When conducting a Kjeldahl analysis, ammonium sulfate is formed from nitrogen which protein consist of.

How in a picture of formation of ammonium sulfate there is involved no nitrogen but only some compounds containing nitrogen.

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Ammonia is basic so: $$\ce{NH3 +H+<=>NH4+}$$ Sulphuric acid is acidic(since it's name is itself so acidic!): $$\ce{H2SO4<=>H+ +HSO4- <=>2H+ +SO4^2-}$$ Now the final reaction would be like: $$\ce{2NH3 +H2SO4<=>NH3 +NH4+ +HSO4- <=>2NH4+ +SO4^2- <=>(NH4)2SO4}$$ And remeber since proteins are polymers of peptides which itself are molecules consisiting of both a carboxylic ($\ce{-COOH/-COO-}$) and amino ($\ce{-NH3/-NH4+}$) group, therefore with strong heating ammonium sulfate is expected.

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