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.