Determine the amount of heavy metals by Method I, unless otherwise specified in the individual monograph. Method I 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 utilized. 

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 mg of lead. A comparison solution prepared on the basis of 100 mL 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 

Standard Preparation:  Into a 50 mL color-comparison tube pipet 2 mL of Standard Lead Solution (20 mg of Pb), and dilute with water to 25 mL. Adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, using short range pH indicator paper as external indicator, 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 water, 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, in percentage. Adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, using short-range pH indicator paper as external indicator, 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. Adjust with 1 N acetic acid or 6 N ammonium hydroxide to a pH between 3.0 and 4.0, using short-range pH indicator paper as external indicator, 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, respectively, add 10 mL of freshly prepared hydrogen sulfide TS, mix, allow to stand for 5 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 intensity of the color of the Monitor Preparation is equal to or greater than that of 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 

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 500º to 600º , 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 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, respectively, add 10 mL of freshly prepared hydrogen sulfide TS, mix, allow to stand for 5 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 

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, and 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 of water, add 2.0 mL of Standard Lead Solution (20 mg of Pb), and mix. Transfer to a 50 mL color-comparison tube, rinse the flask with water adding the rinsing to the tube until the volume is 25 mL and mix. 

Test Preparation:  If the substance is a solid- Transfer the quantity of the test substance specified in the individual monograph to a clean, dry 100-mL Kjeldahl flask [NOTE-A 300-mL flask may be used the reaction foams excessively], clamp the flask at angle of 45º, and add sufficient of a mixture of 8 mL of sulfuric acid and 10 mL of nitric acid to moisten the substance thoroughly. Warm gently until the reaction commences, allow the reaction to subside, and add additional portions of the same acid mixture, heating after each addition, until a total of 18 mL of the acid mixture has been added. Increase the amount of heat and boil gently until the solution darkens. Cool, add 2 mL of nitric aid and again heat until the solution darkens. Continue the heating, followed by addition of nitric acid until no further darkening occurs, then heat strongly to the production of dense, white fumes. Cool, cautiously add 5 mL of water, boil gently to the production of dense, white fumes, and continue heating until the volume is reduced to a few mL. Cool, cautiously add 5 mL of water and examine the color of the solution. If the color is yellow cautiously add 1 mL of 30 percent hydrogen peroxide, and again evaporate to the production of dense, white fumes and a volume 2 to 3 mL. If the solution is still yellow in color, repeat the addition of 5 mL of water and the peroxide treatment. Cool, dílute cautiously with a few mL of water, and rinse into a 50-mL color comparison tube, taking care that the combined volume does not exceed 25 mL.  

If the substance is a liquid- Transfer the quantity of the test substance specified in the individual monograp to a clean, dry 100-mL Kjeldahl flask [NOTE-A 300-mL flask may be used if the reaction foams excessively], clamp the flask at an angle of 45º, and cautiously add a few mL of a mixture of  8 mL of sulfuric acid and 10 mL of nitric acid. Warm gently until the reaction commences, allow the reaction to subside, and proceed as directed under If the substance is a solid, beginning with "add additional portions of the same acid mixture." 

Procedure:  Treat the Test Preparation and the Standard Preparation as follows: Adjust the solution to a pH between 3 and 4 using short-range pH indicator paper as external indicator, with ammonium hydroxide (a dilute ammonia solution may be used, if desired, as the specified range is approached) dilute with water to 40 mL, and mix.  To each tube add 10 mL of freshly prepared hydrogen sulfide TS, mix, allow to stand for 5 minutes, and view downward over a white surface: the color of the Test Preparation is not darker than that of the Standard Preparation.