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Reprinted from Mettler's official website, for learning and communication purposes only, and will be deleted if infringing!

Equipment Operation

This chapter summarizes some recommendations for the daily use of DO electrodes. They are based on generally accepted operating rules.

3.1 DO electrode preparation

Optical DO electrodes do not require any preparation before use. In contrast, electrochemical electrodes require checking the membrane integrity. In addition, if the electrolyte is refillable, it must be ensured that the electrolyte is properly replenished. When using polarographic electrodes, it is necessary to ensure the correct polarization of the electrode.

3.2 DO electrode calibration

DO electrodes can be calibrated with either 1 or 2 points. Saturated water vapor should be used as the first point of calibration (this is equivalent to 100% oxygen saturation). If only a 1-point calibration of the electrode is used, the meter can only adjust the slope of the calibration curve by assuming the absence of 0% signal (Figure 3.1, left). To determine the zero offset, a second calibration point is required. Since the zero offset of most DO electrodes does not deviate too much from zero, a 1-point calibration is sufficient for many applications.

For the second calibration point, a zero oxygen standard solution should be prepared (this corresponds to 0% oxygen saturation). To do this, a zero oxygen tablet is dissolved in water to eliminate all dissolved oxygen in it. With the second point, the offset can be determined (Figure 3.1, right). A 2-point calibration is recommended when measuring samples with an oxygen saturation below 10% or an oxygen concentration below 1 mg/L.

3.3 DO measurement

For most electrochemical DO electrodes,The electrode consumes oxygen during measurement，So you have to stir, and keep the stirring speed constant. In contrast, the optical electrode does not require stirring because it does not consume oxygen.

To reduce measurement time, the electrode should be immersed in the sample before starting the measurement. This process allows the oxygen concentration and temperature to reach equilibrium.

Air bubbles at the electrode tip must be avoided, otherwise the oxygen concentration in the air bubbles will also be measured and lead to erroneous results. In general, any coating of the membrane will affect the reading. Therefore, sample residues (such as oil, algae or slurry) on the electrode should be removed immediately after the measurement. Scratching the membrane should be avoided, as this will cause permanent damage.

The presence of oxidizing gases such as chlorine, nitrous oxide, and nitric oxide can also interfere with the measured oxygen concentration. In addition, sulfur-based molecules such as H2S and SO2 can interfere with DO measurements. While these compounds only directly affect DO measurements at electrochemical electrodes, all DO electrode types are indirectly affected because the oxidizing potential of the gas can damage the electrode material.

3.4 Converting units

Electrochemical DO electrodes measure the amount of oxygen inside themselves. As such, the electrode can only measure how strongly dissolved oxygen is pushed into the electrode through the selective membrane. Therefore, the primary result measured is the partial pressure of oxygen in the solution.

If the solubility of oxygen in the solvent used is known, this value can be converted into oxygen concentration. For the most common solvent, water, this relationship is well understood. Since it depends on the temperature and salinity of the sample, these two values ​​must also be determined or known.

The meter uses formula 3.1 to convert the measured value into oxygen concentration

3.5 Maintenance and storage

After measurement, the electrode should be cleaned with water and wiped with a soft tissue. When measuring biological samples, care should be taken to avoid microbial growth. For best performance, the electrode should be stored in a safe environment at a temperature between 5 and 45°C and avoid rapid temperature changes.

For short-term storage, the galvanic DO electrode should be rinsed with deionized water and stored in storage solution. For long-term storage, it should also be short-circuited (to prevent performance degradation due to continued self-polarization) and stored in a cool place. To avoid the 6-hour polarization requirement of the polarographic DO electrode, it can be connected to the instrument. For long-term storage, it should be separated from the instrument because continuous polarization will gradually reduce its life. If the electrode is filled with internal electrolyte and the protective cap is placed on the membrane, it can be stored for several months. However, to use the electrode again after storage for more than 3 months, the electrolyte should be replaced. If storage is intended for more than 6 months, the electrolyte should be removed.

Exposure to H2S or SO2 can tarnish the electrochemical DO electrode. In addition, the cathode of the polarographic electrode will be covered with AgCl over time due to the chemical reactions involved in its function. The deposits on the electrode can be mechanically removed. After washing with distilled water and drying with a soft paper towel, refilling the electrolyte will allow normal operation.

Generally, the frequency of electrolyte renewal depends on the concentration of oxidizing gases during measurement and the frequency of electrode use. Optical electrodes should be stored dry. Electrodes with replaceable membranes should have their membranes replaced as soon as the electrode shows signs of degradation.

To be continued...