An Introductory Course of Quantitative Chemical Analysis | Page 9

this meniscus is concave, and when the liquids are transparent
accurate readings are best obtained by observing the position on the
graduated scales of the lowest point of the meniscus. This can best be
done as follows: Wrap around the burette a piece of colored paper, the
straight, smooth edges of which are held evenly together with the
colored side next to the burette (Note 1). Hold the paper about two
small divisions below the meniscus and raise or lower the level of the
eyes until the edge of the paper at the back of the burette is just hidden
from the eye by that in front (Note 2). Note the position of the lowest
point of the curve of the meniscus, estimating the tenths of the small
divisions, thus reading its position to hundredths of a cubic centimeter.
[Note 1: The ends of the colored paper used as an aid to accurate
readings may be fastened together by means of a gummed label. The
paper may then remain on the burette and be ready for immediate use

by sliding it up or down, as required.]
[Note 2: To obtain an accurate reading the eye must be very nearly on a
level with the meniscus. This is secured by the use of the paper as
described. The student should observe by trial how a reading is affected
when the meniscus is viewed from above or below.
The eye soon becomes accustomed to estimating the tenths of the
divisions. If the paper is held as directed, two divisions below the
meniscus, one whole division is visible to correct the judgment. It is not
well to attempt to bring the meniscus exactly to a division mark on the
burette. Such readings are usually less accurate than those in which the
tenths of a division are estimated.]
CALIBRATION OF GLASS MEASURING DEVICES
If accuracy of results is to be attained, the correctness of all measuring
instruments must be tested. None of the apparatus offered for sale can
be implicitly relied upon except those more expensive instruments
which are accompanied by a certificate from the !National Bureau of
Standards! at Washington, or other equally authentic source.
The bore of burettes is subject to accidental variations, and since the
graduations are applied by machine without regard to such variations of
bore, local errors result.
The process of testing these instruments is called !calibration!. It is
usually accomplished by comparing the actual weight of water
contained in the instrument with its apparent volume.
There is, unfortunately, no uniform standard of volume which has been
adopted for general use in all laboratories. It has been variously
proposed to consider the volume of 1000 grams of water at 4°, 15.5°,
16°, 17.5°, and even 20°C., as a liter for practical purposes, and to
consider the cubic centimeter to be one one-thousandth of that volume.
The true liter is the volume of 1000 grams of water at 4°C.; but this is
obviously a lower temperature than that commonly found in
laboratories, and involves the constant use of corrections if taken as a

laboratory standard. Many laboratories use 15.5°C. (60° F.) as the
working standard. It is plain that any temperature which is deemed
most convenient might be chosen for a particular laboratory, but it
cannot be too strongly emphasized that all measuring instruments,
including burettes, pipettes, and flasks, should be calibrated at that
temperature in order that the contents of each burette, pipette, etc., shall
be comparable with that of every other instrument, thus permitting
general interchange and substitution. For example, it is obvious that if
it is desired to remove exactly 50 cc. from a solution which has been
diluted to 500 cc. in a graduated flask, the 50 cc. flask or pipette used to
remove the fractional portion must give a correct reading at the same
temperature as the 500 cc. flask. Similarly, a burette used for the
titration of the 50 cc. of solution removed should be calibrated under
the same conditions as the measuring flasks or pipettes employed with
it.
The student should also keep constantly in mind the fact that all
volumetric operations, to be exact, should be carried out as nearly at a
constant temperature as is practicable. The spot selected for such work
should therefore be subject to a minimum of temperature variations,
and should have as nearly the average temperature of the laboratory as
is possible. In all work, whether of calibration, standardization, or
analysis, the temperature of the liquids employed must be taken into
account, and if the temperature of these liquids varies more than 3° or
4° from the standard temperature chosen for the laboratory, corrections
must be applied for errors due to expansion or contraction, since
volumes of a liquid measured at different times are comparable only
under like conditions