-
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/(1000
L
),
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
—
Pipet 4 mL of
the
Standard Lead Solution
into a suitable test tube, and
add 10 mL of 6 N hydrochloric acid.
Test
Preparation
—
Use a quantity,
in g, of the substance to be tested as calculated
by the formula:
4.0/(1000
L
),