Chapter 5: Mastering Laboratory Skills 77D’SOnT UCK
The cardinal rule for using a pipette is to never, ever
pipette by mouth. If you do, the day will come when you
suck something noxious into your mouth. Guaranteed.
Just don’t do it.z
TABLE 5-1:
Densities of distilled water at various temperatures
Temperature (°C) Density (g/mL)15 0.999102616 0.9989460
17 0.9987779
18 0.9985986
19 0.9984082
20 0.9982071
21 0.9979955
22 0.9977735
23 0.9975415
24 0.9972995
25 0.9970479
CLEANING A pIpETTE
The very small bore makes it difficult to clean a pipette
thoroughly. Formal laboratories either use disposable one-use
pipettes or have a pipette cleaner that repeatedly draws cleaning
solution through pipettes to clean them. You can accomplish the
same thing manually by using your pipette pump or pipette bulb.
To clean a pipette, take the following steps:
- The first rule is to clean a pipette immediately after
you use it, or at least rinse it thoroughly. If you allow the
pipette to dry, the solute may crystallize out inside the
bore of the pipette, making it difficult or impossible to
clean properly. - To begin, rinse the pipette inside and out with a thin
stream of warm tap water. - Put a few drops of dishwashing liquid in a beaker and fill
the beaker with warm tap water. - Use your pipette pump or pipette bulb to draw the sudsy
water through the pipette repeatedly, making sure
to clean the outside surface as well. - Rinse the pipette thoroughly with tap water, inside
and out.
6. Fill a beaker with distilled water, and use your pipette
pump or pipette bulb to draw the distilled water
repeatedly through the pipette.
7. Use the pipette pump or pipette bulb to blow out as
much of the final distilled water rinse as possible, and
then set the pipette vertically in a rack to drain and dry.
CALIBRATING A dISpoSABLE pLASTIC pIpETTE
Disposable one-piece soft plastic pipettes, usually called Beral
pipettes, have many uses around the lab. I buy them in bags of
100 or 500 at a time. At a few cents each, they’re cheap enough
to treat as one-use items. But they’re also accurate enough to
substitute for a Mohr pipette when you need to measure small
quantities of liquids.Although some Beral pipettes are roughly calibrated, typically
to 0.5 mL, that’s insufficient for critical work. Fortunately, Beral
pipettes are very consistent from one to another, and all of them
deliver consistent and repeatable drop sizes. For example, if you
know that a particular type of Beral pipette consistently delivers
40 drops per mL, you also know that one drop from that pipette is
0.025 mL. If you need, say, 0.50 mL of a solution, you can use that
Beral pipette to transfer 20 drops.Different models of disposable plastic pipettes are rated to deliver
anything from 20 drops/mL to 50 drops/mL, but those values are
nominal. Just as you can accurately calibrate a volumetric flask
with a balance, you can do the same to calibrate Beral pipettes.
To do so, place a small beaker or similar container on the balance
pan and tare the balance to read 0.00 g. Fill the Beral pipette
completely with distilled water, and then count the drops as you
empty all but the last few drops from the Beral pipette into the
beaker. (Make sure to deliver drops consistently. Hold the pipette
vertically. Don’t squirt drops into the beaker, but allow each drop
to form slowly and fall from gravity alone.)For example, one Beral pipette that I tested was rated to deliver
23 drops/mL. I dispensed 75 drops of water and then set the
pipette aside. The balance told me the mass of those 75 drops of
distilled water was 3.26 g. At room temperature, the density of
distilled water can be taken as 1.00 g/mL for the small quantity
I used, so 3.26 g equals 3.26 mL. Dividing 75 drops by 3.26 mL
gives about 23.01 drops/mL, which is certainly close enough.