A n Apparatus for the Determination of Molecular Weights by the Method of Lumsden OTTO F. STEINBACH'
City College of New York, New York City type of flask was selected as i t is considerably stronger than the ordinary Erlenmeyer flask. The manometer M is made. from 10-mm. capillary tubing, the inner diameter of which is 2 to 3 mm. The manometer is graduated every 0.2 cm. on the lower end for a length of 2 cm. and is thereafter graduated every centimeter. This is readily accomplished by placing the tubing between two meter sticks and marking the divisions with aucihle marking ink. The glass weighing sample bottles B are made from 5-mm. glass tubing. Glass rod, which snugly fits the tubing, is selected for use as stoppers. The weighing bottles hold 0.2 to 0.4 ml. of sample. The apparatus is thoroughly cleaned and assembled. The volume of the apparatus is found by filling it with water and then determining the volume of water present with a graduated cylinder. The apparatus is taken apart, and after it has been thoroughly dried, 50 to 75 ml. of mercury are placed in the flask. Then the apparatus is reassembled. The mercury level on the outside of the manometer is recorded, and the apparatus is placed in a pail of boiling water. The cork stopper a t the top of R is loosely set in place. When the temperature becomes steady, the rubber tubing is removed and the outlet 0 is wiped dry. A sample of known weight (0.18 to 0.30 g.), contained in the weighing bottle, is dropped through the opening, and this in turn is immediately and firmly closed with the rubber stopper S. When the manometer remains steady, the pressure is recorded and likewise the temperature. Successive determinations can he made without taking the apparatus apart, but it is advisable to sweep out the vapors with a stream of dry air which is led in through the top of the manometer before starting the new determination. , The molecular weight is calculated by substituting g the data in the formula P V =-RT. The vapor
T
HE apparatus generally employed for determining the molecular weight of a volatile substance by Lumsden's method requires calibration with a substance of known molecular weight before it can be used to find the molecular weight of an unknown substance. In the apparatus described here, this difficulty is avoided, and the molecular weight may be calculated directly from the measured dimensions of the apparatus and the experimental conditions. The assembled apparatus is shown in the accompanying diagram. It consists of a 250-ml. side-neck pyrex flask which has been sealed off a t the side neck. This Present address: Adelphi College, Garden City. New York.
M
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occupies the volume V which is the initial volume of the flask minus the volume of mercury added. The volume occupied by the glass weighing bottle should also be subtracted from this volume in order to obtain the final volume available to the vapor. While this latter correction is small, it does become appreciable after four or five determinations have been made. The thermometer which is used to obtain the temperature of the vapor need only he divided in whole de-
grees. It should be calibrated a t the temperature of boiling water, for obvious reasons. The mass, g, of sample taken is weighed to a tenth of a milligram. The weighing bottle is conveniently mounted upon a cork placed on the balance pan. I t is inadvisable to allow too much time to elapse between the weighing and the addition of sample to the apparatus, as the loss in weight is appreciable for volatile substances with the type of weighing bottle described. The initial reading of the manometer is probably the most uncertain reading which must be made. It is possible to make a visual estimate of the position of the mercury in the capillary. Another method is to select a small section of the capillary tubing and actually measure the depression in the capillary. A third ,procedure is to calculate the depression by applying the simple surface tension formula. Reasonable agreement between all three procedures is obtained. The
pressure in the flask is, therefore, the final height minus the initial height multiplied by 0.98. The correction factor is used to allow for the change in density of mercury a t 100' and 0%. and hence to obtain the true, corrected pressure. The correction for the expansion of the glass scale on the manometer is neglected. Expressing the pressure in centimeters of Hg, the volume in cubic centimeters, the temperature in absolute degrees, the mass in grams, and employing the value of 6237 cc. X cm./'A. for R, the molecular weight M is obtained by substituting the above quantities in the g RT. Molecular general equation of state PI.'= -
M
weights which are accurate to 5 per cent can be obtained. The author wishes to thank Mr. Harold Wilson for supplying the drawings.
