AOXY5000 Polarographic Dissolved Oxygen Analyzer

AOXY5000 dissolved oxygen Analyzer

Dissolved oxygen meter measures the amount of oxygen dissolved in an aqueous solution. Oxygen dissolves in water through the surrounding air, air flow, and photosynthesis. It can be used to measure and monitor processes that affect reaction speed, process efficiency, or the environment by measuring oxygen content, such as aquaculture, biological reactions, environmental testing (lakes, streams, oceans), water/wastewater treatment, and wine production.

Dissolved oxygen is consumed in water through respiration and decomposition, mainly supplemented by air and photosynthesis. The oxygen content in water mainly depends on temperature. The oxygen concentration in warm water should be lower than that in cold water. However, excessive dissolved oxygen content can be harmful to animals and plants.

Doppler flow meter brings you professional service,the dissolved oxygen electrode can be used to measure the dissolved oxygen content in the aqueous solution of the tested sample on site or in the laboratory. Due to dissolved oxygen being one of the main indicators of water quality, dissolved oxygen electrodes can be widely used for measuring dissolved oxygen content in various situations, especially in aquaculture water, photosynthesis and respiration, and on-site measurements. When evaluating the ability of streams and lakes to support biological survival, a biochemical oxygen demand (BOD) test should be conducted to measure the dissolved oxygen concentration and the temperature of the sample water solution containing organic matter that consumes oxygen when it decays.

Doppler FLOW METER brings you professional service,the dissolved oxygen concentration is usually measured in mg/L (dissolved oxygen per liter of water) or ppm (parts per million). There are two ways to determine dissolved oxygen, polarographic and electrochemical. The polarographic electrode requires an instrument to input a voltage to polarize the electrode. Due to the fact that the applied voltage may take 15 minutes to stabilize, the polarographic electrode is usually preheated before use to ensure proper polarization. The two poles of the primary battery are composed of two different metals that can spontaneously polarize and generate voltage. Due to the fact that the voltage generated by the primary battery is spontaneous rather than externally provided, there is no need for the "preheating" required for polarographic electrode polarization when using the primary battery electrode.

There are two types of practical membrane electrodes for dissolved oxygen meters: polarography and galvanic cell. Polarography: In an electrode, a gold (Au) ring or platinum (Pt) gold ring is used as the cathode; Silver silver chloride (or mercury mercurous chloride) is used as the anode. The electrolyte is potassium chloride solution. The outer surface of the cathode is covered with an oxygen permeable film. The film can be made of breathable materials such as polytetrafluoroethylene, polyvinyl chloride, polyethylene, silicone rubber, etc. Apply a polarization voltage of 0.5-1.5 volts between the anode and cathode electrodes. Some polarization voltages are 0.7 volts. When dissolved oxygen passes through the film and reaches the surface of the gold cathode, the following reaction occurs on the electrode.

Cathode reduction: O2+2H2O+4e → 4OH ˉ

At the same time, the anode is oxidized: 4Cl ˉ+4Ag-4e → 4AgCl

Under normal circumstances, the diffusion current i ∞ generated by the above reduction oxidation reaction is directly proportional to the dissolved oxygen concentration. It can be expressed as follows:

i∞=nFA（Pm/L）Cs

In the formula: i ∞ - steady-state diffusion current

N - number of gain and loss electrons

F-Faraday constant (96500 Coulomb)

A - Cathode surface area (square centimeters)

Pm - permeability coefficient of the film (cm2/s)

Thickness of L-film (cm)

Cs dissolved oxygen concentration (ppm)

After the electrode structure and thin film are determined, A, Pm, L, n, etc. in the equation are all constants. If K=nFA (Pm/L), then in the above equation: i ∞=KCs.

Therefore, it can be seen that as long as the diffusion current i ∞ is measured, the dissolved oxygen concentration can be measured. To eliminate the influence of temperature, salinity, and air pressure factors, each model of product adopts its own technology for compensation.

Galvanic Cell: When external oxygen molecules enter the electrode through the thin film and reach the three-phase interface of the cathode, the following reaction occurs.

Silver cathode is reduced: O2+2H2O+4e → 4OH ˉ

At the same time, the lead anode is oxidized: 2Pb+2KOH+4OH ˉ -4e → 2KHPbO2+2H2O

Namely, oxygen is reduced to hydroxide ions on the silver cathode and simultaneously gains electrons from the external circuit; The lead anode is corroded by potassium hydroxide solution, generating potassium hydrogen lead acid and outputting electrons to the external circuit. After connecting the external circuit, a signal current passes through, and its value is proportional to the dissolved oxygen concentration.

1、 Overview

The online dissolved oxygen analyzer is a continuous monitoring instrument developed by our company and is one of the online electrochemical analyzers. It can be equipped with T401 polarographic electrode to automatically achieve wide range measurement from ppb level to ppm level. It is a specialized instrument for detecting oxygen content in liquids in industries such as environmental wastewater, boiler feedwater, and condensate water.

T401ppb is suitable for large-scale use in thermal power plants due to its fast response, stability, reliability, and low operating costs. Equipped with T401ppm electrode for measuring ppm level, suitable for use in the environmental protection industry.

2、 Function and Features

Beautiful interface, fully displayed in Chinese: using high-resolution LCD display module, all data, status and operation prompts are displayed in Chinese without any symbols or codes defined by the manufacturer.

Simultaneous display of multiple parameters: oxygen concentration value, input or output current, temperature, and status are displayed simultaneously on the same screen.

Three calibration methods: In addition to traditional zero calibration and slope calibration methods, there are also manual input of zero or slope.

3、 Technical indicators

1. Standard: JJG 291-1999 "Coated Electrode Dissolved Oxygen Tester";

2. Chinese LCD display, Chinese operation interface;

3. Measurement range: 0-200.0 ug/L; 0-20.00 mg/L (automatic switching);

0～60℃；

Resolution: 0.1 ug/L, 0.01 mg/L, 0.1 ℃; 0.1%；

4. Basic error of the whole machine: ug/L: ± 1.0% FS; mg/L: ± 0.5% FS,

Temperature: ± 0.5 ℃;

5. Repeatability of whole machine indication: ± 0.5% FS;

6. Stability of whole machine indication: ± 1.0% FS;

7. Automatic temperature compensation range: 0-60 ℃, with 25 ℃ as the reference;

8. Response time:<60 seconds (98% of final value, 25 ℃)

37 ℃: 98% final value<20 seconds;

9. Output current error: ≤± 1.0% FS;

10. RS485 communication interface (optional); RS232 communication can also be optionally selected, please indicate when placing an order;

4、 Use and maintenance of electrodes

4.1 Working principle of electrodes

This table uses a polarographic electrode, with the anode electrode composed of Ag/AgCl and the cathode electrode composed of platinum (Pt). The electrolyte is filled with a special component between the two. Wrapped around the electrode by a silicone rubber permeable membrane. During measurement, a polarization voltage of 675mV is applied between the electrodes. Oxygen permeates through the membrane and is consumed at the cathode, while an equal amount of oxygen is generated at the anode. This dynamic process occurs until the oxygen partial pressures on both sides reach equilibrium simultaneously At this point, the current between the two electrodes is proportional to the oxygen partial pressure. The secondary meter detects this current, and after a series of transformations, the oxygen concentration and oxygen content are obtained At the same time, NTC (Negative Temperature Coefficient Thermistor) detects the temperature of the measured liquid, and after sampling with a secondary meter, temperature compensation is performed to convert the oxygen concentration or oxygen content to the value at 25 ℃.

Cathodic reaction: O2+2H2O+4e - → 4OH-

Anodic reaction: 4Ag+4Cl - → 4AgCl+4e-

4.2 Structure of electrodes

The following diagram shows the components and their interrelationships of the oxygen electrode:

★ Remove the protective cover when using

Inside, there is a platinum wire cathode and a thermosensitive glass rod, which is enclosed in a tubular silver anode to form the electrode body. The inner body is embedded in a stainless steel electrode rod.

Due to the reasonable geometric dimensions of the electrode, high membrane permeability, good electrolyte composition and low usage in the electrode cavity, the response speed of the electrode is relatively fast. However, some domestic coated oxygen electrode chambers require the addition of about 20mL of electrolyte, which results in a relatively long consumption time of background oxygen and leads to a prolonged period of inability to supply it.

4.3 Storage of electrodes

When filled with electrolyte and covered with a protective sleeve, the electrode can be stored for several months. The protective cover can reduce the drying of electrolyte.

If the electrode is to be stored continuously for more than six months, the electrolyte in the membrane should be poured out to keep the anode and cathode electrodes dry. At this time, the electrode cannot be connected to the secondary meter for electrical polarization.

4.4 Polarization of electrodes

When first used or continuously powered off for 5-10 minutes or more, polarization occurs after connecting to the instrument and powering it on. To achieve equilibrium in the chemical system of the electrode, reduce the zero oxygen current, and stabilize the electrode. At the beginning, the current of the electrode is relatively high and decreases exponentially. After 6 hours, it reaches a stable state, during which the displayed data will gradually decrease until it stabilizes. Calibration can only be carried out later.

The polarization process takes 6 hours. First, connect the electrode correctly to the secondary meter and turn on the instrument power. If the power off time is not long, the polarization time will be shorter and stabilize faster.

4.5 Calibration of electrodes

Each oxygen electrode has its own zero point and slope, and as the electrolyte is gradually consumed, the zero point and slope will change. Calibration is to obtain the true zero point and slope of the electrode.

Slope calibration: Calibrate the slope of the electrode in air.

Zero point calibration: The electrode zero point is generally calibrated using laboratory comparison method, and customers do not need to calibrate the zero point again after it has been calibrated at the factory.

At the factory, we have already calibrated the zero point and slope of the electrode. In fact, the zero point is very stable and changes very little during actual use, even after replacing the electrolyte or diaphragm, the drift of the zero point is also very small.

Note: 1. Anhydrous Na2SO3 is dissolved in pure water above 35 ℃ to form "anaerobic water" to mark the zero point. This method can only detect whether the electrode can drop to around 10ug/L, and it is difficult to achieve anaerobic water that can reach 0ug/L. Therefore, 'anaerobic water' cannot be used to mark the zero point.

2. Trace oxygen electrodes, each oxygen electrode only needs to be calibrated for slope in air. If there is a large error in online closed measurement, laboratory comparison method and zero point calibration method can be used to calibrate the electrode zero point.

4.6 Maintenance of electrodes

During use, membrane blockage is prone to occur, resulting in unstable and inaccurate measurements. Due to changes in water quality, especially for power plants, when the boiler is shut down and restarted, a large amount of impurities are carried in the water, and in severe cases, visible dirt (sludge, rust, algal substances, etc.) can be seen covering the membrane. It is easy to detect this type of pollution, but difficult for users to detect another type of ion pollution. Due to the small ions adhering to the surface of the membrane, the permeability of the membrane is affected, which is not easily detected by the naked eye. For this type of pollution, the electrode can be removed and soaked in 3% to 5% dilute hydrochloric acid for several hours before use.

Before each calibration, visually observe whether the diaphragm is damaged. If there is dirt on the diaphragm, carefully apply soft paper to remove it.

The diaphragm should be replaced after failure. The following phenomena often indicate diaphragm failure:

The response time becomes longer and the reaction becomes slower; The secondary meter reading is unstable and has a large drift.

4.7 Performance inspection of electrodes

To test the performance of electrodes, zero oxygen measurement can be used to qualitatively assess the quality of the electrodes. First, remove the electrode and place it in the air. After stabilizing for several tens of minutes, record the concentration value. If the concentration values match the temperature and concentration correspondence table in the appendix, slope calibration should be used. Place the electrode in anaerobic water again, and after several minutes, the T401ppb electrode is below 10ug/L, and the T401ppm electrode is below 500ug/L. If the reading exceeds the above range, it is often due to the depletion of electrolyte or damage to the diaphragm, which should be replaced. If it still doesn't work after replacement, it should be a problem with the electrode itself. Please contact us.

4.8 Replacement of Electrolyte and Membrane Body

The electrolyte has a strong alkalinity of 13.0 pH and should be avoided from contact with the skin, mucous membranes, and eyes. If the above situation occurs accidentally, rinse quickly with plenty of water. Suggest wearing gloves for protection.

The electrode is equipped with a membrane and electrolyte when it leaves the factory, and has been tested. Users can use it directly. But if the user stores it for several months before using it, the electrolyte should be replaced first.

If there is a phenomenon of diaphragm failure (such as longer response time, excessive current in zero oxygen environment, or mechanical damage), the sensitive membrane body should be replaced.

When replacing the electrolyte and sensitive membrane, the following points should be followed:

1. Hold the electrode with your hand in a vertical position, with the membrane facing downwards, and unscrew the old membrane;

2. Like shaking a thermometer, shake off residual electrolyte;

3. Clean the membrane body, anode and cathode electrodes, and electrode inner body with clean water, and air dry or wipe dry with soft paper, both of which should not contain water droplets;

4. Visually inspect whether the O-ring is damaged, and if so, replace it.

5. Drop the new original electrolyte into the inner cavity of the (new) membrane body. Do not fill too much (as long as the corresponding space between the electrode inner body and the membrane body is filled), which takes up most of the space of the entire sensitive membrane body.

6. Grasp the electrode vertically and slowly tighten the membrane upwards, making sure to rotate twice and retract once to ensure there are no air bubbles mixed in. Excess electrolyte will seep out, wipe it clean. The membrane should be easily fitted with an O-ring. If it needs to be used before fitting or cannot be fitted at all, it is because the membrane has not been installed correctly and needs to be reinstalled.

7. After replacing the electrolyte or membrane, it should be repolarized and recalibrated.