The Estimation of Acidity. 235» 



It will thus be seen that by the selection of a suitable indicator 

 sensitive over changes of hydrogen ion concentration which may 

 be anticipated, accurate information as to the ionic concentra- 

 tion of an unknown solution may be gained. 



A " ladder " of indicators arranged by the writer, somewhat 

 on the lines of that in S. W. Cole's Practical Physiological Chem- 

 istry, 5th edition, is illustrated in figure 1. The dyes there men- 

 tioned are prepared by the Cooper Laboratory, Watford, who> 

 reproduce the scheme referred to. 



If this " ladder " be examined, it will be seen that the indicators 

 there shown cover a wide range of possible hydrogen ion con- 

 centration, and it may be of interest to note certain of the fea- 

 tures of some of the indicators in this particular series. 



Methyl red and thymol blue, for instance, may be used in 

 the precipitation of an amphoteric substance, such as anthranilic 

 acid from its alkaline solution. In this operation, insufficient 

 mineral acid will not completely precipitate the organic acid, while 

 an excess will redissolve it. By the use of these indicators the 

 exact point is readily found. To do this one employs a test 

 paper saturated with methyl red, making spot tests upon it 

 from time to time, until the acid point is approached ; then the 

 addition of acid is further continued until a thymol blue paper 

 also shows an acid reaction, which is the concentration of acid 

 at which the maximum precipitation takes place. In thymol 

 blue we have an indicator of two useful working ranges, i.e., 

 an acid and an alkaline range, which can be employed in dif- 

 ferential acidimetric and alkalimetric titrations. Any acid with' 

 a dissociation constant equal to or less than acetic acid, can be 

 titrated in the presence of hydrochloric acid if the weaker acid 

 is completely neutralised when the alkaline change of indicator 

 occurs. (2) 



One illustration taken from a communication by Clark and 

 Lubs will serve to show the application of this method to the dif- 

 ferential titration of an organic acid in the presence of hydro- 

 chloric acid. 



Many organic acids are not dissociated at a concentration of" 

 hydrogen ions at which hydrochloric acid is itself completely 

 dissociated. If to a saturated aqueous solution of benzoic acid 

 a known amount of indicator be added, and this is used as an 

 indicator colour standard, the hydrochloric acid content of an 

 unknown mixture may be estimated by titration with normal 

 sodium hydroxide, in the presence of the same indicator, until' 

 the colours are matched. Care must be taken that the indicator- 



