ammonium carbonate is added from
time to time during the final ignition, in order to convert the acid
sulphates into normal. The residue may usually be analyzed by
simple treatment with water; in the case of potassium ferro-
cyanide, for instance, the potassium sulphate dissolves and pure
(alkali-free) ferric oxide remains behind. The test-analyses that
have been communicated show excellent results.

stance with twice or thrice the amount of this salt and ignite the
mixture moderately in a stream of hydrogen (apparatus, Fig. 83).
From the cooled mass water extracts alkali chloride, while the
reducible metals remain in the metallic state. The method is
peculiarly adapted for the analysis of double cyanide of nickel and
potassium and cobalticyanide of potassium, not so for iron com-
pounds, since the iron obtained is not pure, but contains carbon.

If one of the methods described in e is employed, the nitrogen
and carbon (the cyanogen) must be determined by a combustion,
if an estimation by the loss is not sufficient.

f. Determination of the Alkalies, especially of Ammonia, in
Soluble Ferrocyanides.

Mix the boiling solution with a solution of cupric chloride in
moderate excess, filter off the precipitated cupric ferrocyanide,
free the filtrate from copper by means of hydrogen sulphide, and
then determine the alkalies (KEINDEL*). In the case of fixed
alkalies the object may also be obtained by igniting with barium
thiosulphate (FKOHDE f).

g. Volumetric Determination of Ferro- and Ferricyanogen.

a. After E. DE HAEN. This method, devised in my labora-
tory, is founded upon the simple fact that a solution of potassium
ferrocyanide acidified with sulphuric acid (and which may accord-
ingly be assumed to contain free hydroferrocyanic acid) is by ad-
dition of potassium permanganate converted into the correspond-
ing ferricyanide. If this conversion is effected in a very dilute
fluid, containing about 0-2 grin, potassium ferrocyanide in from

*Journ.f. prakt. Chem., LXV, 452.
\Zeitschr. /. analyt. CJiem., Hi, 181.

147.] CYANOGEN. 555

100 to 200 c. c., the termination of the reaction is clearly and
unmistakably indicated by the change of the originally pure yel-
low color of the fluid to reddish-yellow.*

The process requires two test-fluids of known strength, viz. :

1. A solution of pure potassium ferrocyanide.

2. A solution of potassium permanganate.

The former is prepared by dissolving 20 grin, perfectly pure
and dry crystallized potassium ferrocyanide in water to 1 litre;
each c.c. therefore contains 20 mgrm. The latter is diluted so that
somewhat less than a buretteful is required for 10 c.c. of the solu-
tion of potassium ferrocyanide.

To determine the strength of the potassium permanganate solu-
tion in its action upon the potassium ferrocyanide, measure oif, by
means of a pipette, 10 c.c. of the solution of potassium ferrocyanide
(containing 0-2 grm.), dilute with 100 to 200 c.c. water, acidify with
sulphuric acid, place the glass on a sheet of white paper, and allow
the permanganate to drop into the fluid, stirring it at the same
time, until the change from yellow to reddish-yellow indicates that
the conversion is complete.f Repetitions of the experiment always
give very accurately corresponding results. If at any time you
have reason to suspect that the permanganate has suffered altera-
tion, recourse must be had again to this experiment. If after
acidifying the potassium ferrocyanide with sulphuric acid you add
a trace of ferric chloride to produce a bluish-green color, the latter
will disappear at the end of the reaction, which is thus rendered
Very distinct (GiNTL^:).

To determine the amount of real potassium ferrocyanide con-
tained in any given sample of the commercial article, dissolve 5
grm. to 250 c.c. ; take 10 c.c. of this solution, and examine as just
directed. Suppose, in determining the strength of the permanga-
nate, you have used 20 c.c., and you flnd now that 19 c.c. is suffi-
cient, the simple rule-of -three sum,

20 : -2 : : 19 : x

* Instead of the permanganate you may use potassium chromate. The solu-
tion is added till spots of iron sesquichloride on a plate are no longer colored
blue or green, but brownish. E. MEYER, Zeitschr.f. analyt. Chem., vm, 508.

f If you wish at first for some additional evidence besides the change of color,
add to a drop of the mixture on a plate, a drop of solution of ferric chloride;
if this fails to produce a blue tint, the conversion is accomplished.

I Zeitschr.f. analyt. Chem., vi, 446.


wili inform you how much pure potassium ferrocyanide 0-2 grm. of
the analyzed salt contains. And even this small calculation may
"be dispensed with by diluting the permanganate so that exactly 50
c. c. correspond to 0'2 of potassium ferrocyanide, as, in that case,
the number of half-c.c. consumed expresses directly the percentage
of pure ferrocyanide.

Instead of determining the strength of the permanganate by
means of pure potassium ferrocyanide, which is unquestionably
the best way, one of the methods given in 112, 2, may also-be
employed ; bearing in mind, in that case, that 2 inol. potassium
ferrocyanide = 845*256, 2 at. iron = 111-8, and 1 mol. oxalic
acid = 126 '048 are equivalent in their action upon solution of
potassium permanganate.

The analysis of soluble ferricyanides by this method is effected
by reducing them to ferrocyanides, acidifying, and then proceeding
in the way described. The reduction is effected as follows : ^lix
the weighed ferricyanide with a solution of soda or potassa in
excess, boil and add concentrated solution of ferrous sulphate
gradually, and in small portions, until the color of the precipitate
appears black, which is a sign that protosesquioxide of iron has
precipitated. Dilute now to 300 c.c., mix, filter, and proceed to
determine the ferrocyanide in portions of 50 or 100 c.c. of the
fluid. As the space occupied by the precipitate is not taken into
account in this process, the results are not absolutely accurate ; the
difference is so very trifling, however, that it may safely be disre-
garded. GINTL (Loc. cit.) suggests to put the neutral or alkaline
fluid in a tall vessel and add a few lumps of sodium amalgam as
big as peas : in ten minutes the reduction will be effected and with-
out the aid of heat.

Insoluble ferro- or ferricyanides, decomposable by boiling solu-
tion of potassa (as are most of these compounds), are analyzed by
boiling a weighed sample sufficiently long with an excess of solu-
tion of potassa (adding, in the case of ferricyanides, ferrous sul-
phate), and then proceeding as directed above.

ft. After E. LENSSEN. Ferricyanides may also be analyzed
according to the following method, which too was devised in my
laboratory: The method is based on the fact that on bringing
together potassium ferricyanide, potassium iodide, and concen-

147.] CYANOGEN. 557

trated hydrochloric acid, for every eq. of potassium ferricyanide
1 eq. of iodine are precipitated, thus:

K 3 Fe(CN) + KI = K 4 Fe((m). + I.

On estimating the liberated iodine according to 146, the
quantity of potassium ferricyanide is then determined. In four
experiments LENSSEN obtained 99*22, 101'T, 102-1, and 100*5,
instead of 100. The solution may be diluted only after the hy-
drochloric acid has been added. C. MOHR * obtained still more
accurate results, as he avoided the formation of hydroferricyanic
acid by adding zinc-sulphate solution, which is not at all decomposed
by iodine. . He directs adding potassium iodide and hydrochloric
acid in excess to the diluted ferricyanide solution, then to add an
excess of iron-free zinc-sulphate solution, neutralize the free acid
with sodium bicarbonate in slight excess, and to then determine
the liberated iodine according to 146.

y. To determine potassium ferrocyanide in dyers' baths,
which contain oxidizable organic substances, and which, hence,
cannot be estimated with permanganate, H. RHEINECK f recom-
mends a process based on the fact that potassium -ferrocyanide
solution, on gradually adding a solution of a ferric salt, whether
a mineral acid is added or not, yields a clear, blue solution which
becomes turbid and which, when all the ferrocyanogen is thrown
down, forms a clear, colorless liquid containing Berlin blue sus-
pended in flocculent form. Heiice on adding a solution of a fer-
ric salt to equal volumes of a potassium-ferrocyanide solution of
known strength and of the bath, until in both cases the flocculent
precipitate forms, the unknown quantity of ferrocyanide may be
readily calculated.

d. After E. BOHLIG.J

In the case of a fluid containing potassium ferrocyanide, and
also sulphocyanide (for instance, the red liquor of the prussiate
works), the method given in a cannot be employed, as the hydro-
sulphocyanic acid also reduces permanganic acid. The following
method depending on the precipitation of the ferrocyanogen with
solution of cupric sulphate may then be used; it is accurate
enough for technical purposes: Dissolve 10 grin, pure cupric sul-

* Annal. d. Chem. u. Pharm., cv, 62. f Chem. Centralbl, 1871, p. 778.
%Polytechn. Notizblatt, xvi, 81.


phate to 1 litre, also 4 grm. pure dry potassium ferrocyanide to 1
litre. Add to 50 c.c. of the latter solution (which contain 0'2 grm.
potassium ferrocyanide) copper solution from a burette to complete
precipitation of the ferrocyanogen. In order to hit this point
exactly, from time to time dip a strip of filter-paper into the
brownish-red fluid which will imbibe the clear filtrate, leaving the
precipitate of copper ferrocyanide behind. At first the moist strips
of paper, when touched with ferric chloride, become dark blue, the
reaction gradually gets weaker and weaker, and finally vanishes
altogether. We now know the value of the copper solution with
reference to its action on potassium ferrocyanide, and can, there-
fore, by its means test solutions containing unknown amounts of
ferrocyanogen. If alkali sulphides are present, they are first
removed by boiling with lead carbonate. After filtering off the
lead sulphide, acidify with dilute sulphuric acid, and then proceed.


I. Determination.

To determine hydrogen sulphide in a mixture of gases confined
over mercury* it may be absorbed by a ball made of 2 parts precipi-
tated lead phosphate and 3 parts plaster of Paris. The mixture is
made into a paste with water, and pressed into a bullet mould in
which the platinum wire is inserted. The mould should previously
be oiled. The balls are dried at 100, saturated with concentrated
phosphoric acid, ami are then ready for use (LuDwiof).

To determine sulphuretted hydrogen dissolved in water the
following methods are in use :

a. The method of determining hydrogen sulphide volumetri-
cally by solution of iodine, was employed first by DUPASQUIER ; it
is very convenient and accurate. That chemist used alcoholic solu-
tion of iodine. But as the action of the iodine upon the alcohol
alters the composition of this solution somewhat rapidly, it is bet-
ter to use a solution of iodine in potassium iodide. The decom-
position is as follows :

U 3 S + 21 == 2III + S

* When this gas remains long in contact with mercury, sulphide of mercury
is liable to be formed, \ Annul. <l. Chcm. u. Pharm., CLXJI, 55.

148.] SULPHUR. 559

2 at. I 3= 253*70 correspond, hence, to 1 mol. H a S 34 '086.
However, this exact decomposition can be relied upon with cer-
tainty only if the amount of hydrogen sulphide in the fluid does
not exceed 0'04 per cent.(BuNSEN). Fluids containing a larger pro-
portion of hydrogen sulphide must therefore first be diluted to the
required degree with boiled water cooled out of the contact of air.

The iodine solution of 146 may be used for the estimation of.
larger quantities of hydrogen sulphide ; for weak solutions, e.g.,
sulphuretted mineral water, it is advisable to dilute the iodine solu-
tion 5 times, so that 1 c.c. may contain O'OOl grin, iodine.

The process is conducted as follows :

Measure or weigh a certain quantity of the sulphuretted water,
dilute, if required, in the manner directed, add some thin starch-
paste, and then solution of iodine, with constant shaking or stir-
ring, until the permanent blue color begins to appear. The result
of this experiment indicates approximately, but not with positive
accuracy, the relation between the examined water and the iodine
solution. Suppose you have consumed, to 220 c.c. of the sulphu-
retted water, 12 c.c. of a solution of iodine containing 0-000918
grm. iodine in the c.c.* Introduce now into a flask nearly the
quantity of iodine solution required, add the sulphuretted water
in quantity either already determined, or to be determined, by
weight or measure ;f then to the colorless fluid add thin starch-
paste, and after this iodine solution until the blue color just begins
to show. By this course of proceeding, you avoid the loss of
hydrogen sulphide which would otherwise be caused by evaporation
and oxidation. In my analysis of the Weilbach water, 256 c.c. of
the water required, in my second experiment, 16-26 c.c. of iodine
solution, which, calculated to the quantity of sulphuretted water
used in the first experiment, viz., 220 c.c., makes 13*9 c.c., or 1*9
e.c. more. But even now the experiment cannot yet be considered
quite conclusive, when made with a solution of iodine so dilute ; it
being still necessary to ascertain how much iodine solution is required
to impart the same blue tint to the same quantity of ordinary water
mixed with starch-paste, of the same temperature,^: and as nearly
as possible in the same condition as the analyzed sulphuretted

* The numbers here stated are those which I obtained in the analysis of the
Weilbach water. f Compare Experiment No. 82.

\ Annul, d. CJiem. u. Pharm., en, 186.
In this connection I would recommend, in cases where the sulphuretted


water, and to deduct this from the quantity of iodine solution
used in the second experiment. Thus, in the case mentioned, I
had to deduct 0'5 c. c. from the 16-26 c. c. used. If the instruc-
tions here given are strictly followed, this method gives very
accurate results. (See Expt. No. 82.)

I. FR. MOHR'S method slightly modified.

Add to the sulphuretted water a slight excess of sodium-arsen-
ite solution standardized against iodine solution ( 127, 5, #),
then add hydrochloric acid until the liquid is distinctly acid.
Dilute to 300 c. c., pass through a dry filter-paper, make sure
that the solution still contains sodium arsenite by testing a sample
with hydrogen-sulphide water, and then determine in 100 c. c. ,
after adding powdered sodium bicarbonate, the remainder of the
arsenous acid. Deduct the c. c. of iodine solution last used, mul-
tiplied by 3 (because only 100 c. c. of the 300 c. c. have been
operated upon), from that corresponding to the entire quantity of
arsenous acid used ; the remainder will express the quantity of
iodine solution equivalent to the hydrogen sulphide present. In
calculating it must be remembered that here 2 eq. of iodine cor-
respond to 3 eq. of hydrogen sulphide, since 1 eq. of As 2 O,
decomposes 3 eq. of H 3 S on the one hand, forming As a S, and
3H a O, and requires on the other hand 2 eq. of iodine for its con-
version into arsenic acid.

Yery dilute hydrogen-sulphide solutions cannot be estimated
by this method, as the arsenic sulphide formed in them takes a
long time to deposit, and a very small portion always remains in

c. Mix the sulphuretted fluid with an excess of solution of
sodium arsenite, add hydrochloric acid, allow to deposit, and deter-
mine the arsenous sulphide as directed in 127, 4. The results are
accurate, unless the solution is very dilute, in which case the slight
solubility of arsenous sulphide occasions loss. (See Expt. No. 82.)

In an analysis of the Weilbach waters, this method hence
gave 0-006621 and 0-006604 per 1000, whereas water taken

water contains bicarbonate of soda, to add to the ordinary water an equal quan-
tity of this salt, as its presence has a slight influence on the appearance of the
final reaction.

* Hydrogen-sulphide water containing 0'003 grm. TT 9 S in a litre gave with a
solution of arsenous acid in hydrochloric acid a precipitate that could be filtered

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