- The teacher should remind students that in order to carry out the optimized procedure to test the affect of salinity on Halobacterium growth they will have to learn how to properly use two pieces of lab equipment: the micropipette and the spectrophotometer. In order to use these instruments, proper technique needs to be learned and practiced.
Explain that half of the class will be begin working with the spectrophotometer and half the class will be working with the micropipettes. After about 20 minutes, the students will switch. Tell the students before the can complete the two activities you will be giving an overview of proper technique and procedure for both the micropipetting and spectrophotometer activity.
- Introduce the micropipetting activity to the whole class. The teacher should give a brief introduction to what the micropipette is used for.
A short video clip can be shown of a scientist using a micropipette in the lab (CSI or forensic science show). The teacher should emphasize that micropipettes are very expensive and need to be used properly to maintain the instrument and for accuracy in the lab.
- The teacher should show a short video (~7 minutes) on using a micropipette from ISB (Full_pipet.mov). A short video (~ 1 minute) can also be used to review the use of micropipettes (Review_pipet.mov).
- Distribute the student sheet for the micropipetting activity (Student Text micropipette.doc) (Google Doc | Word Doc)
- and give an overview of the activity.
As a class, read the Objectives, Introduction and begin reading through the procedures. Have the students answer the questions on choosing the correct micropipette and setting the volume as you go through the procedures. When you get to Step 2 of Using the Micropipette, demonstrate each step at the front of the class. You may want to project the picture of the micropipette on the projector as well (micropipette.bmp). Be sure to emphasize the following as you demonstrate how to use the micropipette:
- Once the pipet is set to the desired volume, make sure to lock the volume.
- The plunger has two stops. The first stop is for drawing up the desired volume of liquid and the second stop is for completely dispensing the sample.
- After dispensing the sample into its container, keep the plunger depressed until the tip is completely out of the container.
Briefly go over the steps of the micropipetting activity. Be sure to emphasize the following when going over the micropipetting activity:
- Do not leave the micropipette standing in a solution to avoid spilling the solution.
- Do not allow the top of the micropipette tip to be completely submerged into any of the solutions.
Explain that the students will be working in groups of three. Each student will be using all three of the micropipettes. Project on the classroom screen, Micropipette Reminders.ppt, (Google Slides | PowerPoint) during the activity so students can be further reminded of the important steps.
Distribute the student sheet for the spectrophotometer lab (Student Text spectrophotometer.doc (Google Doc | Word Doc)) and give an overview of the activity.
Explain that you are now going to go over how to use the spectrophotometer as a class. Read through the introduction to the spectrophotometer activity. Emphasize the picture showing how a spectrophotometer works. Explain that the light goes through a slit that sets the wavelength. The small beam of light with a specific wavelength passes through the sample in a cuvette. Some of the light is absorbed and some of the light passes through. The amount of light that passes through or the amount of light that is absorbed (this depends on the mode the spectrophotometer is set for, either absorbance or transmittance) is detected and a value is given by the spectrophotometer. Have the students read through the entire procedure. Be sure to emphasize that the spectrophotometer should be set to measure absorbance (not transmittance). Also emphasize that the graduations on the Beral pipets go up to 1 mL. Students will need to measure 1 mL four times in order to put the desired amount of liquid into the cuvette.
Background Note for the Teacher: Students should observe that as the number of particles (food coloring) present in a given volume of the sample increases, the light absorbance increases too.
- Discuss the importance of using a cuvette with water (blank) to zero the spectrophotometer using Spec.ppt. (Google Slides | PowerPoint)
Ask students why the cuvette with water needs to be inserted into the spectrophotometer between each reading. Emphasize the importance of a blank. Show the PowerPoint of what a blank is. First, ask students to explain how they would find the mass of a substance like salt using an electronic balance, a beaker, and salt. Students may say that you can put the beaker on the balance and then zero the balance or they may say that you can find the mass of the beaker alone and the mass of the beaker with salt. You can then find the mass of the salt by subtracting the mass of the beaker from the mass of the beaker with salt. Explain that determining absorbance of a sample is similar to finding the mass of a substance in a beaker. The substance that serves as the blank may absorb some of the light (not all the light may be transmitted). For example, in the activity the cuvette with water may absorb some light. In order to find the absorbance just the food coloring, you need to subtract the initial absorbance of the water from the new absorbance of the water and food coloring. Just like with an electronic balance, the spectrophotometer can be zeroed after the blank (cuvette with water) is placed in the spectrophotometer. Now when the cuvette with the water and food coloring is added, any change in absorbance is due to the addition of the food coloring not the water.
ACTIVITY (Working in Groups).
- Divide the class into two groups. Half of the class will be working with the spectrophotometer and half the class will be working with the micropipettes.
Assign students to work in groups of three. However, during the spectrophotometer activity, students may need to work in larger groups depending on the class size and the number of spectrophotometers available. After 20 minutes, the teacher should have the groups switch.
- Collecting work. Use the Answer Key and Example Data.doc (Google Doc | Word Doc) to check.
The teacher may either want to collect all student work including the paper towel from micropipetting lab or have students attach filter paper to their lab journal. If you are not going to look at the paper towel right away, you may want students to draw a circle around each of their dried samples from the micropipetting activity. The smaller volumes become hard to see.
- As an extension, students can practice converting metric units.