HEAVY METALS

1. INTRODUCTION:

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 terms of the percentage (by weight) of lead in the test substance, as determined by concomitant visual comparison.

2. REAGENTS:
3. 1N Acetic acid: Dilute 60.0 mL of glacial acetic acid with water to make 1000 mL.
4. 6N Ammonium hydroxide: Dilute 400 mL of ammonium hydroxide (strength 29.0%) to 1000 mL with purified water.

• 6N Hydrochloric acid: Add slowly with stirring 510 mL of hydrochloric acid to   450 mL of purified water and dilute to 1000 mL with purified water.

3. pH 3.5 acetate buffer: Dissolve 25.0 g of Ammonium acetate in 25 mL purified water and  add  0  mL of  6N  hydrochloric  acid. Adjust, if necessary with 6N ammonium hydroxide or 6N hydrochloric acid to a pH of 3.5, dilute to 100 mL with purified water and mix.
4. Lead Nitrate Stock Solution: Dissolve 159.8 mg of lead nitrate in 100 mL of purified water to which has been added 1 mL of nitric acid, then dilute with purified water to 1000 mL. Prepare and store this solution in glass containers free from soluble lead salts.
5. 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 µg  of  (A  comparison  solution  prepared on the basis of   100 µL  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.)

• Thioacetamide TS: Dissolve 4 g of thioacetamide in 100-mL of purified water.
• Glycerin base TS: To 200 g of glycerin add purified water to bring total weight to 235 g. Then add 140 mL of 1N sodium hydroxide and 50 mL of purified water.

1. Thioacetamide-Glycerin base TS : Mix 1 mL of thioacetamide TS and 5 mL of  glycerin  base  TS, and  heat  in  a  boiling  water bath for 20 seconds. Use the mixture immediately.
2. 1 N Sodium Hydroxide TS: Dissolve 4.0 g of Sodium hydroxide in 100 ml of purified water and mix.

3. METHOD I
   • Standard preparation:
     • Into a 50-mL color-comparison tube pipette 2 mL of Standard Lead Solution (20 µg of Pb), and dilute with purified water to 25 mL.
     • Using a pH meter or short-range pH indicator paper as external indicator, adjust with 1N acetic acid or 6N ammonium hydroxide to a pH between 3.0 and 4.0, dilute with purified 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 current individual STP; or using  the designated volume  of  acid  where  specified   in   current individual STP, dissolve  in  purified water,   and  dilute  with  purified  water  to  25 mL  the  quantity, in g, of  the sample  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 with 1N  acetic acid or 6N ammonium hydroxide to a pH between 3.0 and 4.0,  dilute  with purified 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  0 mL  of  Standard Lead Solution.
  • Using a pH meter or short-range pH indicator paper as external indicator, adjust with 1N acetic acid or 6N ammonium hydroxide to a pH between 3.0 and 4.0, dilute  with purified 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 purified water  to 50 mL, mix and  allow to stand for 2 minutes.
  • 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 form 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.]

4. METHOD II

            Note: This method does not recover mercury.

• 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 / (1000 L)

     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.

        (Note: 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°C, until the carbon is completely burned off.
• Cool and add 4 mL of 6N 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.

5. 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 10mL of  purified 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 purified 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  0 mL of Standard Lead   Solution  (20 µg of  Pb), and  mix. Transfer to a 50-mL color-comparison tube, rinse the flask with purified water, adding the rinsing to the tube until the volume is   25 mL and mix.
   • Test preparation :

                     Unless otherwise indicated in the current individual STP, 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, as a percentage.

• If the substance is a solid: –
  • Transfer the weighed quantity of the test substance 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 45° and add sufficient of the 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 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 acid, 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 white, dense fumes, and continue heating until the volume is reduced to a few mL.
  • Cool, cautiously add 5mL 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 of 2 to 3 mL.
  • If the solution is still yellow in color, repeat the addition of 5 mL of purified water and the peroxide treatment.
  • Cool, dilute 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 weighed quantity of the test substance 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 45°, and cautiously add a few mL of a mixture of 8 mL of sulfuric acid and 10 mL of nitric acid.
• Proceed as mentioned under If the Substance is Solid from points 2.1.3 to 5.2.1.10.

• Monitor preparation:
  • Proceed with the digestion, using the same amount of sample and the same procedure as directed in the subsection 'If the substance is a solid in the section test preparation until the step no. 5.2.1.9
  • Add 2.0mL of Lead Standard solution (20 mg of Lead), and mix.
  • Transfer to a 50mL color comparison tube, rinse the flask with purified water, adding the rinsing to the tube until the volume is 25mL, and mix.
• Procedure :
  • Treat the  test  preparation, the  standard  preparation and the monitor preparation as follows:
  • Using a pH meter or short-range pH indicator paper as external indicator adjust the solution to a pH between 3.0 and 4.0, 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 2  mL of   pH  5   acetate   buffer, then  add 1.2  mL of  thioacetamide-glycerine base TS,  dilute with  purified  water  to  50 mL, mix,  allow to  stand  for 2 minutes, and   view  downward  over  a  white  surface: the   color   of   the   test  preparation  is  not  darker  than  that  of  the standard  preparation  and the monitor preparation is equal to or darker than that of the standard  preparation.

Heavy metals, hydrochloric, thioacetamide,