14 



BACTERIOLOGICAL CHEMISTRY 



ranges of pH values of the solution into which they are 

 introduced. The colour change may be due to the 

 conversion of an almost non-ionised weak acid or a weak 

 base of one colour to a strongly ionised salt, where the ion 

 has a characteristic colour. For example, methyl orange 

 exists as the yellow non-ionised acid in acid solution, 

 but on addition of alkali the corresponding salt is formed 

 which ionises to give the red anion. Or the colour 

 change may be associated with a change in the internal 

 structure of the molecule producing a coloured quinonoid 

 form, as is held to be the case with phenolphthalein. In 

 acid solution the compound exists in the colourless form : — 



HO 



\- 



CH-CH 



-cf ^C~OH 

 CH=CH 

 



In alkaline solution this is converted into the sodium 

 salt of the tautomeric quinonoid form : — 



H0< 



I CH=CH 



c=c<^ \c=o 



CH=CH 



COONa 



which ionises to give the red anion characteristic of 

 phenolphthalein in alkaline solution. Some of these 

 indicators, for example, Congo red, phenolphthalein, 

 litmus, change colour sharply over a narrow range of pH 

 value, that is, with a comparatively small change in 

 hydrogen ion concentration, and consequently are of use 



