# Pre Lab: Serial Dilutions Practice Exercise

If students are not familiar with operating pipettes, the Pre Lab Serial Dilutions practice exercise is recommended. The exercise will allow students to learn how to pipette, dilute samples and calculate the concentration of their samples. The use of methylene blue gives them a visual model of how samples become less concentrated with each dilution step. This activity was adapted from An Introduction of Biotechnology developed by the Mathematics and Science Education Center, University of Missouri- St. Louis. A student answer sheet and teacher Power Point are attached for use during this lab. The Pre Lab Exercise can be completed in forty five minutes.

## Materials Needed

 Pre Lab Serial Dilutions Equipment and materials for students Equipment and notes Quantity per group (recommend 2-4) □ Test Tube Rack 1 □ Test Tubes 4 □ Flask (recommend 1000mL) 1 □ Pipette Pump (10 mL) 1 □ Pipette (10 mL) 1 □ Methylene Blue (preferably in a dropping bottle) 6 drops □ Water 1 Full Flask □ Lab Tape 2 inches □ Lab Coat or Apron 1 per person □ Safety Glasses 1 per person □ Safety Gloves 1 pair per person □ Calculator (Optional) 1

## Preparation Instructions

Set up the test tube racks with four test tubes per group. Place the other equipment and supplies at the student work stations or in a central location where they can be located by group members and taken to their work stations.

## Class Demonstration/Pre-activities

If your students are not familiar with how to use a pipette and pump, review the proper way to attach the pipette and pump and how to hold the equipment and how to measure accurately with the pipette. Also, review how to mix the solutions in the tube by using the pipette to draw in the contents of the tube and then releasing back down. Repeat that step two times for a full mix.

## Pre Lab Definitions

Use the Power Point to review the pre lab definitions that the students have on their lab sheet.

• Serial: In a series, order or interval. Measured steps.
• Dilution: Water Down.
• Pipette: “Little pipe” for moving and measuring liquids.
• Serial dilutions reduce the concentration of a sample in small steps or fractions.

## Conducting the Lab

The Power Point can be used to reveal instructions to the students during the lab. The teacher may wish to go step by step, making sure that all groups have completed a step before moving on to the next one. This will allow the teacher to view the student’s pipetting technique and correct any problems that arise. Students should wear protective aprons or lab coats, gloves and safety glasses due to the use of methylene blue during this lab.