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231
HEAVY METALS
This test is provided to demonstrate
that the content of metallic impurities that are
colored by sulfide ion,
under the
specified test conditions, does not exceed the
Heavy metals limit specified in the individual
monograph in percentage (by weight) of
lead in the test substance, as determined by
concomitant visual
comparison (see
Visual Comparison in the section Procedure under
Spectrophotometry and
Light-Scattering
851
) with a control prepared
from a Standard Lead Solution.
[note
—
Substances
that typically will respond to this
test are lead, mercury, bismuth, arsenic,
antimony, tin, cadmium, silver,
copper,
and molybdenum.]
Determine
the amount of heavy metals by Method I, unless
otherwise specified in the individual
monograph. Method I is used for
substances that yield clear, colorless
preparations under the specified
test
conditions. Method II is used for substances that
do not yield clear, colorless preparations under
the
test conditions specified for
Method I, or for substances that, by virtue of
their complex nature, interfere
with
the precipitation of metals by sulfide ion, or for
fixed and volatile oils. Method III, a wet-
digestion
method, is used only in those
cases where neither Method I nor Method II can be
used.
Special Reagents
Lead Nitrate Stock
Solution
—
Dissolve 159.8 mg
of lead nitrate in 100 mL of water to which has
been
added 1 mL of nitric acid, then
dilute with water to 1000 mL. Prepare and store
this solution in glass
containers free
from soluble lead salts.
Standard Lead
Solution
—
On the day of use,
dilute 10.0 mL of Lead Nitrate Stock Solution with
water to
100.0 mL. Each mL of Standard
Lead Solution contains the equivalent of 10
?
g of lead. A comparison
solution prepared on the basis of 100
?
L of Standard Lead Solution per g of
substance being tested
contains the
equivalent of 1 part of lead per million parts of
substance being tested.
Method I
pH 3.5
Acetate Buffer
—
Dissolve
25.0 g of ammonium acetate in 25 mL of water, and
add 38.0 mL of 6 N
hydrochloric acid.
Adjust, if necessary, with 6 N ammonium hydroxide
or 6 N hydrochloric acid to a pH of
3.5, dilute with water to 100 mL, and
mix.
Standard
Preparation
—
Into a 50-mL
color-comparison tube pipet 2 mL of Standard Lead
Solution (20 ?
g
of Pb), and
dilute with water to 25 mL. Using a pH meter or
short-range pH indicator paper as external
indicator, adjust with 1 N acetic acid
or 6 N ammonium hydroxide to a pH between 3.0 and
4.0, dilute with
water to 40 mL, and
mix.
Test
Preparation
—
Into a 50-mL
color-comparison tube place 25 mL of the solution
prepared for the test
as directed in
the individual monograph; or, using the designated
volume of acid where specified in the
individual monograph, dissolve in and
dilute with water to 25 mL the quantity, in g, of
the substance to be
tested, as
calculated by the formula:
2.0/(1000L)
in which L is
the Heavy metals limit, as a percentage. Using a
pH meter or short-range pH indicator paper
as external indicator, adjust with 1 N
acetic acid or 6 N ammonium hydroxide to a pH
between 3.0 and
4.0, dilute with water
to 40 mL, and mix.
Monitor
Preparation
—
Into a third
50-mL color-comparison tube place 25 mL of a
solution prepared as
directed for Test
Preparation, and add 2.0 mL of Standard Lead
Solution. Using a pH meter or
short-
range pH indicator paper as external indicator,
adjust with 1 N acetic acid or 6 N ammonium
hydroxide to a pH between 3.0 and 4.0,
dilute with water to 40 mL, and mix.
Procedure
—
To
each of the three tubes containing the Standard
Preparation, the Test Preparation, and
the Monitor Preparation, add 2 mL of pH
3.5 Acetate Buffer, then add 1.2 mL of
thioacetamide
–
glycerin
base TS, dilute with water to 50 mL,
mix, allow to stand for 2 minutes, and view
downward over a white
surface
: the color of the
solution from the Test Preparation is not darker
than that of the solution from the
Standard Preparation, and the color of
the solution from the Monitor Preparation is equal
to or darker
than that of the solution
from the Standard Preparation.
[note
—
If the color of the
Monitor Preparation is
lighter than
that of the Standard Preparation, use Method II
instead of Method I for the substance being
tested.]
Method II
note
—
This method
does not recover mercury.
pH 3.5 Acetate
Buffer
—
Prepare as directed
under Method I.
Standard
Preparation
—
Prepare as
directed under Method I.
Test Preparation
—
Use a quantity, in g, of the substance to be
tested as calculated by the formula:
2.0 / (1000L)
in which L is
the Heavy metals limit, in percentage. Transfer
the weighed quantity of the substance to a
suitable crucible, add sufficient
sulfuric acid to wet the substance, and carefully
ignite at a low
temperature until
thoroughly charred. (The crucible may be loosely
covered with a suitable lid during the
charring.) Add to the carbonized mass 2
mL of nitric acid and 5 drops of sulfuric acid,
and heat cautiously
until white fumes
no longer are evolved. Ignite, preferably in a
muffle furnace, at 500
to
600
, until the
carbon is
completely burned off. Cool, add 4 mL of 6 N
hydrochloric acid, cover, digest on a steam bath
for 15 minutes, uncover, and slowly
evaporate on a steam bath to dryness. Moisten the
residue with 1
drop of hydrochloric
acid, add 10 mL of hot water, and digest for 2
minutes. Add 6 N ammonium
hydroxide
dropwise until the solution is just alkaline to
litmus paper, dilute with water to 25 mL, and
adjust
with 1 N acetic acid to a pH
between 3.0 and 4.0, using short-range pH
indicator paper as an external
indicator. Filter if necessary, rinse
the crucible and the filter with 10 mL of water,
combine the filtrate and
rinsing in a
50-mL color-comparison tube, dilute with water to
40 mL, and mix.
Procedure
—
To
each of the tubes containing the Standard
Preparation and the Test Preparation, add 2
mL of pH 3.5 Acetate Buffer, then add
1.2 mL of
thioacetamide
–
glycerin base
TS, dilute with water to 50
mL, mix,
allow to stand for 2 minutes, and view downward
over a white surface
: the color of the
solution
from the Test Preparation is
not darker than that of the solution from the
Standard Preparation.
Method III
pH
3.5 Acetate Buffer
—
Prepare
as directed under Method I.
Standard
Preparation
—
Transfer a
mixture of 8 mL of sulfuric acid and 10 mL of
nitric acid to a clean, dry,
100-mL
Kjeldahl flask, and add a further volume of nitric
acid equal to the incremental volume of nitric
acid added to the Test Preparation.
Heat the solution to the production of dense,
white fumes; cool;
cautiously add 10 mL
of water; and, if hydrogen peroxide was used in
treating the Test Preparation, add
a
volume of 30 percent hydrogen peroxide equal to
that used for the substance being tested. Boil
gently
to the production of dense,
white fumes. Again cool, cautiously add 5 mL of
water, mix, and boil gently to
the
production of dense, white fumes and to a volume
of 2 to 3 mL. Cool, dilute cautiously with a few
mL
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