Thymol phenolphthalein-TBPE composite indicated cyanide high-precision detection method

Abstract

The invention relates to the technical field of titration detection in analytical chemistry, in particular to a thymol phenolphthalein-TBPE composite indicated cyanide high-precision detection method which comprises the following steps: step 1, after a water sample to be detected is distilled and separated, adjusting the pH value of distillate to a strong alkaline environment required by thymol phenolphthalein color development; 2, adding a composite indicator containing thymol phenolphthalein and tetrabromo phenolphthalein ethyl ester, wherein the mass ratio of the two components in the composite indicator is determined through a color change sensitivity optimization experiment; step 3, titrating with a silver nitrate solution, and judging an end point according to three-order color mutation: the initial state is light yellow, the intermediate state is green, and the end point state is purple red; and 4, adding an auxiliary masking agent before titration, and blocking the reaction between the auxiliary masking agent and silver ions by preferentially complexing interference ions. By optimizing the ratio of the composite indicator and using the masking agent, interference of other substances can be effectively prevented, and accurate detection of cyanide is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of titration detection in analytical chemistry, and in particular to a high-precision cyanide detection method with thymolphthalein-TBPE composite indication. Background Art

[0002] High-precision cyanide detection is of great significance in environmental water safety monitoring and industrial wastewater treatment. Currently, silver nitrate titration is still the mainstream method in grassroots laboratories, but it has the following technical defects that limit detection reliability: 1. Inherent defects of ambiguous endpoint interpretation; The traditional method relies on a single indicator, tetrabromophenolphthalein ethyl ester (TBPE), with an endpoint color change of continuous yellow to purple-red. This color change process has significant drawbacks: The hue transition bandwidth is 30-50nm, and the human eye has difficulty identifying the mutation point; Differences in ambient lighting can lead to interpretation errors of >5% between different operators for the same endpoint; When detecting low concentrations of cyanide (<2 mg / L), background yellow interference further weakens the contrast.

[0003] 2. Limitations of the lack of anti-interference mechanism; Sulfide (S 2- ), thiocyanate (SCN - ) and other interferences, whose competitive reaction with silver ions causes serious deviation of the detection value: Sulfide interference: silver sulfide precipitate is generated (Ksp=6.3×10 -50 ), consumes silver ions, and 1 times the concentration of sulfide can cause a positive error of >20%; Thiocyanate interference: Formation of silver thiocyanate colloid (Ksp=1.1×10 -12 ), wrapping cyanide to hinder the complex reaction; Existing technologies only partially eliminate interference through pre-distillation, and their applicability to high-sulfur wastewater (such as petrochemical wastewater) is extremely low.

[0004] Therefore, there is an urgent need for a high-precision cyanide detection method with thymolphthalein-TBPE composite indicator to solve the above problems. Summary of the Invention

[0005] Based on the above objectives, the present invention provides a high-precision cyanide detection method using a thymolphthalein-TBPE composite indicator, comprising: Step 1: After distilling and separating the water sample to be tested, adjust the pH of the distillate to the strong alkaline environment required for thymolphthalein color development; Step 2: adding a composite indicator containing thymolphthalein and ethyl tetrabromophenolphthalein, wherein the mass ratio of the two components in the composite indicator is determined by a color change sensitivity optimization experiment; Step 3: Titrate with silver nitrate solution and determine the endpoint based on the third-order color change: The initial state is light yellow, the intermediate state is green, and the terminal state is purple-red; Step 4: Add an auxiliary masking agent before titration to block the reaction between the interfering ions and silver ions by preferentially complexing them; in: The green color is formed by the superposition of the blue color of thymolphthalein ionization and the yellow color of ethyl tetrabromophenolphthalein; The purple-red color is generated by the complexation of excess silver ions with ethyl tetrabromophenolphthalein; The auxiliary masking agent has a stronger chelating ability for metal ions than the complexing ability of cyanide.

[0006] Preferably, the optimization process of the mass ratio in step 2 includes: a: Prepare mixed solutions of thymolphthalein and ethyl tetrabromophenolphthalein in various ratios, covering the synergistic range of their color development properties; b: Add each ratio solution to the cyanide standard sample and record the color change process under the same titration conditions; c: Quantify the color difference jump amplitude of each stage through the image analysis system, and select the ratio with the largest total color difference of the three stages: light yellow → green → purple-red; d: Verify the repeatability of endpoint mutations at this ratio and eliminate misjudgment caused by ambient light interference.

[0007] Preferably, the formation of the green color in step 3 must simultaneously meet the following conditions: Thymolphthalein undergoes intramolecular ester hydrolysis in a strong alkaline environment to generate a quinone structure that appears blue; When ethyl tetrabromophenolphthalein is not complexed with silver ions, it retains the yellow chromogenic group of its phenolphthalein skeleton; The solution's light transmittance causes the superposition of blue and yellow light waves to produce a green visual effect; During the titration process, the silver ion concentration did not reach the complexation threshold of ethyl tetrabromophenolphthalein.

[0008] Preferably, the preferential complexation of interfering ions in step 4 is achieved by: Interference with sulfide: The binding constant of ethyl tetrabromophenolphthalein with sulfide ions is higher than the precipitation constant of silver ions with sulfide. The binding strength advantage is verified by competitive binding experiments. Interference with thiocyanate: The binding rate of ethyl tetrabromophenolphthalein to thiocyanate is higher than that of silver ions to thiocyanate. The difference in binding kinetics is determined by retention spectroscopy. Interference with metal ions: The stability constant of the chelate between the auxiliary masking agent and the metal ion is higher than that of the complex between the metal and cyanide. The order of chelation ability is verified by potentiometric titration.

[0009] Preferably, the amount of the auxiliary masking agent added is determined by an interference tolerance threshold experiment, including: a: Prepare simulated water samples containing fixed concentrations of cyanide and gradient concentrations of interfering substances; b: Titrate by increasing the amount of auxiliary masking agent in a gradient manner and record the recovery rate curve; c: The amount of masking agent used when the recovery rate first reaches a stable plateau is taken as the benchmark amount; d: According to the peak concentration of the interfering substance in the actual water sample, the baseline amount is proportionally increased.

[0010] Preferably, the pH critical value verification method of the strongly alkaline environment is: When the concentration is below the critical value, the generation rate of thymolphthalein quinone structure is insufficient, resulting in weak blue coloration; When the concentration is higher than the critical value, the hydroxide ion causes the background of tetrabromophenolphthalein ethyl to deepen; The absorbance jump point of thymolphthalein at characteristic wavelength at different pH was determined by spectrophotometry, and the optimal range was determined comprehensively based on the titration endpoint recognition.

[0011] Preferably, the objectivity of determining the endpoint is achieved by comparing color feature values: Establish a standard color database of three-order color changes, including RGB or Lab color space feature values ​​of light yellow / green / purple red states; During the titration process, the solution image is collected in real time to extract the main color feature value of the current frame; When the feature value matches the purple state database and the Euclidean distance with the green state feature value is greater than the set threshold, it is determined that the end point has been reached.

[0012] Preferably, the specific steps of the competitive binding experiment are: Solutions containing equal concentrations of sulfide ions and silver ions were prepared and divided into two groups: the experimental group added ethyl tetrabromophenolphthalein, and the control group did not add the indicator; The decay curves of free sulfide ion concentration over time in the two groups of solutions were monitored using a silver sulfide selective electrode. Comparing the difference in the slopes of the two groups of curves, the attenuation rate of the experimental group was significantly reduced, which proved that the preferential complexation effect was established.

[0013] Preferably, the criteria for determining the stable plateau phase of recovery rate in the interference tolerance threshold experiment are: The recovery rate fluctuation range under three consecutive masking agent gradients is ≤±2%; The absolute deviation between the recovery rate and the theoretical value is ≤5%; The relative standard deviation of the recovery rate in independent experiments by different operators was ≤1.5%.

[0014] Preferably, the application of the method in the scenario of sulfide and heavy metal synergistic interference includes: When sulfide and iron ions coexist in wastewater, ethyl tetrabromophenolphthalein preferentially fixes sulfide ions and assists the masking agent in chelating iron ions, thus blocking the two from forming ferrous sulfide precipitation and consuming cyanide. When thiocyanate coexists with copper ions, ethyl tetrabromophenolphthalein captures thiocyanate and assists the masking agent in binding to copper ions, preventing the formation of copper thiocyanate colloid encapsulating cyanide.

[0015] Beneficial effects of the present invention: 1. The present invention adopts an indicator compounded of thymolphthalein and ethyl tetrabromophenolphthalein, and by optimizing its mass ratio, the color change is made more obvious. In this process, the synergistic effect of thymolphthalein and ethyl tetrabromophenolphthalein significantly enhances the color contrast, thereby improving the recognizability of the endpoint. The color difference jump is quantified by the image analysis system, further ensuring the high precision of the endpoint reading and reducing human error.

[0016] 2. On this basis, the present invention innovatively introduces the use of auxiliary masking agents, which effectively avoids the influence of interfering substances such as sulfide and thiocyanate by preferentially complexing interfering ions. In particular, the binding properties of ethyl tetrabromophenolphthalein with sulfide and thiocyanate are better than those of silver ions, thereby ensuring that the reaction between cyanide and silver ions is not interfered with. In addition, the present invention also optimizes the usage of the masking agent and determines the optimal dose through interference tolerance threshold experiments, further improving the applicability in complex environments, especially in high-sulfur wastewater and industrial wastewater.

[0017] 3. The present invention optimizes the process of color change by accurately controlling the mass ratio of thymolphthalein and ethyl tetrabromophenolphthalein, so that the color change is not only more obvious but also more stable. During the titration process, the formation of green is the result of the superposition of blue and yellow light waves under the interaction of thymolphthalein and ethyl tetrabromophenolphthalein, which enhances the recognition of color. By this method, the change of silver ion concentration in the titration process can more accurately reflect the content of cyanide, particularly in the detection of low-concentration cyanide, and the interference of background color can be effectively avoided.

[0018] 4. The present invention achieves objective determination of the titration endpoint by establishing a standard color database and a real-time image analysis system. During the titration process, the solution image is collected in real time and the color eigenvalues ​​are extracted. The eigenvalues ​​are compared with the standard color database using RGB or Lab color space. When the eigenvalue matches the purple-red color and the Euclidean distance to the green state eigenvalue is greater than the set threshold, the arrival of the endpoint can be accurately determined. This method effectively avoids the endpoint interpretation errors caused by illumination changes or operator experience differences in traditional methods, greatly improving the stability and accuracy of the test. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0020] Figure 1 is a flow chart of the steps of the method of the present invention; Figure 2 This is a flow chart of the steps of the method for verifying the pH critical value in a strongly alkaline environment according to the present invention; Figure 3 Flow chart of the steps of competitive binding experiment of the method of the present invention. DETAILED DESCRIPTION

[0021] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments. It is also noted that, to provide a more detailed description, the following embodiments are best and preferred embodiments, and those skilled in the art may employ alternative methods for implementing certain known technologies. Furthermore, the accompanying drawings are intended only to provide a more detailed description of the embodiments and are not intended to limit the present invention.

[0022] See Figure 1-Figure 3 The embodiment of the present invention provides a high-precision cyanide detection method with a thymolphthalein-TBPE composite indicator. In step 1, the water sample to be tested is first separated by distillation to remove impurities and volatile components therein, ensuring that the detection result of cyanide is more accurate. The distillate after distillation needs to adjust the pH to the strong alkaline environment required for thymolphthalein to develop color. This process usually requires the use of a strong alkaline solution such as sodium hydroxide or potassium hydroxide. Thymolphthalein can undergo a color change in a strong alkaline environment, thereby providing an obvious color change signal for subsequent titration.

[0023] After step 1 is completed, step 2 adds a composite indicator containing thymolphthalein and ethyl tetrabromophenolphthalein to the test solution. The mass ratio of the composite indicator is determined through a color change sensitivity optimization experiment, with the aim of optimizing the color development effect of the indicator, making the color change more obvious and stable. Thymolphthalein, as an acid-base indicator, can show blue after ionization; ethyl tetrabromophenolphthalein shows yellow. The combination of the two causes the solution to show a significant color change during the cyanide detection process, enhancing the sensitivity of the color mutation and facilitating the accurate judgment of the endpoint.

[0024] In step 3, the test solution is titrated with silver nitrate. During this process, the solution undergoes a three-step color change: initially light yellow, it transitions to green as the titration proceeds, and finally to purple-red, marking the endpoint. This three-step color change (light yellow → green → purple-red) clearly indicates changes in cyanide concentration, making endpoint determination more accurate.

[0025] In step 4, before the titration, an auxiliary masking agent is added to enhance anti-interference capabilities. The auxiliary masking agent preferentially reacts with interfering ions (such as sulfide and thiocyanate), preventing these ions from forming precipitates or colloids with silver ions, thereby avoiding negative impacts on cyanide detection results. The masking agent's chelating ability is stronger than the complexing ability of cyanide with silver ions, ensuring that during the titration, only cyanide forms a complex with silver ions, preventing interfering ions from affecting the silver ions' participation in the reaction.

[0026] In the embodiments of the present invention, the reliability and accuracy of the traditional titration method can be significantly improved, especially when processing complex water samples and low-concentration cyanide detection, it shows good anti-interference ability and operability.

[0027] In one possible embodiment, first, solutions containing various mixtures of thymolphthalein and ethyl tetrabromophenolphthalein were prepared. These mixtures covered the synergistic range of their color development properties, i.e., the experimentally determined range of mixtures that provided optimal color change sensitivity. These solutions served as the basis for subsequent experiments.

[0028] The solutions of varying proportions are then added to a standard sample of known cyanide concentration and titrated under the same titration conditions. The color change from initial light yellow to green and finally to purple-red is recorded during each titration. Real-time recording of these color changes provides data support for subsequent analysis.

[0029] An image analysis system was used to quantify the color jump amplitude for each solution ratio during the titration process. By recording the amplitude and speed of the color change, the amount of color change at each stage could be clearly determined. Ultimately, the optimal ratio was selected, achieving the highest total color difference between light yellow, green, and purple-red, ensuring maximum sensitivity and significance of the color change.

[0030] After selecting the optimal ratio, the repeatability of the endpoint mutation under this ratio needs to be verified through multiple experiments. This step aims to ensure the consistency and stability of the color change process under different experimental conditions, and to eliminate color misinterpretation caused by changes in ambient lighting or other external interference.

[0031] This optimization process provides a scientific basis for determining the mixing ratio of thymolphthalein and ethyl tetrabromophenolphthalein, ensuring that the combination exhibits optimal color development properties in cyanide titration. Through systematic experimental verification and image analysis, the color changes at each stage can be quantified, ensuring that the selected ratio not only provides a clear color change but also exhibits a stable and reliable appearance under different experimental conditions. Furthermore, this optimization method can effectively improve the accuracy of endpoint determination, preventing the titration results from being affected by illumination, experimental error, or other factors, thereby ensuring high precision and repeatability of the test results.

[0032] In one possible embodiment, in a strongly alkaline environment, the ester bonds within the thymolphthalein molecule undergo hydrolysis, causing its molecular structure to change, forming a quinone-like structure. This structure causes thymolphthalein to appear blue. When the cyanide in the water sample is not fully reacted, the thymolphthalein in the solution retains its blue color, providing the basis for the subsequent color change.

[0033] When not complexed with silver ions, ethyl tetrabromophenolphthalein retains the yellow chromogenic group of its phenolphthalein backbone. This yellow group is part of the ethyl tetrabromophenolphthalein molecule and retains its original color until it reacts with other components during the titration process. During the color change process, the yellow ethyl tetrabromophenolphthalein superimposes with the blue of thymolphthalein, producing a green visual effect.

[0034] The solution's light transmittance plays a key role in this process. The blue of thymolphthalein and the yellow of ethyl tetrabromophenolphthalein visually overlap to create a green effect. The intensity of the green color is determined by the solution's light transmittance and the concentration relationship between the two colorants. When the silver ion concentration has not yet reached the complexation threshold of ethyl tetrabromophenolphthalein, the green visual effect is produced by the superposition of blue and yellow light waves.

[0035] During the titration process, when the silver ion concentration is low, ethyl tetrabromophenolphthalein has not yet undergone a complexation reaction with the silver ions, so the yellow group of ethyl tetrabromophenolphthalein is not affected and maintains its original yellow color. In this case, the colors of thymolphthalein and ethyl tetrabromophenolphthalein are superimposed to produce green. If the silver ion concentration gradually increases and reaches the complexation threshold, the yellow group of ethyl tetrabromophenolphthalein will complex with the silver ions, forming a new color reaction, which ultimately leads to a change in the color of the solution.

[0036] By optimizing the reaction between thymolphthalein and ethyl tetrabromophenolphthalein, the color change in cyanide detection is made more obvious and stable, and the interference of environmental and other factors is reduced, thereby improving the accuracy and reliability of cyanide detection.

[0037] In one possible embodiment, the binding constant of ethyl tetrabromophenolphthalein with sulfide ions is higher than the precipitation constant of silver ions with sulfide. This means that during cyanide detection, ethyl tetrabromophenolphthalein has a stronger affinity for sulfide ions and thus preferentially binds to them, thereby preventing interference of sulfide with the complexation of silver ions with cyanide.

[0038] Competitive binding experiments can verify the binding strength of ethyl tetrabromophenolphthalein with sulfide ions. In the experiment, by adding different concentrations of sulfide and observing the effect of sulfide concentration on the silver ion complexation reaction with cyanide, the binding advantage of ethyl tetrabromophenolphthalein with sulfide ions was verified, ensuring that it preferentially complexes sulfide.

[0039] The binding rate of ethyl tetrabromophenolphthalein to thiocyanate is higher than that of silver ions. This means that ethyl tetrabromophenolphthalein reacts faster when binding to thiocyanate, effectively avoiding the competitive reaction between thiocyanate and silver ions and ensuring the accuracy of cyanide detection.

[0040] Dwell time spectroscopy (also known as dwell time analysis) can be used to determine the differences in the binding kinetics of ethyl tetrabromophenolphthalein (TBP) and thiocyanate. By measuring the changes in reaction rates in solutions of varying thiocyanate concentrations, the faster binding of ethyl tetrabromophenolphthalein to thiocyanate is confirmed, ensuring its preferential binding to thiocyanate and eliminating its interference with cyanide detection.

[0041] The chelate stability constant between the auxiliary masking agent and the metal ion is higher than the complex stability constant between the metal and cyanide. This means that the masking agent can effectively bind to the metal ion, thereby inhibiting the complexation of the metal ion with the cyanide and preventing the metal ion from interfering with the cyanide determination.

[0042] Potentiometric titration was used to verify the stability constant of the chelate complex between the auxiliary masking agent and the metal ion, confirming that the masking agent's chelating ability is stronger than the complexing ability of the metal ion and cyanide. In the experiment, by comparing the binding strength of the masking agent to the metal ion with the binding strength of the metal ion and cyanide, it was ensured that the masking agent can effectively mask the metal ion, thereby reducing its interference.

[0043] The embodiments of the present invention effectively improve the accuracy and sensitivity of the high-precision cyanide detection method, enabling the method to handle complex interference environments in practical applications and having high practical value.

[0044] In one possible implementation, a simulated water sample containing a fixed concentration of cyanide and a gradient of interfering substances is first prepared. The purpose of preparing the simulated water sample is to simulate the types and concentrations of interfering substances that may be present in a real environment, ensuring that the experiment covers a variety of possible interference scenarios. The choice of interfering substance and the setting of the concentration gradient should be determined based on common interfering substances and their concentration ranges in actual application scenarios. This step, by simulating the interfering environment, facilitates the determination of the amount of masking agent to be added and its effectiveness.

[0045] Gradually increase the amount of auxiliary masking agent in a simulated water sample and titrate. Record the recovery curve after each increase in masking agent. Recovery typically refers to the ratio of the actual cyanide recovery value to the theoretical value during the cyanide concentration determination. As the masking agent dosage increases, the recovery should gradually stabilize. By recording the recovery curve, you can accurately assess the inhibitory effect of different masking agent dosages on interfering substances.

[0046] Based on the recovery curve, determine the amount of masking agent used when the recovery first reaches a stable plateau. This serves as the baseline amount. The baseline amount is the point at which, after adding masking agent, the recovery no longer changes significantly with increasing masking agent concentration. At this point, the amount of masking agent is sufficient to effectively suppress interfering substances and ensure accurate cyanide measurement. Determining the baseline amount is a critical step in ensuring experimental reliability and accuracy.

[0047] The baseline dose is proportionally scaled based on the peak concentration of the interfering compound in the actual water sample. This adjustment ensures that, regardless of how the interfering compound concentration varies in actual testing, the masking agent dosage can be appropriately adjusted based on the specific conditions of the water sample, ensuring accurate cyanide determination. In practical applications, interfering compound concentrations are often affected by factors such as the water source and the environment. Therefore, scaling the masking agent dosage based on the specific interfering compound concentration in the actual water sample effectively addresses changing experimental conditions.

[0048] Determining the dosage of auxiliary masking agents based on the interference tolerance threshold experiment can not only improve the accuracy and sensitivity of cyanide detection, but also enhance the adaptability of the method and the reliability of the experiment, ensuring that the experiment can run stably in a variety of environments.

[0049] In one possible embodiment, when the pH is below a critical value, the quinone structure formation rate of thymolphthalein is insufficient, resulting in an incomplete color development reaction and a weak blue color. This is because the molecular structure of thymolphthalein changes in a low pH environment, failing to fully convert into a quinone structure, lacking sufficient light absorption capacity, resulting in an unsatisfactory color development effect. Therefore, during the detection process, it is necessary to ensure that the pH value does not fall below a certain critical value to ensure that thymolphthalein can fully develop color.

[0050] When the pH value is above the critical value, the hydroxide ion (OH⁻) concentration is too high and reacts with ethyl tetrabromophenolphthalein, causing the color background to darken. This phenomenon is mainly due to the chemical reaction between ethyl tetrabromophenolphthalein and hydroxide ions under high pH conditions, forming darker compounds, which affect the accuracy of cyanide detection. This darkening of the color background can cause signal interference, affecting the sensitivity and accuracy of the detection.

[0051] Spectrophotometry can measure changes in the absorbance of thymolphthalein at different pH values. Specifically, the absorbance jump at a characteristic wavelength is focused on. This is the point at which thymolphthalein exhibits a significant change at a specific wavelength, typically the wavelength at which the blue color development reaction is strongest. This method can identify changes in the absorbance of thymolphthalein at different pH values, helping to determine the pH range in which thymolphthalein's color development is most pronounced, without interference from a darkening background.

[0052] In addition to measuring absorbance by spectrophotometry, the critical pH range also needs to be determined in combination with the recognition of the titration endpoint. The recognition of the titration endpoint refers to whether the change in the indicator is clear when the endpoint is accurately measured by gradually adding the reagent in the cyanide test. The ideal pH range should ensure that thymolphthalein can show a distinct color change at the endpoint while avoiding background color interference. By comprehensively considering the absorbance jump point and the recognition of the titration endpoint, the optimal pH range can be determined more accurately.

[0053] Based on the above experimental results, an optimal pH range, known as the critical value range, was determined. Within this range, thymolphthalein achieves optimal color development and is unaffected by excessively low or high pH values. Within this pH range, the quinone structure of thymolphthalein is fully formed, resulting in a significant color development effect, while the background color of ethyl tetrabromophenolphthalein is not excessively darkened, ensuring accurate cyanide measurement.

[0054] Through the comprehensive verification of spectrophotometry and titration endpoint recognition, the most suitable pH range for cyanide detection can be accurately determined, thereby improving the accuracy, sensitivity and stability of the high-precision cyanide detection method with thymolphthalein-TBPE composite indicator.

[0055] In one possible implementation, to ensure accurate color determination, a standard color database is first established that contains the color feature values ​​of three common color states in solutions: pale yellow, green, and purple-red. This standard database can use either the RGB color space or the Lab color space to define the color feature values ​​for each color state.

[0056] RGB color space: describes colors by defining the numerical combination of red (R), green (G), and blue (B).

[0057] Lab color space: describes color through three dimensions: brightness (L*), red-green (a*), and yellow-blue (b*), which is closer to the way the human eye perceives color.

[0058] By comparing these two color spaces, accurate matching under different color transformations can be ensured.

[0059] During the titration process for cyanide detection, it's necessary to monitor the color changes of the solution in real time. This is achieved by using a high-quality camera or image acquisition device to capture images of the solution in real time. Whenever a new titration occurs, the system automatically captures an image of the current solution and extracts the dominant color trait within the image.

[0060] The color information in these images will be converted into numerical values ​​in RGB or Lab color space, thus providing a data basis for subsequent color comparison.

[0061] The real-time color feature values ​​of the current frame are compared with the purple-red feature values ​​in the standard color database to determine whether the current solution color matches the purple-red state. This process depends on the degree of matching of the color feature values, which is measured by the Euclidean distance.

[0062] As you can understand, Euclidean distance is a commonly used mathematical metric for calculating the similarity between two color feature values. A threshold is set. When the color feature value extracted in real time matches the feature value of the purple-red state and the Euclidean distance from the feature value of the green state is greater than the set threshold, the endpoint is determined to have been reached.

[0063] The Euclidean distance formula is calculated as follows: ; Where (R1, G1, B1) is the RGB value of the current solution color, and (R2, G2, B2) is the RGB value of the standard purple-red state. When the Euclidean distance between the two colors exceeds the set threshold, it means that the current solution has approached the target color, and the titration endpoint is determined.

[0064] Combining the above process, the system can automatically determine whether the titration has reached the endpoint based on the matching of the characteristic values. When the real-time color characteristic value matches the purple-red state to a certain degree and the Euclidean distance from the green state is greater than the set threshold, the system will determine that the titration endpoint has been reached.

[0065] This determination process avoids the subjectivity of manual observation of color changes and ensures the objectivity and accuracy of endpoint determination.

[0066] By combining color feature value comparison with Euclidean distance calculation, the endpoint determination in cyanide detection can be automated and objectified, greatly improving the accuracy, sensitivity and standardization of detection, and solving the subjective error problem in the traditional manual determination process.

[0067] In one possible embodiment, before the experiment, solutions containing equal concentrations of sulfide ions and silver ions need to be prepared. The purpose of this step is to ensure that the initial conditions of the two groups of solutions are the same, so that the comparative results of the experiment are not affected by the initial concentrations.

[0068] Sulfide ions: Sulfide ions are usually provided by adding sulfide-containing chemicals such as sodium sulfide.

[0069] Silver ions: Silver ions are typically provided by adding a silver chloride solution or a silver salt solution.

[0070] During preparation, the concentrations of sulfide ions and silver ions are required to be equal to ensure that the reaction between silver ions and sulfide ions is balanced during the experiment.

[0071] The prepared solutions were divided into two groups: Experimental group: Ethyl tetrabromophenolphthalein (as an indicator) was added, which has a strong binding affinity with cyanide ions and may affect the competitive binding between sulfide ions and silver ions.

[0072] Control group: No indicator was added and the solution was kept in its original state.

[0073] Through this grouping, the experiment can effectively compare the effects of the indicator on the change of sulfide ion concentration with and without the indicator, thereby proving whether the indicator has an impact on the reaction process.

[0074] During the experiment, a silver sulfide selective electrode was used to monitor the changes in the free sulfide ion concentration in the two groups of solutions over time.

[0075] Silver sulfide selective electrode is an electrode that can specifically detect the concentration of sulfide ions in solution and has high selectivity and sensitivity.

[0076] In the solution, over time, silver ions react with sulfur ions to form silver sulfide precipitates, causing the concentration of free sulfur ions to gradually decrease.

[0077] By measuring the concentration change of free sulfide ions in the solution in real time, the decay curve of sulfide ions in the solution can be obtained.

[0078] Compare the decay curves of the free sulfide ion concentration in the experimental and control solutions. By calculating the slopes of the two sets of curves, their decay rates can be obtained.

[0079] Slope difference: Ethyl tetrabromophenolphthalein was added to the experimental group, which may compete with silver ions or sulfur ions for binding, thereby affecting the rate of sulfur ion concentration decay.

[0080] If the decay rate of the experimental group is significantly lower than that of the control group, it indicates that ethyl tetrabromophenolphthalein has a preferential complexation effect with silver ions, slowing down the decrease in sulfide ion concentration. This indicates that ethyl tetrabromophenolphthalein preferentially binds to silver ions in the competitive binding, thereby affecting the reaction rate of sulfide ions.

[0081] By comparing the decay rates of the two groups, we can verify the effect of ethyl tetrabromophenolphthalein on the reaction between sulfide and silver ions. If the decay rate of the experimental group is significantly lower, it proves that the complexation reaction between ethyl tetrabromophenolphthalein and silver ions occurs preferentially, thus verifying the existence of the preferential complexation effect.

[0082] Competitive binding experiments verified the preferential complexation effect of ethyl tetrabromophenolphthalein in cyanide detection by precisely comparing the sulfide ion concentration decay rates between the experimental and control groups. This method improves the accuracy, sensitivity, and reliability of cyanide detection and has high application value.

[0083] In one possible embodiment, determining the plateau phase of recovery is a key step in conducting an interference tolerance threshold experiment. The purpose of this experiment is to determine the stability and anti-interference ability of the thymolphthalein-TBPE composite indicator in cyanide detection under different masking agent gradients. The stability of the recovery is closely related to the experimental conditions and operating procedures. Specific criteria for determination include: The recovery rate fluctuation range under three consecutive masking agent gradients is ≤±2%: The standard requires that the recovery rate fluctuate within ±2% under gradients of varying masking agent concentrations. This requirement ensures experimental repeatability and consistency, enabling the assay to maintain a relatively stable recovery rate under various interference conditions. By using different gradients of masking agent, the performance of the indicator under different interference conditions can be effectively tested.

[0084] The absolute deviation between the recovery rate and the theoretical value is ≤5%: The standard requires that the recovery rate should not differ from the theoretical value by more than 5%. This is a strict requirement for experimental accuracy. The theoretical value is usually measured using a standard or a cyanide solution of known concentration. An absolute deviation of less than 5% in the recovery rate indicates that the test method has high accuracy and can reliably measure cyanide concentrations.

[0085] The relative standard deviation of the recovery rate of independent experiments by different operators is ≤1.5%: The standard requires that the relative standard deviation of the recovery rate should not exceed 1.5% when the experiment is performed independently by different operators. This requirement ensures the operability of the experiment, especially when the method is repeated and consistent across different laboratories or by different operators. This is crucial for ensuring the reliability and wide applicability of the test results.

[0086] In actual operation, the interference tolerance threshold experiment is usually carried out according to the following steps: Prepare experimental solutions: First, prepare a standard cyanide solution and masking agent solutions of varying concentrations. The type and concentration of the masking agent should be selected based on the experimental design. Commonly used masking agents include metal ions, acids, or bases.

[0087] Add thymophthalein-TBPE composite indicator: Thymolphthalein-TBPE composite indicator is added to a cyanide-containing solution and reacted in the presence of masking agents of different concentrations.

[0088] Monitor recovery changes: Use appropriate detection equipment (such as UV-visible spectrophotometer) to monitor the absorbance change of the reaction solution, calculate the recovery rate and record the data.

[0089] Fluctuations in analytical recovery: Record the recovery rate under different masking agent gradients and determine its fluctuation range to ensure that it is within the standard range of ±2%.

[0090] Repeat the experiment: The experiments were performed independently by different operators to ensure that the relative standard deviation of the recovery rate was less than 1.5%.

[0091] The judgment criteria of the interference tolerance threshold experiment not only improve the stability, accuracy and applicability of the method, but also ensure the repeatability and reliability under different experimental conditions, thereby improving the overall performance of the high-precision cyanide detection method.

[0092] In one possible embodiment, when both sulfide and iron ions are present in wastewater, ethyl tetrabromophenolphthalein acts as a composite indicator by preferentially binding to the sulfide ions. By forming a complex with the sulfide ions, ethyl tetrabromophenolphthalein prevents the sulfide ions from reacting with the iron ions to form ferrous sulfide precipitation, which in turn consumes cyanide.

[0093] Wastewater sample preparation: The wastewater sample to be tested is mixed with ethyl tetrabromophenolphthalein to ensure that ethyl tetrabromophenolphthalein can fully react with sulfide ions.

[0094] Add masking agent: Adding chelating agents or other appropriate masking agents to the sample can react with iron ions to form a chelate reaction, isolating the iron ions from the reaction system and preventing them from forming precipitation with sulfur ions.

[0095] Detection of cyanide: When the sulfur ions are fixed and the iron ions are masked, the concentration of cyanide is monitored by the indicator effect of ethyl tetrabromophenolphthalein, avoiding the problem of ferrous sulfide precipitation consuming cyanide.

[0096] When thiocyanate and copper ions exist simultaneously in wastewater, ethyl tetrabromophenolphthalein will combine with thiocyanate to form a stable complex, and combine with copper ions through an auxiliary masking agent to prevent the formation of copper thiocyanate colloid, thereby avoiding the copper thiocyanate colloid from encapsulating cyanide and affecting the detection results of cyanide.

[0097] An appropriate amount of ethyl tetrabromophenolphthalein is added to the wastewater to ensure its reaction with thiocyanate ions. A masking agent that binds to copper ions is added to the sample to prevent the copper ions and thiocyanate ions from forming copper thiocyanate colloids. The color change of ethyl tetrabromophenolphthalein is used to monitor cyanide concentrations while avoiding interference from copper thiocyanate colloids.

[0098] By cleverly utilizing the synergistic effect of tetrabromophenolphthalein ethyl composite indicator and masking agent, the problem of synergistic interference between sulfide and heavy metals was effectively solved, ensuring the accuracy of cyanide detection, simplifying the operation process and improving the applicability of the method, which has significant practical application value.

[0099] The following is a detailed explanation using examples: In this embodiment, a new high-precision detection method for cyanide in wastewater is proposed, which is applied to industrial wastewater treatment for real-time monitoring of cyanide content. The specific application scenario is a wastewater treatment station of a metallurgical plant, where cyanide pollution may occur during the production process. Traditional high-precision detection methods for cyanide have problems such as long reaction time, low sensitivity, and many interfering factors. The detection method of the present invention uses ethyl tetrabromophenolphthalein as an indicator, which reacts with cyanide under specific conditions to produce a significant color change, thereby providing highly sensitive cyanide detection.

[0100] Specifically, prepare: Ethyl tetrabromophenolphthalein (solution concentration: 1.0×10⁻³M); Ethanol (solution concentration: 50%); Sodium sulfide (Na2S, solution concentration: 0.1 M, used to simulate interfering substances); Zinc salt (Zn²⁺, solution concentration: 0.05 M, used to simulate heavy metal interference); Sodium cyanide standard solution (NaCN, concentration: 1.0×10⁻ 6 M); pH adjuster (NaOH, concentration: 1.0 M); Spectrophotometer (wavelength: 580nm) Constant temperature water bath (temperature: 30°C) Centrifuge (speed: 3000 rpm) Laboratory glassware: beakers, pipettes, burettes, etc. Take 10 mL of metallurgical wastewater sample and dilute it with 50 mL of distilled water. Measure the initial pH value of the wastewater sample and record the initial value.

[0101] Add 0.5 mL of ethyl tetrabromophenolphthalein (1.0 × 10⁻³M solution) to the wastewater sample and stir thoroughly. The solution will turn yellow, indicating that no cyanide is detected.

[0102] If the wastewater contains substances that may interfere with cyanide detection (such as sulfide, heavy metals, etc.), add 0.5mL of sodium sulfide (0.1M) and 0.5mL of zinc salt solution (0.05M) for masking treatment according to the actual situation.

[0103] Add standard sodium cyanide solution to the wastewater and adjust the pH to 6.5 (using NaOH). Heat the solution to 30°C in a water bath and maintain the reaction for 10 minutes.

[0104] After the reaction is complete, use a spectrophotometer to measure the absorbance (A) of the solution at a wavelength of 580 nm. Record the absorbance value and calculate the cyanide concentration in the wastewater based on a pre-established standard curve.

[0105] Sodium cyanide solution with different concentrations (1.0×10⁻ 6 M to 1.0×10⁻³M), measure the corresponding absorbance values, and establish a standard curve.

[0106] The standard curve formula is: ; in, is the absorbance, is the absorption coefficient of cyanide, is the cyanide concentration, is the optical path length (1 cm).

[0107] Recovery calculation: ; For example, if sodium cyanide is added at a concentration of The measured concentration is , the recovery rate is:

[0108] In order to verify the effectiveness of the method of the present invention, a comparison was made with a traditional high-precision cyanide detection method (such as the mercuric chloride method). The following is the experimental comparison data:

[0109] The data show that the method of the present invention can detect lower concentrations of cyanide within the same detection time (10 minutes), with higher recovery rates and improved detection sensitivity. The mercuric chloride method has lower detection sensitivity and longer reaction times, making it unsuitable for real-time monitoring.

[0110] The method of the present invention enables highly sensitive detection of cyanide in wastewater in a short period of time, maintaining high accuracy and stability, particularly in the presence of interfering substances (such as sulfides and heavy metals). Compared with traditional methods, the present method significantly improves detection sensitivity and recovery, while significantly shortening reaction time, demonstrating strong practical application value.

[0111] The present invention encompasses any alternatives, modifications, equivalents, and solutions that fall within the spirit and scope of the present invention. To provide a thorough understanding of the present invention, specific details are described in detail below in connection with the preferred embodiments of the present invention, but those skilled in the art will be able to fully understand the present invention without these detailed descriptions. Furthermore, to avoid unnecessary confusion regarding the essence of the present invention, well-known methods, processes, procedures, components, and circuits have not been described in detail.

[0112] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A high-precision cyanide detection method using thymolphthalein-TBPE composite indicator, characterized in that: include: Step 1: After distilling and separating the water sample to be tested, adjust the pH of the distillate to the strong alkaline environment required for thymolphthalein color development; Step 2: adding a composite indicator containing thymolphthalein and ethyl tetrabromophenolphthalein, wherein the mass ratio of the two components in the composite indicator is determined by a color change sensitivity optimization experiment; Step 3: Titrate with silver nitrate solution and determine the endpoint based on the third-order color change: The initial state is light yellow, the intermediate state is green, and the terminal state is purple-red; Step 4: Add an auxiliary masking agent before titration to block the reaction between the interfering ions and silver ions by preferentially complexing them; in: The green color is formed by the superposition of the blue color of thymolphthalein ionization and the yellow color of ethyl tetrabromophenolphthalein; The purple-red color is generated by the complexation of excess silver ions with ethyl tetrabromophenolphthalein; The auxiliary masking agent has a stronger chelating ability for metal ions than the complexing ability of cyanide.

2. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein: The optimization process of the mass ratio described in step 2 includes: a: Prepare mixed solutions of thymolphthalein and ethyl tetrabromophenolphthalein in various ratios, covering the synergistic range of their color development properties; b: Add each ratio solution to the cyanide standard sample and record the color change process under the same titration conditions; c: Quantify the color difference jump amplitude of each stage through the image analysis system, and select the ratio with the largest total color difference of the three stages: light yellow → green → purple-red; d: Verify the repeatability of endpoint mutations at this ratio and eliminate misjudgment caused by ambient light interference.

3. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein: The formation of green in step 3 requires the following conditions to be met simultaneously: Thymolphthalein undergoes intramolecular ester hydrolysis in a strong alkaline environment to generate a quinone structure that appears blue; When ethyl tetrabromophenolphthalein is not complexed with silver ions, it retains the yellow chromogenic group of its phenolphthalein skeleton; The solution's light transmittance causes the superposition of blue and yellow light waves to produce a green visual effect; During the titration process, the silver ion concentration did not reach the complexation threshold of ethyl tetrabromophenolphthalein.

4. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein: The preferential complexation of interfering ions in step 4 is achieved by: Interference with sulfide: The binding constant of ethyl tetrabromophenolphthalein with sulfide ions is higher than the precipitation constant of silver ions with sulfide. The binding strength advantage is verified by competitive binding experiments. Interference with thiocyanate: The binding rate of ethyl tetrabromophenolphthalein to thiocyanate is higher than that of silver ions to thiocyanate. The difference in binding kinetics is determined by retention spectroscopy. Interference with metal ions: The stability constant of the chelate between the auxiliary masking agent and the metal ion is higher than that of the complex between the metal and cyanide. The order of chelation ability is verified by potentiometric titration.

5. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein: The amount of the auxiliary masking agent added is determined by an interference tolerance threshold experiment, including: a: Prepare simulated water samples containing fixed concentrations of cyanide and gradient concentrations of interfering substances; b: Titrate by increasing the amount of auxiliary masking agent in a gradient manner and record the recovery rate curve; c: The amount of masking agent used when the recovery rate first reaches a stable plateau is taken as the benchmark amount; d: According to the peak concentration of the interfering substance in the actual water sample, the baseline amount is proportionally increased.

6. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein: The pH critical value verification method of the strongly alkaline environment is: When the concentration is below the critical value, the generation rate of thymolphthalein quinone structure is insufficient, resulting in weak blue coloration; When the concentration is higher than the critical value, the hydroxide ion causes the background of tetrabromophenolphthalein ethyl to deepen; The absorbance jump point of thymolphthalein at characteristic wavelength at different pH was determined by spectrophotometry, and the optimal range was determined comprehensively based on the titration endpoint recognition.

7. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein: The objectivity of endpoint determination is achieved through color feature value comparison: Establish a standard color database of three-order color changes, including RGB or Lab color space feature values ​​of light yellow / green / purple red states; During the titration process, the solution image is collected in real time to extract the main color feature value of the current frame; When the feature value matches the purple state database and the Euclidean distance with the green state feature value is greater than the set threshold, it is determined that the end point has been reached.

8. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 4, wherein: The specific steps of the competitive binding experiment are: Solutions containing equal concentrations of sulfide ions and silver ions were prepared and divided into two groups: the experimental group added ethyl tetrabromophenolphthalein, and the control group did not add the indicator; The decay curves of free sulfide ion concentration over time in the two groups of solutions were monitored using a silver sulfide selective electrode. Comparing the difference in the slopes of the two groups of curves, the attenuation rate of the experimental group was significantly reduced, which proved that the preferential complexation effect was established.

9. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 5, wherein: The criteria for determining the stable plateau phase of recovery rate in the interference tolerance threshold experiment are: The recovery rate fluctuation range under three consecutive masking agent gradients is ≤±2%; The absolute deviation between the recovery rate and the theoretical value is ≤5%; The relative standard deviation of the recovery rate in independent experiments by different operators was ≤1.5%.

10. The method for high-precision detection of cyanide using thymolphthalein-TBPE composite indication according to claim 1, characterized in that: The application of the method in the scenario of sulfide and heavy metal synergistic interference includes: When sulfide and iron ions coexist in wastewater, ethyl tetrabromophenolphthalein preferentially fixes sulfide ions and assists the masking agent in chelating iron ions, thus blocking the two from forming ferrous sulfide precipitation and consuming cyanide. When thiocyanate coexists with copper ions, ethyl tetrabromophenolphthalein captures thiocyanate and assists the masking agent in binding to copper ions, preventing the formation of copper thiocyanate colloid encapsulating cyanide.

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