Evaporation in Burettes 159 



EVAPORATION OF SOLUTIONS AND LIQUIDS IN 

 BURETTES. 



M. G. Mellon, Purdue University. 



In very accurate titrametric analyses one may weigh the amount 

 of standard solution reacting with the constituent being determined and 

 from the weight of solution used calculate the percentage of the con- 

 stituent, providing the concentration of the standard solution has been 

 determined upon a weight basis. This procedure avoids the possible 

 errors arising from a change of temperature and from evaporation, if 

 the standard solution is measured rather than weighed. The volumetric 

 method of determining the amount of solution is, however, the one gen- 

 erally employed. 



In the event of a distinct change occurring in the temperature of 

 the standard solution during a determination, which means a corre- 

 sponding change in the volume of the solution, one needs only to note 

 the variation of temperature and then to consult tables for the proper 

 correction in volume. These tables have been prepared for various solu- 

 tions of various concentrations. 



The error arising from evaporation of the solvent of a standard 

 solution and the consequent increase in concentration of solute per unit 

 volume is not so readily found in the usual works of reference on quanti- 

 tative chemical analysis. The experiments reported at this time were 

 made with the object of obtaining information regarding the rate of 

 evaporation of solutions and liquids in burettes. 



Factors Affecting the Rate of Evaporation. No previous work has 

 been found relating directly to the effect upon the accuracy of analytical 

 data resulting from the loss, through evaporation, of the solvent of stand- 

 ard solutions standing in burettes. Many papers have appeared, however, 

 dealing with the general subject of evaporation and with the laws relat- 

 ing to the process,' but we are here concerned with a consideration only 

 of the factors affecting the systems now being studied. From these 

 papers just mentioned and from others dealing with certain aspects of 

 the physico-chemical theory of solutions and liquids, a number of facts 

 may be summarized as having a direct bearing- upon the present work. 



All liquids tend to assume the gaseous phase, and the measure of 

 this tendency is known as the vapor tension of the liquid. For a given 

 liquid there corresponds to each temperature a certain definite pressure 

 of its vapor. This vapor pressure is defined as that pressure at which 

 the rate of escape into the gaseous phase of the liquid molecules is ex- 



^ Livingston — Monthly Weather Report — U.S.A. — 1909. "An Annotated Bibliography 

 on Evaporation." 



Vaillant— Compt. rend. UC 582, 811 : lJ,.s\ 1099 ; lod. 213. 



Jablczynski and Prezmyski — J. chim. phy.s. 10, 241 . 



Marcelin — Ibid. — 10, 680. 



Marcelin — Compt. rend. 1-JS, 1674. 



Thomas and Ferguson — Phil. Mag. 3h 308 (1917). 



Burger — Proc. Acad. Sci. Amsterdam— Si, 271 (1919). 



Weiser and Porter— Jr. Phys. Chem.—3J,, 33.3 (1920). 



"Proc. 38th Meeting, 1922 (1923)." 



