The percent nitrogen (N) in a protein sample can be determined by reacting the protein with sulfuric acid to produce ammonium sulfate. The ammonium sulfate is reacted with concentrated sodium hydroxide to form ammonia gas. The ammonia gas is collected by bubbling it through a known volume of standardized hydrochloric acid solution (all of the ammonia reacts in this step, is consumed). The unreacted hydrochloric acid is then back-titrated with standardized sodium hydroxide solution. The reactions involved are shown below
The percent nitrogen (N) in a protein sample can be determined by reacting the protein with sulfuric acid to produce ammonium sulfate. The ammonium sulfate is reacted with concentrated sodium hydroxide to form ammonia gas. The ammonia gas is collected by bubbling it through a known volume of standardized hydrochloric acid solution (all of the ammonia reacts in this step, is consumed). The unreacted hydrochloric acid is then back-titrated with standardized sodium hydroxide solution. The reactions involved are shown below:
Protein-N + H2SO4 (aq) ⟶⟶ (NH4)2SO4 (aq)
2 NH4+(aq) + 2 NaOH (aq) 2 NH3 (g) + 2 H2O
NH3 (g) + HCl (excess) NH4Cl (aq) + HCl (unreacted)
HCl (unreacted) + NaOH (aq) NaCl (aq) + H2O
A 1.85 g sample of the protein required 25.00 mL of 0.1535 M HCl and 21.55 mL of 0.0986 M NaOH.
How many moles of NH3 were consumed by the standardized HCl?
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