Illustrated Guide to Home Chemistry Experiments

22 DIY Science: Illustrated Guide to Home Chemistry Experiments

Graduated cylinder Pipette

Nominal volumeClass A Class B Class A Class B

1 mLn/a n/a ±0.006 mL ±0.012 mL

5 mLn/a n/a ±0.01 mL ±0.02 mL

10 mL ±0.08 mL ±0.1 mL ±0.02 mL ±0.04 mL

25 mL ±0.14 mL ±0.3 mL ±0.03 mL ±0.06 mL

50 mL ±0.20 mL ±0.4 mL ±0.05 mL ±0.10 mL

100 mL ±0.35 mL ±0.6 mL ±0.08 mL ±0.16 mL

250 mL ±0.65 mL ±1.4 mL n/a n/a

500 mL ±1.10 mL ±2.6 mL n/a n/a

1000 mL ±2.00 mL ±5.0 mL n/a n/a

2000 mLn/a ±10.0 mLn/a n/a

TABLE 3-1: Comparison of Class A and Class B tolerances for volumetric glassware

Class A volumetric glassware provides the highest accuracy,
but is expensive and is used primarily in professional
laboratories. Class A volumetric glassware complies with the
Class A tolerances defined in ASTM E694, must be permanently
labeled as Class A, and is supplied with a serialized certificate of
precision. (Some manufacturers have begun supplying “generic”
Class A vessels, which meet Class A tolerances, but are not
technically Class A because they are not serialized or certified.)
All Class A volumetric glassware is actually glass; volumetric
plasticware is not eligible for Class A status.

Class B volumetric glassware has tolerances twice those of
Class A (except graduated cylinders, which have rules of their
own), but is considerably less expensive and is adequate for most
home laboratories. Class B volumetric glassware must comply
with the Class B tolerances defined in ASTM E694 and must be
permanently labeled as Class B. Many inexpensive volumetric
glassware items, particularly those made in China, are unlabeled
and so cannot be guaranteed to meet Class B standards. In
practice, most of them seem to meet Class B standards, or at
least come close.

voLUmETRIC GLASSwARE

Volumetric glassware is used to measure liquids with accuracy ranging from moderate to very

high. Many standard beakers and flasks have graduations accurate to ±5%. Although this level of

accuracy is sufficient for many routine tasks, you’ll often need to measure liquids more precisely.

Volumetric glassware allows you to measure liquids with accuracy ranging from ±1% to ±0.1% or

less, depending on its type, design, and quality.

Table 3-1 lists the Class A and Class B tolerances for the four

common types of volumetric glassware used in home chem labs.

Depending on its type and design, volumetric glassware may

be rated To Contain (TC) or To Deliver (TD). A vessel marked TC

contains the amount specified when it is filled to the graduation

line. A vessel marked TD delivers the amount specified when it is

filled to the graduation line and then emptied using the proper

procedure. The difference arises because of drainage holdback

error. For example, volumetric flasks are rated TC. If you fill a

500 mL volumetric flask to the graduation line, it contains

exactly 500 mL of solution (within its tolerance, and assuming

that the ambient temperature corresponds to that at which the

flask was rated, usually 20°C). If you empty that flask into

another container, a bit less than 500 mL will transfer. That’s

because some of the solution remains in the flask, wetting its

inner surface.

GADUATEDR CYLInDERS

A graduated cylinder is a tall, slender cylinder with numerous

graduation lines from near the bottom to near the top. You