COLORIMETRY-SPECTROPHOTOMETRY 1 2 1 



light path we find that only a fraction of the light strikes the photocell, 

 and the response is accordingly lower. If we refer to the light passing 

 through the solvent as the initial incident light, L, and the light passing 

 through the colored solution as I., we arrive at the following relationship: 



^ = T (9-2) 



where T is the fraction of light transmitted through the colored solu- 

 tion. The value T arrived at in this way (expressed as a decimal fraction 

 or as a percentage) is commonly called transmittancy. It is related to the 

 concentration of the solution according to an equation of the general 

 form T = fe/conc. A series of measurements is shown in Fig. 9-6. It 

 would be possible to use this curve in measuring concentrations of un- 

 known solutions. We construct the curve for any particular material at 

 a given wavelength and then, by measuring the transmittancy of an 

 unknown solution and referring to the curve, compute its concentration. 

 This is difficult, however, because of the very curvature of this rela- 

 tionship. A relatively simple transformation of the equation leads to a 

 much more convenient curve, shown in Fig. 9-7. This transformation is 

 shown in the following equation : 



A = ( — logio 7^ ) = logio -T-= logio jj (9-B) 



The value A has been given various names. It is directly related to the 

 amount of light absorbed and therefore is commonly called absorbancy. 

 It is also a measure of the extinction of the light and is sometimes 

 called extinction. It may also be referred to as optical density. All these 

 terms are in relatively common use but mean essentially the same thing. 

 Determining concentrations in this manner is much more convenient be- 

 cause absorbancy is directly proportional to concentration. 



A = khc (9-4) 



where fe is a constant, h is the length of the light path through the 

 solution, and c is the concentration of the solution. We make a measure- 

 ment of the light absorbed by a set of standard solutions and then deter- 

 mine the straight line shown in Fig. 9-7. 



The amount of light absorbed by a given molecule at any wavelength 

 is a function of the molecular structure. If we measure the amount of 

 light absorbed at any particular wavelength, this can be related to con- 

 centration by any of several absorption coefficients (k, above). One of 



