PAPERS OX CHEMISTRY AND PHYSICS 251 



MELTING POIXT, LATENT HEAT OF FUSION 

 AND SOLUBILITY OF OEGANIC COMPOUNDS 



F. S. MoETiMEK, luLixois "Wesleyax L'xiversity 



IXTEODTCTIOX 



Cominercial laboratories as well as educational labor- 

 atories wbicli are working with organic compounds are 

 constantly confronted with questions having to do with 

 solubility and choice of solvent for use in purifications. 

 In the great maiority of cases the desired information 

 is not available from the published data. In such cases 

 it is necessary either to determine the solubility experi- 

 mentally or to resort to some method of calculation. The 

 more successful of the various methods used for calcu- 

 lating solubility generally employ an ec[uation involv- 

 ing Eaoult's freezing point law together with the sec- 

 ond law of thermodynamics. Perhaps the simplest and 

 most useful of these expressions is, — 



— L 

 log x= I (1) 



4.5ST 



In this expression N represents the mole fraction of the 

 solute. (By solute is meant, that component which first 

 crystallizes out in the pure state upon cooling the sys- 

 tem). L is the molecular latent heat of fusion, T is the 

 absolute temperature of the melting point of the system 

 and I is an integration constant. 



In general it may be said that these equations have 

 been successful only for the so called ''ideal" mixtures. 

 By ideal mixture is meant those binary systems, the com- 

 ponents of which may be considered to have the same 

 thermodynamic environment when both are in the liquid 

 state and both are at the same temperature. Two of the 

 criteria for such a system are that there shall be neither 

 any heat effect nor any volume change when the two 

 liciuid components are mixed. The complete absence of 

 any secondary molecular effects, such as association and 

 compound formation, is implied in the definition. There- 

 fore, if in any case the heat effect for the solution pro- 

 cess of dissolving a solid in a liquid differs from the 

 latent heat of fusion of the solute at the temperature 



