Common Operations and Wet Chemical Methods in Environmental Laboratories - Fundamentals of Environmental Sampling and Analysis - page 133

volume measurement in preparing solutions. Only volumetric flasks, burets, and
pipets are accurate enough in measuring volume. Volumetric flasks are calibrated to a
specified volume (5, 10, 25, 50, 100, 250, 500, 1000 mL) when filled to the line
etched on the neck. They are commonly used in preparing standard solutions
for calibration purpose. Burets are specialized graduated cylinders where a stopcock
at the bottom is used as a valve to control the release of solution into a receiving
flask. They are particularly used for titration. Pipets are used to transfer known
volumes of liquid from one container to another, which come in two main size
ranges and the prices differ considerably: (a) micropipets with volumes smaller than
1 mL (1, 10, 50, 100, 500 L), which include syringe pipets and automatic pipets

that come with disposable pipet tips. (b) regular glass or plastic pipets with a
larger volume (1, 2, 5, 10, 25, 50, 100 mL), which can be further divided into two
types (Fig. 6.1):

m

Volumetric pipets deliver fixed volumes (1, 2, 5, 10, 25, 50, 100 mL).
Volumetric pipets labeled as Class A have a certain time in second imprinted
near the top. This is the time allowed to elapse from the time the finger is
released until the pipet is empty. When emptying a volumetric pipet, the
liquid is allowed to drain out, but is not forced out. Hence, the volumetric
pipets are not blow-out pipets. They are also more accurate than the
measuring pipets.
Measuring pipets have graduation lines that are calibrated in convenient
units so that any volume up to the maximum capacity (1, 2, 5, 10, 25, 50,
100 mL) can be delivered. They are not as accurate as volumetric pipets due
to the fact that any imperfection in their internal diameter will have a greater
effect on the volume delivered. Measuring pipets are divided into serological
pipets (the graduations marks continue to the tip) and Mohr pipets (the
graduations end before the tip). Note that serological pipets can be blow-out
pipets if they have a frosted band or two thin rings on the top. This means
that after all the liquid has been allowed to drain out by gravity, the last drop
remaining in the tip is also forced out. Most of the disposable pipets
nowadays are the serological ''blow-out'' type. The last drop of the solution
should be blown out with a pipet bulb.

Figure 6.1 Different types of pipets (from the top): 10-mL volumetric pipet,10-mL serological
pipet, and two Mohr pipets (Courtesy of Dr. Ruth Dusenbery, The University of Michigan-Dearborn)

Volumetric glassware is calibrated either to contain (TC) or to deliver (TD).
Glassware designated TD will do so with accuracy only when inner surface is clean