290 Messrs Laby and Garse, On a relation, etc. 



mately the relation between the linear dimensions of an ion and 

 its velocity. The relative volumes of certain organic anions and 

 cathions may be deduced from molecular and atomic volumes. The 

 cube roots of the ionic volumes give a series of numbers propor- 

 tional to the ionic linear dimensions, and these combined with 

 the velocities of the ions enable the relation sought for to be 

 investigated. 



Calculation of Ionic Volumes*. 



The calculation of the ionic volumes may be best explained 

 by two typical examples. 



(1) Volume of anion of acetic acid. 



The molecular volume of acetic acid CH 3 . COOH at 20° C. 



= molecular weight (ill) x volume of 1 gram acetic acid at 20° C. (- J 



molecular weight _ M 60 ^ _ 

 = density at 20° C. ~ J = r05 = 



The anion of acetic acid being CH 3 . COO', its volume is found 

 by deducting from the molecular volume of CH 3 . COOH the 

 atomic volume of hydrogen in carbon compounds at 20° C. Kopp's 

 value of the atomic volume of hydrogen given by Ostwaldf, when 

 corrected for temperature, is 5*3 and this is the value used. Thus 

 the volume of the acetic anion, CH 3 . COO', is 57*2 — 5*3 = 51'9. 



(2) Volume of cathion of diethylammonium hydroxide. 



The molecular volume of diethylamine (CH g ) 2 HN at 20° C. is 



p 70o 

 atomic volume of hydrogen = h= 5 - 3 



.-. ionic volume of (C 2 H S ) 2 H 2 N- = 108"8. 



It may be pointed out that an error of 10 p.c. in the atomic 

 volume of hydrogen produces at most an error of "5 p. c. in the 

 cube root of the ionic volume of any substance quoted. 



A more strictly applicable value for the ionic volume of acet- 

 anion would be obtained if it could be deduced from the density 

 and concentration of the sodium acetate used in determining the 

 ionic velocity, but as zero or negative values are obtained for the 

 molecular volumes of some substances, this method is not trust- 

 worthy. 



* The meaning of the term "ionic volume " in this paper does not include the 

 watery atmosphere, which the ions are usually regarded as carrying, 

 t Lehrhuch d. Allg. Chem. Bd. i. 



