美国药典重金属全新中英文

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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