C. Remigius Fresenius.

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In cases where the application of this method is inadmissible,
the solution of uranium (which, if it contains uranous salts, must
first be warmed with nitric acid, until they are converted into uranyl
salts) is nearly boiled in a platinum or porcelain dish, and pre-
cipitated with ammonia in slight excess. The yellow precipitate
formed, which consists of hyd rated ammonium uranate, is filtered
off hot and washed with a dilute solution of ammonium chloride, to
prevent the fluid passing milky through the filter. The precipitate
is dried and ignited ( 53). To make quite sure of obtaining the
uranous uranate in the pure state, the crucible is ignited for some
time in a slanting position and uncovered ; the lid is then put on,
while the ignition is still continuing ; the crucible is allowed to
cool under the desiccator, and weighed (EAMMELSBEEG).

If the solution from which the uranyl is to be precipitated con-
tains other basic radicals (alkali-earth metals, or even alkali metals),
portions of these will precipitate along with the ammonium uranate.
For the measures to be resorted to in such cases, I refer to Sec-
tion Y.

The reduction of the uranous uranate to the state of uranous

*Chem. CentraM., 1856, 839.



336 DETERMINATION.

oxide (UO,) is an excellent means of ascertaining its purity for the
pin-pose of control. This reduction should never be omitted, since
PELIGOT has found the uranous uranate to be variable in composi-
tion. It is effected by ignition in a current of hydrogen gas, in the
way described 111, 1 (Cobalt). In the case of large quantities
the ignition must be several times repeated, and the residue must
be occasionally stirred with a platinum wire. While cooling
increase the current of gas to prevent reabsorption of oxygen. By
intense heating the property of spontaneous ignition in the air is
destroyed. If after evaporating a solution of uranyl chloride, the
residue is to be ignited in hydrogen, heat gently at first in the gas
to avoid loss by volatilization. The separation of uranyl from
phosphoric acid is effected by fusing the compound with potassium
cyanide and sodium carbonate. Upon extracting the fused mass
with water, the phosphoric acid is obtained in solution, whilst ura-
nium is left as uranous oxide. KNOP and ARENDT* have employed
this method.

Taking 239 -6 as the atomic weight of uranium, uranous uran-
ate, U(UO 4 ) 2 , contains 84-88 percent, of uranium and 15*12 per
cent, of oxygen. UO a , uranous oxide, contains 88'22 per cent,
uranium and 11*78 per cent, of oxygen.

According to BELOHOUBECK^ uranium may be also determined
volnmetrically by reducing the solution of nranyl acetate or sul-
phate to uranous salts with zinc, as in the case of iron ( 113, 3, a).
As the color of the solution is no safe criterion of the end of the
reduction, you must allow the action of the zinc to continue for a
considerable time. BELOIIOITBECK says, a quarter of an hour is
bufficient for small quantities, half an hour for large quantities.
The solution of the uranous salt is diluted, mixed with dilute sul-
phuric acid, and then titrated with permanganate to incipient red-
dening. The permanganate is standardized by 112, 2; 1 at.
uranium = 2 at. iron.

BKLQHQUBEOK obtained good results also in hydrochloric solu-
tions, but experiments made in this laboratory have shown that
tin-so are liable to the error pointed out in the case of iron (Conip.
{). 319, ;/), at least in the presence of considerable quantities of
hydrochloric acid.

* Chem. Centralblatt, 1856, 773. f Zeitechr.f. analyl. Cliem., vi, 120.



115.] SILVER. 337

Fifth, Group.

SILVER LEAD MERCURY IN MERCUROUS COMPOUNDS MERCURY IN

MERCURIC COMPOUNDS COPPER BISMUTH CADMIUM (PALLA-
DIUM).

115.

1. SILVER.

a. /Solution.

Metallic silver, and those of its compounds which are insoluble
in water, are best dissolved in nitric acid (if soluble in that acid).
Dilute nitric acid suffices for most compounds ; silver sulphide,
however, requires concentrated acid. The solution is effected best
in a flask, which should be heated if necessary, and placed in a
slanting position if gas is evolved. In the case of metallic silver,
or silver sulphide, the solution is heated finally to gentle boiling
to drive off nitrous acid. Silver chloride, bromide, and iodide are
insoluble in water and in nitric acid. To get the silver contained
in chloride and bromide in solution, proceed as follows: Fuse the
salt in a porcelain crucible (this operation, though not absolutely
indispensable, had better not be omitted), pour water over it, put
a piece of clean cadmium, zinc, or iron upon it, and add some
dilute sulphuric acid. Wash the reduced spongy silver, first with
dilute sulphuric acid, then with water, and finally dissolve it in
nitric acid. However, as we shall see below, the quantitative
analysis of these salts does not necessarily involve their solution.

1). Determination.

Silver may be weighed as chloride, sulphide, or cyanide, or in
the metallic state ( 82). It is also frequently determined by volu-
metric analysis.

We may convert into

1. SILVER CHLORIDE : All compounds of silver without excep-
tion.

2. SILVER SULPHIDE : 3. SILVER CYANIDE : All compounds so'u.
ble in water or nitric acid.

4. METALLIC SILVER : Silver oxide and some silver salts of readily
volatile acids; silver salts of organic acids; silver chloride, bro-
mide, iodide, sulphide, and sulphate.

The method 4 is the most convenient, especially when con-
ducted in the dry way, and is preferred to the others in all cases



338 DETERMINATION. [ 115.

where its application is admissible. The method 1 is that most
generally resorted to. 2 and 3 serve mostly only to effect the
separation of silver from other metals.

In assays for the Mint, silver is usually determined volumetric-
ally by GAY-LUSSAC'S method. PISANI'S volumetric method is
especially suited to the determination of very small quantities of
silver. H. YOGEL'S method is specially useful to photographers.
The estimation of silver by cupellation will be detailed under
"Analysis of Galena," in the Special Part.

1. Determination of Silver as Chloride.

a. In the Wet Way.

Mix the moderately dilute solution in a beaker with nitric acid,
heat? to about 70, and add hydrochloric acid with constant stirring
till it ceases to produce a precipitate. A large excess of hydro-
chloric acid must be avoided, as the precipitate is not absolutely
insoluble therein. While protecting the contents of the beaker
from the action of direct sunlight continue the heat till the precipi-
tate has fully settled, pour off the clear fluid through a small filter,
rinse the precipitate on to the latter by means of hot water mixed
with some nitric acid, wash with hot water containing nitric acid,
then with pure hot water, dry thoroughly, transfer the precipitate
to a watch-glass as nearly as possible, incinerate the filter in a
weighed porcelain crucible, treat the ash (which always contains
some metallic silver) with a few drops of nitric acid in the heat ;
add two or three drops of hydrochloric acid, evaporate cautiously
to dryness, add the main bulk of the precipitate, using a camel's-
hair brush to transfer the last portions, heat cautiously till it begins
to fuse at the edge, allow to cool, and weigh.

To remove the fused mass without breaking the crucible, lay
a small piece of iron or zinc upon it, and then add very dilute
hydrochloric or sulphuric acid. The chloride will be reduced, and
the silver may now be detached from the crucible with the greatest
ease.

For the properties of the precipitate see vJ. The method
gives very exact results, at all events in the absence of any con-
siderable quantities of those salts in which silver chloride is some-
what soluble; compare 82. To avoid error in this respect, it is
well to test the clear filtrate with hydrogen sulphide.







SILVER.



339



1). In the Dry Way.

This method serves more exclusively for the analysis of silver
bromide and iodide, although it can be applied in the case of other
compounds.




Fig. 87.

The process is conducted in the apparatus illustrated by Fig. 87.

a is a flask for disengaging chlorine ; it is completely filled with
pieces of pyrolusite (native manganese dioxide) of the size of hazel-
nuts, and half filled with strong hydrochloric acid ; J contains
concentrated sulphuric acid ; c contains calcium chloride; d is a
bulb containing the silver iodide or bromide ; e conducts the
chlorine by means of a rubber tube into the open air or into a
flask containing calcium hydroxide. The operation is commenced
by introducing the compound to be analyzed into the bulb, and
applying heat to the latter until its contents are fused ; when cold,
the tube is weighed and connected with the apparatus. Chlorine
gas is then evolved from a : when the evolution of the eras has

O O

proceeded for some time, the contents of the bulb are heated to
fusion, and kept in this state for about fifteen minutes, agitating
now and then the fused mass. The bulb-tube is then removed
from the apparatus, allowed to cool, and held in a slanting position
to replace the chlorine by atmospheric air ; it is subsequently



340 DETERMINATION. [ 115.

weighed, then again connected with the apparatus, and the former
process repeated, keeping the contents of d in a state of fusion for
a few minutes. The operation may, in ordinary cases, be con-
sidered concluded if the weight of the tube suffers no variation
by the repetition of the process. If the highest degree of accuracy
is to be attained, heat the silver chloride again to fusion, passing
at the same time a slow stream of pure, dry carbon dioxide through
the tube, in order to drive out the traces of chlorine absorbed by
the fused chloride. Allow to cool, hold obliquely for a short time,
so as to replace the carbon dioxide by air, and finally weigh.

2. Determination as Silver Sulphide.

Hydrogen sulphide precipitates silver completely from acid,
neutral, and alkaline solutions ; ammonium sulphide precipitates it
from neutral and alkaline solutions. The precipitate does not
settle clearly and rapidly except a free acid or salt be present (such
as nitric acid or an alkali nitrate). Recently prepared perfectly
clear solution of hydrogen sulphide may be employed to precipitate
small quantities of silver ; to precipitate larger quantities, the solu-
tion of the salt of silver (which must not be too acid) is moderately
diluted, and washed hydrogen sulphide gas conducted into it.
After complete precipitation has been effected, and the silver sul-
phide has perfectly subsided (with exclusion of air), it is collected
on a weighed filter, washed, dried at 100, and weighed. For the
properties of the precipitate, see 82. This method, if properly
executed, gives accurate results. The operator must take care to
filter quickly, and to prevent the access of air as much as possible
during the filtration, since, if this precaution be neglected, sulphur
is likely to separate from the hydrogen sulphide water, which, of
course, would add falsely to the weight of the silver sulphide. If
the presence of a minute quantity of sulphur in the precipitate is
suspected, treat it after drying with pure carbon disulphide on the
filter repeatedly, till the fluid running through gives no residue on
evaporation in a watch-glass ; dry and weigh.

The sulphide must, however, never be weighed as just docribed,
unless the analyst is satisfied that no considerable amount of sul-
phur has fallen down with it, as would occur if the fluid contained
hyponitric acid, a ferric salt, or any other substance which decom-
poses hydrogen sulphide. In case the precipitate docs contain
much admixed sulphur, the simplest process is to convert it into



ll5.j SILVER. 341

metallic silver (H. HOSE*). For this purpose it is transferred to a
weighed porcelain crucible, the filter ash is added, and the whole
is heated to redness in a stream of hydrogen, the apparatus
described in 108 being employed. Results accurate.

Should the apparatus in question not be at the operator's dis-
posal, he may, after complete washing of the precipitate, carefully
rinse it into a porcelain dish (without injuring the weighed filter),
heat it once or twice with a moderately strong solution of pure
sodium sulphite, retransfer the precipitate (now freed from admixed
sulphur) to the old filter, wash well, dry and weigh (J. LowEf) ;
or he may treat the dried precipitate, together with the filter-ash,
with moderately dilute chlorine-free nitric acid at a gentle heat,
till complete decomposition has been effected (till the undissolved
siilphur has a clean yellow appearance), filter, wash well, and pro-
ceed according to 1, a.

3. Determination as Silver Cyanide.

Mix the neutral solution of silver with potassium cyanide, until
the precipitate of silver cyanide which forms at first is redissolved ;
add nitric acid in slight excess, and apply a gentle heat. If the
solution contains free acid, tin's must be first neutralized with pot-
ash or sodium carbonate. After some time, collect the precipitated
silver cyanide on a weighed filter, wash, dry at 100, and weigh.
For the properties of the precipitate, see 82. The results are
accurate.

4. Determination as Metallic Silver.

a. In the Dry Way.

Silver oxide, silver carbonate, &c., are easily reduced by simple
ignition in a porcelain crucible. In the reduction of salts of
organic acids, the crucible is kept covered at first, and a moder-
ate heat applied ; after a time the lid is removed, and the heat
increased, until the whole of the carbon is consumed. For the
properties of the residue, see 82. The results are absolutely
accurate, except as regards silver salts of organic acids; in the
analysis of the latter, it not unfrequently happens that the reduced
silver contains a minute portion of carbon, which increases the
weight of the residue to a trifling extent.

If it is desired to transform silver chloride, bromide, or sulphide
* Pogg. Annal, ex, 139. \Journ.f. prakt. Cliem., LXXVII, 73.



342 DETERMINATION. [ 115.

into metallic silver, for the purpose of analysis, they are heated in
a current of pure hydrogen to redness, till the weight remains
constant. The process may be conducted in a porcelain crucible
or a bulb-tube. In the former case, the apparatus described in
108 is used ; in the latter the apparatus represented in Fig. 87,
with the substitution, of course, of hydrogen for chlorine ( 64,
14). If the bulb-tube is used, it must, after cooling and before
being weighed, be held in an inclined position, so that the hydrogen
may be replaced by air. The results are perfectly accurate. Silver
iodide cannot be reduced in this way.

I. In the Wet Way.

If the silver solution is that of a nitrate, as is usual, add a little
sulphuric acid and evaporate till all the nitric acid is expelled, dis-
solve the silver sulphate in hot water, transfer it to a weighed
porcelain crucible, arid immerse in the solution a rod of cadmium.
The silver is rapidly reduced, and the precipitated metal may be
easily removed from the cadmium and collected into a coherent
mass. Warm the latter with the acid liquid until no more
hydrogen is evolved, wash with hot water by decantation, dry, and
ignite. Results accurate (A. CLASSEN*). Cadmium is preferable
to zinc, because the latter usually leaves behind a little lead on
solution in sulphuric acid.

5. Volumetric Methods.

I. GAY-LIJSSAC'S.

This, the most exact of all known volumetric processes, was
introduced by GAY-LUSSAC as a substitute for the assay of silver by
cupellation, was thoroughly investigated by him, and will be found
fully described in his work on the subject. This method has been
rendered still more precise by the researches of G. J. MULDER, to
whose exhaustive monograph f I refer the special student of this
branch. I shall here confine myself to giving the process so far
as to suit the requirements of the chemical laboratory, taking only
for granted that the analyst has the ordinary nu'asuring apparatus,
&c., at his disposal. MULDER'S results will be made use of to the
full extent possible under these circumstances.

a. 1 1 1. >! I-ITES.

a. SOLUTION OF SODIUM CHLORIDE. Take chemically pure

* Zritichr. f. analyt. (Jhem., \, 402. The method given ly MIM.ON and
COMM.MI.I i i/'/V.sr///-. /. umtlyt. Chem., n, 212), in which silver is precipitated
by means of coppcr-aimiuuiium chloride, is not to be recommended, according
to the "investigations of STAB (itiM., vi, 426), as well as those conducted by me.

\ Die Silberproltirmethodc (s< e n<>i<- p. 198).



115.] SILVER. 343

sodium chloride either artificially prepared or pure rock-salt
powder it roughly and ignite moderately (not to fusion*). ]STow
dissolve 5 '4207 grm. in distilled water to 1 litre, measured at 16.
100 c.c. of this solution contains a quantity of sodium chloride
equivalent to 1 grm. of silver, Ag. The solution is kept in a
stoppered bottle and shaken before use.

/?. DECIMAL SOLUTION OF SODIUM CHLORIDE. Transfer 50 c.c.
of the solution described in or to a 500-c.c. measuring flask,
fill up to the mark with distilled water and shake. Each c.c.
of this decimal solution corresponds to O'OOl grm. silver. The
measuring must be performed at 16. The solution is preserved
like the other.

y. DECIMAL SILVER SOLUTION. Dissolve 0'5 grm. chemically
pure silver f in 2 to 3 c.c. pure nitric acid of 1 -2 sp. gr., and dilute
the solution with water exactly to 500 c.c. measured at 16. Each
c.c. contains O'OOl grm. silver. The solution is kept in a stoppered
bottle and protected against the influence of light.

d. TEST-BOTTLES. These should be of colorless glass, holding
easily 200 c.c., closed with well-ground glass-stoppers, running to
a point below. The bottles fit into cases blackened on the inside,
and reaching up to their necks. In order to protect the latter also
from the action of light, a black-cloth cover is employed.

5. PRINCIPLE.

Suppose we know the value of a solution of sodium chloride,
i.e., the quantity that is necessary to precipitate a given amount of
silver, say 1 grm., we are in the position, with the aid of this solu-

* On fusion, if the flame can in the least way act upon it, it takes an alkaline
reaction, since under the influence of vapor of water and carbon dioxide, a little
hydrochloric acid is formed and escapes, while a corresponding quantity of
sodium carbonate remains.

f For the preparation of pure silver STAS recommends the following method :
Take crude silver nitrate containing copper, fuse in order to decompose any
platinum nitrate which may be present, dissolve in dilute ammonia, allow to
stand 48 hours, filter and dilute till the fluid does not contain more than 2 per
cent, silver. Add ammonium sulphite in excess. To ascertain how much sul-
phite will be required make a small preliminary test; as soon as after heating the
blue solution loses all color, you may be sure that enough of the sulphide has
been added. Warm on a water-bath to 60 or 70, when all the silver will be
thrown down as a metallic powder, allow to cool and wash by decantation with
diluted ammonia till the washings are free from copper and sulphuric acid. Now
digest the metal for several days with strong ammonia, wash, dry, and fuse with
a flux of borax and sodium nitrate.



i>44 DETERMINATION. [ 115.

tion, to determine an unknown amount of silver, for if we put x
for the unknown amount of silver, then.

c.c. of solution used for 1 grin. : c.c. used for x : : 1 grin. : x.

But if we examine whether 1 mol. sodium chloride dissolved in
water actually precipitates 1 at. of silver dissolved in nitric acid
exactly, we find that this is not the case.* On the contrary, the
clear supernatant fluid gives a small precipitate both on the addition
of a little solution of sodium chloride, and on the addition of a
little silver solution, as MULDER has most accurately determined.
The value of a solution of sodium chloride in the sense explained
above cannot, therefore, be reckoned from the amount of salt it
contains, by calculating 1 at. silver for 1 mol. sodium chloride, but
it can only be obtained by experiment. MULDER has shown that
the temperature and the degree of dilution have some influence,
and also that this fact is to be explained on the ground of the sol-
vent power of the sodium nitrate produced on the silver chloride.
In the solution thus formed we have to imagine NaNO 3 and NaCl
with AgNO s in a certain state of equilibrium, which on the addition
of either NaCl or AgNO, is destroyed, silver chloride being pre-
cipitated.

From this interesting observation it follows, that if to a silver-
solution we add at first concentrated solution of sodium chloride,
then decimal solution drop by drop, till the exact point is reached
when no more precipitate appears, now, on addition of decimal
silver-solution, a small precipitate will be again produced; and if
we add the latter drop by drop, till the last drop occasions no tur-
bidity, then again decimal solution of sodium chloride w r ill give a
small precipitate. On noticing the number of drops of both deci-
mal solutions which are required to pass from one limit to the
other, we find that the same number of each are used. Let us
suppose that we had added decimal solution of sodium chloride till
it ceased to react, arid had then used i>!> drops-f- of decimal silver-
.-oliitit.n, till this ceased to produce a further turbidity, wo must
now again add 20 drops of decimal solution of sodium chloride, in

* If sodium bromide or potassium bromide is used, completq precipitation
would ensue on addition of an equivalent quantity of silver solution, since bro-
mide of silver is not at all sclublo in the supernatant fluid (STAB, Compt. rend.
LXVII. 1107).

f Twenty drops from MULDER'S dropping apparatus are equal to 1 c. Q.



115.] SILVER. 345

order to reach the point at which this ceases to react. Were we to
add only 10 instead of these 20 drops, we have the neutral point,
as MULDER calls it, i.e., the point at which both silver and sodium
chloride produce equal precipitates.

"We have, therefore, 3 different points to choose from for our
final reaction : , the point at which sodium chloride has just
ceased to precipitate the silver; , the neutral point; c, the point
at which silver-solution has just ceased to precipitate sodium
chloride. Whichever we may choose, we must keep to it, i.e., we
must not use a different point in standardizing the sodium chloride
solution and in performing an analysis. The difference obtained,
by using first a and then & is, according to MULDER, for 1 grm.
silver, at 16, about 0*5 mgrm. silver; by employing first a and
then c, as was permitted in the original process of GAY-LUSSAC, the
difference is increased to 1 mgrm.

For our object, it appears most convenient to consider, once for
all, the point a as the end, and never to finish with the silver-
solution. If the point has been overstepped by the addition of too
large an amount of decimal solution of sodium chloride, 2 or 3
c.c, of decimal silver-solution should be added all at once. The
end-point is then found by carefully adding decimal solution of
sodium chloride again, and the quantity of silver in the silver-solu-
tion added is added to the original amount of silver weighed off.

c. PERFORMANCE or THE PROCESS.

This is divided into two operations ex, the titration of the
sodium-chloride solution ; //, the assay of t e silver alloy to be
examined.

oc. TITRATION OF THE SODIUM- CHLORIDE SOLUTION.

Weigh off exactly from 1-001 to 1'0()3 grm. chemically pure
silver,* put it into a test-bottle, add 5 c.c. perfectly pure nitric
acid, of 1*2 sp. gr., and heat the bottle in an inclined position in a
water- or sand-bath till complete solution is effected. Now blow
out the nitrous fumes from the upper part of the bottle, and after
it has cooled a little, place it in a stream of water, the temperature
of which is about 16, and let it remain there till its contents are
cooled to this degree, wipe it dry, and place it in its case.

Now fill the 100-c.c. pipette with the concentrated solution of
sodium chloride, which is then allowed to flow into the test-bottle

* See note, p. 343.



346 DETERMINATION. [ 115.

containing the silver-solution*. Insert the glass-stopper firmly
(after moistening it with water), cover the neck of the bottle with
the cap of black stuff belonging to it, and shake violently without
delay, till the silver chloride settles, leaving the fluid perfectly



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