About dissolved oxygen sensor in water – Part 3

1.2 How does the dissolved oxygen sensor work

Detection principle Introduction Advantages and disadvantages
Laboratory methods: Manual colorimetry, Winker method or iodometric method Under alkaline conditions, divalent manganese hydroxide is oxidized to tetravalent manganese by dissolved oxygen in water, and the hydroxide precipitate is formed to realize the fixation of oxygen, but in acidic solution, the generated tetravalent manganese compound can also oxidize KI And precipitation I2. The number of moles of iodine precipitated is equal to the number of moles of dissolved oxygen in water, so it can be titrated with a standard solution of sodium thiosulfate. According to the amount of sodium thiosulfate, the dissolved oxygen content in water can be calculated. Although this method has high accuracy, the operation process is complicated and the degree of automation is low, making it difficult to realize the online detection of dissolved oxygen.
Diaphragm electrode method Diaphragm galvanic cell method The shell end of the electrode uses a breathable diaphragm (polyethylene or polytetrafluoroethylene) with high dissolved oxygen permeability, and the positive and negative electrodes and the internally charged electrolyte are sealed in the shell. The positive electrode is a precious metal such as platinum, gold , and silver, and the negative electrode is an easily oxidized metal such as lead and aluminum. The electrolyte is usually sodium hydroxide and potassium chloride solution. The electrode with this structure is immersed in the detection solution. If the measurement circuit is connected, the dissolved oxygen in the solution enters the diaphragm through the diaphragm, and the following reactions occur at the two electrodes electrode.Positive reaction O2+2H2O+4e——→4OH— Negative reaction 2Pb—→2Pb2++4e—2Pb2++4OH——→2Pb(OH)2 2Pb(OH)2+2KOH—→2KHPbO2+2H2O The above reaction is Therefore carried out under the condition of using lead as the negative electrode and potassium hydroxide as the electrolyte. Because under certain conditions, the size of the passing current is related to the concentration of dissolved oxygen. Therefore, the concentration of dissolved oxygen can be obtained by measuring the current. The two electrodes of the galvanic cell are composed of two different metals that can spontaneously polarize to generate a voltage. Since the voltage of the diaphragm battery is generated spontaneously rather than externally provided, the diaphragm battery electrode can be used without the “preheating” required for polarographic electrode polarization.
Diaphragm Polarography The diaphragm polarographic dissolved oxygen electrode uses platinum (Pt) as the cathode, Ag/AgCl as the anode, the electrolyte is 0.1mol/L potassium chloride (KCl), and the silicone rubber permeable membrane is used as the gas-permeable membrane. During the measurement, a polarization voltage of 0.68V is applied between the anode and the cathode, and oxygen is consumed at the cathode through the permeable membrane. The amount of oxygen permeating the membrane is proportional to the dissolved oxygen concentration in the water, so the limiting diffusion current between the electrodes is proportional to the dissolved oxygen in the water. The concentration is proportional, the meter detects this current and converts it into oxygen concentration by operation. At the same time, the thermistor detects the temperature of the solution and performs temperature compensation for the oxygen concentration. The current during air calibration is generally 50 to 200nA. The diaphragm polarographic electrode needs to be polarized from the input voltage of the meter. Since the applied voltage may take 15 minutes to stabilize, the diaphragm polarographic electrode usually needs to be preheated before use to ensure that the electrode can be properly polarized.
Latest Online Technology: Fluorescence Fluorescent substance molecules have a ground state (spin pairing), a singlet state (diamagnetism), a triplet state (paramagnetism), and the like. The ground state is stable and has the lowest energy. Singlet and triplet states are high in energy and both have different vibrational energy levels. When the molecules, atoms or ions of the luminescent substance absorb the same frequency light radiation and resonate, the electron energy state transitions from the stable ground state to the singlet state. Molecules in the excited state return to the lowest-energy ground state through nonradiative transitions, and in the process generate radiation in the form of photons, resulting in fluorescence. Due to the existence of vibrational relaxation, the energy of the emitted light must be lower than that of the excitation light, so the frequency decreases and the wavelength increases, resulting in a Stokes red shift. The loss of energy is characterized by fluorescence efficiency: factors affecting the fluorescence efficiency of photoluminescence include excitation light intensity, dielectric constant of the solvent, system temperature, pH value, and some fluorescence quenching effects, including collision quenching, energy transfer, oxygen Quenching, self-quenching, etc. Among them, oxygen molecules have been confirmed to have a quenching effect on some fluorescent substances. This may be due to the interaction between paramagnetic oxygen molecules and fluorescent molecules in excited states, promoting the formation of paramagnetic triplet fluorescent molecules, that is, accelerating intersystem jumping. According to the Stern-Volmer formula: I0/I=τ0/τ=1+Ksv[Q] Among them, I0 and I are the luminous intensity of the fluorescent substance under anaerobic and aerobic conditions, respectively; τ0 and τ are the fluorescent substance in the absence of oxygen, respectively. The luminescence decay time under oxygen and aerobic conditions; KSV is the Stern-volmer constant, which is related to the detection conditions; [Q] is the oxygen partial pressure. It can be seen from the above formula that oxygen quenching can affect the fluorescence intensity as well as the fluorescence lifetime. Based on the principle of the quenching effect of oxygen molecules on fluorescent substances, the changes in the fluorescence intensity or lifetime of the sample solution can be used to quantitatively determine the content of dissolved oxygen in the sample. The main features of the product are no membrane, no electrolyte, no polarization; no oxygen consumption, not affected by flow rate; built-in temperature sensor, automatic temperature compensation; no interference from chemical substances such as sulfide; small annual drift, fast response, more accurate measurement.

According to the principle of dissolved oxygen sensor

1.2.1 Manual colorimetric detection method (iodometric method) At present, the detection accuracy of this method can reach 0.2 mg/L, and the measurement range is 0-10.0 mg/L. Combined with flow injection analysis technology, the automatic detection of dissolved oxygen can be realized.

1.2.2 Diaphragm electrode method Diaphragm polarography (flow rate and temperature have an effect on it), there are two types of practical membrane electrodes: Polarography and Galvanic Cell. Polarographic dissolved oxygen sensors can be further divided into steady state sensors and fast pulse sensors .

Polarography: In the cathode, a gold ring or platinum ring is used as the cathode, and silver-silver chloride (or mercury-mercurous chloride) is used as the anode. The electrolyte is potassium chloride solution. The outer surface of the electrode is covered with an oxygen-permeable film. The film can be made of breathable materials such as polytetrafluoroethylene, polyvinyl chloride, polyethylene, and silicone rubber. A polarization voltage of 0.5~1.5V is applied between the anode and cathode electrodes. When the dissolved oxygen penetrates the film and reaches the surface of the gold cathode, the following reactions occur on the electrodes:

The cathode is reduced: O2+2H2O+4e→4OHˉ

The anode is oxidized: 4Clˉ+4Ag-4e→4AgCl

As long as the diffusion current is measured, the solubility of dissolved oxygen can be measured. In order to eliminate the influence of factors such as temperature, salinity, and air pressure, relevant techniques are used to compensate.

Galvanic Cell: When the external oxygen molecules enter the electrode through the membrane, the following reactions will occur.

The silver cathode is reduced: O2+2H2O+4e→4OHˉ

Lead anode is oxidized: 2Pb+4KOH+4OHˉ→2KHPbO2+2H2O

Oxygen is reduced to hydroxide ions on the silver cathode, and electrons are obtained from the external circuit at the same time; the lead anode is corroded by potassium hydroxide solution to generate potassium hydrogen lead acid, and electrons are output to the external circuit at the same time. After the external circuit is turned on, there is a signal current passing through, and its value is proportional to the dissolved oxygen solubility.

1.2.3 Fluorescence detection method

The fluorescent dissolved oxygen sensor is based on the quenching principle of active fluorescence by specific substances in physics. The blue light from a light-emitting diode (LED) irradiates the fluorescent material on the inner surface of the fluorescent cap, and the fluorescent material on the inner surface is excited to emit red light. By comparison, the concentration of oxygen molecules is calculated, and the final value is output through automatic compensation of temperature and air pressure.

Fluorescence Dissolved Oxygen The main features of the product are no membrane, no electrolyte, no polarization; no oxygen consumption, not affected by flow rate; built-in temperature sensor, automatic temperature compensation; no interference from chemical substances such as sulfide; small annual drift , fast response, Measurement is more accurate.

Against Fluorescence detection The principle of dissolved oxygen sensor, recommend a representative sensor:

Host

performance parameter
Supply voltage AC220VorDC24V(pick one of two)
Power consumption <2W
show LCDLCD Chinese display/TFTColor LCD (customized version)
Measuring range 0~20mg/L
Resolution 0.01mg/L
precision 2%Calibration accuracy
Operating temperature transmitter-10~65°C
Temperature compensation Automatic temperature compensation
Protection class I65/IP67(Custom Edition)
Signal output 485communication/4-20mAAnalog output/supportHARTprotocol
/NB.GPRS.LORACommunication (optional function)
Installation method wall mount

Sensor

Power supply DC9-DC12V
Protection class ip68
temperature 0-60℃
Sensor material 304Stainless steel
cable length Standard10m (extendable)
Operating temperature 0-60℃
storage temperature -10-60℃
Installation method Bracket drop-in installation

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