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• Acids, Bases, & the pH Scale

Project Summary

Difficulty	7
Time required	Average (about one week)
Prerequisites	Some chemistry knowledge will be helpful.
Material Availability	Specialty items: You will need a lead test kit, which can be ordered online. See the Materials and Equipment list for details.
Cost	Average ($50 - $100)
Safety	Adult supervision is required. The chemicals used in this science fair project are potentially hazardous. The test solutions contain lead and should not come into contact with skin or any part of your body. It also contains chemicals that produce a strong "rotten egg" smell, so the procedure should be carried out in a well-ventilated area.

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Sponsor

Sponsored by a generous grant from the Camille and Henry Dreyfus Foundation

Abstract

Objective

The objective of this chemistry science fair project is to determine if water or vinegar (water with acetic acid) is better for testing lead content in household objects.

Introduction

Lead is a highly toxic metal that was used for many years in common household products. Exposure to lead can cause a variety of health effects, from behavioral problems and learning disabilities to seizures, and even death. Children six years old and under are most at risk because their bodies are developing quickly. Also, young children are more at risk because they explore their environment by putting toys and other objects in their mouths.

Exposure to lead paint dust from older homes is the most common cause of lead poisoning. The exposure to lead paint usually occurs when a house is remodeled or repainted, leaving the old lead paint underneath. Other sources of lead include some toys, jewelry, dishes, cups, and a variety of other items.

The presence of lead in household objects can be determined using test kits. Although not as accurate as tests that are done in professional laboratories, these kits provide a simple way to test for the presence of lead. In this science fair project, you will use a kit that detects lead using soluble sulfide. Soluble indicates that the sulfide is dissolved in a solution. The word sulfide refers to the bivalent (double-charge) form of sulfur. The chemical equation for the formation of the sulfide ion from sodium sulfide is shown in Equation 1, below. Although it's not shown in the equation, chemicals, such as sodium hydroxide, are usually added to sodium sulfide solutions to help the sodium sulfide dissolve.

Equation 1:

In order for the lead to react with the sulfide, it has to be dissolved, too. Lead ions that have a double positive charge react with sulfide ions to form lead sulfide. Lead sulfide is a black chemical, so the amount of lead in the sample correlates with the darkness of the reaction product. Equation 2 shows the chemical equation for the reaction:

Equation 2:

The kit you will use has a color key that shows the approximate concentration of lead in the sample, based on the color of the reaction products. A faint yellow indicates that lead is present in 1–3 ppm (part per million), whereas a black reaction product indicates that lead is present at over 50 ppm. Using ppm to describe the concentration is similar to using percent. Just as percent means "out of 100," so parts per million (or ppm) means "one out of 1 million." One ppm of lead is equivalent to 1 milligram (mg) of lead per liter (L) of water (mg/L) or 1 mg of lead per kilogram (mg/kg) of sample.

The kit can be used in two different ways. In the first way, a cotton swab is dipped into a solution containing sulfide ions, and the swab is rubbed on the surface to be tested. If lead is present, some of the lead reacts with the sulfide ions to form a colored product that sticks to the cotton on the swab. The kit also allows you to test objects for the presence of lead by immersing them in vinegar, and then testing the vinegar for dissolved lead. This has the advantage that you are testing the entire surface of the object, even regions you could not reach with the cotton swab. Vinegar contains acetic acid. It is usually composed of about 5 percent acetic acid and 95 percent water. In other words, vinegar is a 5 percent (approximately) aqueous solution of acetic acid. Because vinegar contains acetic acid, it is an acidic solution. Acids are solutions that have a higher concentration of hydrogen ions (H⁺ ) than hydroxide ions (OH^-). Solutions that have a higher concentration of hydroxide ions are called basic.

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic. The pH scale is logarithmic. Each whole number value of pH below 7 is 10 times more acidic than the next higher value. For example, a solution with a pH of 4.0 is 10 times more acidic than a solution with a pH of 5.0. A solution with a pH of 3.0 is 1,000 times more acidic than a solution with a pH of 6.0. The same holds true for pH values above 7, each of which is 10 times more alkaline (another way to say basic) than the next lower whole value. For example, pH 9.0 is 10 times more alkaline than pH 8.0 and 100 times more alkaline than pH 7.0. For more information, see the Science Buddies page Acids, Bases, & the pH Scale.

When an object containing lead is dipped in the vinegar solution, the hydrogen ions in the acid react with the lead, resulting in the presence of lead ions in solution. This reaction is shown in Equation 3.

Equation 3:

The goal of this chemistry science fair project is to determine how the acidity of the vinegar solution affects the rate at which lead dissolves. You will test various solutions with different pH's to see how the pH's of each affect the solutions' ability to form lead ions from solid lead.

Terms, Concepts and Questions to Start Background Research

• Lead
• Toxic
• Soluble
• Sulfide
• Solution
• Ion
• Bivalent
• Sodium hydroxide
• Aqueous
• Lead sulfide
• Acidic
• Basic
• Parts per million (ppm)
• pH scale
• Logarithmic
• Control
• Dissociation

Questions

• What are the two ways that the lead test can be performed?
• What are some of the health risks associated with exposure to lead?
• Based on your research, what are some other ways to test for lead that do not involve sulfide chemistry?
• What items have been subject to government recall due to the presence of lead?
• In what mineral is lead usually found in nature?

Bibliography

• Centers for Disease Control and Prevention. (2007, August 31). Toys and Childhood Lead Exposure. Retrieved April 21, 2009, from http://www.cdc.gov/nceh/lead/tips/toys.htm
• Environmental Protection Agency, USA. (2009). Lead in Toy Jewelry. Retrieved April 21, 2009, from http://www.epa.gov/lead/pubs/toyjewelry.htm
• Consumer Reports. (2008). Rating of Lead Test Kits. Retrieved April 22, 2009, from http://www.consumerreports.org/cro/home-garden/home-improvement/home-security/lead-radon-test-kits/lead-ratings/lead-radon-tests-kits-lead-test-kits-ratings.htm
• Thomas Jefferson National Accelerator Facility, Office of Science Education. (2009). The Element Lead. Retrieved April 22, 2009, from http://education.jlab.org/itselemental/ele082.html

Materials and Equipment

• Lead Inspector Test Kit, "24 Test Pack"; available online from www.leadinspector.com. One kit will probably be sufficient, but you should order two kits if you want to pursue experiments suggested in the Variations section.
• Lead sinkers, about 6 grams (g) each (18). Use identical, new sinkers. Sinkers can be purchased at any sporting goods store.
• Plastic cups, clear, 12-ounce (oz.) size (40). You can use fewer cups if you wash and dry them out between trials.
• Vinegar, white
• Permanent marker
• Latex gloves; available at most hardware stores
• Safety goggles
• pH paper; available online from stores like Science Kit and Boreal Laboratories at www.sciencekit.com, item # 69642-03
• Distilled water
• Graduated cylinder; available at science supply stores, such as Carolina Biological: www.carolina.com. As an alternative, you can use measuring cups.
• Plastic wrap
• Eyedropper
• Stopwatch or timer
• Resealable baggies (3)
• Scissors
• Lab notebook
• Optional: Extra cotton swabs (in addition to those that come with the lead inspector kit)

Disclaimer: Science Buddies occasionally provides information (such as part numbers, supplier names, and supplier weblinks) to assist our users in locating specialty items for individual projects. The information is provided solely as a convenience to our users. We do our best to make sure that part numbers and descriptions are accurate when first listed. However, since part numbers do change as items are obsoleted or improved, please send us an email if you run across any parts that are no longer available. We also do our best to make sure that any listed supplier provides prompt, courteous service. Science Buddies receives no consideration, financial or otherwise, from suppliers for these listings. (The sole exception is any Amazon.com or Barnes&Noble.com link.) If you have any comments (positive or negative) related to purchases you've made for science fair projects from recommendations on our site, please let us know. Write to us at scibuddy@sciencebuddies.org.

Experimental Procedure

Preparing Your Samples

1. Put on the latex gloves.
2. Read the directions and the safety precautions that came with the kit.
3. Label eight plastic cups, as shown below. Cups # 1 and 2 will not have sinkers, so they should have no color reaction when tested. They will serve as controls for both the 4 hour and 24 hour sets of samples.
   • 1) Water (control)
   • 2) Vinegar (control)
   • 3) Water (4 hours)
   • 4) Vinegar (4 hours)
   • 5) 50 percent (4 hours)
   • 6) Water (24 hours)
   • 7) Vinegar (24 hours)
   • 8) 50 percent (24 hours)
4. Fill each cup with 50 mL of the following liquids (using the graduated cylinder to measure):
   • 1) Water
   • 2) Vinegar
   • 3) Water
   • 4) Vinegar
   • 5) 50% vinegar in water
     • Mix 25 mL of water with 25 mL of vinegar.
   • 6) Water
   • 7) Vinegar
   • 8) 50% vinegar in water
     • Mix 25 mL of water with 25 mL of vinegar.
5. Place one lead sinker into cups #3 to #8.
6. Each cup has a sinker, except the controls #1 and #2.
7. Record the time at which the sinkers were added in your lab notebook.
8. Cover all eight cups with plastic wrap.
9. Store the cups in a place where they won't be disturbed.
10. Following the directions that came with the kit, one set of tests will be done after 4 hours, and another set after 24 hours.

Testing pH

1. While you are waiting to test the samples for lead, you will determine the pH of the water, the vinegar, and the 50 percent water-vinegar solutions.
2. Label three cups, as follows:
   1. Water pH
   2. Vinegar pH
   3. 50:50 pH
3. Add water, vinegar, or the 50 percent solution to each of the cups, as labeled. These should be fresh ingredients—you aren't using any of the solutions that are already in the previously labeled cup #'s 1–8.
4. Read the instructions for the pH paper and then use it to determine the pH of each solution. Record the pHs in your lab notebook.
   1. Note: Pure water will have a pH of 7, but you may find that the pH of the water that you are using is less than 7.0, which is okay. In other words, it is slightly acidic. This is because water absorbs carbon dioxide from the air, and carbon dioxide is an acid. After carbon dioxide is absorbed by water, it is slowly converted into the weak acid, carbonic acid, which then dissociates to liberate hydrogen ions, making the water acidic.

Testing Your Prepared Solutions

Note: This procedure should be done in a well-ventilated area, because the sulfur in the test solution causes a rotten egg smell.

1. After the lead sinkers have been in the solutions for 4 hours, test the solutions in cup #'s 1–5. Put on gloves and safety goggles.
2. Using the eyedropper, fill the plastic test tube that came with the kit about one-fourth full with test solution from cup #1.
   1. First mark a line on the test tube at approximately one-fourth full. Add test solution to this line.
   2. You can also just pour the solution into the tube if you prefer.
   3. Rinse out the eyedropper with clean water after each use.
3. Activate a cotton swab from the kit by dipping it into the indicator (sulfide) solution.
   1. Place the cotton swab into the test tube with the solution.
   2. Cut the swab in half with scissors so it fits in the tube, if needed.
   3. Replace the cap and invert the tube (tip over and back) once to mix.
   4. Wait 90 seconds.
   5. If lead is present, the solution and swab will turn yellow, brown, or black in color.
   6. If the solution stays clear, or becomes milky white, lead is not present in the sample.
4. Compare the color of the solution with the color key that is in the instructions.
5. Record the color and the approximate lead concentration, in ppm, for the sample in your lab notebook.
6. Pour out the solution from the test tube in the sink and wash it down with running water.
7. Be careful not to get the solution on your skin.
8. Place the used cotton swab in a resealable plastic bag, to be discarded later.
   1. The swab will have a rotten egg smell. This is caused by the sulfur in the test solution.
9. Rinse the tube with water.
10. Repeat steps 2–8 for the solutions in cup #'s 2–5.
11. Now test the sinkers after 24 hours.
12. After the lead sinkers have been in the solutions in cups #6, #7 and #8 for 24 hours, test the solutions.
13. Put on gloves and safety goggles.
14. Repeat steps 2–9 for the solutions in cup #'s 6–8.
15. Repeat the entire procedure two more times, with fresh materials and solutions, so that you have a total of three trials. This ensures that your results are accurate and repeatable.

Analyzing Your Results

1. Graph the sample number on the x-axis vs the lead concentration on the y-axis.
Add a label to each sample on the graph showing the pH of the solution. Discuss how the length of time the sinker was in the solution affected the results. Discuss how the pH of the solution affected the results.

Variations

• Use more dilutions of the vinegar to test more pH values. For example, 0, 5%, 10%, 25%, 50%, 75%, and 100% vinegar in water for 24 hours. Graph pH vs lead concentration.
• Vary the time the lead is leached from the lead sinkers. For example, place them in vinegar for 15 minutes, 1 hour, 4 hours, 16 hours, and 24 hours.
• Use a pH meter to obtain the precise pH of the test solutions. A pH pen is an inexpensive (less than $50.00) pH meter. These are available online, from sites such as www.amazon.com.
• Lead sinkers have been used for many years by fishermen. Devise a way to determine the environmental impact of lead sinkers that end up on the bottom of a lake or ocean. For example, will a sinker affect the health of nearby organisms? Would the water's pH be a factor?

• For more science project ideas in this area of science, see Chemistry Project Ideas.

Credits

David B. Whyte, PhD, Science Buddies

Last edit date: 2009-05-22 09:18:00

Career Focus

If you like this project, you might enjoy exploring careers in Chemistry.

	Chemist Everything in the environment, whether naturally occurring or of human design, is composed of chemicals. Chemists search for and use new knowledge about chemicals to develop new processes or products.			Chemical Engineer Chemical engineers solve the problems that affect our everyday lives by applying the principles of chemistry. If you enjoy working in a chemistry laboratory and are interested in developing useful products for people, then a career as a chemical engineer might be in your future.
	Chemical Technician The role that the chemical technician plays is the backbone of every chemical, semiconductor, and pharmaceutical manufacturing operation. Chemical technicians conduct experiments, record data, and help to implement new processes and procedures in the laboratory. If you enjoy hands-on work, then you might be interested in the career of a chemical technician.

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