An Introductory Course of Quantitative Chemical Analysis | Page 8

a few tenths of a milligram (depending upon the total
weight involved) are found, the weights should be returned for
correction. The rider should also be compared with a 5 or 10-milligram
weight.
In an instructional laboratory appreciable errors should be reported to
the instructor in charge for his consideration.
When the highest accuracy is desired, the weights may be calibrated
and corrections applied. A calibration procedure is described in a paper
by T.W. Richards, !J. Am. Chem. Soc.!, 22, 144, and in many large
text-books.
Weights are inevitably subject to corrosion if not properly protected at
all times, and are liable to damage unless handled with great care. It is
obvious that anything which alters the weight of a single piece in an
analytical set will introduce an error in every weighing made in which
that piece is used. This source of error is often extremely obscure and
difficult to detect. The only safeguard against such errors is to be found
in scrupulous care in handling and protection on the part of the analyst,
and an equal insistence that if several analysts use the same set of
weights, each shall realize his responsibility for the work of others as
well as his own.
BURETTES
A burette is made from a glass tube which is as uniformly cylindrical as
possible, and of such a bore that the divisions which are etched upon its
surface shall correspond closely to actual contents.
The tube is contracted at one extremity, and terminates in either a glass

stopcock and delivery-tube, or in such a manner that a piece of rubber
tubing may be firmly attached, connecting a delivery-tube of glass. The
rubber tubing is closed by means of a glass bead. Burettes of the latter
type will be referred to as "plain burettes."
The graduations are usually numbered in cubic centimeters, and the
latter are subdivided into tenths.
One burette of each type is desirable for the analytical procedures
which follow.
PREPARATION OF A BURETTE FOR USE
The inner surface of a burette must be thoroughly cleaned in order that
the liquid as drawn out may drain away completely, without leaving
drops upon the sides. This is best accomplished by treating the inside of
the burette with a warm solution of chromic acid in concentrated
sulphuric acid, applied as follows: If the burette is of the "plain" type,
first remove the rubber tip and force the lower end of the burette into a
medium-sized cork stopper. Nearly fill the burette with the chromic
acid solution, close the upper end with a cork stopper and tip the
burette backward and forward in such a way as to bring the solution
into contact with the entire inner surface. Remove the stopper and pour
the solution into a stock bottle to be kept for further use, and rinse out
the burette with water several times. Unless the water then runs freely
from the burette without leaving drops adhering to the sides, the
process must be repeated (Note 1).
If the burette has a glass stopcock, this should be removed after the
cleaning and wiped, and also the inside of the ground joint. The surface
of the stopcock should then be smeared with a thin coating of vaseline
and replaced. It should be attached to the burette by means of a wire, or
elastic band, to lessen the danger of breakage.
Fill the burettes with distilled water, and allow the water to run out
through the stopcock or rubber tip until convinced that no air bubbles
are inclosed (Note 2). Fill the burette to a point above the zero-point
and draw off the water until the meniscus is just below that mark. It is

then ready for calibration.
[Note 1: The inner surface of the burette must be absolutely clean if the
liquid is to run off freely. Chromic acid in sulphuric acid is usually
found to be the best cleansing agent, but the mixture must be warm and
concentrated. The solution can be prepared by pouring over a few
crystals of potassium bichromate a little water and then adding
concentrated sulphuric acid.]
[Note 2: It is always necessary to insure the absence of air bubbles in
the tips or stopcocks. The treatment described above will usually
accomplish this, but, in the case of plain burettes it is sometimes better
to allow a little of the liquid to flow out of the tip while it is bent
upwards. Any air which may be entrapped then rises with the liquid
and escapes.
If air bubbles escape during subsequent calibration or titration, an error
is introduced which vitiates the results.]
READING OF A BURETTE
All liquids when placed in a burette form what is called a meniscus at
their upper surfaces. In the case of liquids such as water or aqueous
solutions