Laboratory 19.8: Analysis of Paint Samples for Lead

This article is a slightly modified version of the one published in the May 2009 issue of the HomeChemLab.com newsletter. This article also serves as the instruction manual for the MAKE Detection of Lead Paint Test Kit (WCHEKA).

As I was browsing the web one evening, I came across an article about consumer lead testing kits. These kits are supposed to allow homeowners to determine if the paint in their homes is lead-based by using a simple color test. The conclusion of that article and several others I found was that these tests are marginally useful at best. False negatives occur frequently, as do false positives. It’s difficult to say which is worse.

Most homes built before about 1977 have at least some lead-based paint present. Our home was built in 1969, so I thought it likely some of the original paint would be lead-based. Not that that’s particularly worrisome. Lead-based paint in situ ordinarily presents little hazard, particularly if it has been overcoated with non-lead paint. The primary danger of lead-based paint occurs if the paint begins to chip and children or pets eat those chips. A secondary danger is the lead-bearing airborne dust that occurs if old painted surfaces are sanded.

So, despite the lack of any real danger from the old paint in our house, I decided to see if the resources of a home chemistry lab would allow me to make more reliable lead tests than the consumer lead testing kits. Ideally, I’d like to do quantitative tests for lead, or failing that, at least semi-quantitative tests. But even a reliable qualitative lead test would be an improvement over the consumer tests, at least if the articles I read were correct about their unreliability.

Although various lead compounds were used in residential paints, by far the most common was lead carbonate. It’s difficult to obtain reliable figures for the amount of lead carbonate present in paints by mass percentage, but my research results indicated that mass percentage varied from as little as 1% or less (typically in more recent paints) to 50% or more. Obviously, the lower the mass percentage, the less reason for concern.

Like most lead salts, lead carbonate is extremely insoluble in water, and so must be extracted (leached) from the paint samples before a test can be made. Because lead acetate and lead nitrate are both freely soluble in water, this leaching is usually done by soaking the sample in a dilute solution of acetic acid or nitric acid, either by immersing a sample chip in the acid solution, or by scuffing the painted surface to break the surface skin and then wetting it with the acid solution. In either case, the tightly-bonded lead in the carbonate salt is released as free lead ions that can react with the test reagents.

Having extracted the lead as the soluble acetate or nitrate salt, the next question is which reagent to use to detect the presence of lead ions in the extract. Home testing kits use one of the following reagents:

• Sodium sulfide – reacts with lead ions to produce lead sulfide, an extremely insoluble salt that is black or dark brown. (Hydrogen sulfide gas in the air causes lead paint to darken for the same reason.) Sodium sulfide is cheap, readily available, reasonably sensitive, stable for months in solution, and yields semiquantitative results (you can observe the intensity of the stain and how long it takes to develop to get an idea of how much lead is present). Unfortunately, sodium sulfide is also prone to yielding false positives because several other metal ion species yield stains similar to those of lead. In particular, the cobalt present in small amounts in many paints, including lead-free formulations, may yield a stain that is impossible to differentiate from a stain produced by lead ions.
• Sodium rhodizonate – forms a bright pink complex with lead ions, which turns blue when a drop of hydrochloric acid is applied to the pink stain. On the plus side, rhodizonate is more selective for lead than sulfide, and after-treatment with hydrochloric acid to turn the pink stain blue is extremely selective for lead. Unfortunately, sodium rhodizonate is expensive and lasts only a few minutes to an hour or so in solution. It is also effectively qualitative-only, because the lead concentration needed to produce even a slight pink stain is not much smaller than the concentration needed to produce an intense pink stain.

In essence, sodium sulfide is useful as a preliminary screening reagent because it yields a positive result if lead is present in any significant concentration. But because sodium sulfide is subject to false positives from many other common ion species, a positive sodium sulfide test should be followed by confirmatory tests. For those confirmatory tests, I decided to use two common laboratory chemicals, each of which produces a characteristic yellow or yellow-orange precipitate if lead ions are present. A positive result with either of these reagents is strongly indicative that lead is actually present; a positive result with both is nearly conclusive.

• Potassium chromate – forms a bright yellow to yellow-orange precipitate of lead chromate in the presence of lead ions. The extreme insolubility of lead chromate-about 0.2 μg/mL at room temperature-means that lead chromate precipitates even from very dilute solutions of lead.
• Potassium iodide – forms a bright yellow precipitate of lead iodide in the presence of lead ions. Although it is quite insoluble in absolute terms-only about 0.7 mg/mL at room temperature–lead iodide is much more soluble than lead chromate. That means we can use potassium iodide to approximate the concentration of lead in our sample by comparing its precipitation behavior with the sample solution against solutions that contain known concentrations of lead ions.

If we want to run this experiment qualitatively, the procedure is simple: obtain a small suspect paint chip, break it up and soak it in a small amount of dilute acetic acid (distilled white vinegar works), and treat the extract solution with a few drops of potassium chromate (or potassium dichromate) solution. A yellow to yellow-orange precipitate indicates the presence of some amount of lead.

If we want to run this experiment semi-quantitatively, a bit more work is required. First, we want to make sure we extract all of the lead from the sample. (Actually, it’s seldom possible to extract all of the lead, but we want to make sure that acetic acid is in excess so that we’ll get as much of the lead as possible into solution as lead acetate.) Assuming a paint chip sample of 1 g, and a maximum of 50% lead carbonate in that paint chip, we calculate as follows:

• We start with 0.5 g of lead carbonate. The gram molecular mass of lead carbonate is 267.21 g/mol, so we start with [0.5 g / 267.21 g/mol] = 0.00187 mol of lead carbonate.
• One mole of lead carbonate reacts with two moles of acetic acid to form lead acetate, so we need [2 * 0.00187 mol] = 0.00374 mol of acetic acid to form 0.00187 mol of lead acetate.
• The gram molecular mass of lead acetate is 325.29 g/mol, so we’ll produce [0.00187 mol * 325.29 g/mol] = 0.60868 g of lead acetate.
• The solubility of lead acetate at room temperature is about 0.443 g/mL, so we’ll need at least [0.60868 g / 0.443 g/mL] = 1.37+ mL of water to dissolve the lead acetate.
• For convenience, it would be nice simply to use distilled white vinegar, which is a 5% solution of pure acetic acid. The gram molecular mass of acetic acid is 60.053 g/mol. One liter of distilled white vinegar contains 50 g of acetic acid, or [50 g / 60.053 g/mol] = 0.83 mol, so 1 mL of vinegar contains [0.83 mol/L / 1,000 mL/L] = 0.00083 mol. We need 0.00374 mol of acetic acid to react with our paint chip sample, so we need [0.00374 mol / 0.00083 mol/mL] = 4.5 mL of vinegar. That gives us about three times the minimum volume of water needed to dissolve the lead acetate product, enough to provide samples for several tests without reducing the lead concentration excessively. However, instead using about half that volume (~ 2.3 mL) of 10% acetic acid solution improves the sensitivity of the test.

If you want to avoid any unnecessary dilution of the lead ions, you can simply dilute glacial acetic acid with water to provide an acetic acid solution that contains about 0.00374 moles of acetic acid in about 2.25 mL of water. Given the uncertainties in the amount of lead present in the sample and the percentage of the lead present that would actually be extracted, we didn’t think it necessary to work to this level of precision, so we simply used distilled white vinegar for our initial testing.

• Assuming 500 mg (0.5 g) of lead carbonate fully extracted into 4.5 mL of solution, we calculated the lead concentration in mg/mL, as follows: lead carbonate is 77.54% lead by mass, so our sample contains [500 mg * 0.7754] = 388 mg of lead in 4.5 mL of solution, or [388 mg / 4.5 mL] = 86 mg/mL. (Note that if we’d used the minimum volume of water needed to dissolve the lead acetate, we’d have a lead concentration of about 280 mg/mL.) Because extraction with acetic acid is unlikely to extract more than about 50% of the lead actually present in a high-lead sample, our maximum lead concentration is likely to be [86 mg/mL * 0.5] = 43 mg/mL.
• Finally, we’ll want a range of solutions of known lead ion concentration for comparison purposes. Given our calculations above, a standardized solution that contains 50 mg/mL of lead ions is a good starting point. (Even if we do the extraction with the more concentrated acetic acid solution, the extract is very unlikely to contain much more than 50 mg/mL of lead ions.) We can then serially dilute that standard lead solution to provide solutions with lead ion concentrations of 25 mg/mL, 12.5 mg/mL, 6.25 mg/mL, and so on. Testing those diluted lead solutions, both with spot tests and precipitation tests, will give us both a good indication of the sensitivity range of the tests and a comparison standard to judge the approximate lead concentration of our unknown extract solutions.

We can make up our lead standard solution with either lead acetate or lead nitrate, as follows:

• Anhydrous lead acetate has a molar mass of 325.3 g/mol and therefore a lead mass percentage of 63.70%. To achieve a lead ion concentration of 50 mg/mL, we’ll need to dissolve [50 mg/mL / 0.6370] = 78.5 mg/mL. The solubility of lead acetate at room temperature is about is about 440 mg/mL, so this presents no problem. To make up 10 mL of lead acetate solution that is 50 mg/mL with respect to lead, we can dissolve 0.78 g of anhydrous lead acetate in water and make up the solution to 10 mL.
• Lead acetate trihydrate has a molar mass of 379.3 g/mol and therefore a lead mass percentage of 54.62%. To achieve a lead ion concentration of 50 mg/mL, we’ll need to dissolve [50 mg/mL / 0.5462] = 91.5 mg/mL. To make up 10 mL of lead acetate solution that is 50 mg/mL with respect to lead, we can dissolve 0.92 g of lead acetate trihydrate in water and make up the solution to 10 mL.
• Lead nitrate has a molar mass of 331.2 g/mol and therefore a lead mass percentage of 62.56%. To achieve a lead ion concentration of 50 mg/mL, we’ll need to dissolve [50 mg/mL / 0.6256] = 79.9 mg/mL. The solubility of lead nitrate at room temperature is about is about 520 mg/mL, so this presents no problem. To make up 10 mL of lead nitrate solution that is 50 mg/mL with respect to lead, we can dissolve 0.80 g of lead nitrate in water and make up the solution to 10 mL.

The actual extract solution from the paint chip samples contains lead acetate rather than lead nitrate. However, we’re really only interested in the lead cation concentration. The anions take no part in the test reactions.

Required Equipment and Supplies

• goggles, gloves, and protective clothing
• extraction vial, 1 dram (3.7 mL)
• pipette, disposable (as required)
• reaction plate, two 12-well or one 24-well (see Substitutions and Modifications)
• sharp knife or fine sandpaper
• acetic acid, glacial (a few drops per paint sample to be tested)
• lead standard solution, 50 mg/mL (see Introduction and Substitutions and Modifications)
• potassium chromate reagent (see Substitutions and Modifications)
• potassium iodide reagent (see Substitutions and Modifications)
• sodium sulfide reagent (see Substitutions and Modifications)
• distilled water
• white copy paper and pencil
• suspect paint samples (see Substitutions and Modifications)
• can of spray paint (see Substitutions and Modifications)

Items listed in bold are included in the MAKE Detection of Lead Paint Test Kit (WCHEKA).

Substitutions and Modifications

• If you do not have an extraction vial, you can substitute a test tube, small bottle, or similar small container.
• If you do not have a reaction plate, you may substitute test tubes at the expense of needed larger volumes of the reagents. You may also substitute an egg carton, old white ice cube tray, or a split piece of bubble wrap.
• If you do not have glacial acetic acid, you can substitute ordinary distilled white vinegar from the supermarket, which is 5% acetic acid. If you do have glacial acetic acid, make up a 10% extraction solution, which may yield better results than the 5% solution. To do so, add 1 part of glacial acetic acid to 9 parts distilled water (for example, one teaspoon of glacial acetic acid to 9 teaspoons of distilled water). Either the 5% or 10% acetic acid solution keeps indefinitely, so you can make up the 10% solution in advance and store it. The advantage of using the 10% solution is that you can use half as much liquid, which increases the concentration of the extracted lead and increases the sensitivity of the test. [The MAKE Detection of Lead Paint Test Kit (CHEKA) includes 25 mL of glacial acetic acid, which can be stored indefinitely. Diluted to 10%, this provides sufficient extraction solution to treat about 100 suspect paint samples.]
• [The MAKE Detection of Lead Paint Test Kit (CHEKA) includes 10 mL of premixed 50 mg/mL lead standard solution, which can be stored indefinitely.]
• You can make up 10 mL of potassium chromate reagent by dissolving 1.25 g of potassium chromate in distilled water and making up the solution to 10 mL. [The MAKE Detection of Lead Paint Test Kit (CHEKA) includes 10 mL of premixed potassium chromate reagent, which can be stored indefinitely, and is sufficient to test about 100 suspect paint samples.]
• You can make up 10 mL of potassium iodide reagent by dissolving 5.00 g of potassium iodide in distilled water and making up the solution to 10 mL. [The MAKE Detection of Lead Paint Test Kit (CHEKA) includes 10 mL of premixed potassium iodide reagent, which can be stored indefinitely, and is sufficient to test about 100 suspect paint samples.]
• You can make up sodium sulfide reagent by dissolving about 1 g of anhydrous sodium sulfide or about 3 g of sodium sulfide nonahydrate (concentration is not critical) in 25 mL of distilled water. If you store this reagent in a tightly capped dark bottle it remains usable for at least several weeks. [The MAKE Detection of Lead Paint Test Kit (CHEKA) includes 5 capsules of sodium sulfide, each sufficient to make up 5 mL of sodium sulfide reagent. Dissolve each capsule in 5 mL (1 teaspoon) of distilled water before use and store the solution in the amber glass bottle that comes with the kit. Because this reagent is used dropwise, the 25 mL of reagent supplied with the kit is sufficient to screen two hundred or more suspect paint samples.]
• You can obtain suspect paint samples from any home built before 1977. The most likely locations for exposed lead-based paint are unfinished basement areas, attic stairwells, and other seldom-painted areas. If you expose suspect paint, apply two or three coats of spray paint to recover them until more permanent lead-abatement action can be taken.

No Sodium Sulfide?

If you don’t have any sodium sulfide handy, you can make some up. Under strong heat, a mixture of sodium sulfate and carbon reacts as follows:

Na₂SO₄ + 4 C → Na₂S + 4 CO

Mix 2.5 g of anhydrous sodium sulfate (or 5.4 g of sodium sulfate decahydrate) with about 1 g of activated charcoal (we want carbon in slight excess). Grind the mixture to a fine powder, transfer it to a crucible, cover the crucible, and heat it to red heat for five minutes or so. (If you used hydrated sodium sulfate, the mixture will melt and bubble as the water of hydration is driven off.) Only a tiny amount of carbon monoxide is produced by this reaction, but work in a well-ventilated area anyway to be safe.

Allow the crucible to cool to room temperature, and then carefully break up and remove the mass from the crucible. Transfer the product to a small flask or other small vessel that contains about 25 mL of water. (Some hydrogen sulfide gas is evolved, which has the odor of rotten eggs.) Stir or swirl the liquid until most of the solid has gone into solution. Decant off or filter the resulting sodium sulfide solution into a small storage bottle and discard the solid residue in your hazardous waste container.

Procedure

This lab has four parts. In Part I, we determine the sensitivity of our reagents by testing them against solutions that contain various known concentrations of lead ions. In Part II, we use sodium sulfide solution to do a preliminary screening test for lead. In Part III, we obtain samples of those specimens that tested possibly positive for lead in Part II, and treat them with dilute acetic acid to extract any lead actually present. In Part IV, we test those sample extracts to determine if lead is actually present in them and, if so, in what approximate amounts.

Part I – Testing sensitivity of the reagents

In this part of the lab, we’ll do two sets of serial dilutions of our standard 50 mg/mL (50,000 ppm) lead solution 1:1 with distilled water, each in 12 wells of our reaction plate (or plates), to provide two sets of known lead ion concentrations from 50,000 ppm down to about 24 ppm. We’ll place one drop of each dilution in a separate circle on a sheet of paper that we’ll later use to test our sodium sulfide reagent. We’ll then test one set of dilutions with potassium chromate solution and the other set of dilutions with potassium iodide solution to determine the minimum lead ion concentration at which each of those reagents provides a positive result.

1. On a sheet of copy paper, draw 12 circles, each about 2 to 3 cm in diameter (use a coin), and labeled #1 through #12.
2. If you have not done so already, put on your goggles, gloves and protective clothing.
3. Using a disposable pipette, transfer 20 drops of distilled water to each well in the reaction plate except wells A1 and C1.
4. Using a disposable pipette, transfer 20 drops of the 50 mg/mL standard lead solution to wells A1, A2, C1, and C2. Transfer one drop of the standard lead solution to circle #1 on the paper.

The spots of different lead concentrations should occupy the same space on the paper. The lead solutions from wells A1 through A6 do not soak in and spread at all, but merely bead up. The solutions in B1 and B2 spread a bit, and those in B3 through B6 spread completely. As best you can, use the tip of the pipette to spread the concentrated solutions to cover about the same area as the dilute solutions. Otherwise, it will simply bead up and take hours to dry to a dense little spot on the paper.

5. Mix the solution in A2 thoroughly by drawing it up into the disposable pipette and expelling it into the well several times. Then draw up the solution from well A2 and transfer 20 drops of it to well A3. Place 1 drop of the unused A2 solution in circle #2 on the paper, and return the remainder of the unused solution from the pipette to well A2. At this point, you have a 50,000 ppm lead solution in well A1, a 25,000 ppm solution in well A2, and a 12,500 ppm solution (as yet unmixed) in well A3.
6. Repeat step 4 with well C2, so that you end up with a 50,000 ppm lead solution in well C1, a 25,000 ppm solution in well C2, and a 12,500 ppm solution (as yet unmixed) in well C3.
7. Repeat steps 4 and 5 with the remaining wells until all 24 wells of your reaction plate are populated with the serial dilutions of the lead solution summarized in Table 1 and all 12 circles on the paper have been spotted with the appropriate dilutions of the lead solution.

Table 1. Lead ion concentration in reaction plate wells

Well	Pb2+ Concentration	Well	Pb2+ Concentration	Paper Spot #
A1	~ 50,000 ppm	C1	~ 50,000 ppm	1
A2	~ 25,000 ppm	C2	~ 25,000 ppm	2
A3	~ 12,500 ppm	C3	~ 12,500 ppm	3
A4	~ 6,250 ppm	C4	~ 6,250 ppm	4
A5	~ 3,125 ppm	C5	~ 3,125 ppm	5
A6	~ 1,563 ppm	C6	~ 1,563 ppm	6
B1	~ 781 ppm	D1	~ 781 ppm	7
B2	~ 391 ppm	D2	~ 391 ppm	8
B3	~ 195 ppm	D3	~ 195 ppm	9
B4	~ 98 ppm	D4	~ 98 ppm	10
B5	~ 49 ppm	D5	~ 49 ppm	11
B6	~ 24 ppm	D6	~ 24 ppm	12

8. Transfer 2 drops of the potassium chromate solution to well B6 and observe the well closely for several seconds, looking for the appearance of a yellow precipitate. If a precipitate appears, you know that the potassium chromate solution yields a positive result under these conditions with a lead concentration of about 24 ppm (0.0024%).
9. If no precipitate appears, repeat step 7 with well B5. Continue testing wells with increasing concentrations of lead ions until you determine the minimum lead ion concentration that yields a positive test under these conditions. Note the approximate lower limit for lead concentration determinable by this test.
10. Repeat steps 8 and 9, transferring two drops of potassium iodide solution to well D6, looking for the formation of a precipitate, and, if necessary, repeating with subsequent wells until you have determined what minimum lead concentration yields a positive test.
11. After allowing all spots on the sheet of paper to dry completely, put one drop of the sodium sulfide solution on spot #12 (the least concentrated spot). Observe the spot for 30 seconds. If a stain forms, note the approximate time required before it first becomes visible, and write the time required beside the spot.
12. If no stain appears on spot #12 after 30 seconds, repeat step 11 for spot #11.
13. Continue repeating the procedure for each of the remaining 10 spots, noting the time required in each case for the spot to first become visible. Write this time next to each spot. The most concentrated spots produce a black lead sulfide stain instantly, as shown in Figure 1. The less concentrated spots produce lighter brownish stains, and for the least concentrated spots these stains may take several seconds to a minute to become visible. Because even our most dilute lead solution produced a spot, we did further serial dilutions down to 0.75 ppm (750 ppb). At that level (with our setup, at least) the sulfide test reagent produced a barely visible very light tan stain after 30 seconds or so.

Part II – Preliminary screening

In this section of the lab session, we’ll use sodium sulfide solution to do a quick preliminary screening of suspect paint samples. If lead is present in significant concentration in the specimen, the sulfide test will almost certainly be positive. However, a positive sulfide test tells us only that lead may be present, because many other metal ion species may yield false positives.

The following procedure may not detect lead-based paint that has been covered by one or more coats of non-lead paint. In this situation, lead-based paint presents a minimal hazard because it is in effect sealed by the overlying paint. However, you can detect lead-based paint under one or more coats of non-lead paint simply by using a sharp knife to break off a small chip of paint, exposing the underlying old paint.

1. Use a sharp knife or fine sandpaper to lightly abrade the surface of the suspect paint sample. Rough up an area about the size of a coin.
2. Put a few drops of dilute acetic acid solution on the roughened area and allow it to soak for several minutes. Use enough solution that liquid remains present on the surface.

If the painted surface is vertical, use a small spray bottle to mist the roughened area until it is just moist. Repeat the misting as necessary as the patch dries. You want the patch exposed to liquid acetic acid solution for at least several minutes.

3. Allow the patch to dry, and then apply a drop or two of sodium sulfide solution. If lead is present, a brown or black stain will appear within a few seconds to a few minutes. The intensity of the stain and the time it takes to appear provide a rough indication of the lead concentration present.

If the paint is dark, it can be difficult or impossible to see any stain produced by the sulfide reagent. In that case, you can transfer a drop of the extraction solution from the painted surface to a sheet of white paper. Allow the spot to dry and then apply a drop of sulfide reagent. If lead is present, the spot will turn brown to black over a period of a few seconds to a few minutes.

Part III – Extracting lead from suspect paint samples

In this section, we’ll treat small samples of the suspect paint specimens–those that tested possibly positive for lead in Part II–with dilute acetic acid to extract any lead actually present in the sample into solution.

None of the paint samples we tested had very high levels of lead. In fact, none of the paint samples from our home had any detectable lead at all. We ended up having to obtain other samples. We couldn’t find any high-lead samples, but we suspect if we had those high-lead samples would have reacted vigorously with the acetic acid solution, foaming and bubbling as the reaction produced carbon dioxide gas. The low-lead samples we tested produced only very gentle bubbling, with tiny bubbles forming gradually on the surface of the paint chip fragments. At very low lead levels, no bubbling may occur at all. If you test an unknown sample, we suggest that you add the acetic acid very gradually at first, until you determine how vigorous the reaction will be.

1. If you have not done so already, put on your goggles, gloves, and protective clothing. If there is any chance of paint dust being released into the air, also put on a disposable respirator mask rated to protect against lead paint dust.
2. Use a sharp knife at a shallow angle to pry up a small chip of paint from the suspect surface. The mass of a paint chip depends on its area, the number of coats of paint applied to the surface, and the composition of those paint coatings, but try to get a sample that weighs about 1 g. (About the size of a coin.)
3. Break up the sample paint chip into pieces small enough to fit into reaction vial or other container. The smaller the pieces, the more complete the extraction will be.
4. If you are using the reaction vial, transfer about 2.5 mL of 10% acetic acid to the vial, gradually at first. Considering the volume occupied by the paint sample, this amount of liquid will fill the vial about 2/3 to 3/4 full. If you are using 5% acetic acid, transfer about 4.5 mL to a test tube, making sure that all the bits of paint are immersed. Once any obvious evolution of gas ceases, gently stopper the test tube or cover the mouth with a small plug of cotton or piece of plastic wrap, and place the tube in a test tube rack.
5. Allow the paint chips to soak in the acetic acid for at least several hours, swirling occasionally. Allowing the paint chips to soak overnight will probably give better extraction, and allowing them to soak for several days will do no harm.
6. 6. If you have more than one specimen to test, repeat steps 2 through 5 for each of the other specimens. You can run several extractions simultaneously by substituting labeled test tubes or other small containers for the extraction vial.

Part IV – Testing the sample extracts

In this section of the lab session, we’ll test the extracts we made in Part III.

1. If you have not done so already, put on your goggles, gloves, and protective clothing.
2. Using a clean, dry disposable pipette, transfer one drop of the first extract solution to a labeled circle on a sheet of paper, 20 drops of the solution to the first well of a clean reaction plate and 20 drops of the solution to the second well of the reaction plate.
3. While the spot on the paper is drying, use a clean disposable pipette to transfer two drops of potassium chromate solution to the first well and a second clean disposable pipette to transfer two drops of potassium iodide solution to the second well. A yellow precipitate in the first well confirms the presence of lead in the extract solution. If lead is present, a precipitate may or may not form in the second well, depending on the concentration of lead ion in the extract solution.
4. Repeat steps 2 and 3 for each of your other extract solutions, using the third and fourth wells, fifth and sixth wells, and so on.
5. When the spots on the paper have all completely dried, apply one drop of sodium sulfide solution to the first spot. Observe the spot carefully, and note the time required for a brownish or grayish stain to first appear. Compare the results here with your tests of solutions of known lead concentration in Part I and estimate the approximate lead concentration in the lead extract solution spot.
6. Repeat step 5 for each of the remaining extract spots on the paper.
7. If the potassium iodide produced a precipitate in step 3, you know the lead concentration in your extract solution is at least as high as the minimum concentration you determined in step 1 produced a positive iodine test. You can confirm the semi-quantitative sulfide test results from step 5 by doing a serial dilution of the extract solution and determining the lowest concentration of that extract that yields a positive iodide test.

Figure 2. Precipitates of lead chromate (left) and lead iodide

Disposal

Treat all of the solid and liquids produced in this lab session as hazardous waste. Place them in your hazardous waste container, and dispose of them in accordance with your local laws and regulations.

August 17, 2009