Chemical Oxygen Demand Calculator

Chemical oxygen demand measures the amount of dissolved oxygen required to chemically oxidize organic and inorganic compounds in water. The calculation uses the volume of titrant consumed by a blank versus the sample, adjusted for the normality of the titrant and the volume of the water sample tested. The standard formula is expressed as:

COD (mg/L) = [(V_blank - V_sample) * N * 8000] / V_sample

Where V_blank is the titrant volume for the blank, V_sample is the titrant volume for the water sample, N is the normality of the titrant (such as ferrous ammonium sulfate), 8000 is the milliequivalent weight of oxygen multiplied by 1000 to convert to milligrams per liter, and V_sample is the volume of the water sample in milliliters.

Imagine you are analyzing an industrial wastewater effluent sample. You run a dichromate reflux titration and record the following laboratory values:

• Titrant Blank Volume (V_blank): 25.0 mL
• Titrant Sample Volume (V_sample): 14.5 mL
• Normality of Titrant (N): 0.10 N
• Volume of Water Sample: 50.0 mL

First, find the difference in titration volumes: 25.0 mL - 14.5 mL = 10.5 mL. Next, multiply this difference by the normality and the oxygen equivalent constant: 10.5 * 0.10 * 8000 = 8400. Finally, divide this product by the sample volume: 8400 / 50.0 = 168 mg/L. The chemical oxygen demand of your wastewater sample is 168 mg/L.

Ensure all glassware is meticulously cleaned and free of organic residues before starting titration to avoid falsely elevated COD readings. Always run a blank determination alongside your sample batches to account for any impurities in your reagents. If your sample has a very high organic load that exceeds the calibration range, be sure to dilute the sample appropriately before running the digestion process.