Before performing a membrane electrode DO test, the zero calibration uses a reading on a sample containing an excess of _____ and a trace of cobalt chloride, CoCl2.

Before performing a membrane electrode DO test, the zero calibration uses a reading on a sample containing an excess of _____ and a trace of cobalt chloride, CoCl2.

• A. Sodium Chloride
• B. Sodium Sulfite
• C. Potassium Permanganate
• D. Magnesium Sulfate

Answer: (B)

In membrane DO meter calibration, the zero point is set by creating an oxygen-free solution. Sodium sulfite is used because it chemically reacts with dissolved oxygen to form sulfate, effectively removing O2 from the solution. A trace of cobalt chloride is added as a catalyst to speed this reaction, so the zero reading is reached quickly and remains stable. Sodium chloride would not remove oxygen; it just adds salt. Potassium permanganate is a strong oxidizer and could interfere with the measurement chemistry rather than reliably producing a true zero DO. Magnesium sulfate is simply a salt and also won’t remove dissolved oxygen. Therefore, the combination of an excess of sodium sulfite with a trace of cobalt chloride provides a reliable zero calibration by eliminating dissolved oxygen.

In membrane DO meter calibration, the zero point is set by creating an oxygen-free solution. Sodium sulfite is used because it chemically reacts with dissolved oxygen to form sulfate, effectively removing O2 from the solution. A trace of cobalt chloride is added as a catalyst to speed this reaction, so the zero reading is reached quickly and remains stable.

Sodium chloride would not remove oxygen; it just adds salt. Potassium permanganate is a strong oxidizer and could interfere with the measurement chemistry rather than reliably producing a true zero DO. Magnesium sulfate is simply a salt and also won’t remove dissolved oxygen.

Therefore, the combination of an excess of sodium sulfite with a trace of cobalt chloride provides a reliable zero calibration by eliminating dissolved oxygen.