Rkilly and Hickinbottom — Mctliuil of Distillation in Steam. 135 



the solvent Jind solute, using water as solvent. When such disturhing influences 

 have been elimmated, it might be expected that the distillation constants and 

 the vapour-pressures should be in proportion. The distillation constants of the 

 fatty acids and the alcohols should reach a maximum, and then decrease with 

 increasing molecular weight. Such has been found to occur in the fatty acid 

 series. Distillations of aqueous suspensions of lauric and n)yristic acids were 

 carried out. These acids are practically insoluble in water, consequently the 

 effect of the solvent is eliminated. It was found that the distillate from the 

 lauric acid suspension was richer in acid than that from the myristic acid- 

 water mixture. 



The results obtained by distilling dilute solutions of the volatile fatty 

 acids show that with the method it is possible to detect alterations in the 

 state of molecular aggregation. The success of the method depends on : — 



{a) Constant temperature of distillation, and freedom from variations 

 caused by irregular heating or condensation. 



(S) Convenient and accurate method of estimating the solute. ' 



(e) Choice of a suitable solvent. 



To maintain a constant temperature, the solution must necessarily- be of 

 such a dilution that changes in concentration do not affect the boiling-point 

 appreciably. This can be achieved by allowing only a small change in 

 concentration, or by employing very dilute solution, such that the boiling-' 

 point approximates to that of the solvent. It is, however, possible to correct 

 for change of temperature, and an apparatus has been devised so that 

 distillation can be carried out over a considerable range of concentration and 

 temperature of ebullition. It is intended to describe this at some future 

 date. 



The solute need not necessarily be a volatile fatty acid, and the choice of 

 a solute is restricted only by the solubility in the solvent and its volatility. 

 It is, however, essential that an accurate method should be available for the 

 estimation of the solute. By using the same solute in a series of different 

 solvents, and employing a considerable range of concentration, evidence may 

 be obtained concerning the actual state of substances in solution. This 

 applies more particularly to concentrated solutions, which cannot be con- 

 veniently investigated by methods involving the principles of osmotic 

 pressure. 



The method of distillation of dilute aqueous solutions has been applied 

 chiefly for the detection and estimation of mixtures of fatty acids. It is 

 from this point of view that most of the investigations have been carried 

 out, and the conditions determined for obtaining accurate results.^ It follows 



. 1 Upson, Plum, and Schott. J. Amer, Chem. Soc, 1917, 39, 731 ; Lamb, ihid., 1917, 

 39, 746. Gillespie and Walters, ibid., 1917, 39, 2027 ; Richmond, Analyst, 1919, 44, 255. 



