In this analysis, you will determine the concentration of lead in groundwater samples taken from Lake County.

Using the Atomic Absorption Spectrophotometer (AA), you will be able to measure the concentration of trace amounts of metals in a water sample. We will be measuring lead levels at the parts per million (ppm) level. This requires that the glassware used in the analysis is as uncontaminated as possible. We will therefore use a method called “acid washing” to remove any oxidizable contaminants.

This is a two-day lab. On the first day, glassware will be cleaned. On the second day, the five lead standards will be prepared and these and the groundwater sample will be run on the AA.

The first step in this lab is to wash all glassware used so that it is completely uncontaminated. Acid washing is done to remove any oxidizable contaminants. Six-molar nitric acid is used. It is crucial that you wear safety glasses or goggles at all times during this lab to protect your eyes! You will be wearing latex gloves, and it is recommended that you purchase and wear a lab apron (available in the bookstore) to do your acid washing. This will protect your clothing.

Acid Washing (Day One)

Each group will wash the following equipment at one sink:

• DI water bottle (clean this first so that you can use it to rinse all of your other equipment)
• 1-mL graduated pipet
• 1 5-mL graduated pipet
• 1 10-mL volumetric pipet
• 5 200-mL volumetric flasks
• 1 1-L volumetric flask
• 1 150 mL beaker
• 1 25-mL beaker
• 1 Pasteur pipet

Place approximately 50 mL of 6M nitric acid into a 150 mL beaker. Label a 250 mL beaker (does not need to be acid washed) to contain all acid waste.

For each piece of glassware that you wash, add only a small amount of acid. Do not fill to capacity. Rinse glassware with the acid thoroughly by moving the glassware so that the acid completely coats the inside surfaces of each piece. The outside surfaces do not need to be cleaned with the nitric acid.

Use a pipet bulb to draw a small amount of acid into the pipet. Be careful not to draw it into the pump! Acid will damage the pipet bulb.

Rinse well with tap water at least three times (do not re-use the rinse water). Your final rinse is done at least three times with DI water.

Dispose of all acid into the waste container provided under the hood.

Standard and Sample Prep (Day Two)

The solvent for all standards, 1% nitric acid, will be prepared.

Lead standards will also be prepared for a calibration curve on the AA. They will be made up in the following concentrations: 0 ppm, 2.5 ppm, 5 ppm, 7.5 ppm, and 10 ppm. Five 200-mL volumetric flasks will be labeled with each of the concentrations. These “secondary” standards will be prepared from a certified “primary” 1000 ppm standard.

First determine the volume of the 1000 ppm standard required to make a 200-mL secondary standard in the above concentrations. Show your calculations to the instructor and get the instructor’s initials before preparing the standards.

Next, prepare 1 liter of 1% nitric acid solvent. This can be made by pipetting 10 mL of concentrated nitric acid into a labeled 1-L volumetric flask already filled with about 600 mL of deionized water. Swirl to mix, and only then should you dilute the solution to the mark with deionized water. Stopper and invert to mix the solution about 25 times to mix well. Remember the rule for making up acid solutions: Always add acid to water—never the reverse.

Obtain approximately 10 mL of the primary standard (1000 ppm). It should be placed in a 25 mL beaker and covered with parafilm when not in use.

Label each of the five 200 mL volumetric flasks with one of the following labels: 0 ppm, 2.5 ppm, 5 ppm, 7.5 ppm, and 10 ppm.

The solvent for each of these standards will be 1% nitric acid.

Your 0 ppm standard will be only 1% nitric acid. Using a pipet, add the correct amount of primary standard (1000 ppm) to each of the remaining four volumetric flasks. Dilute to the line with the solvent finishing up with a Pasteur pipet. This is a disposable pipet and can be thrown away when you have prepared all the standards. Stopper and invert each of the flasks 30 times to ensure complete mixing.

You will be provided with a groundwater sample to test. Pour it into your acid-washed 150 mL beaker and cover with parafilm until you are ready to analyze it.

Running the Standards and Sample on the AA

The AA will be set up, turned on, and allowed to warm up for at least 15 minutes.

Aspirate your 0 ppm sample when the instrument prompts you and zero the instrument. Each standard run will be analyzed three times and the average value will be reported on your screen along with the relative standard deviation (RSD) of the three absorbance readings.

When prompted, wipe off the aspirator with a kimwipe and aspirate your 2.5 ppm standard. Follow this procedure with each of the remaining 3 standards. A calibration line will be displayed when you have finished running your standards.

Next you will be prompted to aspirate your groundwater sample. After it has been analyzed, you will print up a report that will display all of your group’s pertinent information.

Dispose of all standards and groundwater samples into the waste container provided under the hood.

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