138 



Dr. T. Ewaii. On the 



yellow, and a saturated aqueous solution is bright yellow. A drop of 

 potash solution added to either the alcoholic or aqueous solutions 

 increases the depth of colour. The addition of hydrochloric acid 

 diminishes it, till, with some excess of acid, it disappears almost 

 entirely. The fact that the solutions of dinitrophenol in benzene or 

 ether are only faintly coloured, suggests that the undissociated 

 dinitrophenol molecule is devoid of any great absorbing power in the 

 visible spectrum. The action of the hydrochloric acid in decoloris- 

 ino- the solutions of dinitrophenol is also due to its converting the 

 dissociated molecules into undissociated molecules.* In order to 

 obtain the spectrum of the undissociated dinitrophenol a solution of 

 dinitrophenol to which excess of hydrochloric acid had been added 

 was used. 



The dinitrophenol had been recrystallised from water, notwith- 

 standing which the solutions obtained from it were never sufficiently 

 clear. They had, therefore, to be filtered through the porous earthen- 

 w.are and analysed. The analysis was made by titration with a 

 dilute ammonia solution, using litmus as indicator. The change of 

 colour not being very sharp, standard hydrochloric acid was added 

 till the liquid was acid, and then ammonia till alkaline, and so on 

 several times. Tests with solutions of known concentration showed 

 that accurate results could be obtained in this way. The following 

 table contains the extinction coefficients of the solution containing 

 excess of hydrochloric acid, and also for comparison, the mean of the 

 numbers obtained with the potassium and ammonium salts. 



Table IX. 



It is evident that the absorption of the acid solution is almost 

 negligible compared with that of the solutions of the salts. 



With the help of these numbers it is possible to calculate the frac- 



Arrheiiius, ' ZeitscLrift Phys. Chem.,' vol. 2, p. 284, 1888, and vol. 5, p. 1, 



1890. 



