THEORY OF LIGHT ABSORPTION 13 



where C is expressed in grams per liter and egp is called the specific 

 extinction coefficient, or: 



where C is expressed in gram molecules per liter and e^^oi is called 

 the molar extinction coefficient. The millimolar extinction coeffi- 

 cient, where C is in millimoles per liter, is written e^M ^^nd is one- 

 thousandth of this. Since the values of the millimolar extinction 

 coefficient for most substances are convenient small numbers, we 

 shall use this constant throughout this book. Where this cannot be 

 done because of uncertainty in the molecular weight of the sub- 

 stance, the specific extinction coefficient will be substituted. 



Beer's law is frequently, but not always, obeyed. When it is not, 

 the deviation may be attributed to failure of the solution to comply 

 with the requirements of independence of the solute molecules, and 

 constant infiuence of the solvent with concentration. In other words 

 we may conclude that association, dissociation, complex formation, 

 or change in solvation has occurred. Beer's law holds for very many 

 of the substances dealt with in this book. 



A variety of symbols has been used for recording spectrophoto- 

 metric data. There is no uniformity from country to country nor even 

 among authors in one particular country. The particular set of 

 symbols chosen is therefore a matter of personal preference. There 

 is no doubt that international standardization would be highly 

 desirable, but since this has not yet been accomplished we shall use 

 here the symbols given in the relationships above. These are shown, 

 together with alternative symbols, in Table I. Also shown are a 

 number of other constants sometimes found in spectrophotometric 

 work, but the list is not exhaustive. It should be noted that specific 

 extinction coefficients are sometimes given in terms of concentration 

 in grams per milliliter, and that Warburg uses the constant molar 

 absorption coefficient in the same way, i.e., with C expressed in 

 gram moles per milliliter. 



Since the laws of light absorption are strictly followed only with mono- 

 chromatic light, the various spectrophotometric constants are true constants 

 only under the same conditions. In practice measurements are made using 

 light sources giving a continuous spectrum, portion of which is selected at 

 each setting of the spectrometer. The band width transmitted is a function 

 of the width of both entrance and exit slits, and, unless these are continu- 

 ously varied, will be greater in the red than in the blue part of the spectrum, 

 since the dispersion changes with wavelength. The effect of spectral impurity 

 will be most marked on very sharp absorption bands, the extinction coeffi- 



