A PIPET FOR THERMOCHEMISTRY ALLEN L. HANSON St. Old College, Northfield, Minnesota
TRE undergraduate course of physical chemistry often includes as an experiment in thennochemistry the measurement of the heat of neutralization. As usually performed, directions call for pouring together acid and base (or other reacting electrolytes) in a Demvu flask and noting the temperature change as the reaction progresses. Students are often frustrated through inability to control the initial temperature of the reactants so as to make them identical a t the start of the reaction. An error of a mere hundredth of a degree (which would be unusually good agreement) will render futile the precise use of a Beckmann thermometer which the student is expected to read t o the nearest thousandth of a degree. One obvious solution to the problem is t o immerse one reagent in the other, thereby to insure thermal equilibrium. There remains, however, the problem of transfer of liquids without loss and without change of either temperature. Such errors arise if one solution is momentarily removed in order t o
VOLUME 35, NO. 5, MAY, 1958
pour it into the other. The accompanying figure shows the design of a vessel which solves the problem. The device is a transfer pipet which will contain one of the reactants (e.g., acid), and is set into the other (basic) solution. When thermal equilibrium has been reached the contents of the pipet are discharged by blowing into tube A without removal from the bath. The pipets can be made from 3-em. X 25-em. test tubes by drawing out the tops, sealing on mouthpipes, and sealing in the delivery tubes which extend to the bottom of the vessel. The pipet thus made is about 33 cm. over-all and is suitable for use with a one-quart Dewar flask. There is room in the flask for a ring stirrer made from plastic rod and for a Beckmann thermometer. About 100 ml. of reagent is used satisfactorily in this manner. Since there is of course not a complete discharge of the contents of the pipet, some of the reagent to be used is first introduced and is blown out. Then 100 ml. are accurately measured and introduced through tube A. A little practice will insure reasonable duplication of the amount of run-back and thus give sufficient precision.
