Soil Digestion and Spectral Analysis for the Determination of
Lead Concentration in an Unknown Sample
Daniel A. Bolt, Samantha J. Paluck, L. Pierre, Matt K. Williams A
Department of Chemistry, Mercer University, 1400 Coleman Avenue, Macon, GA 31207
The lead concentration of a soil sample was found using a nitric acid digestion technique. The sample was analyzed at three
different wavelengths. Likewise, the standard solutions generated
three calibration curves, respectively. ICP-OES spectra were taken of all the standard solutions and the intensity by concentration
of lead was plotted. Those curves were then used to find the lead
concentration of the sample. The lead level present in the sample
was below the accepted value of 1,200 ppm.
Introduction
According to the Environmental Protection Agency (EPA), the
affects of lead range from causing violent behavior and learning
disabilities to even seizures and death.i Children under six years
of age are the most at-risk population, which is why there is a
more strict regulation for the lead content of soil in areas that
children play in (400 ppm) than in areas where they do not (1,200
ppm).2 The purpose of this experiment was to determine if a soil
sample taken fro m the yard of a restored house in Macon, Ga,
complied with these regulations.
Experimental
Before usage, all glassware, volumetric and otherwise,
was soaked in a 10% nitric acid bath for at least a week. All
chemicals were obtained from the Mercer University Chemistry
Department Stockroom. The water used to make solutions was
obtained from a Millipore system, which ensures ultra-purity.
Glassware was rinsed with deionized (DI) water after being removed from the acid bath, and dried with Kim Wipes so that no
particulate matter from paper towels would be left behind. All
mass measurements we performed using an analytical balance. A
soil sample was collected between 7.0-7.5 in below the surface of
the ground, and small rocks were removed from the sample.
1.0075 g of this sample was weighed out and placed in a 50 mL
beaker, which was then placed in an oven to dry for a week. The
temperature of the oven was ~110 °C. After drying, the sample
weighed 0.9917 g, which implies that 0.0158 g was lost due to
moisture content. 0.6028 g of the dried sample was weighed out
and then placed in a ceramic mortar. It was ground up by means
of a ceramic pestle for around 10 minutes. 10 mL of 50% nitric
acid was then added to a beaker, along with the ground sample.
The beaker was then placed in a heated sand bath, covered with a
watch glass, and heated for around 10 minutes, with the final
temperature being 117 °C. The beaker was then removed, an additional 5 mL of the 50% nitric acid solution was added, and the
solution was returned to the sand bath and left to reflux for 30
minutes. Once the solution had evaporated down to a final volume of ~5 mL, it was removed from the sand bath and transferred
to a 100 mL volumetric flask, where it was diluted to the mark
with ultrapure water. The resulting solution was then filtered us-
A
matt.kenrich.williams@live.mercer.edu
ing two sheets of Whatman No. 2 filter paper into a 250 mL beaker. A method blank was also constructed using the above procedure.
Standard solutions were worked up for the generation of
a calibration curve. 1 mL of 1000 ppb lead solution was added to
a volumetric flask. Dilutions were then performed, generating lead
solutions 0, 15, 25, 50, and 100 ppb concentrations. 0.723 mL of
15.8 M (concentrated) nitric acid was added to each solution,
which ensured 5% nitric acid content. These solutions were then
analyzed using Mercer University’s ICP-OES.
Results and Discussion
The soil sample was taken from 1493 Chestnut St.,
Macon, GA 31207. The sample was obtained from an area without vegetation, from the yard of an antique home that was previously painted with lead-based paint. In 2007, the house was restored, and complies with the current EPA standards. Though the
ICP-EOS analysis of the sample was done at three different wavelengths, 220.353 nm, 217.000 nm, and 261.48 nm, the wavelengths were then adjusted to align with the wavelengths corresponding to the maximum value of lead intensity. These adjusted
wavelengths were 220.355 nm, 216.999 nm, and 261.422 nm,
respectively. The standard solutions were then analyzed at each of
these wavelengths and the resulting spectra showed the relative
intensities (Figure 1).
Figure 1. The intensities of lead emission for the calibration solutions at the three adjusted wavelengths (From L to R: 220.355
nm, 216.999 nm, 261.422 nm).
This data was then converted into actual calibration curves (Figure 2), which were then used to back-calculate the concentration
of lead present in the sample based on the samples intensities at
the varying wavelengths.
Figure 2. Calibration Curve for the Corrected Wavelengths.
Likewise, the method blank was also analyzed in this manner to
see if there were any sources of lead in our method, which could
lead to a systematic error in our data analysis. The concentration
of the entire soil sample (1.0075 g) was then calculated based on
the known concentration of the 0.6028 g aliquot that was analyzed. The data for all three wavelengths is summarized below in
Table 1.
Table 1. Lead Concentration of Aliquot, Total Sample, and
Method Blank.
220.355
nm
216.999
nm
261.422
nm
Average*
Aliquot Concentration
142.616
ppm
134.333
ppm
14.611
ppm
138.474
ppm
Total Sample
Concentration
140.413
ppm
132.258
ppm
14.385
ppm
136.335
ppm
Method Blank
Concentration
10.237
ppb
9.022
pbb
-4.471
ppb
9.630 ppb
*The average concentration was found using only data from the
first two columns, as the data from 261.422 nm was deemed inaccurate, due to the presence of the nearby iron peak, circled below
in Figure 3.
Figure 3. The interferring iron peak is circled in orange. Notice
how its intensity is much greater at 261. 418 nm than the adjacent
lead peak.
Based on the current EPA guidelines, our sample falls well below
the legal limit of 400 ppm for children’s play area, as well as the
1,200 ppm limit for non-play areas. The lead present in the soil
can either be attributed to the former paint job, or the fact that
leaded gasoline was previously used in the United States.
Conclusion
The soil sample taken from historic Macon, GA, was
disgested by means of a nitric acid bath and the concentration of
lead present in the soil was found using ICP-OES analysis. The
sample’s emission spectra was compared to a generated
calibration curve to find the concentration of the lead present. The
the concentration of the soil ranged between 132. ppm and 143.
ppm, well below both EPA standards for soil.2
Acknowledgements
I would like to thank Dr. Jennifer Look for her
oversight of the projet, as well as for soil her collection. I would
also like to thank Mr. Gary McInvale, Mrs. Evelyn Prater, and the
handful of other stockroom workers for their continual help in
retrieving the required chemicals, additional glassware, and
apparatuses. Lastly, I would like to thank Mercer University for
funding the Chemistry Department and providing the needed
instrumentation, glassware, chemicals, etc.
1United
States
Environmental
Protection
Agency.
http://epa.gov/lead/index.html (15 February 2011).
2Environmental
Protection
Agency.
http://www.epa.gov/fedrgstr/EPA-TOX/2001/January/Day-05/t84.pdf
(15
February
2011).
Table of Contents Figure
220.355 nm
216.999 nm
261.422 nm
Average*
Aliquot Concentration
142.616 ppm
134.333 ppm
14.611 ppm
138.474 ppm
Total Sample Concentration
140.413 ppm
132.258 ppm
14.385 ppm
136.335 ppm
Method Blank Concentration
10.237 ppb
9.022 pbb
-4.471 ppb
9.630 ppb
3