Kjeldahl nitrogen is a mixture of

Kjeldahl Nitrogen (TKN) is the total amount of nitrogen present in the form of organic nitrogen and ammonia in wastewater.

Think of wastewater as containing nitrogen in different forms. Some nitrogen is still present inside waste materials such as food particles, proteins, human waste, and plant or animal remains. This is called organic nitrogen.

As these waste materials start decomposing, a part of the organic nitrogen is converted into ammonia (NH₃/NH₄⁺). Therefore, wastewater mainly contains nitrogen in these two forms—organic nitrogen and ammonia.

The Kjeldahl method is a laboratory test that measures the combined amount of organic nitrogen and ammonia nitrogen. This combined value is known as Total Kjeldahl Nitrogen (TKN).

Easy Trick to Remember:
TKN = Organic Nitrogen + Ammonia Nitrogen

The test does not measure nitrate (NO₃⁻) or nitrite (NO₂⁻). These are different forms of nitrogen and are analyzed using separate laboratory tests.

Hence, Kjeldahl Nitrogen is a mixture of Organic Nitrogen and Ammonia, making Option C the correct answer.