4 ABSORPTION SPECTRA OF SOLUTIONS. 



ing way: From tables giving the values of x for different values of c the 

 products ex and c(lx) are calculated for all values of c. Two curves 

 are then plotted, one between ex and c, the other between c(l x*) and c. 

 Letting c ly x v and d^ represent the values of c, x, and d for the greatest 

 concentration to be used, the values for any other concentration being 

 represented by the same letters without subscripts, we have, respectively, 



c l (\x^)d l = c(l x)d 

 or, 



The terms on the right are both known, and hence the products ex and 

 c(l x) for any chosen value of d are known; and from the two curves the 

 corresponding values of c may be read off directly. 



Since more is known about dissociation than about association or 

 solvation, it is only natural to try to see whether the observed changes 

 in absorption can be explained by dissociation alone. If dissociation does 

 not suffice, then we must conclude that other factors come in. The pres- 

 ent work is devoted largely to a study of the absorption spectra of a large 

 number of salts from the standpoint of dissociation. 



Let us consider for a moment the kind of evidence obtained by the 

 methods outlined, and what conclusions may be drawn from them. Con- 

 sider first the possibility that an absorption band does not change in 

 width or position with concentration, when the product of depth of layer 

 and concentration is kept constant. The simplest explanation is that 

 the absorption of the molecule and of the ions into which it breaks down 

 on dissociation is the same. An excellent example of this type is fur- 

 nished by the ultra-violet band of nickel sulphate (see Plate 28); also by 

 the more dilute solutions of neodymium and praseodymium chloride. 



Let us now consider cases \vhere we have deviations from Beer's law. 

 Take first the possibility that an absorption band widens with dilution, 

 when the product of concentration and depth of layer is kept constant. 

 This would indicate either that the band is due to ions, or that the ions 

 have stronger absorption in the region considered than the undissociated 

 molecu'es. By making a series of exposures, keeping the number of ions 

 in the path of the beam of light constant, we can decide between the two 

 possible explanations. If the band now remains of constant width and 

 position, it is most likely due to ions alone; if it narrows on dilution, the 

 ions have stronger absorbing power than the undissociated molecules; 

 while if the band should widen with decrease in concentration, dissocia- 

 tion would in no way suffice to explain it. 



The other case is where the band narrows with dilution, when the 

 product of concentration and depth of layer is kept constant. If disso- 

 ciation can account for this we must either have the undissociated mole- 

 cules absorbing more strongly than the ions formed from them, or else 

 the ions not absorbing at all. In the former case the band should widen 

 with dilution when "undissociated molecules" in the path of the light are 



