CHAPTER III. 



POTASSIUM SALTS. 



Potassium chromate. Potassium dichromate. Potassium ferro- 

 cyanide. Potassium ferricyanide. 



Most potassium salts in solution are colorless, and for this reason it is 

 considered that the potassium atoms do not themselves absorb any light 

 in the visible portion of the spectrum. Several colored potassium salts 

 are known and the colors of these are due in some way to the other atoms 

 in the salt molecules. In the present work the absorption spectra of potas- 

 sium ferricyanide, potassium ferrocyanide, potassium chromate, and potas- 

 sium dichromate have been studied. 



Using a 3-mm. length of solution of potassium ferricyanide in water 

 we find that for a normal concentration there is complete absorption of all 

 the shorter wave-lengths of light beyond X 4800. As the concentration is 

 decreased the edge of transmission moves continually towards the violet. 

 It should be noticed that the region between complete absorption and 

 complete transmission for the more concentrated solutions is quite narrow, 

 being less than 40 Angstrom units; solutions of this salt being thus quite 

 good screens for absorbing light. Continually decreasing the concentration 

 we reach a 0.0156 normal solution, when a transmission band begins to 

 appear. For a certain range of concentration there appears an absorption 

 band in the region X 4200. Further decrease in concentration results in 

 increasing transmission throughout the violet and ultra-violet. For dilu- 

 tions greater than 0.00195 normal there is almost complete transmission 

 throughout the ultra-violet. Very faint bands appear in the regions XX 2500 

 to 2600, XX 2950 to 3050 and XX 3200 to 3250. 



Several spectrograms were made of solutions for which the product 

 of concentration and depth of layer were kept constant. In this case the 

 spectrograms will be identical if Beer's law holds. According to this method 

 of testing, Beer's law was found to hold within the ranges of concentration 

 over which the spectrum was mapped. 



The absorption of aqueous solutions of potassium ferrocyanide was 

 investigated in the same way. A half-normal solution 3 mm. deep shows 

 that all light of wave-lengths shorter than X 3950 is absorbed. Keeping the 

 depth of layer the same, it is found that with decrease in concentration the 

 transmission gradually moves towards the ultra-violet, and for dilutions 

 greater than 0.0078 normal there is transmission throughout the whole 

 spectrum. Beer's law was found to hold. 



A 2 normal aqueous solution of potassium chromate 3 mm. in thickness 

 shows complete transmission of wave-lengths greater than X 4950. Decreas- 

 ing the concentration causes the transmission to move gradually towards 

 the violet, and for a 0.01 normal solution a transmission band appears at 

 X 3100, or, in other words, there appears an absorption band whose center 

 is about X 3700. As the concentration decreases this absorption band fills 



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