Question

Determine the total Kjeldahl nitrogen (TKN) and total nitrogen (TN) concentration in mg-N/L of a water sample containing 0.25 mM ammonia, 0.02 mM nitrite, 0.75 mM nitrate, and 1.3 mg/L organic nitrogen

Answer 1

To solve the problem we will need to look up molar masses/molecular weights of ammonia, nitrite and nitrate.

(ammonia = 17 g/mol, nitrite = 46 g/mol, nitrate = 62 g/mol)

Let's begin by converting ammonia-N from mM to mg/L.

ammonia-N = 0.25 millimoles/L X 17 g/1 mole X 1 mole/10³ millimoles X 1000 mg/1 g

ammonia-N = 4.25 mg/L

Kjeldahl nitrogen = ammonia-N + organic nitrogen

Kjeldahl nitrogen = 4.25 mg/L + 1.3 mg/L

Kjeldahl nitrogen = 5.55 mg/L

Total nitrogen = Kjeldahl nitrogen + nitrate-N + nitrite-N

Once again we need to convert nitrate and nitrite forms of nitrogen into mg/L.

nitrite = 0.02 millimoles/L X 46 g/1 mole X 1 mole/1000 millimoles X 1000 mg/1 g

nitrite = 0.92 mg/L

Let's do the same thing with nitrate-N.

nitrate = 0.75 millimoles/L X 62 g/1 mole X 1 mole/1000 millimoles X 1000 mg/1 g

nitrate = 46.5 mg/L

Total nitrogen = 5.55 mg/L + 0.92 mg/L + 46.5 mg/L

Total nitrogen = 52.97 mg/L