( 171 ) 



quantity of the latlcr is tlieu in I rod need into the measuring tut)e M 

 and when the colours are e(iual, a reading is taken. In many eases 

 it is not possible to take a reading, owing to the difference in shade 

 of colour of the two liquids, but still we are able to see at which 

 heights of the standard li(|iiid this is decidedly darker or lighter 

 than the measuring liquid. 



The measuring tube is then tilled with water np to the average 

 of those heights and detlnitive determinations are now made. The 

 difference in concentration between the two liquids is now in most 

 cases so slight, that a difference in shade is no longer perceptible. 



In any case it is desirable to dilute the contents of the measuring 

 tube np to the height found and to take a fresh reading, even when 

 determinations may be readily made witliout dilution. 



In the case of acid colours this mode of working can sometimes not 

 be ai)plied. The measuring liquid should then be gradually diluted 

 until, the colours being equal, the height of the measuring liquid is 

 about the same as that of the standard liquid. 



In the case of such small differences \n the concentration it may 

 be safely assumed that the concentrations are inversely proportional 

 to the height of equally-coloured layers. 



The great advantage of this method of working is this, that at the 

 final determination a series of readings can be taken, also that the 

 standard liquid can be alternately changed from darker to equality 

 of colour and from lighter to equality, as is done in polarisation. 

 This renders each determination ver>^ certain. 



The readings may be rendered much more delicate by placing a 

 coloured piece of glass on the ocular. It is necessary to choose such 

 a colour that the rays of light, transmitted through the measui-ing 

 liquids are also transmitted through the coloured glass. A trial with 

 a pocket spectroscope or a consultation of FokmAni-ik's work "Der 

 spectralanalyti'iclie iVachweis k'dnstlicJier orgmiischer Farhsto[f'e'\ renders 

 the choice eas}-. 



These glasses are readily made by dyeing old photographic plates 

 with basic colours, which is easily done in the cold. 



A colorhnetrlc method for deterniinbKj the dissociation constant 

 of acids. 



Acid colours whose anions possess a colour different from that of 

 the acid itself, and which we will call indicator-acids, may be used to 

 determine the dissociation constant of the indicator-acids themselves 

 in the lirst place, and also of all other coloui'less acids, if we have 



12 



Proceedings Royal Acad. Amsterdam. Vol. VIII. 



