In the Kjeldahl method, the ammonia is absorbed in the acid trap where it reacts with boric acid to form ammonium ions:
NH3 + H2O + B(OH)3 → NH4+ + B(OH)4-
It’s important to note here that n(NH3) = n(B(OH)4-),
where n is the amount in moles.
A color change from pink to green is observed, as this reaction leads to an increase in pH. When titrating with 0.10 M HCl, where the strong acid HCl has reacted with water to form H3O+, the following reaction takes place:
H3O+ + B(OH)4- → B(OH)3 + H2O
This lowers the pH further, and the color of the mixture changes back to pink from green when:
n(HCl) = n(B(OH)4-)
In titration, the measurement gained is the volume of titrant, here mL of 0.10 M HCl. To determine the amount of acid used, the following relationship can be used:
n(Acid) = C(Acid) * V(Acid),
where C is the concentration and V is the volume. Remember that the unit M (molar concentration) is the same as mol/L.
A final very useful equation is the relationship between amount, mass and molar mass:
n = m/M ⇔ m = n * M,
where m is the mass and M is the molar mass.