276 REPORTS ON THE STATE OF SCIENCE. 



' equivalent ' point is 10~ 7 , the curve is very nearly a straight line, and 

 litmus is the only indicator that might possibly give moderately accurate 

 results. But as it is never necessary to titrate a weak acid by a weak base, 

 it is unnecessary to discuss this and similar cases further. 



This method of regarding the problem may of course easily be extended 

 to such examples as the titration of a dibasic acid, or of one acid in presence 

 of another (C0 2 ), but it is impossible within the limits of this paper to 

 consider these in detail. 



The general conclusions to be drawn from the curves are these : firstly, 

 that if we can find an indicator which will give a sharp and satisfactory 

 ' end-point ' in any particular volumetric operation, then the end-point so 

 indicated will probably be accurate to within two parts in a thousand ; 

 secondly, that if no indicator will give a satisfactory end-point, then the 

 only way to obtain really accurate results is to find an indicator the colour 

 of which is sensibly affected at the concentration of hydrions obtaining in 

 a pure solution of the salt of the strong and weak electrolytes, and then, 

 using a comparison solution of the salt containing the same amount of 

 indicator as the titrating solution, titrate to the same colour. In extreme 

 cases results obtained by judging the colours with the unassisted eye will 

 be too inaccurate, and a form of colorimeter should be used. 



