A Simplified Cottrell Pump for the Determination of Molecular Weights OTTO F. STEINBACH1 and GEORGE F. CONERY' City College of Nero York, New York City
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HILE many modifications of the Cottrell boiling pomt apparatus have been suggested, the authors believe the apparatus described here will give satisf a c t ~ r yresults for molecular weight determinations in the hands of students. It is easily made, exceptionally cheap and simple, readily cleaned, and is not fragile. The Cottrell pump and the assembled apparatus are shown in the accompanying drawings. The Cottrell pump B is assembled from three or four individual pumps which have the appearance and dimensions shown in A . If the pump is to be used with organic liquids such as COHO, CHCla, or CCla, it should be made from tubing 2 to 3 mm. in diameter. If the pump is to be used with aqueous solutions, it should be made from 6-mm. tubing. Three or four of these individual pumps are assembled around a piece of glass tubing 8 mm. in diameter and 30 mm. in length, using 2- to 3-mm. glass tubing of the same length as spacers. The whole assembly is held tightly together with copper wire. The general appearance of the assembled pump is shown in B. The boiliug chamber is made of pyrex glass and measures 6 cm. in diameter and 18 cm. in length. A side arm (10mm.) was sealed to the chamber and, when surrounded by a condenser jacket, functioned as a water reflux condenser. The Beckmann thermometer is mounted in a rubber stopper when aqueous solutions are employed. As rubber stoppers cannot be used with organic solvents, a cork stopper was coated with sodium silicate solution and dried in an oven for several days. The pump is readily cleaned by suspending it in a tall boiling chamber containing the boilinz solvent used in the exoerimental ~rocedure. In this way, pure boiling solvent condenses on the cooler pump and washes it clean. Compressed air finishes the job of drying. When the Cottrell pump is placed in the boiling chamber, i t should be within 10 to 15 mm. of the bottom. This clearance can be obtained by selecting a smaller or larger central tube, or by bending the flare on the individual pumps. The pumps can be temporarily assembled with rubber bands. The amount of superheating depends upon the nature of the solvent. Solvents such as CeHs or CClr show very little tendency to superheat. When water is employed as the solvent, the Beckmann thermometer will change as much as O.Ol°C. from superheating. A simplified experimental procedure is employed for the determination of molecular weights. Fifty milli-
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' Present address:
Adelphi College. Garden City, New York. Present address: Columbian Carbon Company. Brooklyn. New York. a
liters of 'solvent are measured by means of a pipet and placed in the boiling chamber. The boiling point is determined by taking six readings about one minute apart and averaging the readings. If care is taken in adjusting the flame so that the solvent condenses a t the same point in the condenser, the boiling point is readily duplicated. Then a 2- to 5-g. sample of solute is weighed on glazed paper and brushed into the cooled solvent. The boiling point is determined again by averaging six readings taken about one minute apart. The diierence in temperature is the boiling point elevation from which the molecular weight can be calculated by the usual procedure. The barometric pressure should be recorded at the beginning and end of each determination. The molecular weight of 10 samples of naphthalene was measured in CC4 and CsHs. The average molecular weight obtained was 124, as contrasted with the (Conlinued on @age538)
A SIMPLIFIED COTTRELL PUMP FOR THE DETERMINATION OF MOLECULAR WEIGHTS (Continuedfrom Page 535)
accepted value of 128, with a maximum deviation of 7 per cent. The molecular weight of four samples of Pane sugar was found to be 337 (accepted value of 342), with a maximum deviation of 7 per cent.
The authors wish to thank Professor Ross A. Baker for his many useful suggestions and assistance. We also wish to thank Mr. Harold Wilson for supplying the drawings.
