Question

Which of the following options is/are CORRECTLY listing the species included in Total Kjeldahl Nitrogen (TKN) estimation of an aqueous sample?

• A. Ammoniacal - N
• B. Nitrite - N
• C. Nitrate - N
• D. Organic - N

Hint:

TKN measures reduced forms of nitrogen only; it does not include the already-oxidized nitrite and nitrate forms.

Answer 1

The Correct Option is A, D

Step 1: Recall the definition of Total Kjeldahl Nitrogen (TKN).
Total Kjeldahl Nitrogen is a parameter used in water and wastewater analysis that measures the sum of organic nitrogen and ammoniacal nitrogen present in a sample. It is determined by the Kjeldahl digestion method, in which the sample is digested with sulfuric acid and a catalyst, converting organic nitrogen compounds into ammonium sulfate, and the total ammonia, both originally present and generated from organic nitrogen, is then measured by distillation and titration.

Step 2: Examine option (A) Ammoniacal-N.
Ammoniacal nitrogen is directly measured as part of the TKN test, since after digestion all nitrogen, both the pre-existing ammonia and the organic nitrogen converted to ammonia, is quantified together as ammonium. So Ammoniacal-N IS included in TKN.

Step 3: Examine option (D) Organic-N.
Organic nitrogen, from proteins, amino acids, urea, and similar compounds, is converted into ammonium sulfate during the acid digestion step of the Kjeldahl method, and is thus captured in the final measured ammonia value. So Organic-N IS included in TKN. By definition, TKN = Organic-N + Ammoniacal-N.

Step 4: Examine option (B) Nitrite-N.
Nitrite is an already oxidized form of nitrogen, an intermediate oxidation state. The Kjeldahl digestion process does not convert nitrite into ammonia, and nitrite nitrogen is NOT captured by the TKN test.

Step 5: Examine option (C) Nitrate-N.
Nitrate is the fully oxidized form of nitrogen. It is chemically stable under Kjeldahl digestion conditions and is not reduced to ammonia during the test, so it is also excluded from the TKN measurement.

Step 6: Conclude.
TKN by definition and by test procedure includes only Organic-N and Ammoniacal-N, and excludes the oxidized forms Nitrite-N and Nitrate-N. Hence the correct answers are options (A) and (D).