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As we know that if you mix lead nitrate and sodium iodide solutions together, a yellow precipitate forms. Can you maybe add nitrate in water that may or may not have lead in it and then mix it with a sodium iodide solution and get the same results if lead was present in the water?

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closed as off-topic by Todd Minehardt, user55119, Mathew Mahindaratne, Mithoron, F'x Jul 2 at 8:36

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First, the iodide precipitates with lead ions, $\ce{Pb+}$, called cations because of the positive charge. It doesn't matter what the anions are in solution, whether nitrate, acetate or other, so there's no need to add nitrates or nitric acid for this test.

Second, yes, this is a test for lead in solution, but it is not at all sensitive. At room temperature, about 0.75 g of the product, lead iodide or $\ce{PbI2}$, could dissolve in a liter of water. That means that there could be a fatal amount of lead in water, yet that wouldn't be enough to form a yellow precipitate. One approved test for lead in drinking water is an atomic absorption (AA) spectrophotometer.

This reaction is not sensitive enough to be a useful test for lead, and the amount of lead needed to show a positive reaction is dangerous, so this should not be done by unwary students (unless the teacher is trying to reduce class size). Finally, safe disposal of the chemicals would be an issue.

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