Determination of COD using the Coulometric Method

The Chemical Oxygen Demand (COD) test is a crucial water quality parameter that measures the amount of oxygen required to chemically oxidize organic and inorganic matter in a water sample. The coulometric method is a modern, efficient, and precise technique for COD determination. It operates by measuring the quantity of electricity (coulombs) required to oxidize the sample, which is directly proportional to the oxygen demand. This method is known for its miniaturized scale, reduced chemical usage, and rapid analysis.

In the coulometric method, the sample is digested with a known amount of potassium dichromate (K₂Cr₂O₇) and strong sulfuric acid (H₂SO₄) under high temperature. During digestion, organic matter reduces dichromate ions (Cr₂O₇²⁻) to chromic ions (Cr³⁺). After digestion, the remaining, unreacted Cr₂O₇²⁻ is titrated coulometrically. 

The instrument generates ferrous ions (Fe²⁺) at the anode through the electrolysis of an iron(III) ammonium sulfate solution. These Fe²⁺ ions immediately reduce the remaining Cr₂O₇²⁻ in the solution. The electrolysis current and the time required to complete this reduction are measured. The total charge (coulombs) consumed is used to calculate the amount of unreacted dichromate, from which the original COD of the sample is determined.

Apparatus and Reagents

Apparatus: Coulometric COD analyzer, Digestion block or thermostat, Analytical balance, Pipettes and micropipettes.

Reagents:

Potassium Dichromate Digestion Solution

Concentrated Sulfuric Acid (H₂SO₄)

Catalyst (e.g., Silver Sulfate, Ag₂SO₄)

Mercuric Sulfate (HgSO₄) - to eliminate chloride interference

Electrolyte Solution (for the coulometric cell)

Operational Steps

Step 1: Sample Preparation

Weigh accurately a small volume of a well-homogenized sample (typically 1-10 µL for low-range COD or 1-2 mL after dilution for high-range COD) into a small glass vial. For samples with high chloride content, add a sufficient amount of mercuric sulfate (HgSO₄) to complex the chloride ions and prevent interference.

Step 2: Digestion

Using a pipette, add a precise volume of potassium dichromate digestion solution and catalyst to the vial. Carefully add a measured volume of concentrated sulfuric acid, which will generate significant heat. Cap the vial securely and mix the contents thoroughly.

Place the vial into a pre-heated digestion block or thermostat. Digest the sample at a high temperature (e.g., 150°C - 165°C) for a specified period, typically 10-15 minutes. This step ensures complete oxidation of the organic matter.

Step 3: Coulometric Titration

After digestion, remove the vial and allow it to cool to room temperature. Transfer the entire digested solution (or a defined aliquot) into the measurement cell of the coulometric analyzer, which contains the electrolyte solution.

Start the titration process on the analyzer. The instrument will automatically generate ferrous ions and measure the charge required to reduce the remaining dichromate. The titration continues until an endpoint is detected, usually by a potentiometric sensor.

Step 4: Calculation and Reporting

The COD analyzer's internal software automatically calculates the COD concentration based on the total charge passed, the sample weight/volume, and pre-calibrated factors. The result is typically displayed and reported in milligrams per liter (mg/L) of oxygen (O₂).