Cod Requirements [new] (LIMITED — 2027)

The first major requirement is proper sample collection and preservation. Because biological activity can alter the sample’s organic content, samples must be analyzed as soon as possible. If a delay is unavoidable, preservation is required: the sample is acidified to a pH of less than 2 using sulfuric acid and stored at 4°C. Even then, analysis should occur within seven days. This step prevents microorganisms from breaking down organic matter, which would lead to falsely low COD readings. Additionally, samples containing high levels of suspended solids must be homogenized, as particulates contribute significantly to the COD value.

At its core, COD measures the total amount of oxygen (expressed in milligrams per liter, mg/L) required to chemically oxidize all oxidizable substances in a water sample. Unlike its counterpart, Biochemical Oxygen Demand (BOD), which measures oxygen consumed by microorganisms over five days, COD provides a faster, more comprehensive result—often in just two to three hours. This speed makes COD indispensable for real-time process control in treatment plants. However, obtaining reliable COD data depends on meeting strict procedural, safety, and equipment requirements. cod requirements

Safety is a non-negotiable requirement when handling COD reagents. Potassium dichromate is a known carcinogen and strong oxidizer; mercuric sulfate is highly toxic to aquatic life; and concentrated sulfuric acid causes severe burns. Therefore, laboratories must enforce requirements for personal protective equipment (PPE)—including acid-resistant gloves, safety goggles, and lab coats—along with adequate fume hoods to capture acid vapors. Proper hazardous waste disposal protocols are also mandatory to prevent environmental contamination from spent digestion solutions. The first major requirement is proper sample collection

Second, the analytical procedure demands precise chemical reagents and equipment. The standard dichromate method requires a digestion solution of potassium dichromate ((K_2Cr_2O_7)) in strong sulfuric acid, along with a catalyst (silver sulfate) to oxidize straight-chain hydrocarbons, and mercuric sulfate to mask chloride interference. These chemicals must be of analytical grade, and the digestion must occur in a closed reflux system or in a pre-heated COD reactor block at 150°C for two hours. Requirements for accuracy also include using certified standards for calibration and running blanks and duplicates to control for contamination. Even then, analysis should occur within seven days