96 



THE ABSORPTION SPECTRA OF SOLUTIONS. 



The wave-lengths of the uranyl nitrate bands are considerably smaller than 

 those of uranyl chloride. This has been shown to be due to the presence of 

 NO., and H 2 groups in the absorbing centers of the nitrate in solution. By 

 adding hydrochloric acid to an aqueous solution of uranyl nitrate, the uranyl 

 nitrate bands shift gradually to the positions of the uranyl chloride bands. 

 The addition of more hydrochloric acid causes most of the uranyl bands to 

 continue to shift towards the red. On the other hand, the addition of nitric 

 acid to an aqueous solution of uranyl nitrate causes the uranyl bands to shift 

 towards the violet. We thus obtain a gradual shifting of the uranyl bands of 

 a nitric acid solution of uranyl nitrate into the position of the uranyl bands 

 of uranyl chloride dissolved in concentrated hydrochloric acid. The magni- 

 tude of this shift is shown by the following wave-lengths : 



These changes are quite different as compared with those that take place 

 when the solvent is changed, and if it is assumed that every characteristic 

 absorption spectrum corresponds to a more or less stable system, then the 

 above changes indicate the existence of a series of systems or compounds. 

 These compounds will be called aggregates. There will be, accordingly, 

 nitric acid aggregates of uranyl nitrate, uranyl nitrate and chloride aggregates, 

 and hydrochloric acid aggregates of uranyl chloride. 



(2) Whether there is a gradual change in the frequency of vibration of 

 the absorbers for a series of uranyl or uranous aggregates, or whether there 

 is simply a relative change in the intensity of a number of finer bands which 

 blend so as to form the uranyl or uranous bands, can not be decided from the 

 data now in hand. In the case of the neodymium salts, the latter effect seems 

 to manifest itself in the spectrograms that have been made. 



(3) It is probable that the. presence of free acid in solvents other than 

 water will lead to the discovery of many new bands of neodymium, erbium, 

 etc. A striking example of this kind is the effect of dissolving hydrochloric 

 acid gas in a neodymium chloride solution in ethyl alcohol. The effect of the 

 free acid is to bring out very strongly bands at X 3695 and X 3760. The pres- 

 ence of sodium perchlorate has a similar effect. 



(4) Mixtures of varying amounts of two salts in the same solvent may 

 result in a gradual shift from the bands of one salt into the bands of the other, 

 the shift depending on the amount of each salt present. As an example of this, 

 uranyl sulphate and uranyl nitrate were studied. On the other hand, there 

 are examples where the two salts do not seem to form any intermediate aggre- 

 gates, and the bands of each simply vary in intensity without showing any 

 change in frequency. 



(5) Acid aggregates of uranous salts are found to be much more stable 

 than the neutral aggregates. Uranous nitrate, for instance, is very unstable; 

 but uranous chloride dissolved in concentrated nitric acid (the absorption 

 spectrum is entirely different from that of uranous chloride) will stand for hours 



