Proceedings of the American Academy of Arts and Sciences. VOL. XLII. No. 21. - MARCH, 1907. CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF HARVARD COLLEGE. CONCERNING THE ADIABATIC DETERMINATION OF THE HEATS OF COMBUSTION OF ORGANIC SUBSTANCES, ESPECIALLY SUGAR AND BENZOL. BY THEODORE W. RICHARDS, LAWRENCE J. HENDERSON, AND INVESTIGATIONS ON LIGHT AND HEAT made and published, WHOLLY OR IN PART, WITH APPROPRIATION FROM THE RUMFORD FUND. CONTRIBUTIONS FROM THE CHEMICAL LABORATORY OF CONCERNING THE ADIABATIC DETERMINATION OF THE HEATS OF COMBUSTION OF ORGANIC SUBSTANCES, ESPECIALLY SUGAR AND BENZOL. BY THEODORE W. RICHARDS, LAWRENCE J. HENDERSON, AND HARRY L. FREVERT. Presented January 9, 1907. Received December 4, 1906. INTRODUCTION. As a preliminary step to the determination of the heats of combustion of an extended series of organic substances, desired for certain theoretical considerations, measurements were made on two common substances, cane-sugar and benzol. These substances were chosen in order that one might represent solids and the other volatile liquids, and because they may be easily obtained in a high state of purity, as well as because their heats of combustion have been carefully studied by other investigators. This investigation offered the opportunity for further testing and improving the adiabatic calorimetric method recently proposed and tested by Richards, Henderson, and Forbes,1 by means of which corrections for accidental loss of heat and for the lag of the thermometer are experimentally eliminated. The method was devised in the hope that its use might increase the accuracy of thermochemical work; and this hope is justified by the present experience. The principle of the method is to cause the temperature of the surroundings of the calorimeter to change in the same direction and at the same rate as the calorimeter itself. This is accomplished by surrounding the calorimeter with vessels in which a suitable warming reaction can take place in a manner fulfilling the above conditions. A reaction easily regulated and well suited to this purpose, namely, the neutralization of an alkali with an acid, was chosen for this purpose. 1 These Proceedings, 41, 3 (1905); Zeit. phys. Chem., 52, 551 (1905). THE APPARATUS. A vertical section of the apparatus is shown in the accompanying diagram. The large outer vessel (A) and the covering vessel (B), designed for holding the alkaline solution, were made of sheet copper. On account of the corrosive action of the caustic solution to which the vessels are continually exposed, the joints must be very thoroughly soldered, otherwise the corrosion may give rise to annoying leaks. These are of serious consequence if they occur in the cover, because then they may allow the solution to pass into the calorimeter or into the narrow air-space surrounding the calorimeter. The inner vessel (C) protecting the calorimeter itself from the alkaline surrounding liquid was a heavy nickel-plated copper can well burnished in the interior and firmly adjusted in the outer vessel several inches above the bottom, so as to allow a free circulation of the liquid beneath it. The calorimeter proper (D) was placed inside of this inner vessel, resting on several bits of cork and separated by an air-space of about two millimeters from its burnished nickel inner surface. This calorimeter was made of pure silver; it weighed 1357 grams and had a capacity of about 4432 milliliters; in operation it was filled with water, completely submerging the combustion bomb which rested upon points bearing on its base. The German-silver stirrer (E) which agitated the water in the silver calorimeter consisted of two perforated rings on upright supporting wires, and was moved up and down at a perfectly regular rate by means of an electric motor with a worm-gear attachment. This stirring arrangement was found to be very satisfactory, as it produced a complete and rapid adjustment of the temperature of the calorimetric system during a combustion, and the comparatively slow motion gave rise to no warming correction during the seven or eight minutes necessary for the actual combustion. The copper pan (B) used as a cover to both calorimeter and outside jacket, was provided with copper tubes for the stirrers and thermometers projecting below it; its temperature also was changed in the same manner as that of the outer jacket by adding acid to its alkaline contents, so as to follow the effect of the heat of the combustion. The liquid in the cover was stirred by a large oscillating perforated copper ring (F) actuated by the same motor which raised and lowered the stirrer of the calorimeter. It was found unnecessary to follow the change in temperature of the calorimeter as closely in the cover as in the jacket, although had this been necessary it might as easily have been done. |