2 ABSORPTION SPECTRA OF SOLUTIONS. 



in other words, the absorption spectrum of a mixture of simple solutions 

 will be the sum of the absorption spectra of the constituents. 



If the concentration is very great, the absorbers may be so close 

 together that they cease to act independently of each other; hence, even 

 if the solution is a simple absorbing one, Beer's law may cease to hold if 

 the concentration is very great. In general, however, we may say that 

 for simple absorbing solutions or mixtures of these, Beer's law will hold. 



Actual solutions always differ more or less from the ideal simple solu- 

 tions on account of the changes produced by dilution. These changes 

 are due to association, dissociation, and solvation. 



The molecules of the dissolved substance may combine with each 

 other, forming more or less complex aggregates, each of which will, in 

 general, have its own peculiar power of absorbing light. The composi- 

 tion of the aggregates will depend upon temperature and concentration, 

 and hence, if a solution containing such aggregates is diluted, we should 

 expect to find deviations from Beer's law even if the temperature is kept 

 constant. 



The molecules of a great number of substances when dissolved disso- 

 ciate into two or more ions, the amount of dissociation depending upon 

 the concentration. It is to be expected that the absorption of the ions 

 into which a molecule dissociates will be different from that of the mole- 

 cule itself, and, consequently, on diluting an electrolyte we should expect 

 to find deviations from Beer's law, unless the solution is so dilute that it 

 may be considered as completely dissociated. 



We may also have various combinations of molecules, aggregates of 

 molecules or ions, not only with each other, but also with the molecules 

 of the solvent, the nature of which will depend both on temperature and 

 concentration, and each of which may have a different power of absorbing 

 light. If a simple, colored electrolyte like cobalt chloride, for example, 

 is dissolved in water, we see at once what a complicated system the solu- 

 tion is; and it is not surprising that, in spite of the great amount of work 

 which has already been done on this one salt alone, we are still far from 

 able to give a satisfactory account of its absorption spectrum. 



A satisfactory account of the absorption of any salt in solution 

 requires a knowledge of the kinds of absorbers the salt forms, and the 

 amount of each for any given set of conditions. Given this knowledge, 

 it would be necessary to determine what would be the absorption spec- 

 trum if the solution contained only one kind of absorber. This could 

 be done as follows: Suppose a salt in solution gives rise to the absorbers 

 A, B, C, D, E, etc., the amount of each of which is supposed to be known 

 under all conditions of temperature, pressure, concentration, etc. Vary 

 the conditions in such a way that all of the absorbers except, say, A are 

 kept constant, and note the change in the spectrum; this change is due 

 to A alone, since by hypothesis no other is varied. Repeat, only keep all 

 except B constant, and so on. By this process of elimination we should 

 eventually arrive at a complete knowledge of the absorption due to each 

 absorber, and could hence predict beforehand exactly what would be the 

 absorption of any solution whatever of the salt in question. Unfortu- 



