MOLECULAR-WEIGHT APPARATUS FOR THE BOILING-POINT METHOD EDWARD M. HOSHALL, U. S. DEPARTMBNT OF AGRICULTURE, FOOD AND DRUGAD-ISTRATION. B A L T I M O MARYLAND ~,
A new apparatus for the determination of molecular weights by the boilingpoint method is described. Two Beckman thermometers, m e measuring the boiling point of the solution, the other measuring that of the solvent, are read simultaneously when equilibrium is reached, obtaining thus the boilingpoint rise directly. Simplicity and ruggedness of construction with ease of operation and accuracy of determinations are the features of the apfiaratus. Of the many types of apparatus for the determination of molecular weights by the boiling-point method, few are found to combine simplicity with ease of operation commensurate with results of high accuracy. With these points in view the following apparatus was devised, consisting essentially of a modified type of Cottrell pump (1) discharging over the bulb of a Beckman thermometer, measuring thus the boiling temperature of the solution, while a connecting side-arm equipped with another Beckman thermometer measures the boiling temperature of the pure solvent. The assembled apparatus is sketched in the accompanying diagram. I t is constructed of Pyrex, and consists of a 32-mm. test tube A with a 20-mm. side-neck B , the latter having an arm C for a condenser; the inner tube D is a 24-mm. test tube with a socket E from which the modified Cottrell pump F is suspended, the latter consisting of a "U"of 4-mm. tubing closed at the upper end and having a 3mm. orifice H 10 mm. from the top. The vertical tube F is 6 mm. when water is used as the solvent, while for low-boiling solvents 4-mm. tuhinx is more suitable. The Beckman thermom&s are held in place by foiled corks, and should be provided with some form of a mechanical tapper to prevent sticking of the mercury threads. Glass beads are used to prevent bumping. By the mechanical pumping of the solution in a thin layer over the thermometer bulb, the errors arising from superheating are reduced to relative insignificance. In the determination of the boiling point of the pure solvent there is some danger of superheating occurring, but this 353
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JOURNAL OF CHEMICAL EDUCATION
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can be reduced to a minimum by surrounding the bulb of the Beckman with a small cylinder of platinum gauze and maintaining an even ebullition rate throughout the determination. To this end, the apparatus should be well shielded from drafts: an asbestos shield with a mica window, for observing the pumping action, will be found suitable. The simplicity of operation is apparent. The apparatus is cleaned and dried, assembled and charged with 25 to 35 cc. of the solution and, by means of gentle heating, brought to boiling so that the liquid is being pumped and discharged over the bulb of the thermometer, and a t the same time in the connecting side-arm the pure solvent vapor should be just condensing within the condenser. The Beckman thermometers, which have been compared with each other by noting the difference of their readings when both are a t the same temperature, are then read when equilibrium is reached and the net temperature rise thus determined. Although designed for use with aqueous solutions, other solvents may be used, provided the diameter of the tube F is commensurate with the physical properties of the solvent in order that maximum pumping action a t a gentle ebullition rate be obtained. With a minimum of care and operation this simple apparatus will give quite accurate results. The author desires to express his appreciation to Professor B . P . Caldwell of the Polytechnic Institute of Brooklyn, Brooklyn, N m York, for many suggestions dealing with this apparatus. Literature Cited J. Am. Chem. Soc., 41,721 (1919). (I) COTTRELL,
