Factors Affecting Dissolved Oxygen Measurements
The solubility of oxygen depends on temperature, pressure and the salt dissolved in water. In addition, oxygen diffuses faster through the solution than through the membrane. If the flow rate is too slow, it will cause interference.
1. The influence of temperature As the temperature changes, the diffusion coefficient of the membrane and the solubility of oxygen will change, which will directly affect the current output of the dissolved oxygen electrode. A thermistor is often used to eliminate the influence of temperature. As the temperature rises, the diffusion coefficient increases, but the solubility decreases instead. The influence of temperature on the solubility coefficient a can be estimated according to Henry's law, and the temperature's effect on the membrane diffusion coefficient β can be estimated through Arrhenius' law.
(1) Solubility coefficient of oxygen: Because the solubility coefficient a is not only affected by temperature, but also by the composition of the solution. Under the same oxygen partial pressure, the actual oxygen concentration of different components may also be different. According to Henry's law, it can be known that the oxygen concentration is proportional to its partial pressure. For a dilute solution, the change of the solubility coefficient a is about 2%/°C when the temperature changes.
(2) Diffusion coefficient of the film: According to the Arrhenius law, the relationship between the solubility coefficient β and the temperature T is: C=KPo2·exp(-β/T), where K and Po2 are assumed to be constant, then it can be calculated β is 2.3%/°C at 25°C. After the solubility coefficient a is calculated, the diffusion coefficient of the membrane can be calculated by comparing the instrument indication and the laboratory analysis value (the calculation process is omitted here). The diffusion coefficient of the membrane is 1.5%/℃ at 25°C.
2. Effect of atmospheric pressure According to Henry's law, the solubility of a gas is proportional to its partial pressure. The partial pressure of oxygen is related to the altitude of the area. The difference between the plateau area and the plain area can reach 20%, and it must be compensated according to the local atmospheric pressure before use. Some instruments are equipped with a barometer inside, which can be automatically corrected during calibration; some instruments are not equipped with a barometer, and should be set according to the data provided by the local weather station during calibration. If the data is wrong, it will lead to large measurement errors.
3. Salt content in the solution Dissolved oxygen in brine is significantly lower than that in tap water. For accurate measurement, the influence of salt content on dissolved oxygen must be considered. Under the condition of constant temperature, the dissolved oxygen decreases by about 1% for every 100mg/L increase in salt content. If the meter uses a solution with a low salt content during calibration, but the actual measured solution has a high salt content, errors will also result. In actual use, the salt content of the measurement medium must be analyzed for accurate measurement and correct compensation.
4. The flow rate of the sample Oxygen diffusion through the membrane is slower than that through the sample, so it is necessary to ensure that the electrode membrane is in full contact with the solution. For the flow-through detection method, the oxygen in the solution will diffuse into the flow cell, causing the loss of oxygen in the solution close to the membrane, resulting in diffusion interference and affecting the measurement. In order to measure accurately, the flow rate of the solution flowing through the membrane should be increased to compensate for the oxygen lost by diffusion, and the minimum flow rate of the sample is 0.3m/s.
