What is the function of boric acid in Kjeldahl method?

Answer 1

Boric acid (H3BO3 or HBO2.H2O) is added (in excess) to form a stable and soluble complex with the steam-distilled ammoniagas:

NH3(g) + HBO2.H2O(aq) --> NH3.HBO2(aq) + H2O

which then can be titrated directly with 0.05M sulfuric acid till pH = 5:

2 NH3.HBO2(aq) + H2SO4(aq) --> 2 NH4+ + HBO2.H2O(aq) + SO42-

Note: Boric acid (HBO2.H2O) is a very weak acid pKa=9.3 and is not interfering with the acid/base titration of ammonia.

Answer 2

Boric acid functions as a non-volatile buffer in the Kjeldahl method. It helps to control the pH during digestion by neutralizing any acidic fumes produced. Additionally, boric acid traps any excess ammonia vapors formed during the distillation step, preventing loss of analyte.