Dissolve the specified quantity of the substance under examination in water, or prepare a solution as directed in the monograph, and transfer to a Nessler cylinder. Add 2 ml of a 20% w/v solution of iron-free citric acid and 0.1 ml of thioglycollic acid, mix, make alkaline with iron-free ammonia solution, dilute to 50 ml with water and allow standing for 5 minutes. Any color produced is not more intense than that obtained by treating in the same manner 2.0 ml of iron standard solution (20 ppm Fe) in place of the solution under examination.
The limit for lead is indicated in the individual monograph in terms of ppm, i.e., the parts of lead, Pb, per million parts (by weight) of the substance under examination.
The following method is based on the extraction of lead by solutions of dithizone.
All reagents used for the test should have as low a content of lead as practicable. All reagent solutions should be stored in containers of borosilicate glass. Glassware should be rinsed thoroughly with warm dilute nitric acid followed by water.
Transfer the volume of the prepared sample directed in the monograph to a separator and, unless otherwise directed in monograph, add 6 ml of ammonium citrate solution and 2 ml of hydroxylamine hydrochloride solution. (For the determination of lead in iron salts use 10 ml of ammonium citrate solution). Add two drops of phenol red solution and make the solution just alkaline (red in color) by the addition of strong ammonia solution. Cool the solution if necessary and add 2 ml of potassium cyanide solution. Immediately extract the solution with several quantities, each of 5 ml, of dithizone extraction solution, draining off each extract into another separating funnel, until the dithizone extraction solution retains its green color. Shake the combined dithizone solution for 30 seconds with 30 ml of a 1 per cent v/v solution of nitric acid and discard the chloroform layer. Add to the acid solution exactly 5 ml of dithizone standard solution and shake for 30 seconds; the color of the chloroform layer is not more intense than that obtained by treating in the same manner a volume of lead standard solution (1ppm Pb) equivalent to the amount of lead permitted in the substance under examination, in place of the solution under examination.
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