SPECTROPHOTOGRAPHY OF CHEMICAL REACTIONS. 



63 



There are solutions of the different colored salts of a certain metal, for 

 which the wave-lengths of the absorption bands are different. It will be 

 remembered in the case of neodymium that the absorption spectra of the 

 acetate, chloride, and nitrate are quite different from one another. The 

 spectra of these salts should all be the same, in dilute solution, if the absorp- 

 tion of light were due solely to the ions. 



Jones and Guy are now studying very dilute solutions of certain salts, 

 to see whether there is any detectable difference between the spectrograms 

 in very dilute and in more concentrated solutions. 



There are not many cases known where the wave-lengths of the absorp- 

 tion bands of different salts of the same metal in the same solvent are very 

 different. Uranyl nitrate in water, however, gives bands that are of quite 

 different wave-lengths from those given by other uranyl salts in water. The 

 following will illustrate this point : 



While previous work of Jones and Strong 1 indicated that there was little 

 or no change in the positions of the nitrate and sulphate bands with change 

 in concentration, recent experiments made with the concentrations 0.5 and 

 0.003 normal indicate that in the more dilute solutions the sulphate bands 

 have been slightly shifted towards the violet, while the nitrate bands have 

 been slightly shifted towards the red. It may be possible that, with sufficient 

 dilution, the sulphate and nitrate bands would occupy the same position, as 

 would be the case if the ions were the only absorbers in the solutions; yet the 

 shifts are so small with the dilution that this seems improbable. 



Indeed, it has recently been shown by Jones and Guy, working in very 

 dilute solutions and with a depth of layer of 250 cm., that the uranyl nitrate 

 bands are only very slightly shifted towards the red, and the uranyl sulphate 

 bands are only very slightly shifted towards the violet. The evidence is there- 

 fore convincing that the presence of the N0 3 and the S0 4 groups influ- 

 ences the periods of the absorbing centers in the uranyl atoms or groups. 

 Another good example that can be tested in a similar way is that of uranyl 

 chloride and uranyl nitrate in acetone. These uranyl bands are quite strong, 

 and the difference in the wave-lengths of corresponding bands is at least from 

 50 to 60 Angstrom units. 



Uranyl nitrate in acetone . 

 Uranyl chloride in acetone . 



4840 

 [4960 

 \4935 



double 



4665 



<*} double 



4515 

 4610 



Such facts as the above can be readily explained in terms of the theory 

 of aggregates. A uranyl nitrate aggregate loses an NO a group, and becomes 

 an ionized aggregate or complex ion. The loss of the N0 3 group causes a 

 decrease in the hypsochromous effect, but this would be smaller if the aggre- 

 gate were large. There would be a similar result in the case of the uranyl 



'Proc. Amer. Phil. Soc, 48, 192 (1909). 



