Biology Lab: Measurements and Measuring

The Serological Pipette

The serological pipette is graduated all the way to the tip.
A 10 ml serological pipette has a line at the top marked 0. When the pipette is filled to this line, it contains 10 ml.
The ten smaller graduations underneath the line represent 0.1 ml.

Working with DNA and enzymes frequently involves measuring very small volumes, often in the microliter range.
Microliter (μl): one millionth of a liter (10-6 L) and one thousandth of a milliliter (10-3 mL).
To make precise measurements of so small a volume, micropipettes are used.
Liquid is never drawn into the barrel/shaft of the micropipette itself. An appropriate tip is always be placed firmly on the end.
When you push down gently on the plunger of the micropipette, you will feel resistance
Plunger: dispenses the set volume.
Volume adjustment knob: enables the user to change the amount of liquid being dispensed as shown by the volume indicator.
The user can control the volume being dispensed unless the pipette is a fixed pipette, meaning it has one set volume.
Body of the pipette: where you hold the instrument
Shaft: the piece of the instrument in which a disposable tip is placed.
Polypropylene tips: often sterile and is the only part of the pipette that contains the liquid.
First stop: used to measure and take up the correct volume.
Second stop: used to completely expel the liquid you are measuring.

Dilute: to make a solute less concentrated by the addition of a solvent.
If a volume of solute-A is added to a volume of solvent-B then you will have a more dilute solution than you started with.
The dilution can be calculated using a simple formula:
- Dilution = (Vol A) / (Vol A + Vol B)

Under optimal conditions it is possible for microorganisms to grow to very high densities.
It is often necessary to dilute these organisms to a lower concentration for other applications.
A set of serial dilutions may be used in order to create a range of concentrations of your culture (serial dilutions can be used to dilute any solution.)
Serial dilutions are usually made in 1:10 steps, although other increments can be used.
A 1:10 dilution is made from the original culture, and then a second 1:10 dilution is made from the first 1:10 dilution, making it a 1:100 dilution relative to the original culture.
- Each 1:10 dilution is made from the previous dilution and results in another ten-fold decrease from the original stock.
Dilution Factor = C2/C1 = V2/V1
- C1V1=C2V2
- C1 = concentration of the stock solution
- C2 = concentration of the working solution (desired concentration)
- V1= volume of the stock solution
- V2= final volume of the working solution (desired volume).