á251ñLEAD
The imposition of stringent limits on the amounts of lead that may be present in pharmaceutical products has resulted in the use of two methods,of which the one set forth following depends upon extraction of lead by solutions of dithizone.For determination of the content of heavy metals generally,expressed as a lead equivalent,see Heavy Metals á231ñ.
Select all reagents for this test to have as low a content of lead as practicable,and store all reagent solutions in containers of borosilicate glass.Rinse thoroughly all glassware with warm dilute nitric acid (1in 2),followed by water.

Special Reagents—
AMMONIA-CYANIDE SOLUTION— Dissolve 2g of potassium cyanide in 15mLof ammonium hydroxide,and dilute with water to 100mL.
AMMONIUM CITRATE SOLUTION— Dissolve 40g of citric acid in 90mLof water.Add 2or 3drops of phenol red TS,then cautiously add ammonium hydroxide until the solution acquires a reddish color.Remove any lead that may be present by extracting the solution with 20-mLportions of Dithizone Extraction Solution(see below),until the dithizone solution retains its orange-green color.
DILUTED STANDARD LEAD SOLUTION— Dilute an accurately measured volume of Standard Lead Solution(see Heavy Metals á231ñ)[containing 10µg of lead per mL],with 9volumes of dilute nitric acid (1in 100)to obtain a solution that contains 1µg of lead per mL.
DITHIZONE EXTRACTION SOLUTION— Dissolve 30mg of dithizone in 1000mLof chloroform,and add 5mLof alcohol.Store the solution in a refrigerator.
Before use,shake a suitable volume of the dithizone extraction solution with about half its volume of dilute nitric acid (1in 100),discarding the nitric acid.
HYDROXYLAMINE HYDROCHLORIDE SOLUTION— Dissolve 20g of hydroxylamine hydrochloride in sufficient water to make approximately 65mL.Transfer to a separator,add 5drops of thymol blue TS,then add ammonium hydroxide until the solution assumes a yellow color.Add 10mLof sodium diethyldithiocarbamate solution (1in 25),mix,and allow to stand for 5minutes.Extract this solution with successive 10-to 15-mLportions of chloroform until a 5-mLportion of the chloroform extract does not assume a yellow color when shaken with cupric sulfate TS.Add 3Nhydrochloric acid until the solution is pink (if necessary,add 1or 2drops more of thymol blue TS),and then dilute with water to 100mL.
POTASSIUM CYANIDE SOLUTION— Dissolve 50g of potassium cyanide in sufficient water to make 100mL.Remove the lead from this solution by extraction with successive portions of Dithizone Extraction Solution,as described under Ammonium Citrate Solutionabove,then extract any dithizone remaining in the cyanide solution by shaking with chloroform.Finally dilute the cyanide solution with sufficient water so that each 100mLcontains 10g of potassium cyanide.
STANDARD DITHIZONE SOLUTION— Dissolve 10mg of dithizone in 1000mLof chloroform.Keep the solution in a glass-stoppered,lead-free bottle,suitably wrapped to protect it from light,and store in a refrigerator.

Test Preparation— [NOTE—If,in the following preparation,the substance under test reacts too rapidly and begins charring with 5mLof sulfuric acid before heating,use instead 10mLof cooled dilute sulfuric acid (1in 2),and add a few drops of the hydrogen peroxide before heating.]Where the monograph does not specify preparation of a solution,prepare a Test Preparationas follows.[Caution—Exercise safety precautions in this procedure,as some substances may react with explosive violence when digested with hydrogen peroxide. ] Transfer 1.0g of the substance under test to a suitable flask,add 5mLof sulfuric acid and a few glass beads,and digest on a hot plate in a hood until charring begins.Other suitable means of heating may be substituted.(Add additional sulfuric acid,if necessary,to wet the substance completely,but do not add more than a total of 10mL.)Add,dropwise and with caution,30percent hydrogen peroxide,allowing the reaction to subside and again heating between drops.Add the first few drops very slowly,mix carefully to prevent a rapid reaction,and discontinue heating if foaming becomes excessive.Swirl the solution in the flask to prevent unreacted substance from caking on the walls of the flask.[NOTE—Add peroxide whenever the mixture turns brown or darkens.]Continue the digestion until the substance is completely destroyed,copious fumes of sulfur trioxide are evolved,and the solution is colorless.Cool,cautiously add 10mLof water,evaporate until sulfur trioxide again is evolved,and cool.Repeat this procedure with another 10mLof water to remove any traces of hydrogen peroxide.Cautiously dilute with 10mLof water,and cool.

Procedure— Transfer the Test Preparation,rinsing with 10mLof water,or the volume of the prepared sample specified in the monograph to a separator,and,unless otherwise directed in the monograph,add 6mLof Ammonium Citrate Solutionand 2mLof Hydroxylamine Hydrochloride Solution.(For the determination of lead in iron salts use 10mLof Ammonium Citrate Solution.)Add 2drops of phenol red TS,and make the solution just alkaline (red in color)by the addition of ammonium hydroxide.Cool the solution if necessary,and add 2mLof Potassium Cyanide Solution.Immediately extract the solution with 5-mLportions of Dithizone Extraction Solution,draining off each extract into another separator,until the dithizone solution retains its green color.Shake the combined dithizone solutions for 30seconds with 20mLof dilute nitric acid (1in 100),and discard the chloroform layer.Add to the acid solution 5.0mLof Standard Dithizone Solutionand 4mLof Ammonia-Cyanide Solution,and shake for 30seconds:the color of the chloroform layer is of no deeper shade of violet than that of a control made with a volume of Diluted Standard Lead Solutionequivalent to the amount of lead permitted in the sample under examination,and the same quantities of the same reagents and in the same manner as in the test with the sample.