Materials Included In Kit

Additional Materials Required

Prelab Preparation

Several days in advance

1. Prepare soil sample A by mixing 75 mL of 0.5 M lead nitrate solution with 150 g of soil in a large weighing dish and allow to air dry.
2. Prepare soil sample B by mixing 75 mL of 1 M lead nitrate solution with 150 g of soil in a large weighing dish and allow to air dry.
3. Prepare soil sample C by mixing 75 mL of deionized water with 150 g of soil in a large weighing dish and allow to air dry.

The day of the laboratory prepare the 0.2% w/v% sodium rhodizonate solution.

4. Use a clean graduated cylinder to add 100 mL of deionized water to the bottle containing 0.2 g of sodium rhodizonate.

Safety Precautions

Lead nitrate is moderately toxic by inhalation and ingestion, a body tissue irritant and a possible carcinogen. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The waste lead nitrate, filtrate and contaminated soil may be disposed of by a licensed hazardous waste disposal company according to Flinn Suggested Disposal Method #27f. Excess sodium rhodizonate solution may be disposed of down the drain with an excess of water according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the Post-Lab Questions may be completed the day after the lab.
• Excess 0.2% sodium rhodizonate solution can be used to test for lead on other substances. The solution must come in contact with the source of lead. For example, a wall with one or more layers of lead-free paint will show a negative result even though lead paint exists under the top layers. Sinkers, metal strips, or minerals such as galena are just a few possible samples.
• For best results use the 0.2% sodium rhodizonate solution the day that it was prepared.

Teacher Tips

• Extend the activity by having students research the health problems associated with lead, lead testing procedures, lead abatement procedures or historical uses of lead.
• For more information about lead in the home, the EPA published a 67-page pamphlet in 1998 titled Lead in Your Home: A Parent’s Reference Guide. EPA publication number 747-B-98-002. The publication can be found online at http://www.epa.gov/lead/pubs/leadrev.pdf (Accessed April 2008).

Answers to Prelab Questions

1. List the four lead testing procedures discussed in the Background section. In your own words, describe the advantages and disadvantages of each testing procedure.

   Atomic absorption testing is accurate but the instrumentation is expensive and the samples must be sent away to an analytical testing laboratory.
   Inductively coupled plasma mass spectrometry is accurate but the instrumentation is expensive and the samples must be sent away to an analytical testing laboratory.
   Portable X-ray fluorescence is convenient since the testing is conducted in the field. XRF is able to accurately test for lead; however, the instrument contains a radioactive source so it can only be used by trained field analysts.
   Sodium rhodizonate testing is an inexpensive field test but it is not always accurate and the solution has a short lifespan.

2. Many field methods for lead include soaking the sample in an acid solution prior to testing with a 0.2% sodium rhodizonate solution. Explain why this step is included in many field protocols.

   Acid is added to reduce the pH to the point at which the lead will leach from the matrix.

Sample Data

Answers to Questions

1. Based on the color of the filtrate and rhodizonate indicate on the data table whether or not each soil sample contained lead.
2. Real estate listing sheets include a line that states if a house was built before 1978. Speculate why this information is important for a potential buyer.

   If the house was built prior to 1978, the potential buyer may want to have lead testing completed on the home before purchasing the house. Houses built after 1978 are less likely to contain lead paint so testing would not be necessary.

3. Design one day of a diet plan that would hinder the amount of lead absorbed by the body.

   The diet should be high in calcium, iron, zinc, and vitamin C, for example, lean red meat, fish, chicken, iron-fortified cereals, dried fruit, milk, yogurt, cheese, green leafy vegetables, citrus fruit and juice.
   Breakfast—iron-fortified cereal, milk and orange juice
   Lunch—spinach salad with orange wedges and a yogurt-based dressing plus a glass of milk.
   Dinner—grilled chicken breast with marinara sauce, broccoli with cheese sauce and a bowl of strawberries.

4. Assuming the soil samples analyzed in this laboratory had been collected from an empty lot where a neighborhood plans to start a community vegetable garden and playground. Would you recommend they continue with their plans? Justify your suggestion with information from the Background section and other sources.

   Student answers will vary.

References

Environmental Protection Agency website http://www.epa.gov/lead/ (accessed April 2008).

National Library of Medicine Tox Town website http://toxtown.nlm.nih.gov/text_version/chemicals.php?id=16 (accessed April 2008).

National Safety Council website http://www.nsc.org/issues/index.htm (accessed April 2008).

U.S. Department of Housing and Urban Development, Residential Lead-Based Paint Hazard Reduction Act of 1992. http://www.hud.gov/offices/lead/enforcement/disclosure.cfm (accessed April 2008).

Introduction

Lead is a naturally-occurring element. It is a heavy, soft, bluish-gray metal. Lead is a stable element; it does not decompose over time. Instead, lead accumulates in the soil and sediment around areas where lead is an airborne particulate byproduct of manufacturing or where deteriorating lead paint settles.

Concepts

• Metal solubility
• Bioaccumulation
• Analysis of metals

Background

The processes of life are a series of chemical reactions that occur in the body of the living organism. Living things eat or absorb compounds composed of elements from the environment surrounding them. Many elements are beneficial while other elements can be harmful to the organism. Lead is an example of an element that is harmful to human beings. The body absorbs lead into the bloodstream. From the bloodstream, lead mimics calcium and bioaccumulates in bones where it can slowly leach out into the bloodstream for an entire lifetime. Lead also affects other systems that depend on calcium such as nerves, muscles and kidneys. Lead is able to cross the blood–brain barrier where it causes damage to all areas of the brain. Lead exposure can lead to learning disabilities, behavioral problems, decreased intelligence, as well as speech, language, hearing, and muscle coordination issues (see Figure 1). Children are especially vulnerable to lead poisoning. Children need a lot of calcium for both bone and brain growth. If not enough calcium is available, lead will be absorbed instead. Children tend to absorb larger doses of lead due to both their smaller size and the habit of putting objects or hands into their mouths. Very high doses of lead can cause mental retardation, coma and even death. In adults, lead exposure can cause nerve, muscle and memory problems, increased blood pressure and even fertility problems.

Lead exposure can come from a work environment or from around the house. Adults who work around lead should be aware of the hazard and take precautions to ensure the lead does not come home with them. Sources of lead around the house are deteriorating lead paint, lead in water or lead in soil around the home. Lead was added to paint to brighten colors and as an additive to provide longevity for the paint. As lead paint deteriorates due to age or abrasion, small particles of paint become airborne before settling as dust over surfaces such as the floor or the soil around the outside of the house. Lead house paint was manufactured until 1978.

Also until 1978, lead was added to gasoline to help stop engine knocking and created a more efficient combustion engine. The lead was not consumed in the combustion reaction however. Instead it was expelled as small particles into the atmosphere as part of the exhaust. Precipitation caused the small lead particles, called particulate, to wash off buildings into the soil or storm drains where it ran into streams, rivers and lakes.

Due to the persistence of lead, most children have their blood tested for lead before the age of two. If a high level of lead in the blood is discovered, the child will undergo medical treatment and the parents are advised to determine the source of the lead so that the lead exposure can be diminished. If the blood lead levels are high enough the doctor may prescribe treatment to reduce the level of lead in the body as well as changing the diet of the affected individuals. A diet high in calcium, iron, zinc and vitamin C will reduce the amount of lead absorbed by the body.

Analytical testing of toys, water, paint and soil all involve acid extraction of lead from the substrate followed by analysis using an analytical instrument such as an atomic absorption spectrophotometer (AA) or an inductively coupled plasma mass spectrometer (ICP-MS). These analytical instruments capitalize on the physical and chemical properties of the lead. Lead becomes soluble at low pH so treatment with acid will leach lead from the non-lead matrix. The analytical instrument converts lead ions into an atomic state where they absorb light at a specific wavelength. The light absorbed by the lead in the unknown samples are compared to the amount of light absorbed by samples with a known amount of lead. These analytical tests are very accurate and the labs that perform the tests are often certified by a national accreditation organization.

It would be very expensive to test all areas around a house for lead using laboratory testing. A field screening process is often used to decrease the number of samples that must be sent to the laboratory for analysis. Two common methods are portable X-ray fluorescence (XRF) and rhodizonate testing. A portable lead-testing XRF looks similar to a radar speed detector. The XRF is pressed against a painted surface and the trigger is pulled causing a shielded radioactive source to be exposed. The source excites any lead atoms which are sensed by the detector and a reading appears on the data screen. Lead paint located below several layers of lead-free paint will still yield a positive result. Because it employs a radioactive source, portable XRF instruments may only be used by highly trained individuals.

The second screening test for lead is rhodizonate testing. Rhodizonate testing uses a simple colorimetric method. The yellow-orange colored sodium rhodizonate reacts with lead to form a pink complex. The lead testing swabs that are commercially available are often rhodizonate tests. There are limits with rhodizonate screening tests. First, other metals such as cadmium, silver, tin, and barium give a false positive response by also reacting with sodium rhodizonate to form a pink complex. Second, rhodizonate tests are qualitative, not quantitative. Third, the rhodizonate solution degrades quickly; commercial products must be used within a few minutes of mixing.

Experiment Overview

The purpose of this experiment is to determine the presence or absence of lead in a sample of soil using the sodium rhodizonate screening procedure.

Materials

Prelab Questions

1. List the four lead testing procedures discussed in the Background section. In your own words, describe the advantages and disadvantages of each testing procedure.
2. Many field methods for lead include soaking the sample in an acid solution prior to testing with a 0.2% sodium rhodizonate solution. Explain why this step is included in many field protocols.

Safety Precautions

Lead nitrate is moderately toxic by inhalation and ingestion, a body tissue irritant and a possible carcinogen. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. Label three 50-mL beakers as soil sample A, B or C.
2. Use the balance to weigh 10 g of soil sample A. Transfer the soil into the beaker labeled soil sample A.
3. Use the graduated cylinder to add 25 mL of deionized water to the sample beaker.
4. Use a stirring rod to stir the soil and water sample. Allow the sample to sit undisturbed for 5 minutes.
5. Fold filter paper as shown in Figure 2.
   {10927_Procedure_Figure_2}
6. Set up the gravity filtration apparatus (see Figure 3) and filter the mixture through a piece of wetted filter paper. Collect the liquid (filtrate) in a clean beaker.
   {10927_Procedure_Figure_3}
7. Use a clean graduated pipet to add 1 mL of the 0.2% sodium rhodizonate solution to the filtrate.
8. Use a clean stirring rod to stir the mixture.
9. Observe the color of the filtrate after the addition of sodium rhodizonate. Record the color in the data table on the Lead in Soil Worksheet.
10. Repeat steps 2–8 for soil samples B and C.
11. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

10927_Student1.pdf