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Testing for Lead can prove to be difficult when you are dealing with aqueous solutions, mixtures of liquids within water. In Flint, Michigan, lead is part of liquid compounds that are immersed in the water-stream, so one way to test for lead is to take a sample and form a precipitate, an insoluble(doesn’t dissolve in water) solid that emerges from a chemical reaction. Follow the chart below to see how our class used precipitation reactions to isolate lead from a sample.
Potassium Iodide - 2KI
To start out, a precipitation reaction must mix two aqueous compounds together. Each compound contains a positive ion(charged atom) and a negative ion. Lead's ion is positive, so we combined it with the negative substance nitrate. The other compound was a mix between potassium(+) and iodide(-). Note that it really doesn't matter what the negative compounds are, as long as the resulting reaction will lead to a solid compound with lead.
Aqueous Solution - A solution with water as its solvent
Aqueous Solution - A solution with water as its solvent
Lead Nitrate - Pb(NO3)2
Full Equation: 2KI + Pb(NO3)2 → 2KNO3 + PbI2(solid)
Next, a reaction occurs when these two substances are combined, forming a Precipitate, or insoluble solid. The positive ions, potassium and lead, switch with each other, forming new compounds which interact differently with water. In this case, the negative iodine ions and the positive lead atoms formed a compound that didn't dissolve in water, allowing us to measure the amount of lead as a solid.
The lead and the iodide react and form a precipitate. Here's the underlying equation:
2I- + Pb2+ → PbI2
Precipitate
Precipitate
An insoluble (doesn't dissolve in water) solid that emerges from a liquid solution (mixture between a solid and a liquid)
Precipitation Reaction
Precipitation Reaction
Formation of a precipitate (an insoluble salt) when two solutions containing soluble salts are combined
To learn more about precipitates, try this fun simulation
Double Replacement Reaction
Double Replacement Reaction
The reactants cations switch places to form a solid
AB + CD --> AD + CB
In order to identify how much lead we originally had, we need to know the limiting reactant, the reactant that runs out first when combined with the other reactant. In our case, lead nitrate was the limiting reactant because we were testing for the amount of lead present, which means that we had an excess of potassium nitrate.
Limiting Reactant
Limiting Reactant
The reactant that runs out first and limits the amount of product formed
Finally, we used a technique called gravimetric analysis to determine how much lead we originally had. To do this, we started by taking the solid product that formed, lead iodide, and filtering it, drying it, and weighing it for its mass. From there, we converted the mass from grams to moles, a universal unit of measure that allows for comparison between reactants and products. And once we knew the molar mass of lead iodide, we knew how much lead was originally in our sample of lead nitrate.
Gravimetric Analysis
Gravimetric Analysis
A method of analysis that determines how much of a given ion is present based on mass. It uses a precipitation reaction to form an insoluble product form of the ion of interest
Molar Mass
Molar Mass
The mass of one mole(6.02*1023)of an element or compound
Testing for Lead in Flint:
Although gravimetric analysis and precipitation reactions are great to measure the amount of lead in a small sample, it would be hard to do so for all of Flint Michigan. Researchers there used different, but similar, methods to find out the lead concentration, in parts per billion, in Flint water. Though the EPA warned that levels of 15ppb or higher were dangerous, researchers from Virginia Tech found levels that were much higher.
Parts Per Billion
Parts Per Billion
Parts per billion, or ppb, is a ratio that compares how many parts of something there are in a billion units of math
Click here to see the results to the Flint Water study:
Work Cited
Work Cited
Green Facts. "Parts per Billion." Green Facts. Accessed November 17, 2020. https://www.greenfacts.org/glossary/pqrs/parts-per-billion.htm.
Ingraham, Christopher. "This is how toxic Flint's water really is." Washington Post (Washington, DC), January 15, 2016. Accessed November 17, 2020. https://www.washingtonpost.com/news/wonk/wp/2016/01/15/this-is-how-toxic-flints-water-really-is/.
Mantha, Anurag, and Siddhartha Roy. "Hazardous Waste-levels of Lead found in a Flint household's water." Flint Water Study Updates. Last modified August 24, 2015. Accessed November 17, 2020. http://flintwaterstudy.org/2015/08/hazardous-waste-levels-of-lead-found-in-a-flint-households-water/.
Parks, Jeffery, and Anurag Mantha. "[Complete Dataset] Lead results from Tap Water Sampling in Flint, MI." Flint Water Studies Updates. Last modified December 1, 2015. Accessed November 17, 2020. http://flintwaterstudy.org/2015/12/complete-dataset-lead-results-in-tap-water-for-271-flint-samples/.
Smith, Lindsey. "This mom helped uncover what's really going on with Flint's water." Michigan Radio (Ann Arbor, MI), December 14, 2014. Accessed November 17, 2020. https://www.michiganradio.org/post/mom-helped-uncover-what-was-really-going-flint-s-water#stream/0.
Bonnier Corporation. "How Did Lead Get Into Flint River Water?" Popular Science. Last modified January 26, 2016. Accessed November 29, 2020. https://www.popsci.com/whats-wrong-with-flint-river/.
Compound Interest. "Lead in the Water – The Flint Water Crisis." Compound Interest. Last modified January 25, 2016. Accessed November 29, 2020. http://www.compoundchem.com/2016/01/25/flint-water/.
The Electrochemical Society. "Corrosion and Corrosion Prevention." The Electrochemical Society. Accessed November 29, 2020. https://www.electrochem.org/corrosion-science/.
Oram, Brian, PG. "Pipe Corrosion Lead Pipe Metal Water Pipes Corrosive Drinking Water." Water Research Center. Accessed November 29, 2020. https://water-research.net/index.php/corrosive-water#:~:text=Corrosion%20is%20a%20complex%20series,their%20more%20stable%20ore%20state.
Torrice, Michael. "How Lead Ended Up in Flint's Tap Water." Chemical & Engineering News, February 11, 2016. Accessed November 29, 2020. https://cen.acs.org/articles/94/i7/Lead-Ended-Flints-Tap-Water.html.
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