In volumetric analysis the method of taking the readings of the burette is an operation of great importance, requiring considerable method and practice.
The first proceeding is to bring the eye to a level with the fluid, and to adopt a fixed and unalterable standard of what is to be considered the surface.
If you hold a burette partly filled with water between the eye and a strongly illumined wall, the surface of the fluid presents the appearance shown in fig. a. If you hold close behind the tube a sheet of white paper with a strong light falling on it, the surface of the fluid will present an appearance similar to that shown in fig. b.
In both cases you have read off at the lower border of the dark zone, this being the most distinctly marked line. Its distinctness may be heightened by adopting Mohr’s contrivance which consists in pasting on a sheet of very white cardboard a broad strip of black paper, and when reading off holding this close behind the burette in a position to place the border line between white and black from 2 to 3 millimetres below the lower border of the dark zone as shown in figure c.
a. and b.
c.
Great care must be taken to hold the paper invariably in the same position, since if it be held lower down, the lower border of the black zone will move higher up.
To test the correctness of the graduation of a burette proceed as follows:—Fill the instrument up to the highest division with water at 60·8° F., then let the cubic centimetres of the liquid flow out into an accurately weighed flask, and determine the weight of these ten cubic centimetres in the usual way; then let another quantity of ten cubic centimetres flow out, and weigh again, and repeat the operation till the contents of the burette are exhausted. If the instrument is correctly graduated, every