What is the preservation approach for COD samples?

• A. Analyze ASAP or add H2SO4 to pH<2; cool <=6C
• B. Cool <=6C; 7D/28D
• C. No preservation required; analyze within 28D
• D. Freeze; 60 days

Answer: (A)

COD samples must be kept in a state that prevents changes before digestion. The best preservation is to acidify the sample to a pH below 2 with sulfuric acid and keep it cold, at about 0–6°C. This combination stabilizes the sample for the dichromate digestion used to measure COD and slows any biological or chemical changes that could alter the result. Analyzing the sample as soon as possible after collection is ideal; if immediate analysis isn’t feasible, preserving with acid and refrigeration maintains integrity for the holding period specified by the method.

Cooling alone doesn’t stop all potential changes, and not preserving at all leaves the sample vulnerable to microbial activity and other alterations. Freezing is not a standard preservation approach for COD and can introduce issues with the digestion process or sample matrix. So the recommended practice—acidify to pH < 2 with sulfuric acid and refrigerate—best maintains the sample’s original COD until analysis.