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21
C.
Volume Measurements
Because of surface tension and capillary
attraction, liquids will not form a flat surface in contact
with walls of a container. The curved
surface is called the meniscus. Water
in a graduated
cylinder has a concave meniscus and the
correct reading is taken at the lowest point, with the
meniscus at eye level. A white card
marked with a colored strip (red, blue or green) is a valuable
aid in determining the bottom of the meniscus.
Place the card against the buret, graduated cylinder
or pipet with the colored strip
slightly below the
bottom of the meniscus to prevent distortion of
readings resulting from illumination problems.
If the colored line is in the right place, the meniscus
will reflect the color of the line and
will be outlined clearly. Your 10 mL graduated cylinder can be
read with a precision of 0.05 mL, your
100 mL graduated cylinder with a precision of 0.50 mL, and a
buret with a precision of 0.04 mL.
As a general rule, the cylinder chosen for measuring should not
have a capacity more than 10 times the
volume to be measured.
1.
Burets and Titration Procedures
Burets are used to measure the volume
of a liquid reagent required to react with a carefully
measured (liquid or solid) sample of another
substance. Burets are calibrated from top to bottom.
Fifty milliliter burets are calibrated
in 0.1 mL increments and thus it is possible to estimate
volumes to within 0.02 mL.
a)
Cleaning the buret - A detergent and a
long-handled brush will serve to remove grease and
dirt from the buret. Several rinsings
with tap water and then distilled water (drained through
the stopcock) will remove the cleaning
solution. Distilled water will drain from a clean buret
in a continuous film leaving no droplets
adhering to the inner wall.
b)
Preparation of the buret - To avoid diluting
the solution to be used, rinse the clean buret with
two 5-8 mL portions of solution with which
the buret is to be filled. Drain the buret after
each rinsing. Then fill it almost
to the top (above the graduations) with the solution. Allow
the solution to fill the delivery tip
by slowly opening the stopcock. Force out any trapped air
bubbles in the tip by letting the liquid
flow while gently tapping the tip. Discard enough
solution to bring the meniscus below the
zero mark. Wait 30 seconds for drainage before
taking a reading.
c)
Reading a buret - The reading of the liquid
level should be at the lowest point of the
meniscus. To avoid parallax error,
you must make your reading by looking at the
graduations on the buret directly at 90
degrees. The bottom of the transparent meniscus can
be more easily determined by placing a
colored strip on a white card, just below the
