Puffenbarger, R. 2011. Understanding Solution Chemistry the “Southern Way” With Sweet Tea. Pages 117-126, in Tested Studies for Laboratory Teaching, Volume 32 (K. McMahon, Editor). Proceedings of the 32nd Conference of the Association for Biology Laboratory Education (ABLE), 445 pages. http://www.ableweb.org/volumes/vol-32/v32reprint.php?ch=10
Introduction
The pipette is designed to measure and transfer very s,mall volumes of liquid with accuracy and precision. Accuracy refers to the margin of error between the dialed volume and the volume actually delivered by the instrument. Precision refers to the ability of the instrument to deliver a given dialed volume repeatedly. The pipette is far more accurate than glass or plastic serological pipettes especially when volumes less than a milliliter (ml) are desired.
Parts of the Pipette
Figure 1. External parts of the pipette
-Plunger button
-Tip ejector button
-Volume adjustment dial
-Volume indicator
-Shaft
-Tip ejector
-Disposable tip
A plastic tip placed on the bottom of the shaft to prevent cross-contamination between samples and to prevent liquid from being pulled into the shaft.
Volume is adjusted by turning the volume adjustment knob or dial.
A pipette function by air displacement. When the plunger is depressed, all air is expelled from inside the shaft by a metal rod or piston, creating a vacuum. As the plunger is released, the rod retract to a point set by the volume dial and the liquid sample flows into the plastic disposable tip replacing the air that was displaced form the shaft. The rod is controlled by a spring mechanism. Dialing the pipette above the maximum volume rage of the pipette distorts the spring leading to inaccurate measurements. If the pipette dial is turned too far past the max volume for the unit the spring can snap entirely.
Reading the scales on the pipette
For the P10 and the P20, the last digit in the window is a decimal. The P200 has no decimal places. On the P1000 the "ones" place or the final digit is the number 1000 is not represent on the scale.
Choosing the correct pipette for the task
| Pipettor | Theoretical Range (ml) | Recommended Range (ml) |
| P10 | 0-10 | 1-10 |
| P20 | 0-20 | 2-20 |
| P200 | 0-200 | 20-200 |
| P1000
|
0-1000 |
100-1000 |
2.) Attach a disposable tip
3.) Load the sample
4.) Expel the sample
Learning Objectives:
1. Defining molarity
2. Dilute solution by on fold change
3. Use the C1V1=C2V2 equation to create solution of the proper molarity using give stock solutions
4. Determine how much of a compound is needed to make a solution of the desired molarity.
5. Determine how to make a solution described in therm of % w/v or % v/v
Background:
Expressing solution concentrations:
The Percent (%) Solution:
Molarity Concentrations:
Converting Between Units:
Lab Exercises/Procedure:
Reference to Department of Biology and Biochemistry Fall 2013, BCHS 3201 Biochemistry 1 Laboratory Manual, University of Houston.
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Title: Making Solutions and Determining Sugar Tasting Threshold
Preparing and Determining molar concentrations
Prelab
a.) Scientific concept(s)
- Learn 3 ways concentration are expressed. X Dilutions, Molarity and percent
- Understand what molarity and % mean in terms of solution chemistry.
b.) Laboratory techniques
- Preparing a solution by dissolving a solid solute in a solvent.
c.) Objective (goal)
- In this lab we will determine the lowest molar concentration (threshold) of sugar that the class can taste
Results
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This graph illustrates the sugar concentration (M) and the amount of students that could taste said concentration. The lowest threshold is .01M
Discussion/PostLab
#2) *note the placebo effect
