POTASSIUM SALTS. 



25 



It will be seen that the deviation here from Beer's law is in the opposite 

 direction from that found by Settegast. Grunbaum finds that e and there- 

 fore A depend on the depth of layer. 



An example will be given where the same concentration and different 

 depths of the solution were used: 



Our work indicates that Beer's law holds for all dilute solutions, and 

 usually the deviations for concentrated solutions are very small. Of 

 the potassium salts above described, only potassium chromate between 2 

 normal and 0.25 normal showed any considerable deviation from Beer's 

 law, and in this case the absorption of the concentrated solution was greater 

 by about 40 Angstrom units than would be expected if Beer's law held. 



The present method is a very good qualitative test of Beer's law, and 



gives the results for each wave-length, whereas most photometric methods 



only give integrated results over a more or less wide region of wave-length. 



A very good review of the work upon Beer's law has been published by 



G. Rudorf .* A more detailed account of the work upon the potassium salts 



will now be given. 



Potassium Chromate. 



Potassium chromate (K 2 O0 4 ) (Plates 1 and 2) was mapped for 

 ranges of concentration between 2 normal and 0.00049 normal concentra- 

 tion. In every case the length of exposure to the Nernst glower was {|{]} 

 seconds, the current being 0.8 ampere, and when there was transmission in 

 the ultra-violet, the length of exposure to the spark was 2 minutes. The 

 slit-width was 0.08 mm. and the depth of cell in each case was 3 mm. The 

 following table gives the limits of absorption bands. The limit is usually 

 taken as midway between complete transmission and complete absorption. 

 For the more concentrated solutions the region between complete absorption 

 and transmission was quite narrow, not being more than 20 or 30 Angstrom 

 units. For the more dilute solutions the edges of the bands were much 

 more diffuse. 



This work here also (Plates 3 and 4) indicates that Beer's law holds for 

 all dilute solutions which have thus far been tested, and usually the devia- 

 tions from Beer's law for concentrated solutions is small. The present 

 method is very crude as far as dealing with relative intensities. In some 

 cases, however, it is quite sensitive, although it never gives absolute values 

 for A. As an example, we will take that of potassium chromate between 

 concentrations 2 normal and 0.25 normal. When the depth of cell is kept 

 constant at 3 mm., the position of the edge of transmission for the 2 normal 

 solution is X 4970, for the 0.25 normal solution 1 4750. In the run for Beer's 

 law (where cl is kept constant) it is found that the absorption is about 40 



1 Jahrb. Rad. u. Elek., 3, 422 (1906); and 4, 380 (1907). 



