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at 0111- disposal a colourless acid the dissociation constant of which 

 is known with certainty. 



Principle of the method. 



If tiie aqueous solution of an indicator-acid is diluted with water 

 the colour will change in the direction of tlie coloui- of the anions. 

 If for the dilution of an indicator-acid solution an isohydric solution 

 of a colourless acid is used, the degree of dissociation will not alter 

 and the colour of the solution will remain the same. 



If the solution of the indicator-acid is diluted with water, we may 

 titrate back with an acid of wliicli the concentration of the H-ions 

 is larger than that of the solution of the indicator-acid, until tiie 

 original-colour is restored. We have then prepared from the water 

 and the acid solution a mixture, which is isohydric with the solution 

 of the indicator acid. 



Starting from an acid with a known dissociation constant — stan- 

 dard acid — and an arbitrary solution of an indicator-acid we may 

 in the same manner determine the dissociation constant of a second 

 colourless acid, by preparing as directed, from the standard acid as 

 well as from the unknown acid solutions, which are isohydric with 

 the same solution of the indicator-acid. The acid solutions are then 

 mutually isohydric and the calculation of the dissociation constant 

 is readily made from the above data. 



The operation. 



The above described colorimeter is best suited for tiiis method. 

 The solution of the indicator-acid is introduced both in the standard 

 tube and the measuring tube. The amount of indicator-acid does not 

 matter, provided the quantity of it, in both tubes, is exactly the 

 same. After most carefully adjusting the colours, the contents of the 

 measuring tube are diluted witli an accurately known volume of 

 water, say, a cc. If now of a standard acid the dissociation constant 

 is Ka and if from this is prepared a solution of a dilution t'j, we 

 then titrate with tliis solution tiie contents of the measuring tube until 

 the colours are again tlie same. If tliis should require b cc the dilution 

 at whicli the solution of the standard acid is isohydric with the 

 given solution of the indicator-acid is : 



a-\-b 



—^XfA=VA. 







If now the dilution of the indicator-acid is known, or if we have 

 found in the same way the dilution at which an unknown dissocia- 

 tion constant yields an isohydric solution, then, calling both dilutions 



