The Kjeldahl method is used to determine the amount of nitrogen present in an unknown sample. Classically, this method is used to estimate the protein content of food and the amount of nitrogen in soil samples. The method depends on the conversion of ammoniacal compounds like amines and amides into free ammonia gas by digestion with strong solutions of NaOH. This ammonia gas is then bubbled by distillation through a known amount of a standard acid solution which neutralizes some of the acid. The amount of ammonia produced is then determined by back titration of the remaining acid with a standardized NaOH solution. The formula you seek is just the math required to do a back titration.

If HA is the standard acid used to trap the ammonia and the concentration and volume of a standardized solution of NaOH used in the back titration are known, then:

(MHA)(vol. of HA) – (Mof NaOH)(vol. of NaOH) = moles of NH3 produced

moles of NH3 x 1 mol N/1 mol NH3 x 14.01 g/mol N = grams of nitrogen in sample

grams of nitrogen/grams of sample x 100% = percentage of nitrogen in the unknown sample

It should be noted that compounds containing oxidized nitrogen (NO3- or NO2), or nitrogen found in ring structures are not determined by the Kjeldahl method