INDICATORS AND THEIR EMPLOYMENT 119 



If, for instance, we so dilute any N acid with distilled 

 water that each successive dilution will be weakened by a 

 tenfold dilution we can by comparison with a known stand- 

 ard solution arrange a scale extending, say from pHl to pH8, 

 and by trying various indicators at various points on the 

 range, determine at which point each indicator functions 

 best. That, in fact, is what has been done and it is in that 

 manner that the colorimetric estimation of H.I.C. as symbo- 

 lyzed by pH is determined. 



INDICATORS AND THEIR EMPLOYMENT. 



If one prepare a normal solution of an electrolyte in pure 

 water and from such solution prepare successive dilutions 

 by negative multiples of ten, it is possible, as said above, 

 by appropriate electrical methods to detect the ions result- 

 ing from dissociation in .dilutions reaching the millionth or 

 the billionth part, as expressed by the appropriate decimals. 

 If, for instance, such dilutions be prepared for a NHC1 

 solution, we shall not find that the hydrogen-ion content 

 of each tenth strength of solution is precisely one-tenth of 

 that of the next stronger dilution, for the higher the dilution 

 of an electrolyte, the greater the ionization, i. e., the greater 

 the dissociation. It is necessary, therefore, to establish for 

 each successive tenth dilution the H.I.C., as symbolized by 

 pH for that dilution, and it is found that in the interval 

 between such dilutions the pH gradually increases from the 

 low to the high dilutions so that such increase may also 

 be expressed in decimals thus a substance whose pH is 

 between 5 and 6 may be expressed as pH5.5, or pH5.8 as 

 the case may be. 



When a solution has been so standardized by the electrical 



