Volumentric Glassware and Other Devices Intended "to Deliver"
Pipets are recommended for the delivery of all volumes <5
ml and are required for delivering volumes <1 ml (in the absence of
an appropriate syringe). There are two basic types of pipets:
- The single volume or transfer pipet is the most accurate and simplest
to use type, but is, obviously, limited to measurement of a fixed, single
volume. Generally, these pipets deliver their inscribed volume by complete
drainage of the pipet from an etched mark. They are normally used for the
accurate transfer of 1.0, 2.0, 5.0, 10.0, and 25.0 ml of liquid.
- The Mohr, or graduate multiple volume pipet, is graduated from a point
near the tip to the nominal capacity of the pipet. Thus, it can delivery
multiple volumes of liquid with good volumetric precision. From a practical
economic stand point, the Mohr pipet is generally the preferred instrument
A pharmacist can accurately compound any prescription requiring relatively
small volumes of liquid if he/she has just three basic sizes of Class III
Mohr pipets available:
1 ml Mohr pipet, subdivided in 1/100 ml
2 ml Mohr pipet, subdivided in 1/10 ml
5 ml Mohr pipet, subdivided in 1/10 ml
1. Single volume pipets
- Using a respirator (rubber bulb) for suction, draw the liquid into
the pipet until it is above the graduation. Caution: Since
we may be using corrosive or poisonous liquids mouth pipeting is
strictly prohibited in the pharmaceutics lab.
View a video demonstration on how to use a pipet bulb
- Remove the pipet from the stock solution.
- Wipe the end of the pipet with a tissue or Kim wipe.
- While holding the pipet in a vertical position, release pressure, allow
the liquid to flow into a beaker (or other vessel) until the bottom of
the meniscus coincides with the etched line. Droplets which remain suspended
from the tip can be removed by touching the inside of the beaker with the
tip of the pipet.
- Allow the pipet to drain for 30 seconds (or up to 5 minutes for viscous
liquids) while touching the tip of the pipet to the inner side of the receiving
2. Mohr (graduated or calibrated) pipets
The manipulations are essentially the same as with single volume pipets
except that fractional volumes may be transferred by noting the meniscus
level before and after delivery. Thus, 1.50 ml may be delivered after observing
an initial reading of 8.50 ml by allowing the liquid to flow until the
meniscus reaches 7.00 ml. When working with viscous materials, it is necessary
to check the final reading after waiting approximately 1 minute. (This
can be very tricky!) It should be noted that the final graduation of
a Mohr pipet is usually some distance above the pipet tip so that delivery
is performed from graduation to graduation and not from graduation to tip
as with the single volume pipet. Make certain that you know which type
of pipet you are using to ensure accurate delivery.