METHODS FOR QUANTITATIVE DETERMINATIONS. 411 



Other indicators used at times in acidimetry are solutions of bra- 

 zil-wood, cochineal, methyl-violet, alizarin, iodeosin, Congo red, turmeric, 

 etc. 



Ionic explanation of the action of indicators. The substances used to indicate 

 the neutralization point are themselves very weak acids or bases, capable of 

 forming salts with the bases or acids that are brought together in solution for 

 the purpose of neutralization. The undissociated molecules of the indicator 

 have a different color from its ions, and it must have feeble dissociating power 

 in the uncombined state. The latter is a characteristic of feeble acids in gen- 

 eral. The salts of the indicators are easily dissociated into ions. Substances 

 that are strong acids or bases cannot be used as indicators, because they disso- 

 ciate in the free state, and thus give no different color when they are neutral- 

 ized. The indicators that dissociate least in the free state are the most sensitive 

 in color changes when combined with traces of acids or bases. The neutral 

 point in neutralization experiments is really overstepped by the amount 

 of acid or alkali required to produce change of color with the indicator, 

 but in the case of sensitive indicators this amount is a mere trace and is 

 negligible. 



Litmus is an acid with slight dissociating power, which in pure water gives 

 a violet color. Addition of acids represses the slight dissociation and the color 

 changes to red, which is the color of the undissociated molecules of litmus. 

 Alkalies form salts with litmus, which dissociate easily, the negative ions show- 

 ing a blue color. 



Phenolphthalein is an acid of less dissociating power than litmus. In 

 pure water or acids it is colorless, in alkaline solutions it is red, which is 

 the color of the negative ions of the dissociated molecules of the salt of the 

 indicator. 



Methyl-orange is an acid of greater dissociating power than litmus. It dis- 

 sociates slightly when greatly diluted in water, giving a yellow color, which is 

 the color of the negative ions. With less water the color is orange, which is 

 the resultant of the yellow color of the ions and the red color of the un- 

 dissociated molecules. In acid solutions the dissociation of the indi- 

 cator is repressed, and the color is pure red. In alkaline solutions, salts 

 of the indicator are formed which dissociate freely and give an intense 

 yellow color. 



Because different indicators, as well as different acids and bases, show great 

 differences in their degrees of dissociation, marked variations in the sensitive- 

 ness of indicators to acids and bases are observed. Hence, indicators must be 

 chosen to suit the particular case of neutralization in hand for accurate work. 

 For example, if an active acid is titrated with ammonia and phenolphthalein 

 as indicator, color is not produced sharply at the neutral point, because am- 

 monium hydroxide is so little dissociated that it requires an appreciable excess 

 beyond the neutral point to produce the dissociated salt with the indicator. In 

 this case litmus or methyl-orange is suitable to use. 



Titration. This term is used for the process of adding the 

 volumetric solution from the burette to the solution of the 



