The Signer Method for Determining Molecular Weights E. P. CLARK Bureau of Entomology and Plant Quarantine, United States Department of Agriculture, Beltsville, Rld.
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FIGURG 1. SIGNERMOLECULAR-WEIGHT APPARATUS
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stant and equimolar. These volumes may then be read by tilting the apparatus and draining the solutions into the graduated side arms. Five minutes are arbitrarily taken for this purpose. With the data thus available, it follows from Raoult's law that
HE purpose of this communication is to call attention to
and elaborate upon an extremely accurate but little known method for determining molecular weights. The procedure published by Signer (I) involves the principle of isothermal distillation and has been shown experimentally to be practical. However, the original directions are such that more specific information is desirable. For this reason a workable outline for making the determination is presented.
where M , 8, and G are, respectively, the molecular weight, volume of solution, and weight of the standard, and M1, VI, and GI are the corresponding values of the unknown. In practice, the volumes are read every 1 to 3 days, depending upon the solvent used, until they become constant. The results thus obtained may be plotted (volume against time) and, if the experiment is progressing normally, smooth typical curves are obtained as presented in Figures 2 and 3.
The experiment consists in permitting two solutions in an evacuated system, with solvent vapors in contact, to arrive at vaporpressure equilibrium by isothermal distillation. Arrangements must be available for determining the volumes of each solution. The apparatus used. to realize this is shown in Figure 1. It is smaller than the original, to make possible accurate measurements of 1.5 t o 1.7 cc. of liquid. The solutions usually employed are approximately 0.1 molar, from which it follows that the quantity of substance necessary for a determination is small. The samples of standard and unknown material, in the form of pellets, are weighed and dro ped through the open side arms of the apparatus, so that one i u l b receives the standard and the other the unknown. The filling tubes are then constricted near their bases to facilitate subsequent sealing. As soon as the glass cools, 2 cc. of solvent are added to each bulb, after which one tube is sealed at its constriction. As this seal is made, a very gentle stream of dry air should be blown through the tube to prevent vapors of the solvent from coming in contact with the hot glass. The system is then evacuated from a line in which is interposed 1 meter of 1-mm. capillary tubing, and in this manner approximately 0.3 cc. of solvent is distilled from each bulb. While the distillation continues, the constricted part of the connecting tube is sealed with a soft gas-oxvgen flame. The closed evacuated system then contains two solutions containing definite quantities of standard and unknown material arranged as outlined above. Therefore, if the entire apparatus is isothermally insulated, solvent will distill from the solution of greater vapor pressure to the one of less, until equilibrium is established. When this occurs the volumes of the two solutions will be con-
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D A Y S
FIGURE 2. AZOBENZENE-o-CHLOROBENZOIC ACID Solvent, ether. solutions 0.1416 molar a t equilibrium. Molecular weight of o-chlorobenzoic acid, 156.5; found, 158
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TABLE I. MOLECULAR WEIGHTS DETERMINED BY METHOD DESCRIBED Substance a-Nitronaphthalene, CloHiNOs Rotenone, CznHziOa 2-(3-Chlorobensoyl)-benzoic acid, C14H90sCl o-Chlorobensoic acid, ClHsOzCl Pyrotenulin, CliHzaO4 Isotenulin, CliHnO6 Ferulic acid CIOHIOOI p,p'-Dibrododiphenyl, CnHaBrz
420-
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821
ANALYTICAL EDITION
November 15, 1941
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400-
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Molecular Weight Found Calculated 173.7 173.1 393.3 394.3 260.7 156.5 288.3
255 157.4 288.4
195.5 313
312
304
306.4 198.1
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FIGURE 3. AZOBENZENE-PYROTENULIN Solvent, chloroform: solutions 0.1367 molar a t equilibrium. weight of pyrotenulin, 288.3: found, 288.4
Molecular
The essential experimental factor in this determination is the maintenance of the apparatus in an isothermal condition. A very simple way to do this is to conduct the experiment a t room temperature in a heavy metal container, such as an aluminum pressure cooker, which has a high thermal conductivity. Under these conditions the time necessary for a pair of solutions to reach equilibrium is greater than a t elevated temperatures, but the simplicity of the procedure and the accuracy of the results warrant its use. The duration of the
experiment is also dependent upon the concentration of the solutions, their relative molarity when prepared, and the solvent used. The best solvents are those with high vapor pressures. Ether, acetone, ethyl bromide, and chloroform have been employed, but others would undoubtedly be as good. These factors can be judged by a study of the curves in Figures 2 and 3, where ether and chloroform are the respective solvents. Aeobenzene is an excellent standard where organic solvents areused. It is easily purified, is permanent in the air, and is readily soluble in most solvents, and the color of its solution distinguishes it from the unknown. The procedure as outlined has given uniformly good results with all classes of compounds studied and appears to be generally applicable. It is thus one of the best methods available for the determination of molecular weights. Several results obtained by it are presented in Table I.
Literature Cited (1) Signer, R., Ann., 478, 246 (1930).
Determination of Hydroxyl Content of Organic Compounds Acetyl Chloride as a Reagent BERT E. CHRISTENSEN, LLOYD PENNIMGTON, AND P. KEENE DIMICK Oregon State College, Corvallis, Ore.
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H E simplest methods for the determination of the hydroxyl radical in organic compounds are those based on esterification procedures. Acetic anhydride with and without pyridine has been widely used in this connection @ , 3,4) 6), but although acetyl chloride is a much more active acetylating agent, little attention has been given to the possibility of using it in quantitative work (6). It is difficult to measure and handle small amounts of this reagent. Smith and Bryant (5) were the first to recognize its possibilities and employ i t for analytical purposes. These workers avoided the problem of volatility and activity by employing it in the form of an acetyl pyridinium chloride suspension in toluene; in other respects their method was similar in principle to the acetic anhydride-pyridine procedures. I n this indirect use of acetyl chloride i t is probable that ketene (8) was the acetylating agent. Much would be gained if pure acetyl chloride rather than a suspension of acetyl pyridinium chloride in toluene were
used as the esterifying agent. This would eliminate the annoying pyridine vapors, the diluent and solubility effects of the toluene, and the use of a solid acetylating agent. I n order to use pure acetyl chloride a way must be found to measure and control it. Acetyl chloride reacts very slowly at low temperatures even with methanol; consequently solid carbon dioxide offers a convenient means of controlling its reaction rate. Linderstrom-Lang and Holter (1) have d e scribed an ingenious pipet for the precise measurement of small volumes. Using a modified form of this instrument, a simple technique has been developed for using pure acetyl chloride in determining the hydroxyl content of organic compounds.
Experimental
REAGENTS. Acetyl chloride, Eastman (practical), sodium hydroxide, 0.3 N (carbonate-free),phenolphthalein indicator, and dry ice.
