SIMPLE PHASE EQUILIBRIUM EXPERIMENT H. S. VAN KLOOSTER Rensselaer Polytechnic Institute, Troy. New York
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N DEALING with phase equilibria (either liquid- ing the apparatus upside down and shaking the contents liquid, liquid-solid, or solid-solid) in condensed gently for half a minute (care being used not to warm systems of two components, which are present in a the liquids) will displace the boundary several cm. upgiven proportion by weight, i t is necessary to know, a t ward, until after three or four consecutive manipulathe temperature of the experiment: (1) the composi- tions a stationary position of the boundary is attained tion of each of the two phases and (2) the relative quan- (mark c), indicating an increase of approximately 17 tities of these phases. These compositions have to cc. in the volume of the bottom layer. The total volbe established by analysis, after which it is a simple ume has decreased by about 3 cc. (mark d ) so that the matter to indicate the relative weights of the two top layer occupies a volume of 110 cc., a decrease of 20 cc. when compared with the original volume of 130 phases. Since the equilibrium at a stated temperature is cc. for the anhydrous ether. By withdrawing with a most readily established in the case of two liquid phases, pipet about 20 cc. of the top layer, its specific gravity can be readily ascertained with a Westphal balance as the writer has used, for a being 0.72. Withdrawal of a sample of the bottom number of years, the follayer with the aid of a pipet having a long, narrow stem lowing arrangement for enables us to determine the specific gravity of the showing the phase equiaqueous layer as being 0.99. The relative quantities librium-at room temperof the two equilibrium phases are, therefore, in the ratio a t u r e i n t h e binary of SW1:SE' = (110 X 0.72):(257 X 0.99) = 23.7:76.3. system water-ether (FigTo establish the location of W' (water dissolves 7yo ure 1). A flat-bottomed ether a t room temperature) and E' (ether dissolves 1% flask of about 240-cc. water a t room temperature), quantitative analysis of capacity is sealed, by the two layers would be necessary. means of a graduated Quelitetiwely one can readily show the presence of t u b e (part of a n old ether in the bottom layer by boiling some of the aqueous buret) measuring about layer in a small 125-cc. Erlenmeyer flask provided with 20 cc., to a round bulb a wide glass tube about 50 cm. long. The first vapor of 100-cc. capacity fitted escaping from the tube is pure ether and can be ignited. with a graduated neck This affords an illustration of the principle of separating closed by a tight-fitting the constituents of a liquid phase by fractional distillastopper. tion. The presence of water in the top layer is demonA measured volume of strated by adding, first, 10 cc. of carbon disulfide (or water, say 240 cc., which carbon tetrachloride) to an equal volume of anhydrous fills the flask completely ether, when a clear solution results. Addition of carbon to the mark a; is intro- w 5 E disulfide (or carbon tetrachloride) to 10 cc. of the duced. Next a volume ethereal top layer will produce a cloudy liquid due to of 130 cc. of anhydrous ether colored yellow by an organic dye insoluble in the separation of another liquid phase. The apparatus* sketched herewith can also be water (butter yellow, for instance) is carefully added so that it fills the stem to the mark b about 5 cm. above satisfactorily used to demonstrate the relative decrease the top of the bulb. in solubility of one solvent in another by the addition If W (Figure 1) represents 10070 water and E 100% of a solid, easily soluble in one of the two solvents. ether, the composition of the system is indicated by Nernst,' in 1890, found that the relative lowering of the the point S, the ratio of the relative quantities of ether solubility of a solvent (e. g., ether) in a second solvent and water being SW:SE = (130 X 0.716) (240 X 1) = (e. g., water) due to the addition of a third substance 27.9:78.1. The specific gravity of water is assumed to (e, g., naphthalene) is equal to the number of dissolved molecules of the added solute divided by the number of be 1 while that of ether is 0.716. Now these two phases, although at the same tem- *Made lor the author by H. A. Wayringer, 10 Frank St.. perature, are not in equilibrium. Shaking, however, ASchenectady, N. Y, will readily bring about a state of equilibrium. Turq1 NBRNST, Z. physik. Chcm., 6, 16 (1890). 438
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molecules of the solvent (ether). This principle has equal weights of naphthalene (say 0.25.g.) into the flask.4 been worked out by Kiister2 and by Tolloczko3 in a Each time, after vigorous shaking, the boundary is procedure for the determination of molecular weights. displaced downward for about the same number of The last-named author 'used apparatus similar to that scale divisions, showing that the addition of the solid sketched in the accompanying diagram. In carrying causes a decrease in the solubility of ether in water out this second ex~eriment.one introduces, successivelv, r o.~ o r t i o nto a l the weight of the dissolved substance. - - ~KWSTER, Bn., 27, 324 (1894). 'VAN KLOOSTER, "Lecture experiments in physical chemisTOLLOCZKO, 2.physik. Chcn., 20, 389 (1896). try," 2nd ed., Chemical Publishing Co., Easton, Pa.. 1925,p. 145. A
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