C. Remigius Fresenius.

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* Ilandb. der analyt. Chem. von II. ROSE, C. Aufl. von FINKEKER, 933.



116.] LEAD. 357

may also be ignited according to 53, but in this case care must be
taken tlfat hardly any of the salt remains adhering to the paper,
and that the heat is not too high. For the properties of the
precipitate, see 93, 2. The results are accurate (Expt. No. 68).

5. Determination as Lead Chloride.

Add an excess of hydrochloric acid to the solution, concentrate
on a water-bath, treat the residue with absolute alcohol to which
a little ether has been added, allow to settle, filter, and wash
with ether-alcohol. The lead chloride may be either dried at
100 after being collected on a dried and weighed filter, or it may
be carefully treated as in 53. In the latter case use a porcelain
crucible, and take care to leave no lead chloride in the filter and
to avoid a temperature above 200.

6 . Determination as Lead Oxide -f- Lead.

Gently heat the organic lead compound (1 to 2 grm.) in a small
weighed porcelain dish, allowing the heat to play first upon the
margins of the dish so that the decomposition may begin on one
side, and thence proceed slowly. When the entire mass is decom-
posed, heat more strongly till no glowing particles are percepti-
ble and the residue appears to be a carbon-free mixture of lead
oxide and lead globules. "Weigh the residue, then warm it with
acetic acid unjtil the oxide is completely dissolved, which is
soon effected, wash the lead by decantation with water, and finally
heat the residual lead to remove all water, and weigh. On de-
ducting the weight found from that of the residue first weighed,
the weight of the oxide contained is found. On calculating the
freight -of metal contained in the oxide found, and adding it to
the weight of the metallic lead directly found, the total metal
eontained in the compound is obtained.

In carrying out this process, two points must be observed.
First, the decomposition must be allowed to proceed very
fclowly, because rapid combustion of the carbon and hydrogen of
the compound at the expense of the oxygen of the lead oxide
causes so high a temperature to develop as to volatilize some lead,
which passes off in visible fumes. Secondly, care must be taken
that no carbon remains in the residue ; and this can with cer-
tainty be ascertained with acetic acid. Neglect of the first point
gives results which are too low ; neglect of the second will give



DETERMINATION. [ 116.

results too high. The method is otherwise very convenient, and
when carefully executed gives accurate results.

DOLK has proposed the following modification of the method
first proposed by BERZELIUS: The compound is very gently
heated in a covered porcelain crucible until the organic matter is
completely carbonized. Then remove the lid and stir the con-
tents with an iron wire. The mass begins to ignite, and a mix-
ture of lead oxide with metallic lead results, which may contain
some unconsumed carbon. Remove the crucible from the flame,
throw into it a few pieces of recently fused ammonium nitrate
and then replace the cover. The salt fuses, oxidizes the lead,
and converts it partly into nitrate. The crucible is now ignited
until vapors of hyponitric acid are no longer visible. The
residual oxide is then weighed. This rapid method insures com-
plete combustion of all the carbon and saves the time and labor
of one weighing. The results are very satisfactory.

T. Determination as Metallic Lead.

a. This method is applicable to lead oxide and most lead com-
pounds, such as lead sulphate and phosphate, but not chromate ;
to lead sulphide it is applied with difficulty. The substance is
fused with 4 to 5 times its weight of potassium cyanide (prepared
according to LIEBIG'S process) in a well-covered, well-glazed por-
celain crucible. After cooling, treat the mass with water, rap-
idly decant the solution from the reduced lead, wash the latter
first with water, then with diluted and finally strong alcohol, dry
and weigh. Sometimes the result is a single lead globule ; usu-
ally, however, there are obtained a number of globules mixed
with lead powder. After weighing, dissolve the lead in warmed
dilute nitric acid. Any residue (portions of the glaze of the
crucible) is determined, and its weight deducted from that first
found (II. KOSE).*

I. Lead may be precipitated from soluble as well as insoluble
lead salts (lead chloride and sulphate) by means of zinc or cad-
mium. To effect this, warm the lead compound with water and
a little hydrochloric acid on a water-bath, and add a smooth piece
of pure zinc or cadmium (soluble without residue in hydrochloric

* Pogg. Annal., xci, 104.



116.] LEAD. 359

acid). Keduction begins at once. The lead precipitating on the
zinc is removed from time to time with a glass rod, and more
acid added occasionally if necessary. The operation is at an end
when lead no longer deposits on the zinc, and when a small quan-
tity of the solution gives no reaction with hydrogen sulphide.
The zinc or cadmium is then removed, the water decanted, and the
spongy lead rapidly and completely washed by decantation. Dis-
tilled water must not be used, as this dissolves traces of lead;
spring water should be used, and to prevent it from precipitating
the zinc or cadmium add a little tincture logwood, and then so
much very dilute sulphuric acid, until the red color just gives
place to a yellow. The spongy lead cannot be dried without the
formation of some hydroxide; hence it may be dried at 150-
200, the weight of the mixture of lead and lead oxide determined,
and the oxide estimated as under 8 <?, and the weight of oxygen
found deducted from the weight of the mixture first found ; or
the spongy lead may be dissolved in nitric acid and the lead deter-
mined as in 3 &, as a sulphate (STOLE A *).

8. Determination of Lead ~by Volumetric Analysis.

Although there is no lack of proposed methods for the volu-
metric estimation of lead, we are still without a really good method
for practical purposes, that is, a method which can be generally
employed and which is at the same time simple and exact. For
the present, therefore, in almost all cases the gravimetric deter-
mination of lead is to be preferred to the volumetric. On my own
part at least I cannot see that it is easier or any better, when one
has the precipitate washed, to subject it to a volumetric process
whereby the accuracy is necessarily diminished instead of igniting
it gently and weighing. For this reason the better volumetric
methods will be but briefly described, the rest being altogether
omitted.

a. The solution of the normal lead salt must be free from
alkali salts, more especially from ammonium salts. It is precipi-
tated with oxalic acid (not with ammonium oxalate), the well-
washed precipitate is dissolved in nitric acid, sulphuric acid added,

*Journ.f. prakt. Ckem., ci, 150; Zettschr.f. analyt. Chem., vn, 102.



360 DETERMINATION. [ 116.

and the oxalic acid in the solution determined by potassium
permanganate (137) HEMPEL.

b. H. SCHWARZ'S method.* To the nitric acid solution add
ammonia or sodium carbonate, as long as the precipitate redissolves
on shaking, mix with sodium acetate in not too small quantity, and
then run in from a burette a solution of potassium dichroinate
(containing 1-i -721 grm. in the litre) till the precipitate begins to
settle rapidly. Now place on a porcelain plate a number of drops
of a neutral solution of silver nitrate, and proceed with the addition
of the chromate, two or three drops at a time, stirring carefully
after each addition. When the precipitate has settled tolerably
clear, which takes only a few seconds, remove a drop of the super-
natant liquid and mix it with one of the drops of silver solution
on the plate. A small excess of chromate gives at once a distinct
red coloration ; the precipitated lead chromate does not act on the
silver solution, but remains suspended in the drop. The number
of c. c. of solution of chromate used (minus O'l, which SCHWARZ
deducts for the excess) multiplied by O0207 = the quantity of lead.
If the fluid appear yellow before the reaction with the silver salt
occurs, sodium acetate is wanting. In such a case first add more
sodium acetate, then 1 c. c. of a solution containing 0'0207 lead in
1 c. c., complete the process in the usual way, and deduct 1 c. c.
from the quantity of chromate used on account of the extra lead
added. Any iron present must be in the form of a ferric salt ;
metals the chromates of w r hich are insoluble must be removed
before the method can be employed.

<?. The lead is precipitated according to 1, a, the carbonate (its
composition is a matter of indifference in the present case) is
washed, dissolved in a measured quantity of standard nitric acid
( 215), and a neutral solution of sodium sulphate added, whereby
lead sulphate is precipitated and an equivalent quantity of sodium
nitrate formed. If the nitric acid still free is now determined
with standard alkali, we shall find the quantity of acid that has
been neutralized by means of the lead, from which the amount of
lead may be calculated, each c. c. of standard nitric acid being the
equivalent of 0*10346 lead. You may also determine the free nitric
acid by adding standard sodium carbonate till, the vessel being on

* Dingl. polyt. Journ., CLXIX, 284; Zeitschr.f. analyt. Chem., u, 378.



117.] MERCURY IN MERCUROUS COMPOUNDS. 361

a black surface, a permanent turbidity is visible. Results good
(F. MOHB*).

117.
3. MERCURY IN MERCUROUS COMPOUNDS.

a. Solution.

Mercurous oxide and mercurous salts may generally be dissolved
by means of dilute nitric acid, but without application of heat if
conversion into mercuric compounds is to be avoided. If all that
is required is to dissolve the mercury, the easiest way is to warm
the substance for some time with nitric acid, then add hydrochloric
acid, drop by drop, and continue the application of a moderate heat
until a perfectly clear solution is produced, which now contains all
the mercury in form of mercuric salts. Heating the solution to
boiling, or evaporating, must be carefully avoided, as otherwise
mercuric chloride may escape with the steam.

b. Determination.

If it is impracticable to produce a solution of the mercurous
compound without formation of mercuric salts, it becomes neces-
sary to convert the mercury completely into mercuric salts, when
it may be determined as directed 118. But if a solution of a
mercurous compound has been obtained, quite free from mercuric
salts, the determination of the mercury may be based upon the
insolubility of mercurous chloride, and effected either gravimetri-
cally or volumetrically. The process of determining mercury,
described 118, 1, a, may, of course, be applied equally well in the
case of mercurous compounds.

1. Determination, as Mercurous Chloride.

Mix the cold highly dilute solution with solution of sodium
chloride, as long as a precipitate forms ; let the precipitate subside,
collect on a weighed filter, dry at 100, and weigh. For the
properties of the precipitate, see 84. Results accurate. If the
mercurous solution contains much free nitric acid, the greater part
of this should be neutralized with sodium carbonate before adding
the sodium chloride.

* Lehrbuch der TitrirmetJiode, 3. Aufl. 115.



362 DETERMINATION. [ 117.

2. Volumetric Methods.

Several methods have been proposed under this head: the
following are those which are most worthy of recommendation :

a. Mix the cold solution with decinormal solution of sodium
chloride ( 141, J, <*), until this no longer produces a precipitate,
and is accordingly present in excess ; filter and wash thoroughly,
taking care, however, to limit the quantity of water used ; add a
few drops of solution of potassium chromate, then pure sodium
carbonate, sufficient to impart a light yellow tint to the fluid, and
determine by means of solution of silver nitrate ( 141, J, a) the
quantity of sodium chloride in solution, consequently the quantity
which has been added in excess ; this shows, of course, also the
amount of sodium chloride consumed in effecting the precipitation.
One mol. of Hg a O is reckoned for 2 mols. of NaCl, consequently
for every c.c. of the decinormal solution of sodium chloride, '0208
grm. of mercurous oxide. As filtering and washing form indis-
pensable parts of the process, this method affords no great advan-
tage over the gravimetric; however, the results are accurate
(FK. MOIIR*). The two methods, 1 and 2, a, may also be advan
tageously combined.

b. Precipitate the mercurous solution,f according to 1, with
sodium chloride in a stoppered bottle, allow to subside, filter, wash,
push a hole through the bottom of the filter, and rinse the precipi-
tate into the bottle, which usually has some of the washed mercu-
rous chloride adhering to its inside. Add a sufficient quantity of
solution of potassium iodide, together with standard iodine solution
(to 1 grm. Hg 2 Cl 2 about 2*5 grm. Kland 100 c.c. decinormal iodine
solution:):), insert the stopper, and shake till the precipitate has
entirely dissolved (Hg a Cl, + 6KI + 21 = 2[HgI f (KI) f ] + 2KC1).
As iodine is in excess, the solution appears brown. If any mercu-
ric iodide separates, add potassium iodide to redissolve it. Now
add from a burette solution of sodium thiosulphate correspond^
ing to decinormal iodine solution till the fluid is decolorized and
appears like water, transfer to a measuring flask, rinse and fill up
to the mark, shake, take out an aliquot patt, add starch paste to it,
and determine the excess of sodium thiosulphate with decinormai
iodine solution. After multiplying by the proper number, add the
c.c. originally employed, subtract the c.c. of thiosulphate used, and

* Lehrbuch der Tilrirmethode, 3. Aufl. 395.

\ If mercuric oxide is also preseut, see 118, 3. 146, 3.



118.] MERCURY IN MERCURIC COMPOUNDS. 363

calculate the quantity of mercury from the remainder. 2 at.
iodine == 1 inol, Hg a CI a . Results good (HEMPEL *).



118.
4. MERCURY IN MERCURIC COMPOUNDS.

a. Solution.

Mercuric oxide, and those mercuric compounds which are
insoluble in water, are dissolved, according to circumstances, in
hydrochloric acid or in nitric acid. Mercuric sulphide is heated
with hydrochloric acid, and nitric acid or potassium chlorate added
until complete solution ensues ; it is, however, most readily dis-
solved by suspending it in dilute potassa and transmitting chlorine,
at the same time gently warming (H. ROSE). "When a solution
of mercuric chloride is evaporated on the water-bath, mercuric
chloride escapes with an aqueous vapor. This fact must not be
lost sight of in effecting solutions of mercuric compounds. The
methods proposed by YoiiLf give on this account inaccurate
results. FR. MOHR J and R. RIETH also have not given this
source of error proper attention.

1). Determination.

Mercury may be weighed in the metallic state, or as mercu-
TOUS chloride, mercuric sulphide, or mercuric oxide ( 84) ; in
separations it is sometimes determined as loss on ignition. It may
also be estimated volumetrically.

The first three methods may be used in almost all cases ; the
determination as mercuric oxide, on the contrary, is possible
only in mercurous or mecuric nitrates. The methods by which
the mercury is determined as mercurous chloride or mercuric sul-
phide are to be preferred before those in which it is separated in
the metallic form. The volumetric method 5 is of very limited
application. The mercurous chloride obtained by method 2,
instead of being weighed, may be determined volumetrically as
in 117, 2, I.

*AnnaL d. CJiem, u. Pharm., ex, 176. |7Wcf.,xciv, 230.
I Lehrbuch der Titrirmelhode, 3. Aufl., 208.
RIETH'S Volumetrie, 225.



364 DETERMINATION. [ ll8.

1. Determination as Metallic Mercury.

a. In the Dry Way.

The process is conducted in the apparatus illustrated by
Fig. 88.

Take a tube 45 cm. long and about 12 mm. wide, made of
difficultly fusible glass and sealed at one end. First put into the
tube a mixture of sodium bicarbonate and powdered chalk 6 cm.
long, then a layer of quicklime ; these two will occupj the space
from a to 5. Then add. the intimate mixture of the substance
with an excess of quicklime (b-c), then the lime rinsings of the
mortar (c-d), then a layer of quicklime (d-e), and lastly, a loose
stopper of asbestos (e-f). The anterior end of the tube is then
drawn out and bent at a somewhat obtuse angle. The manipu-
lations in the processes of mixing and filling being the same as in
organic analysis, they will be found in detail in the chapter on that
subject.

A few gentle taps upon the table are sufficient to shake the
contents of the tube down so as to leave a free passage through the
whole length of the tube. The tube, so prepared and arranged, is
now placed in a combustion furnace, the point being inserted into
a flask containing water, the surface of which it should just touch,
so that the opening may be just closed.

The tube is now surrounded with red-hot charcoal, in the same
way as in organic analysis, proceeding slowly from e to #, the last
traces of mercurial vapor being expelled by heating the mixture at




Fig. 88.

the sealed end of the tube. "Whilst the tube still remains in a state
of intense ignition, the neck is cut off at/*, and carefully and com-
plctely rinsed into the receiving flask, by means <>f a washing-bottle.
The small globules of mercury which have distilled over are united
into a large one, by agitating the ila>k, and, after the lapse of some
time, the perfectly dear water is decanted, and the mercury poured



118.] MERCURY IN MERCURIC COMPOUNDS. 365

into a weighed porcelain crucible, where the greater portion of the
water still adhering to it is removed with blotting-paper. The
mercury is then finally dried under a bell-jar, over concentrated
sulphuric acid, until the weight remains constant. Heat must not
be applied. For the properties of the metal, see 84. In the
case of sulphides, in order to avoid the presence of vapor of water
in the tube, which would give rise to the formation of sulphuretted
hydrogen, the mixture of sodium hydrogen carbonate and chalk is
replaced by magnesite. Mercuric iodide cannot be completely
decomposed by lime. To analyze this in the dry way, substitute
finely divided metallic copper for the lime (II. ROSE*). The accu-
racy of the results is entirely dependent upon the care bestowed.
The most highly accurate results are, however, obtained by the
application of the somewhat more complicated modification adopted
by ERDMANN and MARCHAND for the determination of the atomic
weight of mercury and of sulphur. For the details of this modi-
fied process, I refer to the original essay,f simply remarking here,
that the distillation is conducted, in a combustion-tube, in a cur-
rent of carbon dioxide gas, and that the distillate is received in a
weighed bulb apparatus with the outer end filled with gold-leaf, to
insure the condensation of every trace of mercury vapor. This
way of receiving and condensing may be employed also in the
analsis of amalams



I. In the Wet Way.

The solution, free from nitric acid, and mixed with free hy-
drochloric acid, is precipitated, in a perfectly clean flask (best
previously washed with hot potassa lye), with an excess of a clear,
recently prepared solution of stannous chloride containing free
hydrochloric acid ; the mixture is boiled for a short time, the flask
loosely stoppered, and then allowed to cool. After some time
the perfectly clear supernatant fluid is decanted from the metallic
mercury, which, under favorable circumstances, will be found
united into one globule ; if this is the case, the globule of mer-
cury may be washed at once by decantation, first with water
acidulated with hydrochloric acid and finally with pure water; it
is dried and estimated as in a.



* Pogg. Annal., cy, 546.

^Journ.f. prukt. Chem., xxxr, 385; also Pharm. Centralbl., 1844, 854.

\Journ.f. prakt. Chem., LXX, 64.



366 DETERMINATION. [ 118.

If, on the other hand, the particles of the mercury have not
united, their union into one globule may as a rule be readily ef-
fected by boiling a short time with some moderately dilute hydro-
chloric acid mixed with a few drops of stannous chloride (having,
of course, previously removed by decantation the supernatant
clear fluid). For the properties of metallic mercury, see 84.

Instead of stannous chloride, other reducing agents may be
used, especially phosphorous acid at a boiling temperature. This
method gives accurate results only when conducted with the great-
est care. In general, a little mercury is lost.

2. Determination as Mercurous Chloride.

a. After H. ROSE.* Mix the mercuric solution (which may
contain nitric acid, but which must then be considerably diluted)
with hydrochloric acid and excess of phosphorous acid (obtained
by the oxidation of phosphorus in moist air), allow to stand for
12 hours in the cold or at a very gentle heat (at all events under
60), collect the mercury, now completely separated as mercurous
chloride, on a weighed filter, wash with hot water, dry at 100,
and weigh. Results perfectly satisfactory.

3. Determination as Mercuric Sulphide.

The solution is sufficiently diluted, acidulated with hydrochloric
acid, and precipitated with clear saturated hydrogen sulphide water
(or in the case of large quantities, by passing the gas) ; filter after
allowing the precipitate a short time to deposit, wash quickly with
cold water, dry at 100, and weigh. Results very satisfactory.

If from any cause (e.g. presence of ferric salts, free chlorine, or
the like) the precipitate should contain free sulphur, the filter is
spread out on a glass plate, the precipitate removed to a porcelain
dish by the aid of a jet from the wash-bottle, and warmed for some
time with a moderately strong solution of sodium sulphite. The
filter, having been in the mean while somewhat dried on the glass
plate, is replaced in the funnel, the supernatant fluid is poured on
to it, the treatment with sodium sulphite is repeated, and the pre-
cipitate (now free from sulphur) is finally collected on the filter,
washed, dried, and weighed. Results very good (J. LowEf).

Should the quantity of sulphur mixed with the precipitate be



*Pogg. Ann,<L, < \, 529.

\Journ.f. prakt. Cfiem., LXXVII, 73.



118.] MERCURY IN* MERCURIC COMPOUNDS. 367

not very large, it may be removed also as follows : The precipi-
tate is first washed with water, then fully dried, then repeatedly
washed with carbon disulphide (which must leave no residue on
evaporation), till a few drops of the washings evaporate on a
watch-glass without leaving a residue. (The precipitate is re-
tained on the filter throughout this operation.)
Properties of mercuric sulphide, 84.

4. Determination as Oxide.

. In the mercurous and mercuric salts of the nitrogen acids, the
metal may be very conveniently determined in the form of mer-
curic oxide (MARIGNAC *). For this purpose the salt is heated in
a bulb-tube, of which the one end, drawn out to a point, dips
under water, the other end being connected with a gasometer, by
means of which dry air is transmitted through the tube as long
as the application of heat is continued. In this way complete
decomposition of the salt is readily effected, without reaching the
temperature at which the oxide itself would be decomposed.

5. Volumetric Methods.

a. Precipitate as mercurous chloride as in 2, and treat the
Washed precipitate as in 117, 2, b.

b. According to LIEBIG f : This method depends upon the
fact that sodium phosphate precipitates mercury from solutions of
mercuric nitrate, but .not from mercuric chloride, in the form of
flocculent, white mercuric phosphate, which soon becomes crys-
talline ; and that therefore sodium chloride readily dissolves the
precipitate (as long as it is still amorphous), sodium phosphate
and mercuric chloride being formed. On knowing the quantity
of sodium chloride necessary to effect the solution of the precipi-
tate, that of the mercury is known also, since 2 eq. of sodium
phosphate are the equivalent of 1 eq. of mercuric oxide (as
phosphate).

a. Sodium-chloride /Solution: Decinormal sodium chloride
may be used. Every c. c. of this containing 0-00585 grm. NaCl
is the equivalent of 0-0108 HgO.

ft. Preparation of Mercuric-oxide Solution : This solution
must, of course, be free from all compounds of chlorine, iodine,
and bromine, and the mercury must be present as a mercuric

* Jahresber. von LIEBIG u. KOPF, 1849, 594.



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