Why we use Boric Acid only in Protein Estimation by Kjeldahl Method???

Ammonia which is liberated during steam distillation of digested protein sample is soluble in boric acid as well as it is sparingly soluble in water.

Suppose, we use water instead of boric acid, then the chemical reaction will be,

NH₃ (g) + H₂O (aq) -----> NH₄⁺ + OH^-

which is a basic solution and it is not a complex to give any indicator reaction. Also, solubility of ammonia in water is temperature dependent so any minute variation of temperature can result in loss of captured ammonia.

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

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

which then can be titrated directly with 0.05 M sulphuric acid till pH = 5.

2 NH₃.HBO₂ (aq) + H₂SO₄ (aq) ----->   2 NH₄⁺ + HBO₂.H₂O (aq) + SO₄^2-

This titration gives colour change for quantitative analysis of protein.

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