ANALYTICAL cHEMISTRY - II

Principle

• Boric acid acts as a weak mono–protic acid (Ka = 6.4×10−10). Thus, it cannot be titrated accurately with standard alkali. However, by addition of certain organic polyhydroxy compounds, such as mannitol, glucose, sorbitol, etc. it acts like a much stronger acid (Ka = 1.5×10−4) and can be titrated to a phenolphthalein endpoint.

• The method may be applied to commercial boric acid, but as this material may contain ammonium salts, it is necessary to add a slight excess of sodium carbonate solution and then boil down to half the bulk to expel ammonia. Any precipitate which separates is filtered off and washed thoroughly, then the filtrate is neutralized to methyl red and after boiling, mannitol is added, and the solution titrated against standard sodium hydroxide solution.

• A mixture of boric acid and a strong acid can be analyzed by first titrating the strong acid using methyl red indicator, and then after adding mannitol or sorbitol, the titration is continued using phenolphthalein as an indicator. Mixtures of Na-tetraborate and boric acid can be similarly analyzed by titrating the salt with standard HCl and then adding mannitol and continuing titration with standard sodium hydroxide solution. It must be remembered that in the second titration the boric acid liberated during the first titration will also react.

Observations of Standardization Experiment

Observations of Conductivity Meter Experiment

Calculations