24 



A STUDY OF THE ABSORPTION SPECTRA. 



up, the violet edge of the transmission band gradually pushes out into the 

 ultra-violet, and for dilutions greater than 0.0005 normal there is complete 

 transmission throughout the spectrum. Beer's law was found to hold for 

 potassium chromate throughout the above ranges of concentration, except 

 in the more concentrated solutions between 2 normal and 0.25 normal. 



Potassium dichromate in water was found to have a much greater 

 absorbing power than the solutions previously described. A one-third 

 normal solution absorbed all wave-lengths shorter than X 5350. As the 

 concentration is decreased the transmission extends farther and farther 

 out into the violet. For a 0.0026 normal concentration a transmission 

 band appears in the violet, thus giving an absorption band whose center 

 is about X 3800. As the concentration is further decreased transmission 

 becomes greater and greater in the violet and ultra-violet, and is practically 

 complete for a 0.0006 normal concentration. Beer's law has been tested 

 between the above ranges of concentration and has been found to hold. 



In photometric measurements to test Beer's law, the equation defin- 

 ing the quantities to be measured is 



where J is the intensity of the light that enters the solution (neglecting 

 any loss due to reflection), J the intensity of the light as it leaves the solu- 

 tion, c the concentration in gram molecules of the salt per liter of solution, 

 I the thickness of layer and A a constant if Beer's law holds. Strictly 

 speaking, the above equation holds for monochromatic light. For ordi- 

 nary white light we would have to integrate this equation over the range 

 of wave-lengths used. The equation would then have the form 



J= Jo I eM'd* 



J Al 



The quantity /? is called the index of absorption and A the molecular extinc- 

 tion coefficient. If the absorption is proportionately greater in the more 



concentrated solutions, then Beer's law fails 

 and A decreases inversely as the concen- 

 tration. 



From photometric measurements Sette- 

 gast 1 and Sabatier 2 conclude that the absorp- 

 tion spectrum of potassium dichromate is 

 the same as that of chromic acid, and that 

 the absorption spectrum of potassium chro- 

 mate is entirely different. This is corrobo- 

 rated by the present work. Settegast finds 

 that Beer's law does not hold for potassium chromate and potassium 

 dichromate, the coefficient A decreasing with increasing concentration. 

 Griinbaum 3 finds the accompanying values of A and s where e = c/A. 



1 Wied. Ann., 7, 242-271 (1879). 



2 Compt. rend., 103, 49-52 (1886). 



8 Ann. Phys., 12, 1004, 1011 (1903). 



