Reilly and HiciaNBOTTOM — Volatile Fatty Acids. 515 



his source of heat and his supply of steam so that the level of the liquid in 

 the flask did not alter appreciably. 



In analyses which depend on the formation of esters followed by fractional 

 distillation, accurate results are difficult to obtain, since esterification is seldom 

 complete and the distillation does not give a sharp fractionation. It is 

 applicable only when relatively large amounts of solution are available and 

 when the particular acids present in the mixture differ widely in molecular 

 weight. 



The relation between the formulae used by various workers will be shown 

 later, after the theory of distillation of a dilute solution has been considered. 

 To obtain a regular rate of distillation the need for uniformity in the, 

 supply of heat to the distilling liquid has always been considered important; 

 In addition, cooling effects which may cause irregular condensation in the 

 stillhead have been recognized by some investigators. The influence of con- 

 centration and the effect of varying the ratio between the distillate and the 

 volume in the flask are factors which do not appear to have received sufficient 

 consideration. During the distillation of an aqueous solution it is obvious 

 that, unless the solute has the same rate of volatilisation as the solvent, the 

 concentration of the solution will alter. This effect of change of concentration 

 has been referred to by Eichmond, who finds that with formic, and to a less 

 extent with acetic acid, the rate of distillation is influenced by the concentra- 

 tion of the solution. For higher acids this effect is not appreciable. These 

 observations are important in the determination of the constitution of 

 unknown mixtures. In the case of an ordinary distillation in which the 

 substance in solution distils at a slower rate than the solvent, the solution 

 will become more concentrated, with the consequent alteration of the 

 constants. In an unknown mixture a correction cannot be applied with 

 certainty in such cases. It is therefore important to prevent, as far as possible, 

 any such change. Variations in concentration, with consequent alteration of 

 constants, will be reduced to smaller limits if the distillation is carried out at 

 a constant volume. In addition, the limits of variation can be reduced to 

 a still narrower range if relatively larger volumes of solution are distilled 

 compared with volumes of distillate collected. 



Stein, in one experiment, found that the distillation constant of acetic acid 

 was increased by the addition of sulphuric acid. He did not investigate this 

 point further or offer any explanation to account for the observed results. 

 It is possible that when a relatively large proportion of sulphuric, acid is 

 added, a hydrate or quasi-compound may be' formed. In this case the 

 " effective concentration " of the solution would be increased. HoAvever^.as 

 small quantities of sulphuric acid also have an appreciable effect, this 



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