What Are the Primary Methods for Measuring Dissolved Oxygen in Water?

The electrochemical probe method operates on the principle that oxygen molecules diffuse through a selective membrane and are reduced at the working electrode, generating a diffusion current proportional to the oxygen concentration. By measuring this current, the dissolved oxygen concentration in the sample can be accurately determined. This paper focuses on the operational procedures and maintenance practices associated with the electrochemical probe method, aiming to enhance understanding of instrument performance characteristics and improve measurement accuracy.

1.Instruments and Reagents
Primary instruments: multifunctional water quality analyzer
Reagents: those required for the iodometric determination of dissolved oxygen

2. Full-Scale Calibration of the Dissolved Oxygen Meter
Laboratory Method 1 (Saturated Air-Water Method): At a controlled room temperature of 20 °C, place 1 L of ultrapure water into a 2 L beaker. Aerate the solution continuously for 2 hours, then cease aeration and allow the water to stabilize for 30 minutes. Initiate calibration by placing the probe in the water and stirring with a magnetic stirrer at 500 rpm or gently moving the electrode within the aqueous phase. Select “saturated air-water calibration” on the instrument interface. Upon completion, the full-scale reading should indicate 100%.

Laboratory Method 2 (Water-Saturated Air Method): At 20 °C, moisten the sponge inside the probe’s protective sleeve until fully saturated. Carefully blot the surface of the electrode membrane with filter paper to remove excess moisture, reinsert the electrode into the sleeve, and allow it to equilibrate for 2 hours before initiating calibration. Select “water-saturated air calibration” on the instrument interface. Upon completion, the full-scale reading typically reaches 102.3%. Generally, results obtained via the water-saturated air method are consistent with those from the saturated air-water method. Subsequent measurements of either medium typically yield values around 9.0 mg/L.

Field Calibration: The instrument should be calibrated prior to each use. Given that ambient outdoor temperatures often deviate from 20 °C, field calibration is best performed using the water-saturated air method within the probe sleeve. Instruments calibrated using this approach exhibit measurement errors within acceptable limits and remain suitable for field application.

3. Zero-Point Calibration
Prepare an oxygen-free solution by dissolving 0.25 g of sodium sulfite (Na₂SO₃) and 0.25 g of cobalt(II) chloride hexahydrate (CoCl₂·6H₂O) in 250 mL of ultrapure water. Immerse the probe in this solution and gently agitate. Initiate zero-point calibration and wait for the reading to stabilize before confirming completion. Instruments equipped with automatic zero compensation do not require manual zero calibration.