BURETTE

A graduated gla ve el employed in volumetric analysis for measuring liquids. 1. The first burette was invented by Gay-Lu ac, a drawing of whose instrument is given below. It rarely, if ever, has a capacity greater than 50 cubic centimetres, and consists of a narrow tube fused on to a wider one. The larger tube is about 33 centimetres long, the graduated portion occupying about 25 centimetres, and its internal diameter measures 15 millimetres; the narrow tube has a diameter of 4 millimetres, which in the upper bent end decreases to 2 millimetres. When used the instrument should be held in the left hand, the bottom part being allowed to lean a little against the chest. The operation is aided by giving the instrument from time to time a slight turn in the direction of its longitudinal axis, thereby placing the curve of the stout alternately in a more vertical, alternately in a more horizontal position. The volume must not be read off before the surface of the liquid has attained a constant height. 2. Gei ler’s burette. This instrument differs from Gay-Lu ac’s in having the narrow tube inside, instead of outside the wider one. It is found very convenient in use, and is le liable to fracture than Gay-Lu ac’s. 3. Mohr’s burette, which can be more easily and readily managed than either of the two preceding ones, is described and figured under Alkalimetry . IMG:596405606043057898_i384-1.png: Gay-Lu ac’s Burette and Gei ler’s Burette. 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 distinctne 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. IMG:596405606043057898_i384-2.png: a. and b. IMG:596405606043057898_i384-3.png: 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 correctne 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 ten cubic centimetres of water at 60·8° F. must weigh 9·990 grammes.