Water hardness detection reagent as well as preparation method and application thereof

Abstract

The invention relates to the technical field of water hardness detection, in particular to a water hardness detection reagent as well as a preparation method and application thereof. Comprising the following raw materials in parts by weight: 0.8-1.5 parts of ethanolamine, 5-7 parts of diethanolamine, 35-40 parts of triethanolamine, 52-57 parts of an ammonia-ammonium chloride buffer solution, 1.5-2 parts of a chrome black T indicator, 1-1.5 parts of ethylene diamine tetraacetic acid disodium magnesium salt, 0.1-0.3 part of sodium chloride and 0.1-0.3 part of a formaldehyde aqueous solution. The prepared water hardness detection reagent has the characteristics of good stability and high sensitivity.

Description

Technical Field

[0001] The present invention relates to the technical field of water hardness detection, and in particular to a water hardness detection reagent, a preparation method and an application thereof. Background Art

[0002] Water hardness is generally determined by the total concentration of dissolved calcium and magnesium ions in the water. The content of these ions determines the degree of hardness or softness of the water. Water with high concentrations of calcium and magnesium salts is considered hard water; conversely, low concentrations are considered soft water. Hard water has a wide range of impacts on daily life and industrial production. For example, in the textile industry, washing with hard water can cause stains on textiles, affecting their appearance and potentially reducing their strength. In food processing, hard water can cause protein precipitation, making it difficult to cook. Furthermore, hard water can negatively impact the effectiveness of pesticides, boiler operation, and heat exchanger efficiency. However, traditional indicators for water hardness testing suffer from issues such as poor stability, susceptibility to oxidation, and unclear endpoint color change.

[0003] Patent CN1584560A discloses a solid water hardness test reagent composed of the following components: disodium ethylenediaminetetraacetic acid (EDTA), sodium tetraborate, chrome black T, sodium carbonate, and sodium chloride. This reagent is portable, simple to operate, and can detect water hardness without the need for specialized personnel. This greatly facilitates water hardness testing in many industries and can be applied to water hardness testing in processes such as boilers, winemaking, beverages, and wool spinning, as well as testing by relevant supervisory inspection departments. Patent CN102749324A discloses a rapid water hardness test reagent and its applicable reagent bottle, test tube, and method. The rapid water hardness test reagent includes an alkaline solution, an indicator, and an EDTA aqueous solution, all packaged separately in different containers. The indicator includes a mixture of calcium red and an alkali metal salt, and a mixture of chrome black T and an alkali metal salt. When testing water hardness using this invention, the reagent components are packaged in separate containers for easy portability. Through the precise design of the reagent bottle and test tube, even non-professionals can quickly test water hardness on-site. By using different alkaline solutions and indicators, the total hardness and calcium hardness of the water can be determined. However, neither of the aforementioned patents considers the susceptibility of the chromium black T used in alkaline solutions to oxidation failure, resulting in a lack of sharpness in the endpoint color change of the water hardness test reagent.

[0004] Therefore, there is an urgent need for a water hardness detection reagent with good stability and high sensitivity in the market. Summary of the Invention

[0005] In view of the problems existing in the prior art, the purpose of the present invention is to obtain a water hardness detection reagent with good stability and high sensitivity.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A first aspect of the present invention provides a water hardness detection reagent, which comprises the following raw materials, in parts by weight: 0.8-1.5 parts of ethanolamine, 5-7 parts of diethanolamine, 35-40 parts of triethanolamine, 52-57 parts of ammonia-ammonium chloride buffer, 1.5-2 parts of chrome black T indicator, 1-1.5 parts of disodium magnesium salt of ethylenediaminetetraacetic acid, 0.1-0.3 parts of sodium chloride, and 0.1-0.3 parts of formaldehyde aqueous solution.

[0008] The water hardness detection reagent prepared by mixing ethanolamine, diethanolamine, triethanolamine, ammonia-ammonium chloride buffer, chrome black T indicator, disodium magnesium salt of ethylenediaminetetraacetic acid, sodium chloride, and formaldehyde aqueous solution has the advantages of good stability and high sensitivity. 3 +、Al3+、Cu 2 +、Mn 2 + and other metal ions to reduce the impact of these ions on the test results. Among them, triethanolamine as the core complexing agent preferentially reacts with Ca 2 +、Mg 2 + combination, while ethanolamine and diethanolamine preferentially complex Fe 3 +, and Al3+ synergistically eliminate interference from individual ions in endpoint determination. Disodium magnesium ethylenediaminetetraacetic acid enhances the color sensitivity of the chrome black T indicator; sodium chloride maintains ionic strength and reduces electrochemical interference. Formaldehyde inhibits the photolytic oxidation of chrome black T, extending the reagent's shelf life.

[0009] In some embodiments, the method for preparing the ammonia-ammonium chloride buffer solution comprises the following steps: dissolving ammonium chloride in deionized water, adding concentrated aqueous ammonia, stirring at room temperature for 3-5 minutes, adding deionized water to make up to volume, stirring at room temperature for 3-5 minutes, and adjusting the pH to 10 to obtain an ammonia-ammonium chloride buffer solution.

[0010] In some embodiments, the method for preparing the chrome black T indicator comprises the following steps:

[0011] A1. Mixing an iron salt with an organic ligand, acetic acid, and a solvent, and ultrasonically mixing to obtain a metal-organic mixed solution; and subjecting the metal-organic mixed solution to a solvothermal reaction to obtain an Fe-MOF suspension;

[0012] A2, centrifuging, washing, and drying the Fe-MOF suspension obtained in step A1 to obtain a Fe-MOF material;

[0013] A3, dissolving chrome black T in 45-55wt% ethanol aqueous solution, adding the Fe-MOF material obtained in step A2, stirring in the dark for 12-24h under the condition of pH 7, filtering, washing, centrifuging, and vacuum drying at 55-65°C to obtain chrome black T indicator.

[0014] Preferably, the iron salt is a ferric salt.

[0015] In some embodiments, the organic ligand is a combination of 2-aminoterephthalic acid and biphenyldicarboxylic acid, and the mass ratio of the two is 1:(0.2-0.5).

[0016] The structure of Eriochrome Black T contains an oxidizing group, a nitro group, and two different reducing groups, an azo group and a phenolic hydroxyl group, which makes it easy to undergo redox reactions within the molecule, thus destroying the indicator. To address the above problem, this application loads Eriochrome Black T on a metal-organic framework material to reduce its direct contact with air / solvent, thereby improving the stability of Eriochrome Black T and improving the accuracy of the test results of water hardness detection reagents. Fe-MOF has a high specific surface area and a controllable pore structure, which can selectively adsorb calcium and magnesium ions in water and react with them through color changes such as changes in ligand structure or Fe 3 + redox reaction amplifies the signal and improves the detection sensitivity. This application further improves the dispersibility of the MOF material loaded with chrome black T in water by grafting amino groups on the MOF material. 3 + and Al3+ are further masked, reducing interference and further improving the accuracy of the detection results. In addition, the rigid ligand biphenyl dicarboxylic acid selected in this application can open the metal nodes to form large pores. By limiting the pore size of the MOF material, the MOF material has a better loading capacity while making it easy for calcium and magnesium ions to complex with chrome black T.

[0017] In some embodiments, the mass ratio of the iron salt to the organic ligand is 1:(0.8-1.2).

[0018] The present application can further improve the stability and sensitivity of the water hardness detection reagent by limiting the mass ratio of iron salt and organic ligand. This may be because the MOF material has a suitable porosity at this ratio, which is conducive to increasing the loading amount of chrome black T.

[0019] In some embodiments, the solvent is a composition of N,N-dimethylformamide and deionized water, and the volume ratio of the two is (2-6):1.

[0020] In some embodiments, the mass ratio of the chrome black T to the Fe-MOF material is 1:(7-11).

[0021] The present application can further improve the stability and sensitivity of the water hardness detection reagent by limiting the mass ratio of the iron metal salt and the organic ligand. This may be because at this ratio, the Fe-MOF nanoparticles can be evenly dispersed in the system to reduce the influence of contrast color.

[0022] In some embodiments, the water hardness detection reagent further contains 3-7 parts by weight of hydroxylamine hydrochloride.

[0023] The addition of hydroxylamine hydrochloride can prevent the influence of external oxidizing substances on chrome black T.

[0024] A second aspect of the present application provides a method for preparing a water hardness detection reagent, comprising the following steps: adding ethanolamine, diethanolamine, and triethanolamine to an ammonia-ammonium chloride buffer solution, stirring at 25°C±2°C for 20-30 minutes, sequentially adding formaldehyde aqueous solution, chrome black T indicator, disodium magnesium ethylenediaminetetraacetic acid, and sodium chloride, ultrasonically dispersing for 20-30 minutes under light-proof conditions, filtering, and packaging in a brown reagent bottle to obtain a water hardness detection reagent.

[0025] The third aspect of the present application provides an application of a water hardness detection reagent, comprising the following steps: placing a water sample in a colorimetric chamber, adding a water hardness detection reagent, continuously adding a standard titration solution of disodium ethylenediaminetetraacetic acid with a concentration of 0.01 mol / L or 0.005 mol / L into the water sample through a pump, and calculating the hardness of the water sample by titrating until the volume of reagent consumed reaches the titration endpoint.

[0026] Preferably, the water sample is one of tap water, boiler water and synthetic water.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The water hardness detection reagent prepared by mixing ethanolamine, diethanolamine, triethanolamine, ammonia-ammonium chloride buffer, chrome black T indicator, disodium magnesium salt of ethylenediaminetetraacetic acid, sodium chloride, formaldehyde aqueous solution and water has the advantages of good stability and high sensitivity. Among them, ethanolamine, diethanolamine and triethanolamine can mask Fe 3 +、Al3+、Cu 2 +、Mn 2 + and other metal ions, reducing the impact of these ions on the test results; disodium magnesium ethylenediaminetetraacetic acid can improve the color sensitivity of chrome black T indicator.

[0029] (2) The present invention loads chrome black T on a metal-organic framework material to reduce its direct contact with air / solvent, thereby improving stability and enhancing the accuracy of the detection results.

[0030] (3) The present invention not only improves the dispersibility of MOF material loaded with chrome black T in water by grafting amino groups on MOF material, but also improves the dispersibility of Fe 3 + and Al3+ are further masked to reduce interference and further improve the accuracy of the detection results. In addition, the rigid ligand biphenyl dicarboxylic acid in the system can expand the metal nodes to form large pores. By limiting the pore size of the MOF material, the MOF material has a better loading capacity while making it easier for calcium and magnesium ions to combine with chrome black T. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a schematic diagram of the colorimetric chamber for water hardness determination;

[0032] Figure numerals: 1. Water sample inlet; 2. Reagent dosing port; 3. 515nm detection light source; 4. Water sample outlet; 5. Light intensity sensor. DETAILED DESCRIPTION

[0033] The present invention will be described below in conjunction with specific embodiments. It should be noted that the following examples are illustrative of the present invention and are intended only to illustrate the present invention and are not intended to limit the present invention. Other combinations and various modifications within the scope of the present invention may be made without departing from the spirit or scope of the present invention.

[0034] In the following examples and comparative examples, except for the chrome black T indicator, all other compounds and related reagents used can be purchased from the market.

[0035] Preparation Example 1

[0036] The preparation method of chrome black T indicator-1 comprises the following steps:

[0037] A1. Mix 1 g of ferric chloride with 1 g of an organic ligand, 1.67 ml of acetic acid, 44 ml of N,N-dimethylformamide, and 11 ml of deionized water, and ultrasonically mix to obtain a metal-organic mixed solution. The metal-organic mixed solution is subjected to a solvothermal reaction at 140°C for 24 h to obtain an Fe-MOF suspension.

[0038] A2. Centrifugally washing and drying the Fe-MOF suspension obtained in step A1 to obtain Fe-MOF material;

[0039] A3, 1g of chrome black T was dissolved in 50g of 50wt% ethanol aqueous solution, 9g of the Fe-MOF material obtained in step A2 was added, and the mixture was stirred in dark at pH 7 for 18h, filtered, washed, centrifuged, and dried in vacuo at 60°C to obtain chrome black T indicator-1.

[0040] The organic ligand is a composition of 2-aminoterephthalic acid and diphenyldicarboxylic acid, and the mass ratio of the two is 1:0.35.

[0041] Preparation Example 2

[0042] The preparation method of Eriochrome Black T Indicator-2 is the same as that of Preparation Example 1, except that the organic ligand is 2-aminoterephthalic acid.

[0043] Preparation Example 3

[0044] The preparation method of Eriochrome Black T Indicator-3 is the same as that of Preparation Example 1, except that the organic ligand is a combination of 2-aminoterephthalic acid and biphenyldicarboxylic acid, and the mass ratio of the two is 1:0.1.

[0045] Preparation Example 4

[0046] The preparation method of Eriochrome Black T Indicator-4 is the same as that of Preparation Example 1, except that the amount of organic ligand added is 1.4 g.

[0047] Preparation Example 5

[0048] The preparation method of Eriochrome Black T Indicator-5 is the same as that of Preparation Example 1, except that the amount of Fe-MOF material added is 13 g.

[0049] Preparation Example 6

[0050] The method for preparing an ammonia-ammonium chloride buffer solution comprises the following steps: dissolving 5.35 g of ammonium chloride in 200 ml of deionized water, adding 7 ml of 26 wt% concentrated ammonia water, stirring at room temperature for 4 minutes, adding deionized water to make the volume 1 L, stirring at room temperature for 4 minutes, and adjusting the pH to 10 to obtain an ammonia-ammonium chloride buffer solution.

[0051] Example 1

[0052] A water hardness detection reagent comprises the following raw materials, measured in parts by weight: 1.2 parts of ethanolamine, 6 parts of diethanolamine, 37 parts of triethanolamine, 54 parts of ammonia-ammonium chloride buffer, 1.7 parts of chrome black T indicator-1, 1.2 parts of disodium magnesium ethylenediaminetetraacetic acid, 0.2 parts of sodium chloride, and 0.2 parts of a 40wt% formaldehyde aqueous solution.

[0053] The preparation method of the water hardness detection reagent of this embodiment comprises the following steps: adding ethanolamine, diethanolamine, and triethanolamine to an ammonia-ammonium chloride buffer solution, stirring at 25° C. for 25 minutes, sequentially adding a 40 wt % formaldehyde aqueous solution, chrome black T indicator-1, disodium magnesium ethylenediaminetetraacetic acid, and sodium chloride, ultrasonically dispersing the solution for 25 minutes in the dark, filtering, and packaging the solution in a brown reagent bottle to obtain a water hardness detection reagent.

[0054] Example 2

[0055] A water hardness detection reagent comprises the following raw materials, measured in parts by weight: 0.8 parts of ethanolamine, 5 parts of diethanolamine, 35 parts of triethanolamine, 52 parts of ammonia-ammonium chloride buffer, 1.5 parts of chrome black T indicator-1, 1 part of disodium magnesium ethylenediaminetetraacetic acid, 0.1 part of sodium chloride, and 0.1 part of a 40wt% formaldehyde aqueous solution.

[0056] The preparation method of the water hardness detection reagent of this embodiment comprises the following steps: adding ethanolamine, diethanolamine, and triethanolamine to an ammonia-ammonium chloride buffer solution, stirring at 23° C. for 30 minutes, sequentially adding a 40 wt % formaldehyde aqueous solution, chrome black T indicator-1, disodium magnesium ethylenediaminetetraacetic acid, and sodium chloride, ultrasonically dispersing the solution for 20 minutes in the dark, filtering, and packaging the solution in a brown reagent bottle to obtain a water hardness detection reagent.

[0057] Example 3

[0058] A water hardness detection reagent comprises the following raw materials, measured in parts by weight: 1.5 parts of ethanolamine, 7 parts of diethanolamine, 40 parts of triethanolamine, 57 parts of ammonia-ammonium chloride buffer, 2 parts of chrome black T indicator-1, 1.5 parts of disodium magnesium ethylenediaminetetraacetic acid, 0.3 part of sodium chloride, and 0.3 part of a 40wt% formaldehyde aqueous solution.

[0059] The preparation method of the water hardness detection reagent of this embodiment comprises the following steps: adding ethanolamine, diethanolamine, and triethanolamine to an ammonia-ammonium chloride buffer solution, stirring at 28° C. for 20 minutes, sequentially adding a 40 wt % formaldehyde aqueous solution, chrome black T indicator-1, disodium magnesium ethylenediaminetetraacetic acid, and sodium chloride, ultrasonically dispersing the solution for 30 minutes in the dark, filtering, and packaging the solution in a brown reagent bottle to obtain a water hardness detection reagent.

[0060] Example 4

[0061] A water hardness detection reagent comprises the following raw materials, measured in parts by weight: 1.2 parts of ethanolamine, 6 parts of diethanolamine, 37 parts of triethanolamine, 54 parts of ammonia-ammonium chloride buffer, 1.7 parts of chrome black T indicator-1, 1.2 parts of disodium magnesium ethylenediaminetetraacetic acid, 0.2 parts of sodium chloride, 0.2 parts of a 40wt% formaldehyde aqueous solution, and 5 parts of hydroxylamine hydrochloride.

[0062] The preparation method of the water hardness detection reagent of this embodiment comprises the following steps: adding ethanolamine, diethanolamine, triethanolamine, and hydroxylamine hydrochloride to an ammonia-ammonium chloride buffer solution, stirring at 25° C. for 25 minutes, sequentially adding a 40 wt % formaldehyde aqueous solution, chrome black T indicator-1, disodium magnesium ethylenediaminetetraacetic acid, and sodium chloride, ultrasonically dispersing the mixture in the dark for 25 minutes, filtering, and packaging the mixture in a brown reagent bottle to obtain a water hardness detection reagent.

[0063] Example 5

[0064] A water hardness detection reagent and a preparation method thereof, wherein the specific implementation manner is the same as that of Example 1, except that an equal amount of chrome black T indicator-1 is replaced by chrome black T indicator-2.

[0065] Example 6

[0066] A water hardness detection reagent and a preparation method thereof, wherein the specific implementation manner is the same as that of Example 1, except that an equal amount of chrome black T indicator-1 is replaced by chrome black T indicator-3.

[0067] Example 7

[0068] A water hardness detection reagent and a preparation method thereof, wherein the specific implementation manner is the same as that of Example 1, except that an equal amount of chrome black T indicator-1 is replaced by chrome black T indicator-4.

[0069] Example 8

[0070] A water hardness detection reagent and a preparation method thereof, wherein the specific implementation manner is the same as that of Example 6, except that an equal amount of chrome black T indicator-1 is replaced by chrome black T indicator-5.

[0071] Example 9

[0072] A water hardness detection reagent and a preparation method thereof, wherein the specific implementation manner is the same as that of Example 1, except that an equal amount of chrome black T indicator-1 is replaced by chrome black T.

[0073] Performance Testing

[0074] The water hardness detection reagents obtained in the above examples were tested for the following properties:

[0075] (1) Preparation of simulated water sample: Dissolve CaCl, FeCl3 and MgCl in deionized water to prepare a simulated water sample with a hardness of 200 mg / L. 3 The concentration of + is 5ppm.

[0076] (2) Determination of water sample hardness: Schematic diagram of the water hardness determination colorimetric chamber is as follows Figure 1As shown, the water sample to be tested in the colorimetric chamber is updated by the automatic detection system sampling control device, and the water sample to be tested is added from the water sample inlet 1. The water hardness detection reagent obtained in Examples 1-9 is stored at 25°C for 0 days and 180 days and then added to the simulated water sample from the reagent dosing port 2. A standard titration solution of disodium ethylenediaminetetraacetic acid with a concentration of 0.01 mol / L is continuously added to the simulated water sample from the reagent dosing port 2 by a plunger pump. Light is emitted from the 515 nm detection light source 3 on the left side of the colorimetric chamber, and the water hardness detection reagent obtained in Examples 1-9 is stored at 25°C for 0 days and 180 days respectively. The light intensity sensor 5 measures the absorbance. As the reagent is continuously added to the water sample, the color will continue to deepen, and the value of the light sensor will continue to decrease until the calcium and magnesium ions in the water sample are completely reacted. The water sample color suddenly changes from red to blue, and the value of the light sensor suddenly rises. The hardness of the water sample can be calculated by measuring the amount of consumed reagent (measured in molar concentration c1, the value is expressed in mmol / L). After the test, distilled water is continuously introduced from the water sample inlet 1 to rinse the colorimetric chamber, and the water sample flows out from the water sample outlet 4. The calculation formula is as follows:

[0077] c1=(V1×c×10 3 ) / V

[0078] Where:

[0079] V1 is the volume of EDTA solution consumed in titrating the water sample, in ml;

[0080] c is the concentration of EDTA solution, in mol / L;

[0081] V is the volume of the water sample in the colorimetric chamber, in ml.

[0082] The hardness of the water sample is converted to CaCO3, and the conversion method is: 1mmol / L=100mg / L (CaCO3).

[0083] (3) Light intensity signal decay rate: The light intensity signal decay rate of the water hardness detection reagent stored at 25°C for 6 months was tested using a spectrophotometer.

[0084] The test results are shown in Table 1:

[0085] Table 1

[0086]

[0087] It can be seen from the data in Table 1 that the water hardness detection reagents in Examples 1-3 of the present invention have high color sensitivity and good stability; from the comparison between Example 4 and Example 1, it can be seen that after adding hydroxylamine hydrochloride, the color sensitivity performance and stability of the water hardness detection reagent are further improved; from the comparison between Examples 5 and 6 and Example 1, it can be seen that when the organic ligand uses only 2-aminoterephthalic acid or changes the ratio of 2-aminoterephthalic acid and biphenyldicarboxylic acid, it will affect the loading amount of the MOF material or the complexation efficiency of calcium and magnesium ions with chrome black T, thereby making the water hardness detection reagent The color development sensitivity decreases; from the comparison of Example 7 and Example 1, it can be seen that changing the ratio of iron salt and organic ligand will affect the porosity and loading amount of MOF material, thereby worsening the color development sensitivity performance of water hardness detection reagent; from the comparison of Example 8 and Example 1, it can be seen that changing the ratio of chrome black T and Fe-MOF material will affect the dispersibility of chrome black T indicator, resulting in a decrease in the color development sensitivity of chrome black T indicator; from the comparison of Example 9 and Example 1, it can be seen that when chrome black T is used directly, the color development sensitivity of water hardness detection reagent is low and the reagent stability decreases.

[0088] The above embodiments are only for illustrating the technical concept and features of the present invention. Its purpose is to enable people familiar with this technology to understand the content of the present invention and implement it. It cannot be used to limit the scope of protection of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A water hardness detection reagent, characterized in that: The preparation method comprises the following raw materials in parts by weight: 0.8-1.5 parts of ethanolamine, 5-7 parts of diethanolamine, 35-40 parts of triethanolamine, 52-57 parts of ammonia-ammonium chloride buffer solution, 1.5-2 parts of chrome black T indicator, 1-1.5 parts of disodium magnesium ethylenediaminetetraacetic acid, 0.1-0.3 parts of sodium chloride, and 0.1-0.3 parts of formaldehyde aqueous solution.

2. The water hardness detection reagent according to claim 1, characterized in that: The preparation method of the ammonia-ammonium chloride buffer solution comprises the following steps: dissolving ammonium chloride in deionized water, adding concentrated ammonia water, stirring at room temperature for 3-5 minutes, adding deionized water to make up the volume, stirring at room temperature for 3-5 minutes, and adjusting the pH to 10 to obtain the ammonia-ammonium chloride buffer solution.

3. The water hardness detection reagent according to claim 1, characterized in that: The preparation method of the chrome black T indicator comprises the following steps: A1. Mixing an iron salt with an organic ligand, acetic acid, and a solvent, and ultrasonically mixing to obtain a metal-organic mixed solution; and subjecting the metal-organic mixed solution to a solvothermal reaction to obtain an Fe-MOF suspension; A2, centrifuging, washing, and drying the Fe-MOF suspension obtained in step A1 to obtain a Fe-MOF material; A3, dissolving chrome black T in 45-55wt% ethanol aqueous solution, adding the Fe-MOF material obtained in step A2, stirring in the dark for 12-24h under the condition of pH 7, filtering, washing, centrifuging, and vacuum drying at 55-65°C to obtain chrome black T indicator.

4. The water hardness detection reagent according to claim 3, characterized in that: The organic ligand is a composition of 2-aminoterephthalic acid and biphenyldicarboxylic acid, and the mass ratio of the two is 1:(0.2-0.5).

5. The water hardness detection reagent according to claim 3, characterized in that: The mass ratio of the iron salt to the organic ligand is 1:(0.8-1.2).

6. The water hardness detection reagent according to claim 3, characterized in that: The solvent is a composition of N,N-dimethylformamide and deionized water, and the volume ratio of the two is (2-6):

1.

7. The water hardness detection reagent according to claim 3, characterized in that: The mass ratio of the chrome black T to the Fe-MOF material is 1:(7-11).

8. The water hardness detection reagent according to claim 1, characterized in that: The water hardness detection reagent further contains 3-7 parts by weight of hydroxylamine hydrochloride.

9. A method for preparing a water hardness detection reagent according to any one of claims 1 to 7, characterized in that: The method comprises the following steps: adding ethanolamine, diethanolamine and triethanolamine to an ammonia-ammonium chloride buffer solution, stirring at 25°C±2°C for 20-30 minutes, sequentially adding a formaldehyde aqueous solution, an chrome black T indicator, disodium magnesium ethylenediaminetetraacetic acid and sodium chloride, ultrasonically dispersing for 20-30 minutes in the dark, filtering and packaging in a brown reagent bottle to obtain a water hardness detection reagent.

10. Use of the water hardness detection reagent according to any one of claims 1 to 8 or the water hardness detection reagent prepared by the preparation method of claim 9, characterized in that: The method comprises the following steps: placing a water sample in a colorimetric chamber, adding a water hardness detection reagent, continuously adding a standard titration solution of disodium ethylenediaminetetraacetic acid with a concentration of 0.01 mol / L or 0.005 mol / L into the water sample through a pump, and calculating the hardness of the water sample by titrating until the titration endpoint is reached based on the volume of reagent consumed.

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