588 Dr. Gr. Hevesy on the Diffusion and 



Diffusion into water of PbCl 2 from a nearly saturated 

 solution — 1/31 normal. 

 (D 16 °. 2 ) is the diffusion coefficient measured at 16°*2 C. 

 (D i8 o) is the diffusion coefficient calculated at 18° C, 

 allowing 2*7 per cent, per degree for the temperature co- 

 efficient. This correction is deduced from the mobility of 

 the ions. 



Di6°-2« Dxgo. 



L 2-02x0-278 1 ' 03 X ' 0S 



1L 2W278 i' 03 1>08 



L r^kre o-99 i-07 



IL ra^raio 1>03 1-" 



IV - F8OT8 094 1-02 



Mean value D 18O =l*08 sq. cm. per day. 



0*0224 

 From the equation D= U, where U is the mobility of 



the lead ions (or more correctly the resultant of the mobi- 

 lities of the lead and chlorine ions, since the diffusion in this 

 case does not take place in excess of anion, and so the 

 chlorine ion somewhat increases the mobility of the lead ion) 

 the valency is calculated to be 1*64. 



This apparently strange result follows from the considera- 

 tions given above, namely, that the diffusion is governed by 

 the mobility and the number of charges on the ion. Ac- 

 cording to the work of v. Ende*, such a result is to be 

 expected, since in a saturated lead chloride solution there 

 are present 6'2 per cent, lead molecules undissociated, 

 47*3 per cent. PbCl' ions and 50 1 per cent. Pb M ions. 

 About one half of the lead present will therefore possess 

 one and the other half will possess two charges, and hence 

 the lead molecules will diffuse as if they had 1*5 charges. 



Calculating on the assumption that PbCl* ions have a 

 velocity similar to that of Pb M ions, one obtains from the 

 diffusion constant of lead chloride a value for the degree of 



* v. Ende, Zeit. Anorg. Chem. xxvi. p. 162 (1903) ; Harkins, J. Ainer. 

 Chem. Soc. xxxiii. p. 1859 (1911). 



