CWEA Grade 2 Lab Analyst Practice Test

Why must the COD digestion be performed in sealed vessels?

To prevent evaporation during heating and maintain reaction conditions. To prevent introduction of air. To allow water to evaporate. To avoid changing color.

Correct Answer:

To prevent evaporation during heating and maintain reaction conditions.

Explanation:

Focusing on what makes COD digestion reliable: you need to keep the reaction conditions constant while heating a strong oxidizing mixture. Sealing the digestion vessels prevents water and volatile components from escaping as the temperature rises, so the concentration of reagents stays the same and the sample can be fully oxidized. This control of temperature, pressure, and composition is essential for an accurate COD measurement. Air ingress is not the primary concern here, and allowing water to evaporate would defeat the purpose by changing concentrations. Color change can occur as a result of the digestion, but sealing is about maintaining the correct reaction environment to ensure complete digestion rather than about avoiding color changes.

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