This experiment is based on an experiment titled Atomic Absorption - Measuring Metals, obtained from the Chemistry Department at Roanoke College, Salem, Virginia. It is used with the department's permission.Introduction:
Older paints (pre-1977) can be as much as 5% lead by weight (50,000 ppm), and a single 1 cm2 paint chip might contain 175 μg of lead. Current standards for paint are much more restrictive, allowing only 600 ppm lead in paint, making the same size paint chip contain only 6 μg of lead. That might still sound like a lot, but it is much less than in old paint, and is in the range of lead amounts that our bodies can handle and successfully excrete.Procedure:You might find some helpful information that you can use in your introduction regarding lead and its dangers to people, especially children at the following URL: http://www.hud.gov/lea/leadhelp.html You can also use a search engine such as www.google.com to look for other sites with information about lead hazards.
Caution: Concentrated acids are used in this experiment to digest the paint samples. Take care to avoid skin contact with any of the solutions in this experiment, and wash your hands immediately, if you do get liquid on them. Safety goggles and a lab apron must be worn at all times during this lab experiment.
Part A. Acid Digestion of the pain chip samplesResults:Weigh out 0.1 to 0.5 grams of paint chip sample to three (3) decimal places in a zeroed 150 mL beaker and then add 30 mL distilled water and 10 mL concentrated HNO3. (Caution: concentrated nitric acid is corrosive to skin and clothing.) Add 3-4 boiling stones and, under your lab station hood, slowly boil on a hot plate until the volume is reduced to about half. Add another 10 mL of concentrated nitric acid and heat until the volume is reduced to about 10-15 mL. Then rinse down the sides of the beaker and filter the sample into a 100 mL volumetric flask and dilute to the mark with distilled water. Make sure that the solution is not cloudy, otherwise it must be filtered again before proceeding.
It is possible that the paint chips have a concentration of lead that will be out of the range for the wavelength chosen for the analysis, therefore, you are going to make further dilutions of your samples. Pipet 1.00 mL of each paint chip solution into a 10 mL volumetric flask and dilute to volume with distilled water to give a 1:10 dilution. Then pipet another 1.00 mL of each paint chip solution into a 100 mL volumetric flask and dilute to the line. This will give a 1:100 dilution.
Part B. Preparation of the lead standards
Use the 1000 ppm lead stock solution provided by the instructor. Prepare dilutions of 5, 10, and 20 ppm in 100 mL volumetric flasks. Use 0.002M nitric acid to dilute the solutions.
Part C. Analysis of the samples
When your group has finished preparing the paint chip samples and the lead standards, notify the instructor that you are ready for the AAS analysis. Be sure to record all of the parameters for the setup of the AAS, e.g. wavelength used, bandwidth, voltage. Record the 3rd - 5th readings for each standard and then for each paint chip sample.
Make a Beer's Law plot using the absorbance of each standard, get the equation of the line, and solve for the concentration of the paint chip samples. Calculate the total amount of lead present in the original sample, multiplying by the dilution factor if needed. Report the final concentrations as μg of lead per gram of paint (μg Pb/g).Collection of the paint chip samples
Find a location where you suspect that the paint is older than 25 years. Collect three samples of the paint chips that are each about 0.5 gram in size. Store your samples in clean "baggies". Label them with an ID Number and keep them well closed until you bring them to class. Fill out the following information for each location on 3" x 5" note cards.Discussion:Person collecting the sample:
Date sample was collected:
Sample ID Number:
Full description of sampling location: Include both location of sampling site and important nearby factors, e.g. condition of paint, nearby metal objects, etc.
Description of sample's physical appearance right after sampling (color, cleanliness, odor, etc.)
Why was this particular sample chosen? What lead results are expected? Why?
What do your results mean? Do the owners of the facility from where the paint chips were obtained need to be concerned about what the results of the analysis show? Why?