OTHER PROPERTIES OF ELECTROLYTIC SOLUTIONS 293 



changes, but these changes are to be ascribed, primarily, to a displace- 

 ment of the hydration equilibrium existing in these solutions. As the 

 solutions become more dilute, this effect disappears. The absence of any 

 difference in the optical effects of the ions and of the un-ionized molecules 

 has led some writers to infer that un-ionized molecules are entirely want- 

 ing in electrolytes. This inference, however, does not appear to be well 

 founded. 



In general, if no reaction takes place which tends to alter the nature 

 of the chromophore group, the absorption of an ion is independent of the 

 nature of the solution, as well as that of other ions with which it may be 

 combined. This is well illustrated in the case of the absorption of acetic 

 acid in the ultra-violet region. 



In Figure 57 is shown the absorption curve for acetic acid 18 in water 

 and in petroleum ether and for the potassium and barium salts of this 

 acid in water. From an inspection of the figure it is evident that the 

 absorption of these solutions is the same within the limits of experimental 

 error. In Figure 58 is shown the absorption curve for ammonium, potas- 

 sium, barium and calcium salts of trichloroacetic acid in water. 19 Here, 

 again, it is evident that the absorption curves are identical within the 

 limits of the experimental error. What holds true in the cases which 

 have just been cited holds true also in solutions of other electrolytes. In 

 general, whenever a variation arises in the absorption curve, as a result 

 of a change in the solvent or a change in the accompanying ion, this 

 effect may be ascribed to some reaction taking place in these solutions 

 which alters the nature of the chromophore group. 



In the following table are given the extinction coefficients for chromic 

 acid and potassium bichromate in water according to the measurements 

 of Hantzsch. 20 



TABLE CXIX. 



EXTINCTION COEFFICIENTS OF CHROMIC ACID AND POTASSIUM 

 BICHROMATE IN WATER. 



H 2 Cr 2 7 K 2 Cr 2 7 



Wave Lengths 405 436 486 543 405 436 486 546 



10 1.9 1.73 



100 89 1.8 .. 291 88.7 1.67 



500 333 275 .. .. .. 292 86.8 



1000 320 269 88.5 . . 332 287 87.2 



"Hantzsch, Ztschr. f. phyg. Chem. 86. 629 (1914). 



19 Idem, loc. cit. 



*>Idem, Ztschr. f. phys. Chem. 63, 370 (1908). 



