What is added to measure CBOD?

To accurately measure Carbonaceous Biochemical Oxygen Demand (CBOD), a nitrogen inhibitor is added. This is crucial because CBOD specifically measures the amount of oxygen that microorganisms will consume while degrading organic matter that is carbon-based. The presence of nitrogen can lead to either a false positive for higher oxygen demand or increased oxygen consumption attributable to nitrogenous materials, which is not what we want to measure in the case of CBOD.

By using a nitrogen inhibitor, it ensures that the oxygen demand being measured is solely due to the carbonaceous matter. Therefore, this allows for an accurate assessment of the CBOD without the interference caused by nitrogenous compounds, which would otherwise skew the results. The other choices do not pertain directly to the measurement of CBOD and would not help in isolating the carbon-associated demand, making the nitrogen inhibitor the correct choice for this application.