24 DIY Science: Illustrated Guide to Home Chemistry Experiments
of graduations, with zero at the top, which makes it easier to
calculate the amount dispensed from the cylinder. Graduated
cylinders are available in capacities from 10 mL to 2000 mL.
The most useful sizes in a home chem lab are the 10 mL and
100 mL versions.
PPETTESI
A graduated pipette (also spelled pipet) is a slender glass tube
that is used to measure and dispense liquids with a very high
degree of accuracy and precision. Three types of graduated
pipettes are commonly used in chemistry labs.
volumetric pipette
A volumetric pipette, also called a transfer pipette, is easily
recognized by the bulge in its middle. Standard volumetric
pipettes have only one graduation line that corresponds to
the nominal capacity of the instrument, and so can be used
only to measure that specific quantity. For example, a 50 mL
volumetric pipette can measure exactly 50 mL (within its
tolerance), but cannot measure smaller quantities because
it has no graduation lines for them. Volumetric pipettes are
available in Class A and Class B, and may be calibrated To
Contain or To Deliver. A few volumetric pipettes are dual-
calibrated TC and TD, with two corresponding graduation
lines. Volumetric pipettes are specialized instruments that
aren’t needed in most home chem labs. Instead, use a Mohr or
serological pipette to measure and transfer small quantities of
liquids, and a volumetric flask for larger quantities.
mohr pipette
A Mohr pipette, also called a measuring pipette, is graduated
from a zero line near the top of the pipette to a baseline near
the bottom. Those graduations allow a Mohr pipette to be used
to measure any amount of liquid from its smallest increment
up to its maximum capacity. The fineness of the graduations
depends on the capacity of the pipette. For example, a 10.0 mL
Mohr pipette is typically graduated by 0.1 mL markings, while
a similar 1.0 mL model is graduated by 0.01 mL markings. You
can interpolate between markings. For example, if the fluid level
in a 10.0 mL Mohr pipette is about halfway between the 5.1 mL
and 5.2 mL marks, you can record that as 5.15 mL, give or take.
FIGURE 3-15: 10.0 mL (top) and 1.0 mL serological pipettes
PETTEPRnEvE I BY MOUTH
In the Bad Olde Days, many chemists used their
mouths to suck liquids into pipettes and to blow out the
last droplet. Many of them dropped dead, too. Never put
your mouth in contact with a pipette. Not even if you’re
pipetting pure water. If you get into the habit of filling
pipettes by mouth, the day will inevitably arrive when you
suck something nasty into your mouth. Just don’t do it.
Use a pipette bulb or pipette pump instead.z
A Mohr pipette is a TD device, in the sense that when a Mohr
pipette has delivered its maximum calibrated volume, some
liquid remains in the tip. Do not blow out this remaining liquid,
which is accounted for in the calibration.Serological pipette
A serological pipette, also called a blow-out pipette, can
be thought of as a TC version of a Mohr pipette. The only
real difference is that the serological pipette is calibrated TC
rather than TD, which means you have to blow out the liquid
that remains in the tip after the serological pipette drains.
Serological pipettes are made in glass and plastic versions,
and have at best Class B accuracy. They are also inexpensive
and sufficiently accurate for nearly any task in a home chem
lab. Serological pipettes are readily available in capacities of
0.2 mL, 0.5 mL, 1.0 mL, 2.0 mL, 5.0 mL, and 10.0 mL. Most
serological pipettes are color-coded for easy identification.
Glass models sell for $0.50 to $3.00 each, depending on
capacity and quantity, and disposable plastic models for less.
Keep at least one 1.0 mL and one 10.0 mL glass serological
pipettes on hand.Various types of ungraduated or roughly graduated pipettes
are also useful in a chemistry lab, including medicine droppers,
the Pasteur pipette (essentially a large medicine dropper with
a long, fine tip), and the Beral pipette, which is described later
in this chapter.