Question

Ammonia nitrogen is present in a given wastewater sample as the ammonium ion (NH₄⁺) and ammonia (NH₃). If pH is the only deciding factor for the proportion of these two constituents, which of the following is a correct statement?
1. At pH 7.0, NH4+ will be predominant.
2. At pH below 9.25, NH₃ will be predominant.
3. At pH below 7.0 NH4+ and NH3 will be found in equal measures
4. At pH 9.25, only NH4+ will be present

Answer 1

Correct Answer - Option 1 : At pH 7.0, NH4+ will be predominant.

Concept:

Nitrogen Contents in sewage: The presence of nitrogen in sewage indicates the presence of organic matter, and may occur in one or more of the following forms: Free ammonia (also called Ammonia nitrogen), Albuminoid nitrogen (also called Organic nitrogen), Nitrites and Nitrates.

 Free ammonia (or Ammonia nitrogen):

• Ammoniacal nitrogen (NH3-N), is a measure for the amount of ammonia present in sewage.
• Ammonia has chemical formula of NH3 in the unionized state and NH4+ in the ionized state. The ammonia nitrogen is the sum of nitrogen present in both NH3 and NH4+.
• The free ammonia indicates the very first stage of decomposition of organic matter and the amount will progressively decrease as sewage gets treated.

Ammonia exist in the form of NH₄⁺ & NH₃

1. · At pH < 8, all ammonia is in the form of NH₄⁺
2. · At pH = 9.5, there are 50% NH₃ & 50% NH₄⁺
3. · At pH > 11, all ammonia is in the form of NH₃

_​∴ Option 1 is correct.

Extra Points 

 Albuminoid nitrogen (or Organic nitrogen): 

• Organic nitrogen is that nitrogen which describes nitrogen compounds that had its origin in living material.
• The nitrogen in protein and urea is organic nitrogen. Organic nitrogen can enter sewage as bodily wastes, discarded food material, or as components of cleaning agents.
• This albuminoid nitrogen indicates quantity of nitrogen present in sewage before the decomposition of organic matter is started.
• If this organic nitrogen reacts anaerobically then it decomposes to ammonia and if this reacts aerobically it converts to nitrites or nitrates.