By Lawrence D. Margerum, University of San Francisco
Title: Post-Lab: Is there Lead in my Paint (using AAS)?
Assignment Goals
In this assignment you will:
1) Plot a standard calibration curve with typical Atomic
Absorption Spectrophotometer (AAS) data for lead calibration standards.
2) Calculate the % weight of lead in a paint sample from the
standard curve.
3) Write an essay in the form of a short formal laboratory report
(Introduction, Experimental, Results & Conclusion.
Read your Chemistry 111 lab handout: Is there Lead in my Paint?
Consult the Web Site on a working curve and linear regression.
Go the Consumer Product Safety Commission Web Site to get answers
about Lead Paint on Playground Equipment:
Source Material Resources:
Working
Curves and Linear Regression -
A web site from the Chemistry Department at Virginia
Tech.
URL: http://www.chem.vt.edu/chem-ed/data/wcurve.html
The
Consumer Product Safety Commission -
Questions and Answers: Lead Paint on Public
Playground Equipment
URL: http://www.cpsc.gov/cpscpub/pubs/lead/6007.html
HTML
Tutor - This website has information
about how to put in HTML tags so that your essay
looks correct when it is displayed. For this essay, pay close attention to
superscripts and paragraph formatting
URL: http://www.cwu.edu/~verheys/howto.html
By now, you have completed your experiment on Lead in Paint. The
purpose of this post-lab assignment is to give you practice analyzing simulated
AAS data so that you will have an easier time analyzing your own experimental
data. You will also be guided in writing a short formal laboratory report on
the simulated data.
First, go the Web sites listed under resources and read about
working curves and linear regression.
Second, you will work with a set of simulated data taken from the AAS.
Please follow these same steps in your lab handout.
Step A. One must make a standard or working plot of absorbance
(y-axis) versus lead concentration (x-axis) to analyze an unknown
concentration. Plot the data shown below using the Plotting Tool that is on the CD-ROM
from your textbook. Create the linear regression or least-squares fit line to
the data.
Lead Standards
Absorbance (no units)---> Lead Concentration (ppm)
0.0132 --------------------------------->2.10
0.0331 --------------------------------->4.90
0.0543 --------------------------------->10.00
0.1264 --------------------------------->19.99
0.2353 --------------------------------->40.10
Consult the directions in your lab handout under Results Sheet, Step A. Print out your
graph and rewrite the equation (by hand on your printout) as Absorbance = m[ppm
Pb] + b, where m is the slope and b is the y-intercept from the computer fit.
You will use this result to write your essay (this plot will not be handed in)
Step B: Unknown Results. You will now calculate the % Pb in the
unknown paint sample. This sample comes from playground equipment in a nearby
park. Start by weighing the sample, then doing an acid digestion directly in
10.0 mL of dilute nitric acid. Assume that the sample is filtered without
losing any volume and run directly on the AAS.
Mass of unknown Paint chip = 0.0245 g
AAS results with Lead Hollow Cathode Lamp
Absorbance Blank Reading=0.0017
Absorbance Reading from paint chip sample = 0.0976
Now, subtract the blank from your paint chip reading and use your
linear regression equation from Step A to solve for [ppm Pb] which is equal to
mg Pb/L.
Use the Wt % Lead = equation in your lab handout to calculate your
unknown answer. If you are having trouble with this equation, try the factor
label method:
[ppm Pb] = __ mg Pb/L X 1 L/1000 mL X 10.0 mL acid = __ mg Pb in
the paint chip.
__ mg Pb in paint chip/ ___ mg of unknown Paint chip X 100 = % wt
Pb in sample.
Significant Figures are based on measured values. Note that the
least significant figure in all the lead standards is the second digit past the
decimal (hundredths of ppm). Therefore, your ppm Pb answer and your % wt Pb
must be reported to the second decimal point (regardless of how many digits
there may be to the left of the decimal point!)
Finally, read the first few answers to the questions on the
Consumer Product Safety Commission: Questions and Answers: Lead Paint on Public
Playground Equipment Web site.
Make sure you can answer all of the guiding questions and proceed
to the writing prompt.
Guiding Questions to think about before writing a short formal
report: In studying the resources and writing your text, consider the issues
raised by the following questions:
1. What are you doing in this experiment and why? (An
Introduction)
2. How are you doing this experiment? (Experimental Section) Only
include the following:
a. What is a working curve and how did you construct yours for the
AAS experiment?
b. How does one use a working curve to find out the lead
concentration in the unknown?
3. What is the outcome of your experiment? How much lead did you
find in % wt Pb? (Results)
4. What have you learned? (Conclusion) What lead level is
identified in Title X of the 1992 Residential Lead-Based Paint Hazard Reduction
Act as a priority for hazard reduction activities (i.e. to remove the paint)?
How does your sample compare?
It turns out that the unknown paint chip came from playground
equipment at an old park. You are a working chemist at a company that tests
paint samples. You just got a new boss who does not know the AAS method you are
using. Write a short formal report to your new boss on your findings.
Your report must have the following headings and paragraphs in
your report:
A. Introduction (What are you doing? And Why?)
Start with a topic sentence and write a short paragraph (2-3 full
sentences) that describes what you are doing and why you are doing it.
B. Experimental Section (How are you doing it?)
Start with a topic sentence for each paragraph and answer
(briefly!) the following questions:
1. Use complete sentences to describe what a working curve is and
how you made it for the standard data given above.
2. Use complete sentences to describe the calculations you carried
out to find out the % wt of Pb in the paint sample (Do not just write out the
math formula, explain what you did).
C. Results and Conclusion (What is the outcome and what have you
learned?)
1. Start with a topic sentence and introduce the reader to the
results of your standard curve (an outcome). Was it a straight line? How good
was the straight line fit? Use the value of the Correlation Coefficient
(R-value) to support your argument.
2. Introduce your reader to your unknown result in % wt using the
correct number of significant figures based on the measured data (an outcome).
See student Instructions for a discussion of significant figures
3. In a final paragraph, answer the question: what have you learned?
Compare your paint chip to the value cited in 1992 Residential Lead-Based Paint
Hazard Reduction Act. Is your paint sample from the playground higher or lower
than that value?
Be brief and concise in your explanations. Remember that
plagiarism (copying word for word) is not acceptable from source material or
from other students.
Copyright )2003 by Lawrence D.
Margerum