What is free acidity and what is titratable acidity.

Free acidity:

• Free acidity of a solution is its instant H ion concentration measured without any manipulation.
• Free equimolar concentration of strong acid and weak acid, the free acidity will be very high in strong acid solution compared to that in weak acid solution because strong acids are completely dissociated and weak acids are partially dissociated into hydrogen ions.

Titratable acidity:

• Titratable acidity of a solution is its total H ion concentration that would be if the acid was allowed to ionize totally to hydrogen ion following its titration with alkali.
• For equimolar concentration of strong acid and weak acid the total hydrogen ion concentration of both the solution will be same at the end of complete ionization following titration with alkali.
• Very small amount of weak acid usually ionize to hydrogen ion and majority fail to ionize. During titration when alkali is added to the medium, the hydrogen ion concentration of the medium decreases which facilitate the unionized weak acid to ionize completely to hydrogen ion.
• Equimolar concentration of strong acid and weak acid differ with respect to free acidity, but they are same the respect to the titratable or total acidity.