CN120609962A

CN120609962A - A high-precision cyanide detection method with thymolphthalein-TBPE composite indicator

Info

Publication number: CN120609962A
Application number: CN202511120290.8A
Authority: CN
Inventors: 李亚琪; 马新凤; 张清云
Original assignee: Weifang Hongrun Petrochemical Technology Co ltd
Current assignee: Weifang Hongrun Petrochemical Technology Co ltd
Priority date: 2025-08-12
Filing date: 2025-08-12
Publication date: 2025-09-09

Abstract

The invention relates to the technical field of titration detection in analytical chemistry, in particular to a method for detecting cyanide with high precision by composite indication of thymolphthalein-TBPE, which comprises the following steps of 1, distilling and separating a water sample to be detected, and adjusting the pH of distillate to a strong alkaline environment required by thymolphthalein color development; the method comprises the steps of (1) adding a compound indicator containing thymolphthalein and tetrabromophenolphthalein ethyl ester, determining the mass ratio of the two components in the compound indicator through a color change acuity optimization experiment, determining an end point according to three-order color mutation by titration with a silver nitrate solution, wherein the end point is light yellow in initial state, green in middle state and purple in end point state, and adding an auxiliary masking agent before titration, and blocking the reaction of the auxiliary masking agent with silver ions by preferentially complexing interference ions. The interference of other substances can be effectively prevented by optimizing the proportion of the compound indicator and using the masking agent, so that the accurate detection of cyanide is ensured.

Description

High-precision detection method for cyanide indicated by thymolphthalein-TBPE (Tunnel boring PE) compound

Technical Field

The invention relates to the technical field of titration detection in analytical chemistry, in particular to a high-precision cyanide detection method for thymolphthalein-TBPE composite indication.

Background

The high-precision detection of cyanide has great significance in environmental water safety monitoring and industrial wastewater treatment. At present, the silver nitrate titration method is still a mainstream method in a basic laboratory, but has the following technical defects that restrict the detection reliability:

1. Inherent defects of fuzzy end point interpretation;

The traditional method relies on a single silver-test agent (tetrabromophenolphthalein ethyl ester, TBPE) indicator, and the endpoint color changes to continuously transition yellow to mauve. The color change process has obvious disadvantages:

the hue transition bandwidth reaches 30-50nm, and the mutation points are difficult to identify by human eyes;

the difference in ambient light results in an interpretation deviation of >5% for different operators for the same endpoint;

upon detection of low concentrations of cyanide (< 2 mg/L), background yellow interference further weakens contrast.

2. The limitation of the lack of an anti-interference mechanism;

Interfering substances such as sulfide (S ^2-), thiocyanate (SCN ^-) and the like commonly exist in environmental water bodies and industrial wastewater, and the detection value is seriously deviated due to the competition reaction of the interfering substances and silver ions:

Sulfide interference, namely generating silver sulfide precipitate (Ksp=6.3X10 ^-50), consuming silver ions, and leading to a positive error of >20% by using sulfide with concentration of 1 time;

Thiocyanate interference, forming silver thiocyanate colloid (Ksp=1.1X10 ^-12), coating cyanide to hinder complexation reaction;

the prior art only partially eliminates interference through pre-distillation, and has extremely low applicability to high-sulfur wastewater (such as petrochemical wastewater).

Therefore, a need exists for a method for detecting cyanide with high precision by thymolphthalein-TBPE composite indication, which solves the above problems.

Disclosure of Invention

Based on the above purpose, the invention provides a cyanide high-precision detection method for thymolphthalein-TBPE composite indication, which comprises the following steps:

Step 1, after distilling and separating a water sample to be detected, regulating the pH of a distillate to a strong alkaline environment required by thymolphthalein color development;

Step 2, adding a compound indicator containing thymolphthalein and tetrabromophenolphthalein ethyl ester, wherein the mass ratio of the two components in the compound indicator is determined by a color change acuity optimization experiment;

Step 3, titrating with silver nitrate solution, and judging an endpoint according to third-order color mutation:

The initial state is pale yellow, the intermediate state is green, and the end state is purple;

Step 4, adding an auxiliary masking agent before titration, and blocking the reaction of the auxiliary masking agent with silver ions by preferentially complexing interfering ions;

Wherein:

The green color is formed by overlapping thymolphthalein ionization blue and tetrabromophenolphthalein ethyl ester yellow;

The mauve is formed by complexing excessive silver ions with tetrabromophenolphthalein ethyl ester;

The auxiliary masking agent has a greater chelating capacity for metal ions than cyanide complexing capacity.

Preferably, the optimizing process of the mass ratio in step 2 includes:

preparing a plurality of mixed solutions of thymolphthalein and tetrabromophenolphthalein ethyl ester, wherein the proportion range covers the synergistic interval of the two chromogenic characteristics;

b, adding the solutions in each proportion into cyanide standard samples respectively, and recording the color change process under the same titration condition;

c, quantifying the color difference jump amplitude of each stage through an image analysis system, and selecting the proportion with the maximum color difference sum of the stages of light yellow, green and purple red;

And d, verifying the repeatability of the end point mutation under the proportion, and eliminating misjudgment caused by ambient illumination interference.

Preferably, the green color is formed in step 3 while satisfying the following conditions:

The thymolphthalein is subjected to molecular lactone bond hydrolysis in a strong alkaline environment to generate a quinoid structure which shows blue color;

A yellow chromogenic group for retaining the phenolphthalein skeleton of tetrabromophenolphthalein ethyl ester when the tetrabromophenolphthalein ethyl ester is not complexed with silver ions;

the light transmittance of the solution enables blue light waves and yellow light waves to be overlapped to generate a green visual effect;

The concentration of silver ions in the titration process does not reach the complexation threshold of tetrabromophenolphthalein ethyl ester.

Preferably, the implementation manner of the preferential complexing interfering ion in the step 4 is as follows:

the binding constant of tetrabromophenolphthalein ethyl ester and sulfur ions is higher than the precipitation constant of silver ions and sulfides, and the competitive binding experiment proves that the binding strength is superior;

the binding rate of tetrabromophenolphthalein ethyl ester and thiocyanate is higher than that of silver ion and thiocyanate, and the difference of binding kinetics is measured by a stay spectrometry;

the chelate stability constant of the auxiliary masking agent and the metal ion is higher than that of the complex of the metal and cyanide, and the sequence of the chelating ability is verified by a potentiometric titration method.

Preferably, the addition amount of the auxiliary masking agent is determined by an interference tolerance threshold experiment, comprising:

a, preparing a simulated water sample containing cyanide with fixed concentration and interferents with gradient concentration;

b, gradually increasing the dosage of the auxiliary masking agent for titration, and recording a recovery rate change curve;

c, taking the using amount of the masking agent when the recovery rate reaches the stable platform period for the first time as a reference amount;

and d, amplifying the reference quantity proportionally according to the peak concentration of the interfering substances in the actual water sample.

Preferably, the method for verifying the pH critical value of the strong alkaline environment comprises the following steps:

Below the critical value, insufficient formation of thymolphthalein quinoid structure results in weak blue coloration;

above the critical value, hydroxide ions cause the color development background of tetrabromophenolphthalein ethyl ester to deepen;

And (3) measuring the absorbance jump points of thymolphthalein at characteristic wavelengths under different pH values by a spectrophotometry method, and comprehensively determining the optimal range by combining the identification degree of the titration end point.

Preferably, objectification of the determination of the endpoint is achieved by color eigenvalue comparison:

Establishing a standard color database of third-order color change, wherein the standard color database comprises RGB or Lab color space characteristic values of light yellow/green/purple red states;

acquiring a solution image in real time in the titration process, and extracting a main tone characteristic value of a current frame;

and when the characteristic value is matched with the mauve state database and the Euclidean distance between the characteristic value and the green state characteristic value is larger than a set threshold value, judging that the end point arrives.

Preferably, the specific steps of the competitive binding assay are:

Preparing a solution containing sulfur ions and silver ions with equal concentration, wherein the solution is divided into two groups, namely an experimental group is added with tetrabromophenolphthalein ethyl ester, and a control group is not added with an indicator;

Monitoring the decay curves of the concentration of free sulfide ions in the two groups of solutions with time by using silver sulfide selective electrodes;

comparing the slope difference of the two groups of curves, and proving that the preferential complexation effect is established when the attenuation rate of the experimental group is obviously reduced.

Preferably, the criterion of the recovery rate stabilization plateau in the interference tolerance threshold experiment is:

the fluctuation range of the recovery rate under the gradient of three continuous masking agents is less than or equal to +/-2 percent;

The absolute deviation between the recovery rate and the theoretical value is less than or equal to 5 percent;

The relative standard deviation of the recovery rates of the independent experiments of different operators is less than or equal to 1.5 percent.

Preferably, the application of the method in the scene of the cooperative interference of sulfide and heavy metal comprises the following steps:

when sulfide and iron ions exist in the wastewater at the same time, tetrabromophenolphthalein ethyl ester preferentially fixes the sulfur ions, and the auxiliary masking agent chelates the iron ions to block the sulfide and the iron ions from generating ferrous sulfide precipitation to consume cyanide;

When thiocyanate and copper ions coexist, tetrabromophenolphthalein ethyl ester captures thiocyanate, and the auxiliary masking agent is combined with copper ions to prevent generation of copper thiocyanate colloid to wrap cyanide.

The invention has the beneficial effects that:

1. according to the invention, the thymolphthalein and tetrabromophenolphthalein ethyl ester compound indicator is adopted, and the color change is more obvious by optimizing the mass proportion of the indicator. In the process, the synergistic effect of thymolphthalein and tetrabromophenolphthalein ethyl ester obviously enhances the color contrast ratio, so that the identifiability of the end point is improved. The image analysis system quantifies the chromatic aberration jump, so that the high accuracy of end point interpretation is further ensured, and the human error is reduced.

2. The invention innovatively introduces the use of the auxiliary masking agent on the basis, and the influence of interferents such as sulfide, thiocyanate and the like is effectively avoided by preferentially complexing the interfering ions. In particular, the binding property of tetrabromophenolphthalein ethyl ester with sulfide and thiocyanate is superior to that of silver ion, so that the reaction of cyanide and silver ion is ensured not to be disturbed. In addition, the invention optimizes the using amount of the masking agent, determines the optimal dosage through interference tolerance threshold experiments, further improves the applicability in complex environments, and has obvious application effect especially in high-sulfur wastewater and industrial wastewater.

3. According to the invention, the quality proportion of thymolphthalein and tetrabromophenolphthalein ethyl ester is accurately controlled, so that the color change process is optimized, and the color change is more obvious and stable. In the titration process, green is formed as a result of superposition of blue and yellow light waves under the interaction of thymolphthalein and tetrabromophenolphthalein ethyl ester, and the color recognition is enhanced. By the method, the change of the concentration of silver ions in the titration process can reflect the content of cyanide more accurately, and particularly in the detection of low-concentration cyanide, the interference of background color can be effectively avoided.

4. According to the invention, the objective judgment of the titration endpoint is realized by establishing the standard color database and the real-time image analysis system. In the titration process, a solution image is acquired in real time, a color characteristic value is extracted, RGB or Lab color space characteristic values are adopted to compare with a standard color database, and when the characteristic value is matched with mauve and the Euclidean distance between the characteristic value and a green state characteristic value is larger than a set threshold value, the arrival of the end point can be accurately judged. The method effectively avoids the end point interpretation error caused by illumination change or operator experience difference in the traditional method, and greatly improves the stability and accuracy of detection.

Drawings

In order to more clearly illustrate the invention or the technical solutions of the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it will be obvious to those skilled in the art that other drawings can be obtained from these drawings without inventive effort.

FIG. 1 is a flow chart of the steps of the method of the present invention;

FIG. 2 is a flow chart showing the steps of a method for verifying pH threshold in a strongly alkaline environment of the method of the present invention;

FIG. 3 is a flow chart showing the steps of a competitive binding assay according to the method of the present invention.

Detailed Description

The invention will now be described in detail with reference to the drawings and to specific embodiments. While the invention has been described herein in detail in order to make the embodiments more detailed, the following embodiments are preferred and can be embodied in other forms as well known to those skilled in the art, and the accompanying drawings are only for the purpose of describing the embodiments more specifically and are not intended to limit the invention to the specific forms disclosed herein.

Referring to fig. 1-3, the embodiment of the invention provides a method for detecting cyanide with high precision by thymolphthalein-TBPE composite indication, in step 1, a water sample to be detected is firstly separated by distillation, impurities and volatile components in the water sample are removed, and the detection result of cyanide is ensured to be more accurate. The distilled distillate requires a pH adjustment to the strongly alkaline environment required for color development of thymolphthalein, and this process usually requires the use of a strongly alkaline solution such as sodium hydroxide or potassium hydroxide. Thymolphthalein can be subjected to color development change in a strong alkaline environment, so that a remarkable color change signal is provided for subsequent titration.

After the step 1 is completed, adding a compound indicator containing thymolphthalein and tetrabromophenolphthalein ethyl ester into the solution to be detected in the step 2. The mass proportion of the composite indicator is determined through a color change acuity optimization experiment, and the aim is to optimize the color development effect of the indicator, so that the color change is more obvious and stable. Thymolphthalein, which is an acid-base indicator, can show blue after ionization, and tetrabromophenolphthalein ethyl ester shows yellow. The combination of the two can lead the solution to show obvious color change in the cyanide detection process, thereby enhancing the sensitivity of color mutation and facilitating accurate judgment of the end point.

In the step 3, the silver nitrate solution is used for titrating the solution to be measured, and in the process, the color of the solution can generate third-order mutation, wherein the initial state is pale yellow, the color of the solution is changed into green along with the progress of titration, and finally, the solution is purple red, and the arrival of a titration end point is marked. The three-stage mutation (light yellow, green, purple) of the color change can clearly indicate the change of cyanide concentration, so that the endpoint determination is more accurate.

In step 4, an auxiliary masking agent is added to improve the anti-interference ability prior to titration. The auxiliary masking agent is used for preferentially reacting with interfering ions (such as sulfide, thiocyanate and the like) to prevent the ions from forming precipitates or colloids with silver ions, so that negative influence on cyanide detection results is avoided. The chelating ability of the masking agent is stronger than the complexing ability of cyanide and silver ions, so that only the cyanide and the silver ions form a complex in the titration process, and the interference ions cannot influence the participation reaction of the silver ions.

According to the embodiment of the invention, the reliability and the accuracy of the traditional titration method can be remarkably improved, and particularly, the method has good anti-interference capability and operability when treating complex water quality samples and detecting low-concentration cyanide.

In one possible embodiment, first, a mixture solution of thymolphthalein and ethyl tetrabromophenolate in various proportions is prepared. The range of these ratios covers the synergistic interval of the two color development characteristics, i.e. the ratio interval experimentally obtained that can provide the best color change sensitivity. These solutions will be the basis for subsequent experiments.

Then, the solutions of different ratios were added to standard samples of known cyanide concentrations, respectively, and treated under the same titration conditions. Each drop time requires recording the color change from an initial pale yellow, to green, to mauve. Real-time recording of these color changes will provide data support for subsequent analysis.

An image analysis system was used to quantify the magnitude of the color difference jump for each solution ratio during titration. The color difference variation of each stage can be clearly obtained by the system recording the amplitude and the speed of the color variation. And finally, selecting the proportion with the maximum color difference sum of the light yellow, green and purple red stages as the optimal proportion, and ensuring the maximum sensitivity and the significance of the color change.

After the optimal ratio is selected, the reproducibility of the endpoint mutation at that ratio needs to be verified by multiple experiments. The purpose of this step is to ensure consistency and stability of the color change process under different experimental conditions, eliminating color misjudgment that may be caused by ambient illumination changes or other external disturbances.

The optimization process can provide scientific basis for the mixing proportion of thymolphthalein and tetrabromophenolphthalein ethyl ester, so that the combination of thymolphthalein and tetrabromophenolphthalein ethyl ester shows optimal color development characteristic in cyanide titration. Through experimental verification and image analysis of the system, the color change of each stage can be quantified, and the selected proportion can not only provide clear color change, but also be stably and reliably represented under different experimental conditions. In addition, the optimization method can effectively improve the accuracy of end point judgment, avoid the influence of illumination, experimental errors or other factors on the titration result, and further ensure the high accuracy and high repeatability of the detection result.

In one possible embodiment, in a strongly basic environment, intramolecular ester bonds of thymolphthalein undergo hydrolysis, resulting in a change in its molecular structure, thereby producing a quinoid structure. This structure is formed such that thymolphthalein appears blue. When cyanide in the water sample is not sufficiently reacted, thymolphthalein in solution remains its blue color, which provides a basis for subsequent color changes.

The tetrabromophenolphthalein ethyl ester still maintains the yellow chromogenic group of the phenolphthalein skeleton when not being complexed with silver ions. The yellow group is a part of tetrabromophenolphthalein ethyl ester molecule, and retains its original color before reacting with other components during titration. During the color change, the yellow tetrabromophenolphthalein ethyl ester is superimposed with the blue phase of thymolphthalein, producing a green visual effect.

The transmittance of the solution plays a key role in this process. The blue color of thymolphthalein and the yellow color of tetrabromophenolphthalein ethyl ester are visually superimposed to form a green effect. At this time, the transmittance of the solution and the concentration relationship of the two color-developing substances determine the intensity of green. When the concentration of silver ions has not reached the complexation threshold of tetrabromophenolphthalein ethyl ester, the visual effect of green is produced by superposition of blue and yellow light waves.

In the titration process, when the concentration of silver ions is low, the tetrabromophenolphthalein ethyl ester and the silver ions do not have complexation reaction, so that the yellow group of the tetrabromophenolphthalein ethyl ester is not influenced, and the original yellow color is kept. In this case, the superposition of the colors of thymolphthalein and tetrabromophenolphthalein ethyl ester produces a green color. If the concentration of silver ions is gradually increased and reaches the complexing threshold, the yellow group of tetrabromophenolphthalein ethyl ester is complexed with silver ions to form a new color reaction, and finally, the color of the solution is changed.

By optimizing the reaction of thymolphthalein and tetrabromophenolphthalein ethyl ester, the color change in cyanide detection is more obvious and stable, and the interference of the environment and other factors is reduced, so that the accuracy and reliability of cyanide detection are improved.

In one possible embodiment, the binding constant of tetrabromophenolphthalein ethyl ester to sulfur ions is higher than the precipitation constant of silver ions to sulfide. This means that during cyanide detection, tetrabromophenolphthalein ethyl ester has stronger affinity to bind with sulfur ions and therefore can preferentially bind with sulfur ions, thereby avoiding interference of sulfide on complexation of silver ions with cyanide.

The binding strength of tetrabromophenolphthalein ethyl ester to sulfide ions can be verified through competitive binding experiments. In the experiment, the binding advantage of tetrabromophenolphthalein ethyl ester and sulfur ions is verified by adding sulfides with different concentrations and observing the influence of the concentration of the sulfides on the silver ion complex cyanide reaction, so that the preferential complex sulfides are ensured.

The binding rate of tetrabromophenolphthalein ethyl ester and thiocyanate is higher than that of silver ions and thiocyanate. This means that tetrabromophenolphthalein ethyl ester reacts faster when combined with thiocyanate, so that competing reaction of thiocyanate and silver ions can be effectively avoided, and accuracy in the cyanide detection process is ensured.

The difference in binding kinetics of tetrabromophenolphthalein ethyl ester to thiocyanate can be determined by residence spectroscopy (also known as residence time analysis). In the experiment, the change of the reaction rate in thiocyanate solutions with different concentrations is measured, so that the combination rate of tetrabromophenolphthalein ethyl ester and thiocyanate is verified to be faster, the tetrabromophenolphthalein ethyl ester and thiocyanate are ensured to be combined preferentially, and the interference of the tetrabromophenolphthalein ethyl ester on cyanide detection is eliminated.

The stability constant of the chelate of the auxiliary masking agent with the metal ion is higher than that of the complex of the metal with cyanide. This means that the masking agent is able to bind effectively to the metal ions, thereby inhibiting complexation of the cyanide by the metal ions and avoiding interference with cyanide determination by the metal ions.

The stability constant of the auxiliary masking agent and the chelate of the metal ions is verified by a potentiometric titration method, and the chelating ability of the masking agent is confirmed to be stronger than the complexing ability of the metal ions and cyanide. In the experiment, the binding strength of the masking agent and the metal ion is compared with the binding strength of the metal ion and cyanide, so that the masking agent can be ensured to effectively mask the metal ion, and the interference of the masking agent and the cyanide is reduced.

The embodiment of the invention effectively improves the accuracy and sensitivity of the cyanide high-precision detection method, so that the method can process complex interference environments in practical application and has higher practical value.

In one possible embodiment, first, a simulated water sample is prepared containing a fixed concentration of cyanide and a gradient of concentration of interferents. The purpose of the preparation of the simulated water sample is to simulate the types and the concentrations of the interferents possibly existing in the actual environment, so that the experiment can cover a plurality of possible interference situations. The selection of the interferents and the setting of the concentration gradient are determined according to the common interferents and the concentration ranges thereof in the practical application scene. This step facilitates the determination of the amount of masking agent added and its effectiveness by modeling the interference environment.

Gradually increasing the dosage of the auxiliary masking agent in the simulated water sample, and performing titration. After each addition of a certain amount of masking agent, a change in recovery rate was recorded. Recovery generally refers to the ratio of the actual recovery value of cyanide measured to the theoretical value during the determination of cyanide concentration. As the masking dose increases, the recovery should gradually stabilize. By recording the recovery rate curve, the inhibition effect on the interferents under different masking agent dosages can be accurately estimated.

And determining the using amount of the masking agent when the recovery rate reaches the stable plateau for the first time according to the recovery rate change curve, and taking the masking agent as a reference amount. The baseline amount refers to the point at which recovery no longer varies significantly with increasing concentration of masking agent after the masking agent is added, at which point the amount of masking agent is sufficient to effectively inhibit interferents and ensure accuracy of cyanide measurements. The determination of the reference quantity is a key step in ensuring the reliability and accuracy of the experiment.

And amplifying the reference quantity proportionally according to the peak concentration of the interfering substances in the actual water sample. The adjustment ensures that in actual detection, no matter how the concentration of the interfering substance changes, the dosage of the masking agent can be reasonably adjusted according to the specific condition of the water sample, and the accuracy of cyanide determination is ensured. In practical application, the concentration of the interfering substance is often influenced by factors such as a water source, environment and the like, so that the masking dosage is amplified according to the specific concentration of the interfering substance of the actual water sample, and the changing experimental conditions can be effectively responded.

The dosage of the auxiliary masking agent is determined based on the interference tolerance threshold experiment, so that the accuracy and the sensitivity of cyanide detection can be improved, the adaptability of the method and the reliability of the experiment can be enhanced, and the experiment can be ensured to stably run in various environments.

In one possible embodiment, at a pH below a threshold, the rate of formation of the quinoid structure of thymolphthalein is insufficient, resulting in incomplete color development and thus weak blue color development. This is because thymolphthalein has a molecular structure that is changed in a low pH environment, is not sufficiently converted into a quinoid structure, and lacks sufficient light absorbing ability, resulting in unsatisfactory color development. Therefore, the pH value must be ensured not to be lower than a certain critical value in the detection process so as to ensure that thymolphthalein can sufficiently develop color.

When the pH value is higher than the critical value, the concentration of hydroxyl ions (OH-) is too high, and the hydroxyl ions can react with tetrabromophenolphthalein ethyl ester, so that the color development background is deepened. The phenomenon is mainly due to the fact that tetrabromophenolphthalein ethyl ester can react with hydroxide ions under the high pH condition to generate a compound with a darker color, and therefore accuracy of cyanide detection is affected. This darkening of the colored background can cause signal interference, affecting the sensitivity and accuracy of detection.

The change in absorbance of thymolphthalein can be measured by spectrophotometry at different pH values. In particular, attention is paid to the point of abrupt absorbance at a characteristic wavelength, which means the point at which thymolphthalein changes significantly at a specific wavelength, typically the wavelength at which the blue color reaction is strongest. By the method, the absorbance change of thymolphthalein under different pH values can be identified, the determination of which pH range is the most obvious in the color development of thymolphthalein is facilitated, and the interference of deepening the color development background is avoided.

In addition to spectrophotometric determination of absorbance, it is also necessary to combine the identity of the endpoint of titration to determine the pH threshold range. The titration endpoint identity refers to whether the change of the indicator is clear or not when the endpoint is precisely determined by gradually dropping the reagent in cyanide detection. The ideal pH range should ensure that thymolphthalein exhibits a clear color change at the end point while avoiding background color interference. By comprehensively considering the identification degree of the absorbance jump point and the titration end point, the optimal range of the pH value can be more accurately determined.

By combining the experimental results, an optimal pH value range, namely a critical value range, can be finally obtained, and the thymolphthalein can realize the optimal color development effect within the range and is not influenced by the excessively low or excessively high pH value. In the pH value range, the quinoid structure of thymolphthalein can be fully generated, the color development effect is obvious, the color development background of tetrabromophenolphthalein ethyl ester is not excessively deepened, and the accurate measurement of cyanide is ensured.

Through comprehensive verification of spectrophotometry and titration end point identification, the most suitable pH value range in cyanide detection can be accurately determined, so that the precision, sensitivity and stability of the thymolphthalein-TBPE compound indication cyanide high-precision detection method are improved.

In one possible embodiment, to ensure the accuracy of the color determination, it is first necessary to build a standard color database containing color characteristic values of three color metamerism common to solutions, namely pale yellow, green, and purple. This standard database may use the RGB color space or Lab color space to define color characteristic values for each color state.

RGB color space-colors are described by defining a combination of values of red (R), green (G), blue (B).

Lab color space describes colors through three dimensions of brightness (L), red green chromaticity (a) and yellow blue chromaticity (b), and is more similar to the perception mode of human eyes.

By comparing the two color spaces, an exact match under different color transforms can be ensured.

During titration for cyanide detection, the color change of the solution needs to be monitored in real time. This is achieved by providing a high quality camera or image acquisition device for real-time acquisition of the solution images. Every time there is a new titration operation, the system automatically captures an image of the current solution and extracts the dominant hue feature value in the image.

The color information in these images is converted to values in the RGB or Lab color space, providing a data base for subsequent color comparisons.

And comparing the color characteristic value of the current frame acquired in real time with the characteristic value of the mauve state in the standard color database, and judging whether the color of the current solution is matched with the mauve state. This process depends on the degree of matching of the color feature values and is measured by euclidean distance.

It will be appreciated that euclidean distance is a common mathematical metric used to calculate the similarity between two color feature values. Setting a threshold value, and judging that the end point is reached when the real-time extracted color characteristic value is matched with the characteristic value of the mauve state and the Euclidean distance between the real-time extracted color characteristic value and the green state characteristic value is larger than the set threshold value.

The Euclidean distance formula is calculated by the following steps:

;

Where (R1, G1, B1) is the RGB value of the current solution color and (R2, G2, B2) is the RGB value of the standard mauve state. And when the Euclidean distance between the two colors is larger than a set threshold value, indicating that the current solution is close to the target color, and judging as a titration end point.

By combining the processes, the system can automatically judge whether the titration reaches the end point according to the matching condition of the characteristic values. When the matching degree of the color characteristic value acquired in real time and the purple red state reaches the standard and the Euclidean distance between the color characteristic value and the green state is larger than a set threshold value, the system judges the arrival of the titration end point.

The judgment process avoids subjectivity of manually observing color change, and ensures objectivity and accuracy of end point judgment.

Through the combination of color characteristic value comparison and Euclidean distance calculation, automation and objectification of end point judgment in cyanide detection can be realized, the accuracy, sensitivity and operation standardization of detection are greatly improved, and the subjective error problem in the traditional manual judgment process is solved.

In one possible embodiment, it is desirable to formulate a solution containing equal concentrations of both sulfide and silver ions prior to the experiment. The purpose of this step is to ensure that the initial conditions of the two sets of solutions are the same, to ensure that the comparative results of the experiment are not affected by the initial concentration.

Sulfide ions are typically provided by the addition of sulfide-containing chemicals, such as sodium sulfide.

Silver ions are typically provided by adding a silver chloride solution or a silver salt solution.

The concentration of sulfur ions and silver ions are required to be equal during formulation to ensure that the reaction of silver ions with sulfur ions is balanced during the experiment.

The formulated solutions were divided into two groups:

experimental group i.e. ethyl tetrabromophenolate was added (as an indicator) which had a strong binding affinity to cyanide ions and which could affect the competitive binding between sulfide and silver ions.

Control group, no indicator was added, maintaining the original state of the solution.

By this grouping, experiments can effectively compare the effect of the presence or absence of the indicator on the change in the concentration of sulfide ions, thereby proving whether the indicator has an effect on the reaction process.

During the experiment, silver sulfide selective electrodes were used to monitor the change in free sulfide concentration in both sets of solutions over time.

The silver sulfide selective electrode is an electrode capable of specifically detecting the concentration of sulfur ions in a solution, and has high selectivity and sensitivity.

In solution, silver ions form silver sulfide precipitates with sulfur ions over time, resulting in a gradual decrease in free sulfur ion concentration.

The attenuation curve of the free sulfide ions in the solution can be obtained by measuring the concentration change of the free sulfide ions in the solution in real time.

The decay curves of the free sulfide ion concentration in the solutions of the experimental group and the control group are compared. By calculating the slopes of the two sets of curves, the rate at which they decay can be derived.

Slope difference tetrabromophenolphthalein ethyl ester was added to the experimental group, which may compete with silver ion or sulfur ion for binding, thereby affecting the rate of decay of the concentration of sulfur ion.

If the decay rate of the experimental group is obviously lower than that of the control group, the tetrabromophenolphthalein ethyl ester and silver ions generate a preferential complexing effect, and the reduction of the concentration of the sulfur ions is slowed down. This suggests that tetrabromophenolphthalein ethyl ester preferentially binds silver ions in competing binding, thereby affecting the reaction rate of sulfur ions.

By comparing the decay rates of the two groups, the effect of tetrabromophenolphthalein ethyl ester on the reaction of sulfur ions and silver ions can be verified. If the attenuation rate of the experimental group is obviously reduced, the complex reaction of tetrabromophenolphthalein ethyl ester and silver ions is proved to occur preferentially, and then the establishment of the preferential complex effect is verified.

Competitive binding experiments the preferential complexing effect of tetrabromophenolphthalein ethyl ester in cyanide detection was verified by accurate comparison of the rate of decay of the sulfur ion concentration in the experimental and control groups. The method improves the accuracy, sensitivity and reliability of cyanide detection, and has higher application value.

In one possible embodiment, the determination of the recovery plateau is one of the key steps in conducting an interference tolerance threshold experiment. The purpose of this experiment was to determine the stability and anti-interference ability of thymolphthalein-TBPE composite indicators in cyanide detection under different masking agent gradients. The stability of recovery is closely related to experimental conditions and operating methods. Specific decision criteria include:

The fluctuation range of the recovery rate under the gradient of three continuous masking agents is less than or equal to +/-2 percent:

The standard requires that the recovery rate does not fluctuate by more than + -2% at different concentrations of masking agent gradient. This requirement ensures the repeatability and consistency of the experiment so that the detection method can still maintain a relatively stable recovery rate under various interference environments. By means of different gradients of the masking agent, the behavior of the indicator under different disturbing conditions can be effectively detected.

Absolute deviation of recovery rate from theoretical value is less than or equal to 5 percent:

The standard requires that recovery not differ from theoretical by more than 5%. This is a strict requirement on the accuracy of the experiment. Theoretical values are usually measured by standard or cyanide solutions of known concentrations, and absolute deviations of less than 5% for recovery indicate that the detection method has a higher accuracy and is capable of reliably detecting cyanide concentrations.

The relative standard deviation of the recovery rates of the independent experiments of different operators is less than or equal to 1.5 percent:

The standard requires that the relative standard deviation of recovery should not exceed 1.5% when experiments are conducted independently by different operators. This requirement ensures that the operability of the experiment, especially when operated by different laboratories or by different personnel, is still ensured. This is important to ensure reliability and wide applicability of the test results.

In practice, the interference tolerance threshold test is generally performed according to the following steps:

preparing an experimental solution:

Standard cyanide solutions and masking agent solutions of different concentrations were prepared. The type and concentration of the masking agent should be selected according to the experimental design, and common masking agents include metal ions, acids or bases, etc.

Adding thymolphthalein-TBPE composite indicator:

thymolphthalein-TBPE composite indicator is added to cyanide-containing solution and reacted in the presence of masking agents of varying concentrations.

Monitoring recovery rate changes:

The absorbance change of the reacted solution is monitored using a suitable detection device (e.g., an ultraviolet-visible spectrophotometer), and the recovery is calculated and the data recorded.

Analysis of recovery rate fluctuations:

the recovery rates under different masking agent gradients were recorded and their fluctuation ranges were judged to ensure that they were within a standard range of + -2%.

The experiment was repeated:

experiments were performed independently by different operators, ensuring that the relative standard deviation of recovery was less than 1.5%.

The judgment standard of the interference tolerance threshold experiment not only improves the stability, accuracy and applicability of the method, but also ensures the repeatability and reliability under different experimental conditions, thereby improving the overall performance of the cyanide high-precision detection method.

In one possible embodiment, tetrabromophenolphthalein ethyl ester acts as a composite indicator to preferentially bind to sulfide ions when both sulfide and iron ions are present in the wastewater. The tetrabromophenolphthalein ethyl ester is combined with sulfur ions through forming a compound, so that the reaction of the sulfur ions and the iron ions to form ferrous sulfide precipitates is avoided, and cyanide is consumed.

Waste water sample preparation:

And mixing the wastewater sample to be detected with tetrabromophenolphthalein ethyl ester to ensure that the tetrabromophenolphthalein ethyl ester can fully react with sulfur ions.

And (3) adding a masking agent:

Chelating agents or other suitable sequestering agents are added to the sample and are capable of chelating the iron ions, sequestering the iron ions from the reaction system, and preventing precipitation with the sulfide ions.

Detecting cyanide:

Under the conditions that the sulfur ions are fixed and the iron ions are masked, the concentration of cyanide is monitored through the indication action of tetrabromophenolphthalein ethyl ester, so that the problem that cyanide is consumed by ferrous sulfide precipitation is avoided.

When thiocyanate and copper ions exist in the wastewater at the same time, tetrabromophenolphthalein ethyl ester can be combined with thiocyanate to form a stable compound, and the auxiliary masking agent is combined with copper ions to prevent the generation of copper thiocyanate colloid, so that the cyanide coated by the copper thiocyanate colloid is avoided, and the detection result of the cyanide is prevented from being influenced.

An appropriate amount of tetrabromophenolphthalein ethyl ester was added to the wastewater to ensure that it was able to react with thiocyanate ions. And adding a masking agent capable of being combined with copper ions into the sample to ensure that copper ions and thiocyanate ions do not generate copper thiocyanate colloid. Cyanide concentration is monitored by the color-changing property of tetrabromophenolphthalein ethyl ester, and meanwhile, the interference of copper thiocyanate colloid is avoided.

By skillfully utilizing the synergistic effect of the tetrabromophenolphthalein ethyl ester compound indicator and the masking agent, the problem of the synergistic interference of sulfides and heavy metals is effectively solved, the accuracy of cyanide detection is ensured, the operation flow is simplified, the applicability of the method is improved, and the method has remarkable practical application value.

The following is a detailed description by way of example:

In the embodiment, a novel method for detecting cyanide in wastewater with high precision is provided, and is applied to industrial wastewater treatment and used for monitoring the cyanide content in real time. The specific application scene is a waste water treatment station of a certain metallurgical plant, and cyanide pollution can be generated in the production process of the plant. The traditional cyanide high-precision detection method has the problems of long reaction time, low sensitivity and more interference factors. The detection method of the invention provides high sensitivity cyanide detection by using tetrabromophenolphthalein ethyl ester indicator to react with cyanide under specific conditions to produce a significant color change.

Specifically, preparation:

tetrabromophenolphthalein ethyl ester (solution concentration: 1.0X10-3M);

ethanol (solution concentration: 50%);

sodium sulfide (Na ₂ S, solution concentration: 0.1M, used to simulate interfering substances);

zinc salt (zn2+, solution concentration: 0.05M, used to simulate heavy metal interference);

Sodium cyanide standard solution (NaCN, concentration: 1.0X10- ⁶ M);

pH regulator (NaOH, concentration: 1.0M);

Spectrophotometer (wavelength 580 nm)

Constant temperature water bath (temperature: 30℃)

Centrifuge (rotating speed: 3000 rpm)

Laboratory glassware such as beaker, pipette, buret, etc

10ML of metallurgical wastewater sample is taken and diluted with 50mL of distilled water. The preliminary pH of the wastewater sample was measured and the initial value was recorded.

To the wastewater sample was added 0.5mL tetrabromophenolphthalein ethyl ester (1.0X10-3M solution) and stirred well. At this point the solution was yellow, indicating no cyanide was detected.

If the wastewater contains substances (such as sulfides, heavy metals and the like) which can interfere with cyanide detection, 0.5mL of sodium sulfide (0.1M) and 0.5mL of zinc salt solution (0.05M) are added according to actual conditions for masking treatment.

Standard sodium cyanide solution was added to the wastewater and the pH was adjusted to 6.5 (using NaOH). The solution was heated to 30 ℃ in a water bath and the reaction was maintained for 10 minutes.

After the reaction was completed, absorbance (A) of the solution was measured at a wavelength of 580nm using a spectrophotometer. The absorbance values were recorded and the cyanide concentration in the wastewater was calculated according to a pre-established standard curve.

Experiments were performed with different concentrations of sodium cyanide solution (1.0X10- ⁶ M to 1.0X10-3M), corresponding absorbance values were measured, and a standard curve was established.

The standard curve formula is:;

Wherein, the In order to be the absorbance, the light is,Is the light absorption coefficient of cyanide, and the light absorption coefficient of cyanide,In the presence of a concentration of cyanide,Is the optical path length (1 cm).

And (3) calculating the recovery rate:;

for example, if sodium cyanide is added at a concentration of The concentration was measured asThe recovery rate is:

to verify the effectiveness of the method of the present invention, it is compared to conventional methods for high precision detection of cyanide, such as mercury chloride. The following are experimental comparative data:

from the data, the method can detect cyanide with lower concentration in the same detection time (10 minutes), and has higher recovery rate and improved detection sensitivity. The mercury chloride method has low detection sensitivity and long reaction time, and is not suitable for real-time monitoring.

By implementing the method provided by the invention, high-sensitivity detection of cyanide in wastewater can be realized in a short time, and particularly, when interfering substances (such as sulfide and heavy metal) exist, higher accuracy and stability can be still maintained. Compared with the traditional method, the method has the advantages of obviously improved detection sensitivity and recovery rate, greatly shortened reaction time and stronger practical application value.

The invention is intended to cover any alternatives, modifications, equivalents, and variations that fall within the spirit and scope of the invention. In the following description of preferred embodiments of the invention, specific details are set forth in order to provide a thorough understanding of the invention, and the invention will be fully understood to those skilled in the art without such details. In other instances, well-known methods, procedures, flows, components, circuits, and the like have not been described in detail so as not to unnecessarily obscure aspects of the present invention.

The foregoing is merely a preferred embodiment of the present invention and it should be noted that modifications and adaptations to those skilled in the art may be made without departing from the principles of the present invention, which are intended to be comprehended within the scope of the present invention.

Claims

1. A high-precision cyanide detection method using 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.

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 in step 2 comprises:

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 the green color in step 3 must simultaneously satisfy 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.

4. The method for high-precision detection of cyanide using a thymolphthalein-TBPE composite indicator according to claim 1, wherein the preferential complexing interfering ions in step 4 are implemented as follows:

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 cyanide detection 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, comprising:

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 high-precision cyanide detection method using a thymolphthalein-TBPE composite indicator according to claim 1, wherein the objective endpoint determination 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.

8. The method for high-precision detection of cyanide using thymolphthalein-TBPE composite indication 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 criterion for determining the stable plateau phase of the recovery rate in the interference tolerance threshold experiment is:

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 high-precision cyanide detection method using thymolphthalein-TBPE composite indicator according to claim 1, wherein the application of the method in the scenario of sulfide and heavy metal synergistic interference comprises:

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.