C. Remigius Fresenius.

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clear. Then take the stopper out, rub it on the neck, so as to
remove all silver chloride, replace it firmly, and by giving the
bottle a few dexterous turns, rinse the chloride down from the
upper part. After allowing to rest a little, again remove the
stopper, and add, from a burette divided into (VI c.c., decimal
sodium-chloride solution, allowing the drops to fall against the
lower part of the neck, the bottle being held in an inclined
position. If, as above directed, 1*001 to 1*003 grm. silver have
been employed, the portions of sodium chloride solution at first
added may be | c.c. After each addition, raise the bottle a little
out of its case, observe the amount of precipitate produced, shake
till the fluid has become clear again, and proceed as above, before
adding each fresh quantity of sodium-chloride solution. The
smaller the precipitate produced, the smaller should be the quan-
tity of sodium chloride next added ; towards the end only tw r o
drops should be added each time ; and quite at-the end read off
the height of the fluid in the burette before each further addition.
When the last two drops give no more precipitate, the previous
reading is the correct one.

If by chance the point has been overstepped, and the time has
been missed for the proper reading off of the burette, add 2 to 3
c.c. of the decimal silver solution (the silver in which is to be
added to the quantity first weighed), and try again to hit the point
exactly by careful addition of decimal sodium-chloride solution.

The value of the sodium-chloride solution is now known.
Reckon it to 1 grm. silver.

Suppose we had used for 1*002 grm. silver, 100 c.c. of concen-
trated and 3 c.c. of decimal sodium-chloride solution ; this makes
altogether 100*3 of concentrated ; then

1-002 : 1*000 :: 100*3 : x

x = 100*0998

We may without scruple put 100-1 for this number. We now

* The pipette, having been filled above the mark, should be fixed in a support,
before the excess is allowed to run out, otherwise the measurings will not be suffi-
ciently accurate.

115.] SILVER. 347

know that 100*1 c.c. of the concentrated solution of sodium
chloride,* measured at 16, exactly precipitates 1 grin, of silver.
This relationship serves as the foundation of the calculation in
actual assaying, and must be re-examined whenever there is reason
to imagine that the strength of the sodium chloride solution may
have altered.


"Weigh off so much as contains about 1 grm. of silver, or better,
a few mgrm. more ;* dissolve in a test-bottle in 5 to T c.c. nitric
acid, and proceed in all respects exactly as in a.

Suppose we had taken 1-116 grm. of the alloy, and in addition
to the 100 c.c. of concentrated sodium-chloride solution, had used
5 c.c. of the dilute (= 0'5 concentrated), how much silver would
the alloy contain ?

Presuming that we use the same sodium chloride solution
which served as our example in a, 100*1 c.c. of which = 1 grm.
silver, then

100-1 : 100-5 : : 1-000 : x

x = 1-003996 (say (1-004).

We may also arrive at the same result in the following manner :

NaCI Solution.
For the precipitation of the silver in the alloy

were used . . . ...... . . 100*5 c.c.

For 1 grm. silver are necessary . ..... 100*1 c.c,

Difference ....... 0*4 c.c.

There are, therefore, 4 mgrm. of silver present more than a grm.,
on the presumption that O'l of the concentrated sodium-chloride
solution (= 1 c.c. of the decimal solution) corresponds to 1 mgrm.

* In coins containing 9 parts of silver and 1 part of copper, therefore take
about 1*115 or 1'120. In weighing off alloys of silver and copper, which do not

correspond to the formula Ag 3 Cu 2 (standard Vo^o"^) == we mus t remember that
they are never homogeneous in the mass ; thus, for instance, the pieces of metal,
from which coins are stamped, often show 1-5 to 1'7 in a thousand more silver in
the middle than at the edges. In assaying alloys, then, portions from various
parts of the mass must be taken, in order to get a correct result. The inaccuracy,
however, proceeding Irom the cause above-mentioned, can only be completely
overcome by fusing the alloy and taking out a portion from the well-stirred mass
for the assay.


silver. This supposition, although not absolutely coriect, may be
safely made, for the inexactness it involves is too minute, as is
evident from the previous calculation.

Before we can execute this process exactly, we must know the
quantity of silver the alloy contains very approximately. In
assaying coins of known value this is the case, but with other silver
alloys it is usually not so. Under the latter circumstances an
approximate estimation must precede the regular assay. This is
performed by weighing off 0'5 grin, (or in the case of alloys that are
poor in silver, 1 grm.), dissolving in 3 to 6 c.c. nitric acid, and
adding from the burette sodium chloride solution, first in larger,
then in smaller quantities till the last drops produce no further
turbidity. The last drops are not reckoned with the rest. The
operation is conducted, as regards shaking, &c., as previously
given. Suppose we had weighed off 5 grin, of the alloy, and
employed 25 c.c. of the sodium-chloride solution taking the
above supposed value of the latter

We have 100-1 : 25 : : 1-000 : x

x = 0-2497

that i?, the silver in 0*5 grm. of the alloy ; and as to the quantity of
alloy we have to weigh oil for the assay proper,

We have 0-2497 : 1'003 : : -5 : so

x = 2-008.

This quantity will, of course, require more nitric acid for solution
than was previously used (use 10 c.c.). In cases where the highest
degree of accuracy is not required, the results afforded by this
rough preliminary estimation will be accurate enough, if the
experiment is carefully conducted, since they give the quantity of
silver present to within T ^ O r ,fo.

With alloys which contain sulphur, and with such as consist of
gold and silver, and contain a little tin, LEVOL* employs concen-
trated sulphuric acid (about 25 grm.) as solvent. The portion of
the alloy is boiled with it till dissolved; after cooling, the fluid is
treated in the usual manner. As, however, concentrated sulphuric
acid fails to dissolve all the silver when there is much copper
present, MAscAzzmif digests the weighed portion of alloy (which

* Annal. de (Jhim. et de Flit,*, (ii), XLIV, Ml ; Jouru.f. prakt,, (Jhem.,
382. j cttctn. Centralbl., 1357, 300,

115.] SILVEB. 349

may contain small quantities of lead, tin, and antimony, besides
gold) first with the least possible amount of nitric acid, as long as
red vapors are formed ; he then adds concentrated sulphuric acid,
boils till the gold has settled well together, adds water after
cooling, and then titrates. In the presence of mercury, the
chloride of that metal is carried down with the silver, rendering
the method inaccurate. If the quantity of mercury is but small,
you may get over the difficulty by adding 25 c.c. ammonia and
20 c.c. acetic acid (LEVOL). The ammonium acetate acts by
decomposing the mercuric chloride, and thus preventing its
precipitation (DEBRAY*). If the quantity of mercury is large the
addition of an alkali acetate is not effective, and DEBKAY recom-
mends to drive oil the mercury by igniting for four hours in a
small crucible of gas carbon in a muffle. The presence of other
volatile metals, such as zinc, does not interfere with this oper-


This process depends on the following reaction : a solution of
iodide of starch added to a very dilute neutral solution of silver
nitrate, forms silver iodide and silver hypoiodite. The blue color
consequently vanishes, and on continued addition of the iodide of
starch, the fluid does not become permanently blue till all the sil-
ver nitrate present is decomposed in the above manner. The
iodide of starch solution used is therefore proportional to the quan-
tity of silver nitrate. Hence, .if the value of the iodide of starcl)
solution be determined, by allowing it to act on a certain amount
of silver solution of known strength, we shall be able to estimate
unknown quantities of silver with the greatest ease, provided that
the silver solution is free from all other substances which exert 3
decomposing action on the iodide of starch. Besides the ordinary
reducing agents, the following salts must be especially mentioned
as possessing this power : Mercurous and mercuric salts, stannous
salts, manganous, ferrous, and antimonous salts, also auric chloride
and arsenites ; lead and copper salts, on the other hand, do not
affect iodide of starch.

The iodide of starch is prepared as follows : make an intimate

* Compt. rend , LXX, 849 ; Zeitschr. f. Ckem., 1870, 349.

f Annal. d. Min. t x, 83 ; Jahresber. von Liebig u. Kopp, 1856, 749,

350 DET J : It M I N ATION. [ 1 1 '">

mixture in a mortar of 2 grin, iodine and 15 grm. starch with the
addition of 6 to 8 drops of water, and heat the slightly-moist mix-
ture in a closed flask in a water-bath, till the original violet-blur
color has passed into dark grayish-blue it takes about an hour.
The iodide of starch thus prepared is then digested with water; it
dissolves completely to a deep bluish-black fluid.

The value of this fluid is determined by allowing it to act on
10 c.c. of a neutral solution of silver nitrate, containing 1 grm. of
pure silver in 1 litre the silver solution is mixed with a little
pure precipitated calcium carbonate before adding the iodide of
starch. The strength of this latter is right, if 50 to 60 c.c. are
used in this experiment. On adding it, at first the blue color dis-
appears rapidly, and the fluid becomes yellowish from the silver
iodide. The end of the operation is attained as soon as the fluid is
bluish-green. The point is pretty easy to hit, and an error of Q'o
c.c. is of no importance, as it only corresponds to about "0001 grm.
silver. The calcium carbonate, besides neutralizing the free acid,
has the effect of rendering the final change of the color more dis-
tinctly observable. To analyze an alloy of silver and copper, dis-
solve about 0-5 grm. in nitric acid, dilute to 100 c.c. to lower the
color of the copper, saturate 5 c.c. with calcium carbonate, and add
iodide of starch till the coloration appears. Or you may deter-
mine very approximately the amount of silver in 2 c.c. of the solu-
tion, then precipitate the greater part (about 99-JJ-) of the silver
from 50 c.c. of the solution with standard solution of potassium
iodide, and without filtering estimate the remainder of the silver
by means of iodide of starch. If the amount of silver to be deter-
mined is more than 0'02grm., it is always better to employ the
latter method. In the case of a nitric acid solution containing sil-
ver with lead, the latter metal is first precipitated witli sulphuric
acid and filtered off, calcium carbonate is added to the filtrate rill
all free acid is neutralized, the fluid is filtered again (if necessary),
and lastly, more calcium carbonate isndded, and then the iodide of
starch. Yery dilute solutions must be concentrated, so that one may
have no more than from 50 to 100 c.c. to deal with. The method is
worthy of notice a n< 1 specially suited for the estimation of small
quantities of silver. With such it has afforded me perfectly satis-
tartory results. Instead of the standard iodide of starch, a dilute
standard solution of iodine in potassium iodide may he equally well

116.] LEAD. 351

employed with addition of starch solution (FIELD *) % If this
is used you must bear in mind that any substance which decom-
poses potassium iodide with separation of iodine will interfere.

H. YOGEL f has modified PISANI'S method for the conve-
nience of photographers. Nitroso-nitric acid (prepared by adding
1 grm. ferrous sulphate to 1000 grm. of nitric acid of sp. gr.
1-2) is added to the silver solution, which may contain free acid;
starch solution is then added, and also standard potassium-iodide
solution, until a permanent blue color. This occurs when all the
silver is precipitated, partly as iodide, partly as iodate. The pre-
cipitation depends upon the following reactions :

KI + AgNO. = KNO 3 + Agl ; and 61 + 6Ag]N T O 3 = AglO,
+ 5AgI + 3N 2 O 5 . In both cases 1 eq. of iodine precipitates 1 eq.
of silver. The potassium-iodide solution is made by YOGEL of such
strength that 1 c. c. is the equivalent of 0*01 grm. silver nitrate,
i.e., 10 grm. of pure, dry potassium iodide are dissolved in suffi-
cient liquid to measure 1024*1 c.c. From my experience, the
method is quite expeditious, but is not very accurate, because
equal volumes of silver solution will require distinctly varying
quantities of potassium-iodide solution if the conditions are
altered, e.g., the concentration and quantity of free acid; the
variation is evidently connected with the formation of silver
iodate, which is not entirely insoluble in the acid liquid.



This is the reverse of the method for the estimation of chlorine,
141 b, a, and will be described in that place.


2. LEAD.

a. /Solution.

Few of the lead salts are soluble in water. Metallic lead, lead
oxide, and most of the lead salts that are insoluble in water dis-
solve in dilute nitric acid. Concentrated nitric acid effects neither
complete decomposition nor complete solution, since, owing to the

* Chem. News, n, 17.

f Pogg. Ann., cxxiv, 347 ; Zeitschr. f. analyt. Chem., v, 227.


insolubility of lead nitrate in concentrated nitric acid, the first por-
tions of nitrate formed protect the yet undecomposed parts of the
salt from the action of the acid. For the solubility of lead chlo-
ride and sulphate, see 83. As we shall see below, the analysis of
these compounds may be effected without dissolving them. Lead
iodide dissolves readily in moderately dilute nitric acid upon ap-
plication of heat, with separation of iodine. Solution of potassa
is the only menstruum in which lead chromate dissolves without
decomposition ; for analysis, it is best converted into chloride.

b. Determination.

Lead may be determined as oxide, sulphate, chromate, or sul-
phide, chloride, lead oxide -f- lead, and metallic lead; also by
volumetric analysis.

We may convert into


a. By Precipitation.

All lead salts soluble in water, and those of its salts which,
insoluble in that menstruum, dissolve in nitric acid, with separa-
tion of their acid.

1). By Ignition.

of. Lead salts of readily volatile or decomposable inorganic acids.

ft. Lead salts of organic acids.


All lead salts in solution.


a. By Precipitation.

The salts that are insoluble in water, but soluble in nitric acid,
and the acids of which cannot be separated from the solution.

b. By Evaporation.

a. All the oxides of lead, and also the lead salts of volatile

/?. Many of the organic compounds of lead.


The compounds of lead soluble in water or nitric acid.

Lead chromate.

Jktany organic lead compound?.

316.] LEAD. 35.3


The oxides and most of the lead salts (compounds of lead with
chlorine, bromine, and iodine).

Lead may be also determined volumetrically, but rarely

The application of these several methods must not be under-
stood to be rigorously confined to the compounds specially enu-
merated under their respective heads ; thus, for instance, all the
compounds enumerated under 1 may likewise be determined as lead
sulphate ; and, as above mentioned, all soluble compounds of lead
may be converted into lead sulphide ; also, the lead in lead sulphate
may be without difficulty determined as sulphide. Lead chloride,
bromide, and iodide may be conveniently reduced to the metallic
state in a current of hydrogen gas in the manner described in 115
(reduction of silver chloride), if it is not deemed preferable to dis-
solve them in water, or to decompose them by a boiling solution
of sodium carbonate, in which case, after cooling, carbonic-acid
gas is passed into the solution, when the small quantity of lead
retained will be precipitated. If the reduction method is resorted
to, the heat applied should not be too intense, since this might
cause some lead chloride to volatilize.

The higher oxides of lead are reduced by ignition to the state
of lead monoxide, and may thus be readily analyzed and dissolved.
Should the operator wish to avoid having recourse to ignition, the
most simple mode of dissolving the higher oxides of lead is to act
upon them with dilute nitric acid with the addition of alcohol.
For the methods of analyzing lead sulphate, chromate, iodide, and
bromide, I refer to the paragraphs treating of the corresponding
acids, in the second part of this section. To effect the estimation
of lead in the oxide arid in many lead salts, especially also in the
sulphate, the compound under examination may be fused with
potassium cyanide and the metallic lead obtained well washed and
weighed. From the sulphide also the greater portion of the lead
may be separated by this method, but never the whole (H, ROSE*).
1. Determination as Oxide,

a. By Precipitation.

Mix the moderately dilute solution with ammonium carbonate
slightly in excess, add some caustic ammonia, apply a gentle heat,
*Pogg. Annal., 91, 144.




allow to cool and filter through a small thin filter. Wash with
pure water, dry, and transfer the precipitate to a watch-glass,
removing it as completely as possible from the filter ; burn the
latter in a weighed porcelain crucible. After the crucible is cold,
moisten the ash with nitric acid, allow it to evaporate, ignite gently,
allow to cool, add the precipitate and ignite gently till all the car-
bonic acid is driven off. For the properties of the precipitate and
residue, see 83. The results are very satisfactory, although gen-
erally a trifle too low, owing to lead carbonate not being absolutely
insoluble, particularly in fluids rich in ammonium salts (Expt. No.
42, 5).

5. By Ignition.

Compounds like lead carbonate or nitrate are cautiously ig-
nited in a porcelain crucible until the weight remains constant.
Lead nitrate must be very completely dried before being ignited,
in order to avoid loss through decrepitation. For the manner of
converting lead salts of organic acids into oxide, see 6.
2. Determination as Sulphide.

Lead may be completely precipitated fron. acid, neutral and
alkaline solutions by hydrogen sulphide, and also from neutral and
alkaline solutions by ammonium sulphide. Precipitation from
acid solution is usually employed, especially in separations. A
large excess of acid and also warming should both be avoided.
The former is prejudicial to complete precipitation ( 83,/ 1 ), the
latter may readily occasion the re-solution of the sulphide that has
already been precipitated. In order to guard against incomplete
precipitation, before filtering, test a portion of the supernatant
fluid by mixing with a relatively large quantity of strong hydrogen
sulphide water ; the fluid must remain clear.

If the fluid contained no hydrochloric acid or metallic chloride,
the lead sulphide is pure. After it has been filtered off, washed
with cold water and dried, it is transferred, together with the
filter-ash, to a porcelain crucible, a little sulphur added, and ignited
\\\ hydrogen at gi-ntk- redness till its weight is constant. It should
always be allowed to cool in a current of the gas, before being
weighed. As ivgards the apparatus, see 108, 2, Fig. 83. For the
properties of the residue, see 83,. f. The results are satisfactory
(II. ROSE). The heat of the ignition must not be too low, or the
residue will contain too much sulphur, ror too high, or the lead
sulphide will heii'in t<> \olat ili/e, and lead disulphide will also be

116.] LEAD. / 355

formed with loss of hydrogen sulphide. Drying the precipitate
at 100 cannot be recommended ( 83, f). If the fluid, on the
contrary, contained hydrochloric acid or a metallic chloride, the
lead sulphide contains chloride which cannot be removed even by
boiling the precipitate with ammonium sulphide. If the precipi-
tate were treated as above, we should obtain a tolerably pure
sulphide, but not without loss from volatilization of chloride. A
precipitate of this kind must therefore be decomposed with strong
hydrochloric acid, the solution evaporated to dry ness, the residue
dissolved by heating with a concentrated solution of sodium
acetate, and this solution diluted and poured with stirring into
excess of strong hydrogen sulphide water. Or the lead chloride
obtained may be evaporated, heated to 200, and weighed as such


3. Determination as Sulphate.

a. By Precipitation.

a. Mix the solution (which should not be over dilute) with
moderately dilute pure sulphuric acid slightly in excess, and add
to the mixture double its volume of common alcohol ; wait a few
hours, to allow the precipitate to subside; filter, wash the precipi-
tate with common alcohol, dry, and ignite after the method
described in 53. Though a careful operator may use a platinum
crucible, still a thin porcelain crucible is preferable. See also the
remarks, 1, a.

P. In cases where the addition of alcohol is inadmissible, a
greater excess of sulphuric acid must be used, and the precipitate,
which is allowed some time to subside, filtered, and washed first
with water acidulated with a few drops of sulphuric acid, then
repeatedly with alcohol. The remainder of the process is con-
ducted as in a.

If the fluid contained nitric acid, whether alcohol is used or
not, it is advisable to evaporate on the water-bath after the
addition of the sulphuric acid, till the nitric acid has escaped,
otherwise the precipitation will not be complete. If the fluid
contained hydrochloric acid or a metallic chloride, lead chloride is
thrown down with the sulphate. In this case you must either
evaporate the fluid with excess of sulphuric acid and heat the
residue till sulphuric acid fumes escape to drive off the hydro-

* Handb. far analyt. Chem. von H. ROSE, 6. Aufl. von FINKENER, 932.


chloric acid, or you must treat the precipitate and filter-ash in the
crucible with concentrated sulphuric acid, evaporate and ignite to
convert it into pure lead sulphate (FnrKKNEB*).

For the properties of the precipitate, see 83. The method a
gives accurate results; those obtained by ft are less exact (a little
too low), but still however satisfactory, if the directions given are
adhered to. If, on the contrary, a proper excess of sulphuric acid
is not added, in the presence, for instance, of ammonium salts, the
lead is not completely precipitated, and if pure water is used for
washing, decided traces of the precipitate are dissolved.

b. By Evaporation.

of. Put the substance into a weighed dish, dissolve in dilute
nitric acid, add moderately dilute pure sulphuric acid slightly in
excess, and evaporate at a gentle heat; at last high over the lamp,
until the excess of sulphuric acid is completely expelled. In the
absence of organic substances, the evaporation may be effected
without fear in a platinum dish ; but if organic substances are
present, a light porcelain dish is preferable. With due care in the
process of evaporation, the results are perfectly accurate.

ft. Organic compounds of lead are converted into the sulphate
by treating them in a porcelain crucible, with pure concentrated
sulphuric acid in excess, evaporating cautiously in the well-covered
crucible, until the excess of sulphuric acid is completely expelled,
and igniting the residue. Should the latter not look perfectly
white, it must be moistened once more with sulphuric acid, and
the operation repeated. The method gives, when conducted with
great care, accurate results ; a trifling loss is, however, usually in-
curred, the escaping sulphur dioxide and carbon dioxide gases
being liable to carry away traces of the salt.

4. Determination as Lead Chromate.

If the solution is not already distinctly acid render it so with
acetic acid, then add potassium dichromate in excess, and, if free
nitric acid is present, add sodium acetate in sufficient quantity to
replace the free nitric acid by free acetic acid; let the precipitate
subside at a gentle heat, and collect on a weighed filter dried at
100, wasli with water, dry at 100, and weigh. The precipitate

Online LibraryC. Remigius FreseniusQuantitative chemical analysis → online text (page 31 of 69)