A method for measuring the thickness of diffusion boundary layer
By configuring electrolyte solutions of different concentrations in a measuring device for diffusion boundary layer thickness and recording voltage data using a displacement platform, the problem of difficult to measure the diffusion boundary layer thickness in the prior art is solved, and the measurement effect with high accuracy and convenient operation is achieved.
Patent Information
- Application Number
- CN202210754972.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-06-30
- Publication Date
- 2025-04-18
- Estimated Expiration
- 2042-06-30
AI Technical Summary
It is difficult for the prior art to effectively measure the magnitude of the diffusion boundary layer thickness in the case of concentration differences.
By configuring two electrolyte solutions of different concentrations, a measuring device for the thickness of the diffusion boundary layer is used, including two sinks placed side by side, an ion exchange membrane, an indicator electrode and a fixed electrode, the indicator electrode is translated using a displacement platform, voltage data is recorded, and the voltage change curve with displacement is calculated by calculating the average value of the voltage and drawing the change curve of the voltage with displacement, the distance from the point where the potential difference changes from non-zero to zero to the film is taken as the thickness of the diffusion boundary layer.
It realizes simple and efficient measurement of the diffusion boundary layer thickness in the case of concentration differences, improving the accuracy of measurement and convenience of operation.
Smart Images
Figure CN115078181B_ABST
Abstract
Description
Technical Field
[0001] The invention belongs to the technical field of electrochemistry, and in particular relates to a method for measuring the thickness of a diffusion boundary layer. Background Art
[0002] Reverse electrodialysis is a method of generating electricity by utilizing salt difference. A general reverse electrodialysis system is composed of anion and cation exchange membranes, anion and anode electrodes, separators, external loads, concentrated solutions and dilute solutions. It is a technology that generates electricity by controlling the mixing of two water bodies with different salt concentrations. In recent years, the reverse electrodialysis method has been widely studied at home and abroad, with the main purpose of improving the power density of the device.
[0003] The concept of diffusion boundary layer was proposed in the system with ion exchange membrane. In the membrane-solution system, because the ion exchange membrane selectively transmits cations or anions, the number of ion migration in the membrane is greater than the number of ion migration in the solution, which causes the concentration of the solution near the membrane to change, forming a diffusion boundary layer with a certain thickness. In the reverse electrodialysis power generation process, the diffusion boundary layer is equivalent to a non-ohmic resistor, and its existence will reduce the power of the device.
[0004] The research on the thickness of the diffusion boundary layer mainly focuses on the case where the electrolyte solution on both sides of the membrane has the same concentration. The diffusion boundary layer is generated by applying a current signal, and the thickness of the diffusion boundary layer is calculated using a theoretical formula. At present, there is no simple and effective method to describe the thickness of the diffusion boundary layer in the case of concentration difference. Summary of the invention
[0005] The purpose of the present invention is to provide a method for measuring the thickness of a diffusion boundary layer, which solves the problem in the prior art that the thickness of the diffusion boundary layer is difficult to calculate under the condition of concentration difference.
[0006] The technical solution adopted by the present invention is a method for measuring the thickness of a diffusion boundary layer, which is characterized by being implemented in the following steps:
[0007] Step 1, preparing two electrolyte solutions of different concentrations;
[0008] Step 2, injecting the two electrolyte solutions of different concentrations prepared in step 1 into two water tanks of a device for measuring the thickness of the diffusion boundary layer respectively, and running for a period of time;
[0009] Step 3, after the liquid surface is stabilized, the displacement platform of the diffusion boundary layer thickness measuring device is used to translate the indicating electrode for a fixed distance each time, and the voltage data at each position is recorded using LabVIEW software in the computer;
[0010] Step 4: Use EXCLE software in the computer to calculate the average voltage, and use Origin to draw the curve of voltage versus displacement;
[0011] Step 5: Observe the changing trend of the curve and take the distance from the point where the potential difference changes from non-zero to zero to the membrane as the diffusion boundary layer thickness.
[0012] The present invention is also characterized in that:
[0013] In step 1, the two electrolyte solutions of different concentrations are NaCl solutions in any proportion, and the water used is deionized water.
[0014] The specific structure of the device for measuring the thickness of the diffusion boundary layer is as follows: it includes two water tanks placed in parallel, the water tanks are filled with electrolyte, and the inner walls of the adjacent sides of the two water tanks are provided with ion exchange membranes. An indicator electrode and a fixed electrode are placed in one of the water tanks, and the indicator electrode and the fixed electrode are connected to a digital multimeter, and the digital multimeter is connected to a computer.
[0015] A silicone gasket is installed between the two sinks.
[0016] The indicator electrode and the fixed electrode are both microelectrodes. The indicator electrode is fixed on the displacement platform through a fixed bracket, and the fixed electrode is placed on a horizontal plane through the fixed bracket to keep the device horizontal and stable.
[0017] The indicator electrode and the fixed electrode are both drawn from glass capillaries. A columnar Ag / AgCl electrode wire is inserted into the glass capillary as an inner electrode, and KCl filling solution is passed into the interior.
[0018] The inner diameter of the glass capillary is 0.4mm-0.8mm, the end of the glass capillary in contact with the solution to be tested is drawn into a sharp needle shape, the diameter of the Ag / AgCl electrode wire is 0.3mm-0.5mm, and the KCl filling solution is a saturated KCl solution.
[0019] The distance between the fixed electrode and the ion exchange membrane is 500um-800um.
[0020] The running time in step 2 is 30s-60s, the initial position of the indicator electrode is close to the surface of the ion exchange membrane and is at the same depth as the fixed electrode, and the displacement platform moves 10μm-50um each time.
[0021] The data recording time at each position in step 3 is 30s-50s.
[0022] The beneficial effect of the present invention is that after the two sides of the ion exchange membrane are additionally provided with silica gel gaskets, they are sandwiched between two water tanks, and then electrolyte solutions of different concentrations are injected into the two water tanks to form a simple reverse electrodialysis device. During the reverse electrodialysis process, the solution concentration difference drives the ions to move from the concentrated chamber to the dilute chamber. Since the ion exchange membrane selectively transmits cations or anions, the number of ion migrations in the membrane is greater than the number of ion migrations in the solution, thereby causing the concentration of the solution near the membrane to change and form a diffusion boundary layer. Therefore, outside the diffusion boundary layer, the ion concentration is almost equal everywhere, and within the diffusion boundary layer, the ion concentration changes continuously. Accordingly, a fixed electrode and an indicator electrode are placed on one side of the solution, and a displacement platform is used to slowly translate the indicator electrode. A digital multimeter is used to measure the voltage data between the two electrodes, and the voltage data is collected using LabVIEW software in a computer and a curve of the change of voltage with displacement is plotted. The distance from the indicator electrode to the membrane when the potential difference changes from non-zero to zero is taken as the thickness of the diffusion boundary layer. The device is simple and easy to operate. It not only reflects the entire reverse electrodialysis process, but also can intuitively obtain the diffusion boundary layer thickness through the voltage data of the digital multimeter and the displacement data of the displacement platform. The comprehensive consideration of voltage and displacement improves the accuracy of measurement. BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Figure 1 is a schematic diagram of a device for measuring the thickness of the diffusion boundary layer;
[0024] Figure 2 This is the principle diagram of diffusion boundary layer measurement;
[0025] Figure 3 The voltage of the dilute chamber and the concentrated chamber changes with displacement when the concentration ratio is 30:1.
[0026] In the figure, 1. water tank, 2. indicating electrode, 3. fixed electrode, 4. displacement platform, 5. fixed bracket, 6. digital multimeter, 7. computer, 8. ion exchange membrane, 9. silicone gasket. DETAILED DESCRIPTION
[0027] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.
[0028] The device for measuring the thickness of the diffusion boundary layer in the present invention has a structure as follows: Figure 1 , Figure 2 As shown, it includes two water tanks 1 placed in parallel, the water tanks 1 are filled with electrolyte, and the inner walls of the adjacent sides of the two water tanks 1 are provided with ion exchange membranes 8. An indicator electrode 2 and a fixed electrode 3 are placed in one of the water tanks 1, and the indicator electrode 2 and the fixed electrode 3 are both connected to a digital multimeter 6, and the digital multimeter 6 is connected to a computer.
[0029] A silica gel gasket 9 is installed between the two water tanks 1 .
[0030] The indicator electrode 2 and the fixed electrode 3 are both microelectrodes. The indicator electrode 2 is fixed on the displacement platform 4 through the fixed bracket 5, and the fixed electrode 3 is placed on a horizontal plane through the fixed bracket 5 to keep the device horizontal and stable.
[0031] The indicator electrode 2 and the fixed electrode 3 are both drawn from glass capillaries. A columnar Ag / AgCl electrode wire is inserted into the glass capillary as an inner electrode. KCl filling liquid is passed into the interior to keep the inner electrode in a stable working environment.
[0032] The inner diameter of the glass capillary is 0.4mm-0.8mm, the end of the glass capillary in contact with the solution to be tested is drawn into a sharp needle shape, the diameter of the Ag / AgCl electrode wire is 0.3mm-0.5mm, and the KCl filling solution is a saturated KCl solution.
[0033] The distance between the fixed electrode 3 and the ion exchange membrane 8 is 500um-800um.
[0034] In the present invention, the water tank is designed to be open at the top, and there are water channels at the upper and lower ends of the tank. The electrolyte solution can be poured directly from the top; it can also be introduced through a pump to study the effect of flow rate on the thickness of the diffusion boundary layer.
[0035] A method for measuring the thickness of a diffusion boundary layer is based on a device for measuring the thickness of a diffusion boundary layer and is specifically implemented according to the following steps:
[0036] Step 1: Prepare two electrolyte solutions of different concentrations; in step 1, the two electrolyte solutions of different concentrations are NaCl solutions of any proportion, and the water used is deionized water. (The greater the concentration difference, the better the experimental effect, and the ratio of this concentration depends on the actual situation of the experiment or project).
[0037] Step 2, injecting the two electrolyte solutions of different concentrations prepared in step 1 into two water tanks 1 respectively, and running for a period of time;
[0038] In step 2, the water tank is cleaned 1 to 2 times with the prepared electrolyte solution before measurement; the running time is 30s-60s.
[0039] Step 3: After the liquid surface is stabilized, the indicator electrode 2 is translated by the displacement platform 4 for a fixed distance each time, and the voltage data at each position is recorded by the LabVIEW software in the computer; the data recording time at each position is 30s-50s.
[0040] In step 3, the fixed electrode 3 remains in the same position, and the distance from the ion exchange membrane 8 is 500um-800um; the indicator electrode 2 is initially located close to the surface of the ion exchange membrane 8 and is at the same depth as the fixed electrode 3, and the displacement platform moves 10μm-50um each time. (The smaller the moving distance each time, the more accurate the data obtained, but if the moving distance is too small, the requirements for the experimental environment are higher, generally 10μm-50um.
[0041] Step 4: Use EXCLE software in the computer to calculate the average voltage, and use Origin to draw the curve of voltage versus displacement;
[0042] Step 5: Observe the changing trend of the curve and take the distance from the point where the potential difference changes from non-zero to zero to the membrane as the diffusion boundary layer thickness.
[0043] The above method is used to obtain the measurement principle of the diffusion boundary layer thickness during reverse electrodialysis. Figure 3 As shown. On the concentrated chamber and dilute chamber sides, since the ion exchange membrane selectively transmits cations or anions, the number of ion migrations in the membrane is greater than the number of ion migrations in the solution, which causes the concentration of the solution near the membrane to change and form a diffusion boundary layer. Outside the diffusion boundary layer, the ions are evenly distributed and the concentration curve is approximately a straight line, while the ion concentration in the diffusion boundary layer is constantly changing. The fixed electrode is fixed outside the diffusion boundary layer at a distance of 500μm from the membrane, and the indicator electrode moves from the membrane surface to the fixed electrode. When the indicator electrode is in the diffusion boundary layer, there is a concentration difference between the two electrodes, and the potential between the two electrodes is not zero; as the indicator electrode moves out of the diffusion boundary layer, the concentration difference between the two electrodes becomes 0, and the potential between the two electrodes also becomes 0. Combined with the displacement data, the thickness of the diffusion boundary layer is obtained. Specific embodiment 1:
[0045] Use ASTOM cation exchange membrane, concentration ratio is 30:1, no flow rate. Fix the membrane between two water tanks, then rinse the concentration chamber with concentrated and diluted solutions 1-2 times respectively, fix the fixed electrode and indicator electrode and put them into the concentration chamber, then connect the two electrodes to the digital multimeter, and connect the digital multimeter to the computer. Then pour the prepared solution into the water tank, and start the measurement after the liquid level stabilizes. First, record the voltage within 30s at the displacement of 0, then slowly move the displacement platform 10μm, and record the voltage at this position within 30s. The measurement ends when the displacement is 500μm, that is, the fixed electrode and the indicator electrode are in the same position. Then use EXCLE software to calculate the average voltage within 30s at each position, use Origin to draw the curve of voltage versus displacement, and take the displacement corresponding to the first time the voltage becomes 0 as the diffusion boundary layer thickness. The dilute chamber is the same as the concentrated chamber.
[0046] The obtained curves of the voltage change of the concentrated chamber and the dilute chamber with displacement are as follows: Figure 3As shown. It can be seen from the figure that the voltage in the dilute chamber decreases with the increase of displacement. When the displacement is 330μm, the first voltage is 0, and the thickness of the diffusion boundary layer is 330μm. Compared with the dilute chamber, the concentrated chamber has a higher initial voltage, and the voltage changes with displacement in a similar trend. When the displacement is 350μm, the first voltage is 0, and the thickness of the diffusion boundary layer is 350μm. Further analysis shows that in the diffusion boundary layer, the concentration distribution of the concentrated and dilute chambers is roughly the same, with the highest concentration near the membrane and gradually decreasing as it moves away from the membrane. The thickness of the diffusion boundary layer in the concentrated and dilute chambers shows that the concentration has little effect on the thickness of the diffusion boundary layer.
[0047] The present invention can conveniently and intuitively measure the thickness of the diffusion boundary layer by combining the numerical change of the digital multimeter with the displacement of the micro-motion platform. The voltage change of the digital multimeter can also reflect the change of ion concentration, and obtain the ion concentration distribution in the water tank driven by concentration difference. The device of the present invention is simple, easy to operate, and suitable for various concentration difference conditions. At the same time, the influence of flow velocity on the diffusion boundary layer can be studied.
Claims
1. A method for measuring the thickness of a diffusion boundary layer, characterized in that: A device for measuring the thickness of a diffusion boundary layer is provided. The device comprises two water tanks (1) placed in parallel. The water tanks (1) are filled with electrolyte. An ion exchange membrane (8) is provided on the inner wall of one side of the two water tanks (1). An indicator electrode (2) and a fixed electrode (3) are placed in one of the water tanks (1). The indicator electrode (2) is fixed on a displacement platform (4) via a fixed bracket (5). The indicator electrode (2) and the fixed electrode (3) are both connected to a digital multimeter (6). The digital multimeter (6) is connected to a computer. The method is specifically implemented according to the following steps: Step 1, preparing two electrolyte solutions of different concentrations; Step 2, injecting the two electrolyte solutions of different concentrations prepared in step 1 into two water tanks (1) of a device for measuring the thickness of a diffusion boundary layer respectively, and running the solution for a period of time; Step 3: After the liquid surface is stabilized, the indicator electrode (2) is translated by a fixed distance each time using the displacement platform (4) of the diffusion boundary layer thickness measuring device, and the voltage data at each position is recorded using LabVIEW software in the computer; Step 4: Use EXCLE software in the computer to calculate the average voltage, and use Origin to draw the curve of voltage versus displacement; Step 5: Observe the changing trend of the curve and take the distance from the point where the potential difference changes from non-zero to zero to the membrane as the diffusion boundary layer thickness.
2. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: In step 1, the two electrolyte solutions of different concentrations are NaCl solutions in any proportion, and the water used is deionized water.
3. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: A silica gel gasket (9) is installed between the two water tanks (1).
4. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: The indicator electrode (2) and the fixed electrode (3) are both microelectrodes, and the fixed electrode (3) is placed on a horizontal plane via a fixed bracket (5) to keep the device horizontal and stable.
5. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: The indicator electrode (2) and the fixed electrode (3) are both drawn from a glass capillary, a columnar Ag / AgCl electrode wire is inserted into the glass capillary as an inner electrode, and a KCl filling solution is passed into the interior.
6. The method for measuring the thickness of a diffusion boundary layer according to claim 5, characterized in that: The inner diameter of the glass capillary is 0.4 mm-0.8 mm, the end of the glass capillary in contact with the solution to be tested is drawn into a sharp needle shape, the diameter of the Ag / AgCl electrode wire is 0.3 mm-0.5 mm, and the KCl filling solution is a saturated KCl solution.
7. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: The fixed electrode (3) is 500 um to 800 um away from the ion exchange membrane (8).
8. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: The running time in step 2 is 30s-60s, and in step 3, the position of the fixed electrode (3) remains unchanged, and the distance from the ion exchange membrane (8) is 500um-800um; the initial position of the indicator electrode (2) is close to the surface of the ion exchange membrane (8) and is at the same depth as the fixed electrode (3), and the displacement platform moves 10μm-50um each time.
9. The method for measuring the thickness of a diffusion boundary layer according to claim 1, characterized in that: The data recording time at each position in step 3 is 30s-50s.