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First you have to identify a compound of lead that is insoluble in water, such as lead carbonate, lead fluoride or lead sulfite. You would then add a solution of a soluble compound that provides the necessary anion to react with lead to produce the insoluble compound. For example you could add sodium carbonate, to precipitate lead carbonate. If the waste sample is acidic however, that would not be a good choice since carbonate is unstable in acid. Then you might use sodium fluoride and precipitate lead fluoride. You also have to make sure that the compound you are adding is not likely to react with other ions to produce other insoluble compounds. Sodium fluoride is a good choice because all sodium salts and almost all fluoride salts are readily soluble in water.