ON THE SENSITIVENESS OF INDICATORS. 



273 



some cases, but our knowledge is at present too imperfect to allow of their 

 being denned with greater precision : — 



Methyl orange 10" 4 ' 5 to 10" 5 ' 5 



Methyl red 10" V7 to 10"° * 



Litmus 10" 6 " 5 to 10" 7 •"' 



Phenolphthalein 10~ 8 2 to 10~ 9 



Thymolphthalein lO" 10 ' 5 to lO" 11 ' 5 



Having arrived at these numbers, all that remains is to consider how 

 the concentration of hydrogen ions changes in a solution when we titrate 

 acids and bases of various strengths. When we know this, we can not 

 only decide upon the best indicator to use, but also estimate, the probable 

 error in using it, and the absolutely certain error in using any different one. 

 If the acid and base used are both ' strong ' electrolytes, for example 



Fig. 1. 



HC1 and NaOH, then when equivalent quantities are present in solution 

 we have an exactly neutral solution of an unhydrolysed salt (NaCl), and 

 the concentration of hydrions will be exactly the same as in pure water, 

 namely 10 7 at 25°. Also, a very slight excess of acid and base makes an 

 enormous difference in the concentration of hydrions in the solution. This 

 can be seen most clearly by the accompanying curve, which represents the 

 change in H° ion concentration when the titration of 50 c.c. of N /100 HC1 

 by N /100 NaOH is almost complete. As ordinates are plotted the number 

 of c.c. of base added ; the abscissa} represent the concentration of H° ions. 

 When 49 - 95 c.c of base are added the concentration of hydrions is 10~° ; 

 when 5005 are present, C„ is 10~ 8 ; thus two drops of the alkali, or two 

 parts in 1,000 present, diminish C H to 100th part. Along the curve are 

 written in the various indicators at points which correspond to the ' end- 

 points ' they indicate. It will be seen that methyl red, litmus, phenol- 

 phthalein, coming as they do on the flat, or most sensitive part of the curve, 

 all give sharp end-points, bv which we mean that a small trace of the 

 1911. T 



