At about pH 9.2 ammonium and ammonia are about equal in concentration solution.  As the pH increases, more ammonium will convert to ammonia and is subject to volatilization.  MgO in solution has a pH of around 11.0.  By adding MgO to the solution being analyzed (likely a soil extract), you are raising the pH of the solution to drive the ammonium to ammonia which volatilizes. as you are distilling the solution with steam, water vapor is also driven off.  Ammonia has a high affinity for water and so it combines with the water vapor to form NH4OH which is trapped in a weak acid solution which maintains the ammonium in its ammonium form due to the excess of H+ ions in the solution.

The Devarda Alloy is a reducing catalyst that once the ammonium is driven off during the first part of the distillation, is added to the solution and helps convert (reduce) nitrate to ammonium.  The solution still has a high pH so that the NO3 converted to NH4 is again distilled off for analysis.

Phenoldisulfonic acid reacts readily in the presence of H2SO4 to form a compound which produces a yellow color  in an alkaline solution.  The compound is an alkaline salt of 6-nitorphenol-2:4-disulfonic acid.

The attached reference is a very solid methods manual!

At about pH 9.2 ammonium and ammonia are about equal in concentration solution.  As the pH increases, more ammonium will convert to ammonia and is subject to volatilization.  MgO in solution has a pH of around 11.0.  By adding MgO to the solution being analyzed (likely a soil extract), you are raising the pH of the solution to drive the ammonium to ammonia which volatilizes. as you are distilling the solution with steam, water vapor is also driven off.  Ammonia has a high affinity for water and so it combines with the water vapor to form NH4OH which is trapped in a weak acid solution which maintains the ammonium in its ammonium form due to the excess of H+ ions in the solution.

The Devarda Alloy is a reducing catalyst that once the ammonium is driven off during the first part of the distillation, is added to the solution and helps convert (reduce) nitrate to ammonium.  The solution still has a high pH so that the NO3 converted to NH4 is again distilled off for analysis.

Phenoldisulfonic acid reacts readily in the presence of H2SO4 to form a compound which produces a yellow color  in an alkaline solution.  The compound is an alkaline salt of 6-nitorphenol-2:4-disulfonic acid.

The attached reference is a very solid methods manual!

Thanks you very much Dear Larry for your valuable explanations. It's very usefull.

Depending on the sequence of N analysis, Devardas and MgO are sometimes added prior to the first distillation, which will yield total inorganic N (NO2/NO3/NH4). We would do this when we were running parallel NH4 and then organic N analysis of the same samples (first ammonia distillation, then addition of acid for organic N digestion). We then could calculate NO3/NO2 by subtraction of the two results.

Thanks Pr Brian for your explanations.

Can we add the acid for organic matter digestion along woth MgO and Devarda alloy for total nitrogen distillation? or should we keep inorganic N distillation before organic N one?

Keep the inorganic distillation separate from the determination of the organic N. There are similar principles for the distillation part but are different for the digestion part.  The inorganic N distillations give you the plant available N fraction but doesn't give you the organic fraction.  Digestion with acid in the presence of a catalyst will convert the organic N (generally plant unavailable) into NH4.  Then you add NaOH (strong alkali) to drive off (distill off) the NH4.  When you do this, you get total N which is the organic plus inorganic N.  Inorganic N is only 1-3% of the total N.  It just depends if you want plant available N or total N.  By subtracting inorganic N from total N, you obtain organic N.  I think that if you look at the reference that you attached, you will find these procedures laid out pretty well. These procedures are very sound.