Measurement Experiment of Saturation Rate of Water Treatment Dissolved Oxygen Analyzer: Electrochemical Probe Method
Measurement experiment of saturation rate of water treatment dissolved oxygen analyzer: electrochemical probe method.
1. Principle of the method. The oxygen-sensitive membrane consists of two metal electrodes in contact with the supporting electrolyte and a selective membrane. The membrane is only permeable to oxygen and other gases, but not to water and soluble substances. Oxygen permeating the membrane is reduced on the electrode to generate a weak diffusion current, which is proportional to the dissolved oxygen content of the water sample at a certain temperature.
2. Scope of application of the method. The lower limit of determination of the electrode method depends on the instrument used, and is generally applicable to water samples with dissolved oxygen greater than 0.1mg/L. When the water sample is colored and contains organic substances that can react with iodine, it is not suitable to use the iodometric method and its correction method for determination, and the electrode method can be used. However, the gas or vapor containing chlorine, sulfur dioxide, iodine, and bromine in the water sample may interfere with the measurement, and it is necessary to frequently replace the membrane or calibrate the electrode. instrument
(1) DO Dissolved Oxygen Meter: The instrument is divided into two types: primary battery type and polarographic type (applied voltage).
(2) Thermometer: accurate to 0.5°C.
Reagent
(1) Sodium sulfite; (2) Divalent cobalt salt (CoCl2 6H2O)
steps
When using the instrument, operate according to the instruction manual.
1. Preparation before the test.
(1) Assemble the probe according to the instrument manual, and add the required electrolyte. For used probes, check whether there are air bubbles or rust-like substances in the probe membrane. If necessary, the film needs to be removed and reassembled.
(2) Zero calibration: immerse the probe in water containing 1g of sodium sulfite and 1mg of cobalt salt per liter for zero calibration.
(3) Calibration: Calibrate according to the requirements of the instrument manual, or take 500ml of distilled water, siphon part of it into the dissolved oxygen bottle, and measure the dissolved oxygen content with the iodometric method. Put the probe into the distilled water (to prevent aeration and oxygenation), and adjust the instrument to the iodometric measurement value. When the instrument cannot be calibrated, the electrolyte and sensitive membrane should be replaced.
2. Determination of water samples. Carry out according to the instrument manual, and pay attention to temperature compensation. Precision and accuracy. After 6 laboratory analysts used different models of dissolved oxygen measuring instruments in the same laboratory, they measured 5 kinds of surface water with dissolved oxygen content of 4.8-8.3mg/L, and the measured values of each sample were relatively standard. The deviation does not exceed 4.7%; the absolute error (relative to the iodometric method) is less than 0.55mg/L. Precautions
(1) The primary battery-type instrument can react spontaneously when exposed to oxygen. Therefore, when not measuring, the electrode probe should be stored in anaerobic water and short-circuited to avoid consuming electrode materials and affecting the measurement. For the probes of polarographic instruments, when not in use, they should be placed in a humid environment to prevent the electrolyte solution from evaporating.
(2) Do not touch the surface of the probe film with your hands.
(3) After replacing the electrolyte and membrane, or when the membrane is dry, make the membrane wet, and calibrate after the reading is stable.
(4) If the water sample contains algae, sulfide, carbonate and other substances, long-term contact with the membrane may cause the membrane to be blocked or damaged.
