Illustrated Guide to Home Chemistry Experiments

78 DIY Science: Illustrated Guide to Home Chemistry Experiments

I wasn’t so lucky with another pipette, which was rated to deliver
43 drops/mL but in fact delivered closer to 44 drops/mL, at least
using my dropping technique. Still, once the actual delivery rate
is known, the nominal delivery rate is immaterial. Because I’ve
calibrated them, I can use those pipettes to deliver accurately
known small amounts of liquid reliably.

The number of drops per mL depends on the viscosity of the
liquid. For example, if you calibrate the number of drops per mL
using distilled water, that number is not valid for a more viscous
liquid such as olive oil or concentrated sulfuric acid, nor for a
less viscous liquid such as acetone or methanol. Fortunately,
the viscosity of the dilute aqueous solutions commonly used in
labs does not significantly differ from the viscosity of pure water.

UBISnG A URETTE
A burette is used to transfer solutions while measuring the
volume transferred with high accuracy. Although burettes are
sometimes used to measure and transfer nominal fixed volumes
(called aliquots) of a solution, for example, by transferring 5.0 mL
of a solution to each of several beakers that are to be used in a
subsequent procedure, that function is ordinarily better done using
a volumetric or Mohr pipette.

By far the most common use of a burette is for titration,
a procedure in which a solution (called the titrant) whose
concentration is known very accurately is dispensed by a burette
and reacted with a known volume of another solution of unknown
concentration (called the analyte). By measuring the amount of

titrant needed to neutralize the analyte, you can determine the

concentration of the analyte accurately.

Here is the proper way to use a burette:

1. Rinse the inside of a clean burette thoroughly with the
   solution it will contain. Allow the solution to run out
   through the stopcock. Drain the burette completely.
   Repeat the rinse at least once.


2. Make sure that the outside of the burette is clean and dry,
   and then mount it securely to a laboratory stand using a
   burette clamp of the proper size.


3. Fill the burette to above the zero mark, using a graduated
   cylinder, small beaker, or other container. Use a funnel if
   necessary to prevent spillage.


4. Run some solution through the stopcock to fill the burette
   tip completely, making sure that there are no air bubbles
   and that the level of the solution falls to or below the zero
   mark. (It’s not necessary to hit the 0.00 mark exactly.)


5. Record the starting volume. When you complete the
   titration, you’ll subtract this starting volume from the
   final volume to determine the amount of solution you’ve
   added. Read the value by placing your eye at the level of
   the solution and reading the value at the bottom of the
   meniscus, as shown in Figure 5-10 (which indicates a
   reading of about 34.37 mL).

When you complete the titration, empty any remaining solution

into a waste container (using the proper disposal method for

NTE NdpoI vERSUS EqUIvALENCE poINT

The end point of a titration is the point at which the

titration is complete, typically when an added indicator

such as phenolphthalein changes color. (The end point

may also be determined by other methods, such as the

conductivity of the analyte reaching a minimum or the

pH of the analyte reaching 7.00.) The equivalence point

is closely related to but not necessarily identical with the

end point. The equivalence point is the point at which the

number of moles (or equivalents) of titrant exactly equals

the number of moles (or equivalents) of analyte.

Ideally, the end point should exactly equal the equivalence

point, but in the real world they are usually slightly

different. For example, you may titrate a hydrochloric

acid analyte with sodium hydroxide titrant, using

phenolphthalein as an indicator. Phenolphthalein is

colorless in acid solutions, and pink in base solutions, but

no color change occurs until the pH of the solution reaches

about 8.2, well into the basic range.

FIGURE 5-9: Calibrating a disposable pipette