Method for measuring dissolved oxygen in aquaculture tail water based on iodine-ascorbic acid-potassium dichromate system spectrophotometric method

Abstract

The invention discloses a method for measuring dissolved oxygen in aquaculture tail water by a spectrophotometric method based on an iodine-ascorbic acid-potassium dichromate system, and belongs to the technical field of water quality detection. The method comprises the following steps: sequentially adding a manganese sulfate solution and an alkaline potassium iodide solution into a to-be-detected culture tail water sample, sealing a container, fully reacting, and standing after the reaction is finished to completely precipitate Mn2O3; adding a citric acid solution, sealing, reacting, completely dissolving the precipitate and releasing the iodine elementary substance, adding water and an ascorbic acid-potassium dichromate mixed color developing agent into the solution after the reaction is finished, fixing the volume, sealing, uniformly mixing, and carrying out a color developing reaction in a dark place; a blank sample is used for reference, the culture tail water subjected to color development is placed in a sample pool of a spectrophotometer, the spectrophotometric value of the sample is measured at the wavelength of 360 nm, and the dissolved oxygen concentration is calculated according to a standard curvilinear equation. The method provided by the invention reduces the detection limit of the traditional spectrophotometric method, and has the advantages of high sensitivity, strong anti-interference capability, high speed and rapidness in operation, wide application range, greenness and environmental protection.

Description

Technical Field

[0001] The invention belongs to the technical field of water quality detection, and particularly relates to a method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system. Background Art

[0002] In the aquaculture industry, the quality of aquaculture tailwater directly impacts the health and profitability of aquacultured organisms. Accurate measurement of dissolved oxygen (DO), a core indicator of tailwater quality, is crucial. Accurately understanding the DO content in tailwater helps optimize stocking density, adjust the operation of aeration equipment, and prevent the death of aquacultured organisms due to lack of oxygen. It also provides guidance for the proper discharge and subsequent treatment of tailwater.

[0003] Among the current mainstream detection methods, the iodine titration method has a complex operating process, the titration process is prone to errors, and the standard solution has poor stability; the electrode method is easily interfered with by various chemical substances in aquaculture tailwater, such as sulfide and heavy metal ions, and requires frequent calibration and maintenance. Although spectrophotometry has the potential to be fast and automated, existing related technologies have problems such as insufficient sensitivity and weak anti-interference ability in aquaculture tailwater detection, making it difficult to meet the needs of high-precision and real-time detection of dissolved oxygen in complex water environments. Aquaculture tailwater often contains a large amount of organic matter, nitrite, ferrous ions and other substances. These interfering substances will seriously affect the accuracy of traditional methods for determining dissolved oxygen, making it difficult to meet the needs of high-precision and real-time detection of dissolved oxygen in aquaculture tailwater. Existing methods often involve sulfuric acid, sodium hydroxide or advanced laboratory equipment, which is not environmentally friendly and increases the difficulty and cost of experiments, while also increasing the difficulty and risk of experiments.

[0004] Existing methods have poor adaptability to different water types (surface water, aquaculture water, aquaculture tailwater, etc.), lack effective interference elimination mechanisms, and cannot guarantee the validity of test results in various scenarios. The dissolved oxygen content in tailwater discharged from fish, shrimp, and crab aquaculture ponds is affected by factors such as stocking density, feed intake, and aquatic plant growth. Furthermore, marine aquaculture tailwater presents even more interfering factors due to its complex composition, including various salts and trace elements.

[0005] Therefore, there is an urgent need to develop a new method for the detection of dissolved oxygen in aquaculture tail water that is efficient, accurate, and has strong anti-interference ability. Summary of the Invention

[0006] In response to the problems of cumbersome operation, large errors and weak anti-interference ability of existing methods, the present invention provides a method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on the iodine-ascorbic acid-potassium dichromate system. This method reduces the detection limit of traditional spectrophotometric detection, and has high sensitivity, strong anti-interference ability, high speed and fast operation, wide application range, and is green and environmentally friendly.

[0007] The present invention is achieved through the following technical solutions: The present invention discloses a method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system, comprising the following steps: (1) Sample pretreatment: Add manganese sulfate solution and alkaline potassium iodide solution to the aquaculture tail water sample to be tested in sequence, seal the container and allow it to react fully. After the reaction is completed, let it stand to allow Mn2O3 to precipitate completely; (2) Color development reaction: After the complete precipitation of Mn2O3 in step (1), add citric acid solution, seal and react to completely dissolve the precipitate and release iodine. After the reaction, add water and ascorbic acid-potassium dichromate mixed color developer to the solution to make it equal to the volume. After sealing, mix evenly and place in a dark place to carry out color development reaction. (3) Spectrophotometry: Using the blank sample as a reference, the aquaculture tail water after color development in step (2) is placed in the sample cell of the spectrophotometer, and the sample spectrophotometry value is measured at a wavelength of 360 nm. The sample spectrophotometry value is substituted into the standard curve equation to calculate the dissolved oxygen concentration.

[0008] Furthermore, in step (1), before adding the manganese sulfate solution, the contents of nitrite and ferrous ion in the aquaculture tail water sample to be tested are tested, and according to the contents of nitrite and ferrous ion, excess aminosulfonic acid or p-aminobenzenesulfonic acid solution is added in sequence to eliminate the interference of nitrite, and excess o-phenanthroline solution is added to eliminate the interference of ferrous ion.

[0009] Furthermore, after adding aminosulfonic acid or p-aminobenzenesulfonic acid to the aquaculture tail water sample to be tested, the container is sealed and reacted for 5 to 10 minutes to eliminate the interference of nitrite; after adding o-phenanthroline, the container is sealed and reacted for 5 to 10 minutes to eliminate the interference of ferrous ions.

[0010] Furthermore, the concentration of the aminosulfonic acid or p-aminobenzenesulfonic acid solution is 10 wt %, and the concentration of the o-phenanthroline solution is 0.1 mol / L.

[0011] Furthermore, the concentration of the manganese sulfate solution in step (1) is 1 mol / L, the alkaline potassium iodide solution is an NH3•H2O-KI solution, and each liter of NH3•H2O-KI solution contains 150 g of KI and 350 mL of a 20% ammonia solution; the preparation method of the ascorbic acid-potassium dichromate mixed color developer in step (2) is to weigh 0.5 g of ascorbic acid and 0.1 g of potassium dichromate, add 500 mL of 0.1 mol / L citric acid solution to dissolve them, and obtain the ascorbic acid-potassium dichromate mixed color developer after complete dissolution. Furthermore, in step (1), 1 mL of manganese sulfate solution and 2 mL of alkaline potassium iodide solution are sequentially added to 50 mL of the aquaculture tail water to be tested; after the Mn2O3 is completely precipitated in step (2), 2 mL of 0.1 mol / L citric acid solution is added, and the solution after the reaction is completed is transferred to a 100 mL volumetric flask, and water and 10 mL of ascorbic acid-potassium dichromate mixed color developer are added to make up the volume.

[0012] Furthermore, the operation for sufficient reaction in step (1) is to mix by inversion 20 times, and to stand for 10 minutes after sufficient reaction; the reaction time after adding citric acid in step (2) is 5 minutes, and the time for color development reaction is 15 minutes.

[0013] Furthermore, the blank sample in step (3) is prepared by taking an amount of water equal to the aquaculture tail water sample to be tested in step (1) and treating it in the same manner as in steps (1) and (2).

[0014] Furthermore, the method for drawing the standard curve equation in step (3) is to provide different water samples, determine the spectrophotometric value using steps (1) to (3), determine the dissolved oxygen concentration using the iodine titration method, and obtain the standard curve by fitting with the spectrophotometric value as the horizontal axis and the dissolved oxygen concentration as the vertical axis.

[0015] Furthermore, the standard curve equation established in step (3) is y = 3.4541x - 2.8747, where x is the spectrophotometric value and y is the dissolved oxygen concentration.

[0016] Furthermore, in step (1), various reagents are added below the liquid level of the aquaculture tail water sample to be tested, and the container is immediately sealed after the addition is completed; after the manganese sulfate solution in step (1) is prepared, the bubbles dissolved in the solution are removed by ultrasonication.

[0017] In the present invention, after collecting aquaculture tailwater samples, an appropriate amount of aminosulfonic acid or para-aminobenzenesulfonic acid is added. Under acidic conditions, nitrite reacts with aminosulfonic acid according to the reaction equation: HNO2 + NH2SO3H = N2↑ + H2SO4 + H2O. The generated nitrogen escapes the system, thereby converting nitrite into a harmless substance, preventing it from undergoing a redox reaction with potassium dichromate and interfering with dissolved oxygen measurement. If para-aminobenzenesulfonic acid is used, it will undergo a diazotization reaction with nitrite in an acidic environment, similarly eliminating nitrite interference with subsequent measurements.

[0018] In the present invention, before adding the manganese sulfate solution, o-phenanthroline is first added to the aquaculture tail water sample. Under the condition of pH value of 3-9, ferrous ions can form a stable orange-red complex with o-phenanthroline. The reaction principle is based on the formation of a coordination bond between the nitrogen atom in the o-phenanthroline molecule and the ferrous ion. The reaction equation can be simply expressed as: Fe 2+ + 3C12 H8N2= [Fe(C 12 H8N2)3] 2+ Due to the formation of a stable complex, the redox activity of ferrous ions is significantly reduced and they can no longer react with potassium dichromate, thus effectively eliminating the interference of ferrous ions on dissolved oxygen determination.

[0019] In the present invention, interference from suspended particulate matter is eliminated: for water bodies containing a large amount of suspended particulate matter, it can be pre-treated by filtering with a 0.45μm filter membrane after sampling; for water bodies with high concentrations of colored substances, the interference of color on spectrophotometric determination is reduced by adding a blank sample subtraction combined with dilution determination.

[0020] The beneficial effects achieved by the present invention are: (1) The present invention constructs a spectrophotometric method for determining dissolved oxygen in aquaculture tail water based on the iodine-ascorbic acid-potassium dichromate system, which uses NH3•H2O-KI to replace the traditional NaOH-KI to fix dissolved oxygen, and uses the buffering effect of ammonia to optimize the reaction environment and reduce the metal ions (such as Fe 3+ 、Cu 2+ ) and OH - The formation of precipitation interferes with the determination; ascorbic acid-potassium dichromate dual-function colorimetric reagent accelerates the conversion of I2 to I3 - The conversion enhances the selectivity and stability of the oxidation reaction, and produces a characteristic absorption peak at a wavelength of 360nm. For the common interfering substances such as nitrite and ferrous ion in aquaculture tail water, aminosulfonic acid or p-aminobenzenesulfonic acid is used to eliminate the interference of nitrite, and o-phenanthroline is used to complex ferrous ions to ensure the accuracy of detection.

[0021] (2) The present invention adopts a one-step acidification-simultaneous color development technology to combine the precipitation dissolution and color development reaction, reducing the number of operating steps and reagent consumption, improving the reaction repeatability, and integrating the interference elimination step into the pretreatment without adding additional complex operations. By accurately optimizing the ratio of each reactant and the reaction conditions, the reaction specificity and sensitivity are enhanced, forming a stable and quantifiable detection signal for the accurate determination of dissolved oxygen in aquaculture tail water.

[0022] (3) When the present invention detects dissolved oxygen in aquaculture tail water, there is no need to manually adjust the pH of the solution, and the accuracy of the detection result of the present invention will not be affected.

[0023] (4) The method of the present invention has a low detection limit, a high apparent molar absorptivity, a small measurement error, and the entire detection process takes less than 40 minutes. It is easy to operate and is suitable for on-site rapid detection of aquaculture tail water and laboratory batch analysis. It is in line with the concept of green chemistry and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the standard curve of spectrophotometry value-dissolved oxygen concentration. DETAILED DESCRIPTION

[0025] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0026] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0027] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0028] The reagents used in the examples and comparative examples of the present invention are: (1) Manganese sulfate solution (1 mol / L): Prepare a 1 mol / L manganese sulfate solution using a volumetric flask. Transfer the prepared manganese sulfate solution to an ultrasonic cleaner and ultrasonicate for 10 minutes to eliminate dissolved bubbles in the solution. This is because the presence of bubbles may affect the full contact between the reagent and the water sample during the subsequent reaction, thereby interfering with the accuracy of the experimental results. After the ultrasonication is completed, transfer the solution to a clean, dry, and sealed reagent bottle for future use.

[0029] (2) Alkaline potassium iodide (NH3•H2O-KI) solution: First, weigh 150g of KI (accurate to 0.1g) and dissolve it in 500mL of distilled water. Stir continuously during the dissolution process to accelerate the dissolution of KI. After the KI is completely dissolved, slowly add 350mL of 20% concentrated ammonia water (perform the operation in a fume hood to avoid inhaling the irritating gas emitted by concentrated ammonia water). Stir while adding to mix the solution evenly. Then transfer the solution to a 1000mL volumetric flask and dilute to the mark with distilled water. The alkaline potassium iodide solution must be prepared and used immediately. Before use, take a small amount of the solution into a test tube, add appropriate amounts of starch solution and dilute citric acid solution, and observe whether the solution turns blue. If the solution turns blue, it indicates that there are oxidizing impurities in the solution, which will interfere with subsequent experiments and need to be re-prepared. If the solution does not turn blue, it indicates that the solution has good reducing properties and no obvious impurity interference, and can be used.

[0030] (3) Ascorbic acid-potassium dichromate mixed color developer: Accurately weigh 0.5g ascorbic acid (accurate to 0.0001g) and 0.1g potassium dichromate (accurate to 0.0001g), place them in a clean beaker, add 500mL 0.1mol / L citric acid solution to the beaker, and stir to completely dissolve the ascorbic acid and potassium dichromate; transfer the dissolved solution to a brown reagent bottle. Because ascorbic acid has strong reducing properties and is easily oxidized under light conditions, it needs to be stored away from light; the ascorbic acid-potassium dichromate mixed color developer should be re-prepared every 7 days to ensure its chemical stability and the reliability of the experimental results.

[0031] (4) Aminosulfonic acid solution (10%): Weigh 10 g of aminosulfonic acid (accurate to 0.1 g), dissolve it in 100 mL of distilled water, and stir until completely dissolved. This solution must be prepared immediately before use, because if the aminosulfonic acid solution is left for too long, it may decompose or react with substances in the air, affecting its ability to eliminate nitrite interference.

[0032] (5) 1.98 g of ... 12 H8N2•H2O, accurate to 0.001g), first place it in a beaker, add a small amount of ethanol, stir to dissolve the 1,1-phenanthroline, after the 1,1-phenanthroline is completely dissolved, dilute it with distilled water to 100mL, transfer it to a reagent bottle and store it for later use.

[0033] The ultraviolet-visible spectrophotometer of the present invention is model UV-5100, produced by Shanghai Yuanxi Instrument Co., Ltd.

[0034] Example 1 (1) Sample pretreatment: Take 50.00 mL of the water sample to be tested (use a 50 mL pipette to transfer, rinse the pipette 2-3 times with the water sample to be tested before transfer to ensure the accuracy of the transfer volume) into a 250 mL iodine volumetric flask; 1) If nitrite is present in the water sample based on prior knowledge of the aquaculture tailwater or preliminary testing, first add sulfamic acid solution or, based on the estimated nitrite content, solid p-aminobenzenesulfonic acid (accurately weighed using an analytical balance). After adding the reagent, immediately secure the stopper on the iodine volumetric flask, shake thoroughly to allow the reagent and water sample to fully mix, and allow the reaction to proceed for 10 minutes. During this process, place the iodine volumetric flask on an oscillator to accelerate the reaction rate and ensure complete reaction. 2) If ferrous ions are present in the water sample, add o-phenanthroline solution (measure accurately using a pipette) after the nitrite reaction is complete. Again, tightly cap the bottle and shake well to allow the o-phenanthroline to fully complex with the ferrous ions. The complexation reaction should last for 5 minutes. 3) If it is determined that the water sample does not contain nitrite or ferrous ions in the early stage, then the corresponding nitrite and / or ferrous ion interference removal operation will not be performed, and step 4) can be directly performed; 4) Pipette the solution below the liquid surface (avoiding contact between the solution and air to prevent oxygen in the air from dissolving into the water sample and affecting the dissolved oxygen determination), and then add 1.0 mL of manganese sulfate solution (accurately measured using a 1 mL pipette) and 2.0 mL of alkaline potassium iodide solution (accurately measured using a 2 mL pipette). After the additions are complete, quickly secure the stopper to the iodine flask. Hold the iodine flask by hand and invert it 20 times to allow the solution to react fully. Let the iodine flask stand for 10 minutes to allow the generated Mn2O3 precipitate to settle completely to the bottom of the flask. 5) Simultaneously prepare a blank sample: Take 50.00 mL of distilled water (using the same pipette size as used for the aquaculture tail water sample) and place it in a 250 mL iodine volumetric flask. Follow the above steps for the aquaculture tail water sample, adding nitrite and ferrous ion interference elimination reagents (if no interfering substances are present, the interference elimination reagents can be omitted), adding manganese sulfate solution and alkaline potassium iodide solution, mixing, and allowing to stand. This will be used for background subtraction in the subsequent detection process to eliminate the influence of factors such as reagents and experimental operations on the test results. (2) Color reaction 1) After the Mn2O3 precipitate has completely settled, tilt the iodine flask and slowly add 2.0 mL of 0.1 mol / L citric acid solution along the flask wall. Immediately stopper and shake well. Gently shake the iodine flask to completely dissolve the precipitate and release the iodine. Allow to react for 5 minutes. During this process, the solution will gradually darken in color, indicating the formation of iodine. 2) Transfer the solution from step 1) to a 100mL volumetric flask (use a glass rod to drain to ensure complete transfer). Rinse the iodine volumetric flask several times with small amounts of distilled water (approximately 10-15mL of distilled water each time, for three rinses). Transfer the rinse solution to the volumetric flask. Add 10.0mL of ascorbic acid-potassium dichromate color developer (use a 10mL pipette to accurately measure). Fill the volume to the mark with distilled water, ensuring that the concave meniscus of the solution is tangent to the mark. Once filled, secure the stopper on the volumetric flask and shake it upside down to mix the solution. Then, place the volumetric flask in a dark place for 15 minutes to allow the color development reaction to proceed. (3) Spectrophotometry 1) Before performing spectrophotometric measurements, preheat the spectrophotometer for at least 30 minutes to ensure stable operation. Then, perform wavelength and baseline calibration using a standard wavelength calibration sheet to ensure the measurement wavelength is accurately 360 nm. Perform baseline calibration by measuring a blank solvent (distilled water) to achieve a spectrophotometer reading of 0. 2) Using the prepared blank sample as a reference, use a 1 cm quartz cuvette; place the cuvette in the sample cell of the spectrophotometer and measure the spectrophotometric value at a wavelength of 360 nm; measure each sample three times in parallel, with an interval of 1-2 minutes between each measurement. Record the data after the instrument reading stabilizes; take the average of the three measurement results as the spectrophotometric value of the sample to reduce experimental error.

[0035] Example 2 Drawing of standard curve: Ordinary ponds, intertidal zones, aquaculture water, and tailwater from grouper aquaculture farms were selected as research subjects (eight samples were taken from each type of water), and measurements and experiments were conducted. The nitrite and ferrous ion levels in ordinary pond water were 0.01 mg / L and 0.05 mg / L, respectively; the nitrite and ferrous ion levels in intertidal zones were 0.02 mg / L and 0.1 mg / L, respectively; the nitrite and ferrous ion levels in aquaculture water were 0.03 mg / L and 0.1 mg / L, respectively; and the nitrite and ferrous ion levels in tailwater from grouper aquaculture farms were 0.6 mg / L and 0.05 mg / L, respectively. First, the iodine titration method according to the national standard (GB 7489-87) was used to accurately test the dissolved oxygen concentration of each water sample and record the dissolved oxygen value (dissolved oxygen concentration). Subsequently, the same batch of water samples was measured again using the spectrophotometry method described in Example 1 of the present invention, wherein the amount of aminosulfonic acid solution added was 2 mL, and the amount of o-phenanthroline solution added was 3 mL, and the spectrophotometric value of each sample was recorded; The spectrophotometric values ​​and dissolved oxygen values ​​of the four types of water bodies measured using the above method were recorded, and the results are shown in Table 1 below: Table 1 Spectrophotometric values ​​and dissolved oxygen values ​​(dissolved oxygen concentration) of four types of water bodies According to the test data in Table 1, the standard curve is fitted with the spectrophotometric value (x) as the horizontal axis and the dissolved oxygen value (y) as the vertical axis, as shown in the following example: Figure 1 As shown, the equation is y=3.4541x-2.8747 (R² =0.9778).

[0036] Example 3 Surface water samples were collected from a farm in the East (the contents of nitrite and ferrous ions in the surface water samples were 0.6 mg / L and 0.05 mg / L, respectively). The water samples were sealed and stored away from light, and the test was completed within 2 hours.

[0037] (1) Detection Steps Referring to Example 1, the amount of aminosulfonic acid solution added was 2 mL, and the amount of o-phenanthroline solution added was 3 mL. The detected spectrophotometric value was 3.213. Substituting the standard curve in Example 2, the dissolved oxygen concentration was calculated to be 8.22 mg / L.

[0038] (2) The result of simultaneous determination by iodine titration was 8.13 mg / L, with a small relative error, proving the accuracy of this method.

[0039] Comparative Example 1 The same water sample as in Example 3 was used, except that no sulfamic acid solution and o-phenanthroline solution were added to remove interference factors in Comparative Example 1. The remaining steps were the same as in Example 4. The detected spectrophotometric value was 4.587. When the standard curve in Example 2 was applied, the dissolved oxygen concentration was calculated to be 12.97 mg / L, indicating that nitrite and ferrous ions had a relatively large impact on the test results.

[0040] In summary, the method of the present invention has the following advantages: (1) significant performance advantages: the detection limit is 0.01 mg / L, which is more sensitive than the traditional spectrophotometric method and meets the requirements of dissolved oxygen detection. It can still maintain high-precision determination in water samples containing common interfering ions. It can meet the requirements of dissolved oxygen detection in aquaculture tail water and can still maintain high-precision determination in aquaculture tail water samples containing common interfering ions. (2) strong anti-interference ability: through reagent system optimization and blank correction, it effectively eliminates the interference of common reducing substances (such as sulfide, nitrite) and metal ions in water bodies. (3) efficient and convenient operation: the entire detection process takes less than 40 minutes and does not require professional skills to operate. It is suitable for rapid detection on the aquaculture site and batch analysis in the laboratory. The interference elimination steps are simple and easy to use, and do not significantly increase the operation time, making it convenient for aquaculture personnel to quickly obtain test results. (4) wide range of applications: it has been successfully applied to the detection of different types of water bodies such as surface water in river basins, aquaculture water, and aquaculture tail water, and the measurement results are consistent with the national standard method (relative error <5%). It can also accurately measure dissolved oxygen in water samples with high interfering ion content, such as complex aquaculture tailwater (such as high-density aquaculture and aquaculture tailwater with serious water quality deterioration), providing reliable data support for aquaculture production management and tailwater treatment.

Claims

1. A method for determining dissolved oxygen in aquaculture tail water using spectrophotometry using an iodine-ascorbic acid-potassium dichromate system, characterized in that: The following steps are involved: (1) Sample pretreatment: Add manganese sulfate solution and alkaline potassium iodide solution to the aquaculture tail water sample to be tested in sequence, seal the container and allow it to react fully. After the reaction is completed, let it stand to allow Mn2O3 to precipitate completely; (2) Color development reaction: After the complete precipitation of Mn2O3 in step (1), add citric acid solution, seal and react to completely dissolve the precipitate and release iodine. After the reaction, add water and ascorbic acid-potassium dichromate mixed color developer to the solution to make it equal to the volume. After sealing, mix evenly and place in a dark place to carry out color development reaction. (3) Spectrophotometry: Using a blank sample as a reference, place the aquaculture tail water after color development in step (2) in the sample cell of a spectrophotometer and measure the sample spectrophotometry value at a wavelength of 360 nm; substitute the sample spectrophotometry value into the standard curve equation to calculate the dissolved oxygen concentration; In step (1), the alkaline potassium iodide solution is an NH3•H2O-KI solution, wherein each liter of the NH3•H2O-KI solution contains 150 g of KI and 350 mL of a 20 wt% ammonia solution; Step (1) Before adding the manganese sulfate solution, the contents of nitrite and ferrous ion in the aquaculture tail water sample to be tested are tested, and according to the contents of nitrite and ferrous ion, excess aminosulfonic acid or p-aminobenzenesulfonic acid solution is added in turn to eliminate the interference of nitrite and excess o-phenanthroline solution to eliminate the interference of ferrous ion.

2. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein After adding aminosulfonic acid or p-aminobenzenesulfonic acid to the aquaculture tail water sample to be tested, the sealed container is reacted for 5 to 10 minutes to eliminate the interference of nitrite; after adding o-phenanthroline, the sealed container is reacted for 5 to 10 minutes to eliminate the interference of ferrous ions.

3. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein The concentration of the aminosulfonic acid or p-aminobenzenesulfonic acid solution is 10wt%, and the concentration of the o-phenanthroline solution is 0.1mol / L.

4. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on the iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein The concentration of the manganese sulfate solution in step (1) is 1 mol / L; the preparation method of the ascorbic acid-potassium dichromate mixed color developer in step (2) is as follows: weigh 0.5 g of ascorbic acid and 0.1 g of potassium dichromate, add 500 mL of 0.1 mol / L citric acid solution to dissolve, and after complete dissolution, obtain the ascorbic acid-potassium dichromate mixed color developer.

5. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein In step (1), 1 mL of manganese sulfate solution and 2 mL of alkaline potassium iodide solution are sequentially added to 50 mL of aquaculture tail water to be tested; in step (2), after Mn2O3 is completely precipitated, 2 mL of 0.1 mol / L citric acid solution is added. After the reaction is completed, the solution is transferred to a 100 mL volumetric flask, and water and 10 mL of ascorbic acid-potassium dichromate mixed color developer are added to make up the volume.

6. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on the iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein The operation for sufficient reaction in step (1) is to mix by inversion 20 times, and then let it stand for 10 minutes after sufficient reaction; the reaction time after adding citric acid in step (2) is 5 minutes, and the time for color development reaction is 15 minutes.

7. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on the iodine-ascorbic acid-potassium dichromate system according to claim 1, characterized in that: The preparation method of the blank sample in step (3) is as follows: take an amount of water equal to the aquaculture tail water sample to be tested in step (1) and treat it in the same way as steps (1) and (2).

8. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein: The method for drawing the standard curve equation in step (3) is to provide different water samples, determine the spectrophotometric value using steps (1) to (3), determine the dissolved oxygen concentration using the iodine titration method, and obtain the standard curve by fitting with the spectrophotometric value as the horizontal axis and the dissolved oxygen concentration as the vertical axis.

9. The method for determining dissolved oxygen in aquaculture tail water by spectrophotometry based on an iodine-ascorbic acid-potassium dichromate system according to claim 1, wherein: In step (1), various reagents are added below the liquid level of the aquaculture tail water sample to be tested, and the container is immediately sealed after the addition is completed; after the manganese sulfate solution in step (1) is prepared, the bubbles dissolved in the solution are removed by ultrasonication.

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