Mr Purvis, The influence of dilution on the colour, etc. 247 



Tfie influence of dilution on the colour and the absorption 

 spectra of various permanganates. By J. E. Purvis, M.A., 

 St John's College. 



[Read 17 May 1909.] 

 [Plate XI.] 



It has always been considered that permanganate solutions 

 show similar absorption bands whatever be the nature of the 

 element or group of elements acting as the positive ion. Ostwald's 

 experiments * comparing the absorption spectra of the solutions 

 of a number of permanganates, whereby there appeared to be no 

 change either in the position or the width or the intensity of the 

 bands, appeared to prove conclusively that such absorptions were 

 independent of the nature of the positive ion. The only criticism 

 which could be brought against the experiments was, and is, that 

 sufficient dispersion of light had not been used. It is probable 

 that any changes would be exceedingly small, and in order to 

 distinguish small differences, if there are any, it would seem 

 to be desirable to use a greater dispersion than was used by 

 Ostwald. 



It is also well known that dilute solutions of permanganates 

 gradually change their colours when they are kept for some time, 

 and it is usual to explain such changes by the action of light. 

 The change is only noticed in very dilute solutions, and, ordinarily, 

 a brown colour is produced. No observations are recorded with 

 regard to changes in the absorption bands during the colour 

 changes ; nor are there any observations to indicate that the 

 changes take place in the dark, or out of contact with the air. 

 The following description gives some account of experiments 

 conducted in these directions. 



Experimental part. 



The separate bands of solutions of permanganates can only be 

 seen when the solutions are very dilute : and, after some preliminary 

 trials, the following standard solutions of the permanganates of 

 barium, zinc and potassium were made : 



0"37.5 gram barium permanganate dissolved in 1000 c.c. distilled 

 water, 



0'262 gram zinc permanganate dissolved in 1000 c.c. distilled 

 water, 



0"316 gram potassium permanganate dissolved in 1000 c.c. 

 distilled water. 



Three separate solutions of each salt were investigated, and 



* Zeits.physik. Ghem. Vol. ix. (1892), p. 579. 



VOL. XV. PT. III. 17 



