An electrolytic cell for determining chemical oxygen demand

Abstract

The present invention relates to an electrolytic cell for determining chemical oxygen demand, and belongs to the field of environmental monitoring. In the electrolytic cell, the sample to be tested is injected from a sample inlet located at the front end of the liquid inlet, and flows into an electrolytic reaction tank with an open top along with the electrolyte from the liquid inlet below, and passes through a porous working electrode located in the middle of the reaction tank and an auxiliary electrode and a reference electrode arranged closely above it. Organic matter in the sample to be tested is oxidized and degraded by electrocatalysis on the porous working electrode loaded with an active electrocatalyst, and the generated electrical signal is used for quantitative determination of chemical oxygen demand. In the present invention, the penetrable porous working electrode can enhance the mass transfer of organic matter and improve the efficiency of electrocatalytic oxidation; the reaction tank with an open top is conducive to the discharge of bubbles, avoiding the interference of bubbles on the reaction and current signals; the smaller inter-electrode distance between the auxiliary electrode and the reference electrode and the working electrode can ensure a higher electric field strength and current efficiency. The above characteristics enable a more stable and reliable electrical signal to be obtained when the electrolytic cell is used for chemical oxygen demand determination, thereby realizing efficient and accurate determination of chemical oxygen demand in the sample to be tested.

Description

Technical Field

[0001] The invention belongs to the field of environmental monitoring and relates to an electrolytic cell for measuring chemical oxygen demand. Background Art

[0002] Chemical oxygen demand refers to the amount of reducing substances that need to be oxidized in water samples measured by chemical methods. It is mainly used to measure the concentration of organic pollutants in water and is an important monitoring and evaluation indicator in the field of water environment protection and water pollution control. For more than a century, the internationally accepted method for determining chemical oxygen demand has been the standard potassium dichromate reflux titration method. However, this method has the disadvantages of being complex and time-consuming, difficult to automatically detect online, large errors in measuring low-concentration chemical oxygen demand, and the need to use a large amount of multiple toxic reagents.

[0003] The electrocatalytic oxidation method does not require the use of toxic reagents. It can mineralize organic pollutants by using hydroxyl free radicals with strong oxidizing ability generated by electrolysis. The chemical oxygen demand of the water sample to be tested can be accurately calculated based on the correlation between the electrical signal and the pollutants. Based on this principle, the electrocatalytic oxidation method has the advantages of being environmentally friendly, fast, highly sensitive, and free of secondary pollution. It is in line with the needs of my country's ecological civilization construction and environmental protection industry upgrades, and has extremely high application potential.

[0004] The electrolytic cell is one of the cores of the determination of chemical oxygen demand based on the electrocatalytic oxidation method. Its main functions are two: one is to ensure that the pollutants react fully, and the other is to obtain accurate and stable electrical signals. The Chinese invention patent (application number: 200510023445.2) discloses a chemical oxygen demand electrochemical analyzer. In the analysis and detection device, the carrier direction in the electrolytic cell is parallel to the surface of the working electrode, and the organic matter diffuses to the surface of the working electrode with the carrier current, thereby reacting and generating an electrical signal. However, the contact between the liquid and the electrode in the electrolytic cell is not sufficient, which is not conducive to the electrocatalytic oxidation and degradation of organic matter in the sample. In the determination device disclosed in the Chinese invention patent (application number 200810010232.X) "A device and method for determining chemical oxygen demand in water based on flow injection sampling", the boron-doped diamond working electrode is fixed on the liquid flow channel, and a protrusion that changes the direction of the fluid is provided opposite to it, so that the liquid can change direction when flowing through the working electrode, thereby enhancing the contact between the liquid and the surface of the working electrode. In addition, in a device for monitoring the chemical oxygen demand of river water disclosed in the Chinese utility model patent (application number 201420232345.5) "Device for monitoring chemical oxygen demand by electrochemical method", the water inlet is arranged lower than the water outlet and the working electrode protrudes from the pipe wall, so that the liquid and the electrode are in more complete contact.

[0005] Although the existing disclosed patents have made several improvements to promote sufficient contact between the liquid and the electrode in the electrolytic cell, which is beneficial to enhancing the oxidation process of organic matter in the sample, there are still some problems: (1) The working electrodes used are all planar structures. When the electrode area is fixed, the loading density of the catalytically active substance is limited, which is not conducive to the detection of samples with higher concentrations; (2) The working electrodes used are all placed parallel to the liquid flow channel. Only part of the sample close to the electrode in the liquid can diffuse to the surface of the working electrode in time and oxidize, which is not conducive to test reproducibility and result reliability; (3) The working electrodes are mostly in contact with the liquid downward. When electrocatalytically oxidizing organic matter, the bubbles generated will adhere to the electrode surface, hindering the contact between the reactants in the liquid and the electrode. The lower flow rate set for sufficient reaction will aggravate this problem; (4) The distance between the working electrode and the reference electrode, and the distance between the working electrode and the auxiliary electrode cannot be controlled within a small range, resulting in problems such as low applied potential and energy efficiency. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides an electrolytic cell for efficiently and accurately measuring chemical oxygen demand, so as to solve the problems of narrow measurement range, poor reproducibility, low energy efficiency, etc. existing in measuring chemical oxygen demand using the existing electrolytic cell.

[0007] The electrolytic cell comprises an electrolytic reaction tank with an open top, a porous working electrode, an auxiliary electrode, a reference electrode, a liquid inlet, a liquid outlet and a sample inlet. The porous working electrode is placed in the electrolytic reaction tank with an open top, and the auxiliary electrode and the reference electrode are placed above the porous working electrode; the liquid inlet is located below the electrolytic reaction tank with an open top, and the liquid outlet is located above the electrolytic reaction tank with an open top; the liquid flows into the electrolytic reaction tank with an open top from the liquid inlet, flows through the electrolytic reaction tank with an open top from bottom to top, passes through the porous working electrode, and flows out from the liquid outlet. The liquid passes through the porous working electrode from bottom to top to enhance the mass transfer of organic matter on the electrode surface, so that the organic matter in the water sample to be tested fully contacts and reacts with the catalytic active components on the surface of the working electrode, and improves the oxidation efficiency of the organic matter, thereby broadening the measurement range of chemical oxygen demand and improving the accuracy and sensitivity of the measurement. The reaction tank with an open top helps to quickly discharge the bubbles generated during the electrocatalytic oxidation reaction, reducing the adhesion of the bubbles on the electrode surface and the disturbance to the electrical signal, thereby obtaining a more stable and reliable electrical signal and chemical oxygen demand measurement value.

[0008] Preferably, the porous working electrode is a combination of one or more porous electrodes of the same or different thicknesses. The porous working electrode has a high specific surface area, which can promote full contact between organic matter and catalytically active components in the sample, and can also increase the loading density of catalytically active components and increase catalytically active sites.

[0009] Preferably, the catalytically active component on the porous working electrode is one or more of non-metallic boron-doped diamond, nitrogen-doped diamond, boron-nitrogen co-doped diamond, metal Pt, Au, Ir, Pd, or metal oxides SnO2, PbO2, TiO2, IrO2, RuO2. The catalytically active component has the advantages of stable physical and chemical properties, high conductivity, and high catalytic oxidation efficiency of organic matter.

[0010] Preferably, the porous working electrode and the auxiliary electrode are placed in parallel, and the vertical distance between the two is 0.1 to 3 cm; the distance between the porous working electrode and the reference electrode is 0.1 to 1 cm. This arrangement is conducive to forming a more uniform electric field distribution and a higher electric field intensity between the porous working electrode and the auxiliary electrode, thereby improving the catalytic oxidation effect of organic matter in the electrolytic cell and improving the accuracy of chemical oxygen demand determination.

[0011] Preferably, the electrolytic reaction tank with an open top is a cylindrical cavity with a volume of 50 μL to 50 mL.

[0012] Preferably, the sample inlet is arranged at the front end of the liquid inlet, and the injection volume is 5 to 200 μL.

[0013] Preferably, the residence time of the liquid in the electrolytic reaction tank with an open top is 10 s to 10 min.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1) The penetration design allows the liquid to pass through the porous working electrode, increasing the contact between the organic matter and the active sites of the catalyst, accelerating mass transfer, and improving the degree of oxidation of the organic matter, thereby improving the accuracy of the chemical oxygen demand measurement;

[0016] 2) The open-top design facilitates the timely discharge of bubbles generated during the electrocatalytic oxidation process, which helps to obtain more accurate and stable electrical signals;

[0017] 3) The micro-compact design can reduce the inter-electrode distance, effectively improve the electric field strength between electrodes and the uniformity of electric field distribution, which is beneficial to improving the efficiency of electrocatalytic oxidation of organic matter. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of the electrolytic cell of the present invention.

[0019] Figure 2 Schematic diagram of the structure of an electrolytic cell for determining chemical oxygen demand in Example 1 of the present invention.

[0020] Figure numerals: 1-electrolysis reaction cell with an open top; 2-porous working electrode; 3-auxiliary electrode; 4-reference electrode; 5-liquid inlet; 6-liquid outlet; 7-sample inlet; 8-1, 8-2, 8-3-cylindrical cavity; 9-0.1M Na2SO4 electrolyte inlet; 10-test water sample inlet; 11-hollow support layer; 12-multilayer porous carbon paper electrode loaded with {001} crystal plane exposed TiO2 single crystals; 13-platinum wire electrode; 14-Ag / AgCl (sat.KCl) electrode; 15-liquid outlet; 16-fastening screw hole. DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not used to limit the present invention.

[0022] like Figure 1 As shown, an electrolytic cell for determining chemical oxygen demand provided by the present invention comprises an electrolytic reaction tank 1 with an open top, a porous working electrode 2, an auxiliary electrode 3, a reference electrode 4, a liquid inlet 5, a liquid outlet 6 and a sample inlet 7. The porous working electrode 2 is placed in the electrolytic reaction tank 1 with an open top, and the auxiliary electrode 3 and the reference electrode 4 are placed above the porous working electrode 2; the porous working electrode 2, the auxiliary electrode 3 and the reference electrode 4 are respectively connected to the electrochemical workstation through electrode clips; the liquid inlet 5 is located below the electrolytic reaction tank 1 with an open top, and the liquid outlet 6 is located above the electrolytic reaction tank 1 with an open top; the liquid flows from the liquid inlet 5 into the electrolytic reaction tank 1 with an open top, flows through the electrolytic reaction tank 1 with an open top from bottom to top, passes through the porous working electrode 2, and flows out from the liquid outlet 6.

[0023] The porous working electrode 2 is a combination of one or more porous electrodes of the same or different thicknesses, and its catalytically active components are non-metallic boron-doped diamond, nitrogen-doped diamond, boron-nitrogen co-doped diamond, metal Pt, Au, Ir, Pd, or one or more of metal oxides SnO2, PbO2, TiO2, IrO2, and RuO2 with electrocatalytic oxidation performance. The porous working electrode 2 has a large specific surface area and abundant catalytic active sites. The liquid passes through the porous working electrode 2 from bottom to top to enhance the mass transfer of organic matter on the electrode surface, so that the organic matter in the water sample to be tested fully contacts and reacts with the catalytically active components on the surface of the porous working electrode 2, thereby improving the oxidation efficiency of the organic matter, thereby broadening the measurement range of chemical oxygen demand and improving the accuracy and sensitivity of the measurement. The reaction tank 1 with an open top helps to quickly discharge the bubbles generated during the electrocatalytic oxidation reaction, reduce the adhesion of bubbles on the electrode surface and the disturbance of the electrical signal, thereby obtaining a more stable and reliable electrical signal and chemical oxygen demand measurement value.

[0024] The distance between the porous working electrode 2 and the auxiliary electrode 3 is 0.1 to 3 cm, and the distance between the porous working electrode 2 and the reference electrode 4 is 0.1 to 1 cm. A smaller inter-electrode distance is conducive to forming a more uniform electric field distribution and a higher electric field strength between the porous working electrode 2 and the auxiliary electrode 3, thereby improving the catalytic oxidation effect of organic matter in the electrolytic cell and improving the accuracy of chemical oxygen demand determination.

[0025] The electrolytic reaction tank 1 with an open top is a cylindrical cavity with a volume of 50 μL to 50 mL.

[0026] The sample inlet 7 is arranged at the front end of the liquid inlet 5, and the injection volume is 5-200 μL to ensure that the trace sample is fully mixed in the electrolyte and then uniformly contacts and reacts with the catalytic active components of the porous working electrode 2.

[0027] The residence time of the liquid in the electrolytic reaction tank 1 with an open top is 10s to 10min. The flow rate of the liquid can be adjusted according to the concentration range of the sample to be tested and the catalytic performance of the porous working electrode 2, thereby changing the residence time of the liquid in the electrolytic reaction tank 1 with an open top.

[0028] Embodiment 1

[0029] A preferred electrolytic cell for determining chemical oxygen demand and a method for using the electrolytic cell are further described below.

[0030] like Figure 2 As shown, the electrolytic cell shell is made of three acrylic plates, including a cylindrical reaction tank with an open top assembled from cylindrical cavities 8-1, 8-2, and 8-3, a 0.1M Na2SO4 electrolyte inlet 9, a water sample inlet 10 to be tested, a hollow support layer 11, a multi-layer porous carbon paper electrode 12 loaded with {001} crystal plane exposed TiO2 single crystals, a platinum wire auxiliary electrode 13, an Ag / AgCl (sat.KCl) reference electrode 14, a liquid outlet 15, and a fastening screw hole 16.

[0031] A multilayer porous carbon paper electrode 12 loaded with {001} crystal plane exposed TiO2 single crystals is placed on a hollow support layer 11 and connected to an electrode clamp of an external electrochemical workstation through copper conductive glue or copper wire; the hollow support layer 11 is located in the middle of the cylindrical cavity 8-1, and a platinum wire auxiliary electrode 13 is fixed parallel to the porous carbon paper electrode 12, and the vertical distance between the two electrodes is 0.3 cm; the vertical distance from the lower end of the Ag / AgCl (sat.KCl) reference electrode 14 to the porous carbon paper electrode 12 is 0.2 cm; the 0.1M Na2SO4 electrolyte inlet 9 is located on the side of the cylindrical cavity 8-1, and the liquid outlet 15 is located on the side of the cylindrical cavity 8-3, and ensures that the platinum wire auxiliary electrode 13 is completely immersed in the liquid. The water sample to be tested flows into the electrolytic reaction tank from the water sample inlet 10 located at the front end of the electrolyte inlet 9, passes through the porous carbon paper electrode 12, and flows out from the liquid outlet 15. By enhancing mass transfer, the liquid and the porous carbon paper electrode 12 are fully contacted and reacted.

[0032] The electrolytic cell for determining chemical oxygen demand is used, and 0.1M Na2SO4 electrolyte is injected into the electrolytic cell at a constant flow rate of 10 μL / s through a flow injector, so that the residence time of the liquid in the cylindrical reaction tank with an open top is 3 minutes. After the liquid fills the entire pipeline and runs stably, the electrochemical workstation connected to the electrolytic cell is started, and a potential of +1.4V is applied to the porous carbon paper electrode 12 used, and the generated background current signal is collected in real time. When the background current signal is stable, the sample to be tested is injected through a micro-injector placed at the water sample inlet 10 to be tested, and the chemical oxygen demand value is obtained by processing the generated current signal.

Claims

1. An electrolytic cell for determining chemical oxygen demand, characterized in that It comprises an electrolytic reaction tank (1) with an open top, a porous working electrode (2), an auxiliary electrode (3), a reference electrode (4), a liquid inlet (5), a liquid outlet (6) and a sample inlet (7); The porous working electrode (2) is placed in the electrolytic reaction tank (1) with an open top, and the auxiliary electrode (3) and the reference electrode (4) are placed above the porous working electrode (2); the liquid inlet (5) is located below the electrolytic reaction tank (1) with an open top, and the liquid outlet (6) is located above the electrolytic reaction tank (1) with an open top; the liquid flows into the electrolytic reaction tank (1) with an open top from the liquid inlet (5), flows through the electrolytic reaction tank (1) with an open top from bottom to top, passes through the porous working electrode (2), the auxiliary electrode (3) and the reference electrode (4) in sequence, and flows out from the liquid outlet (6); The porous working electrode (2) and the auxiliary electrode (3) are placed in parallel, and the vertical distance between the two is 0.1 to 3 cm; the distance between the porous working electrode (2) and the reference electrode (4) is 0.1 to 1 cm.

2. The electrolytic cell according to claim 1, characterized in that One or more porous working electrodes (2) with the same or different thicknesses are used in combination.

3. The electrolytic cell according to claim 1 or 2, characterized in that The catalytically active component on the porous working electrode (2) is one or more of non-metallic boron-doped diamond, nitrogen-doped diamond, boron-nitrogen co-doped diamond, metal Pt, Au, Ir, Pd, or metal oxides SnO2, PbO2, TiO2, IrO2, RuO2.

4. The electrolytic cell according to claim 1, characterized in that The electrolytic reaction tank (1) with an open top is a cylindrical cavity with a volume of 50 μL to 50 mL.

5. The electrolytic cell according to claim 1, characterized in that The sample inlet (7) is arranged at the front end of the liquid inlet (5), and the injection volume is 5 to 200 μL.

6. The electrolytic cell according to claim 1, characterized in that The residence time of the liquid in the electrolytic reaction tank (1) with an open top is 10 seconds to 10 minutes.

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