Method for determining iodine in regional geochemical survey sample

Abstract

The invention provides a method for measuring iodine in a regional geochemical survey sample, and belongs to the technical field of measuring methods. The iodine content in the sample is determined by adopting a spectrophotometric method. The method comprises the following steps: mixing a sample with an Eschka mixed solvent, heating and melting through a program, extracting in a water bath, and fixing the volume; taking supernate, sequentially adding sodium carbonate, acetic acid, tetraalkali and chloramine T solution for color development, and measuring the absorbance in a cuvette with the wavelength of 600nm and the length of 1cm or 3cm. Drawing a standard curve through an iodine standard solution, and calculating the iodine content of the sample according to the absorbance after the blank is deducted. The method is low in detection limit, high in accuracy and good in precision. The method is easy to operate, low in cost, superior to the existing national standard in efficiency, suitable for analysis and test of large-batch geochemical sample iodine and wide in application prospect.

Description

Technical Field

[0001] This invention belongs to the field of measurement method technology, and in particular relates to a method for determining iodine in regional geochemical survey samples. Background Technology

[0004] Various methods exist for the determination of iodine. Currently, trace iodine in regional geochemical survey samples is mainly determined using ion chromatography, inductively coupled plasma mass spectrometry (ICP-MS), and catalytic spectrophotometry. Ion chromatography typically requires an amperometric detector, resulting in high overall equipment costs, and the detection limit is relatively high, leading to few reports in recent years. ICP-MS offers excellent sensitivity, with detection limits as low as 0.00X mg / kg and a wide linear range, making it widely used in soil, food, environmental, and agricultural testing. However, this method is costly, the resin treatment and exchange steps are time-consuming, and the instrument exhibits a strong memory effect. Classical spectrophotometry remains widely used due to its low cost, economical equipment, and ease of implementation. However, Fe... 3+ -CNS - -NO 2- Traditional spectrophotometric methods using catalytic systems suffer from numerous interfering factors, poor stability, long operation cycles, and practical detection limits exceeding 1.0 mg / kg, making them inaccurate for determining the content of samples with concentrations <1.0 mg / kg. This fails to meet the detection requirements of the "Specifications for Multi-Target Regional Geochemical Survey (1:250000)" (DZ / T 0258-2014). This study employs an optimized chloramine T-tetraalkali catalytic spectrophotometric method, providing a low-cost, highly accurate, precise, and low-detection-limit method suitable for batch detection of regional geochemical samples. Summary of the Invention

[0005] This invention provides a method for determining iodine in regional geochemical survey samples, comprising the following steps:

[0006] S1 Sample preparation: Crush the sample, sieve it, and then dry it;

[0007] S2 Sample Pretreatment: Mix the sample with Esc flux and stir evenly in a container, then cover with another layer of Esc flux; calcine using a programmed temperature rise method; cool after calcine; transfer the molten mass to a container with hot water, place in a boiling water bath, cool and bring to volume to the mark after the water bath, and let it stand to clarify; thus obtaining the sample solution.

[0008] The Aiska mixed flux is a mixture of sodium carbonate and zinc oxide in a mass ratio of 3:2;

[0009] Preparation of blank sample S3: Prepare at least two blank samples using the same sample pretreatment method as S2; obtain blank experimental solution;

[0010] Determination of S4 sample: Take the supernatant from the sample solution and add water to obtain a fixed volume solution; add sodium carbonate solution and acetic acid solution quantitatively in sequence to homogenize, add 4,4'-tetramethyldiaminodiphenylmethane solution to homogenize, add chloramine T solution to homogenize; then transfer the solution to a cuvette, use water as a reference solution and measure the absorbance at a wavelength of 600 nm using a spectrophotometer; at the same time, perform the same operation on the blank experimental solution and record the blank absorbance;

[0011] S5 curve determination: Take iodine standard solutions of different concentrations and follow the same steps as the sample determination in step S4 to determine the maximum absorbance value and plot the absorbance-iodine content standard curve.

[0012] S6 Iodine content calculation: The iodine content of the sample is calculated based on the absorbance-iodine content standard curve after subtracting the blank absorbance.

[0013] The formula for iodine content is:

[0014] In the formula:

[0015] A 样品 —Absorbance measured from uncalibrated samples;

[0016] A 空白 —The average absorbance of the blank sample;

[0017] a — The slope of the absorbance-iodine content standard curve;

[0018] b — Intercept of the absorbance-iodine content standard curve;

[0019] S — Dilution factor;

[0020] m — Mass of the sample, in grams (g).

[0021] Preferably, in the preparation of the S1 sample, the particle size of the sample should be <74μm; and it should be dried at (105±5)℃ for 2h.

[0022] Preferably, in step S2 sample pretreatment, when the sample is mixed with the EAST mixed flux, the amount of EAST mixed flux used is three times the mass of the sample; when covering with the EAST mixed flux, the amount of EAST mixed flux used is three times the mass of the sample.

[0023] Preferably, the specific method for programmed temperature rise in the S2 sample pretreatment is as follows:

[0024] Phase 1: Raise the room temperature to 400℃ and maintain it for 30 minutes;

[0025] Second stage: Increase from 400℃ to 700℃ and maintain for 60 minutes.

[0026] Preferably, in the S2 sample pretreatment, the boiling water bath time is 60 min.

[0027] Preferably, in the sample determination in step S4, sodium carbonate solution, acetic acid solution, 4,4'-tetramethyldiaminodiphenylmethane solution, and chloramine T solution are added sequentially and quantitatively. The absorbance measurement is completed within 60 minutes after adding the 4,4'-tetramethyldiaminodiphenylmethane solution, and the ambient temperature during the measurement is between 18-23°C.

[0028] Preferably, the concentration of the 4,4'-tetramethyldiaminodiphenylmethane is 0.1 g / L; the concentration of the sodium carbonate solution is 36 g / L; the concentration of the acetic acid solution is 8% (v / v); and the concentration of the chloramine T solution is 1.0 g / L.

[0029] Preferably, when the iodine content in the sample is ≤1.0 mg / kg, in step S4, 2.0 mL of supernatant is taken and colorimetrically measured in a 1 cm cuvette, or 1.0 mL of supernatant is taken and colorimetrically measured in a 3 cm cuvette.

[0030] When the iodine content in the sample is >1.0 mg / kg, take 1.0 mL of the supernatant and measure the color in a 1 cm cuvette;

[0031] The total volume of the diluted solution is 10.0 mL.

[0032] This invention employs an optimized ammonium chloride T-tetraalkali catalytic system for spectrophotometric determination of iodine. It is the first to propose a segmented testing approach based on content, establishing a catalytic spectrophotometric method applicable to different content ranges. This method is used for the determination of trace iodine in samples from regional geochemical surveys, effectively solving the technical challenge of accurate quantification in samples <1.0 mg / kg. The results of numerous sample and standard material analyses demonstrate that, within the temperature range of 18–23℃, stable and reliable analytical results can still be obtained without strictly accurate volume determination of the solution involved in the catalytic colorimetric reaction.

[0033] The detection limit of this invention is 0.12 μg / g, and the method precision is 3.6-8.5%. Validated by national primary standard reference materials and external quality control samples, the accuracy and precision meet geological specifications. This method is applicable to the determination of trace iodine in regional geochemical survey samples (rocks, soils, and stream sediments, etc.).

[0034] The detection method employed in this invention features a low detection limit, high accuracy, and good precision. The method is easy to operate, low in cost, and more efficient than existing national standards. It is suitable for the analysis and testing of iodine in large batches of geochemical samples and has broad application prospects. Detailed Implementation

[0035] To better understand the present invention, the present invention will be further described below with reference to specific serial numbers. The terminology used in the serial numbers is for describing specific embodiments and does not constitute a limitation on the scope of protection of the present invention.

[0036] In the specific implementation methods, unless otherwise specified, the experimental methods used are all conventional methods, and the materials and reagents used are all commercially available unless otherwise specified.

[0037] Unless otherwise specified, percentages (%) in the specific implementation method are assumed to be volume percentages.

[0038] Example 1

[0039] I. Instruments, Reagents and Materials

[0040] 1.1 Instruments

[0041] Visible spectrophotometer (wavelength range: 340 nm~1000 nm); electronic balance (sensitivity 0.1 mg); constant temperature water bath; box-type resistance furnace.

[0042] 1.2 Reagents and Materials

[0043] Unless otherwise stated, all reagents used in the analysis were of analytical grade, and the water was Grade I water as specified in GB / T 6682.

[0044] 1.2.1 Glacial acetic acid; anhydrous ethanol; anhydrous sodium carbonate; zinc oxide; chloramine T; potassium iodate; 4,4'-tetramethyldiaminodiphenylmethane (0.1 g / L); sodium carbonate solution (36 g / L); acetic acid solution (8%); chloramine T solution (1.0 g / L).

[0045] 1.2.2 Escal mixed flux: Sodium carbonate (100 mesh) and zinc oxide are mixed at a mass ratio of 3+2.

[0046] 1.2.3 Iodine standard solution [ρ(I) = 1000 μg / mL]: Commercially available certified single-element standard stock solution, stored protected from light. Use this as a stock solution to dilute and prepare iodine working solution (ρ(I) = 100 μg / L).

[0047] II. Sample Preparation

[0048] The sample should pass through a 0.074 mm sieve. The sample should be pre-dried at (105±5)℃ for 2 h and then cooled to room temperature in a desiccator.

[0049] 2.1 Sample Pretreatment

[0050] Weigh 0.5 g (accurate to 0.1 mg) of the sample and place it in a porcelain crucible pre-filled with a 3:2 mixture of sodium carbonate and zinc oxide. Stir well, then cover with another 1.5 g of the mixture. Place the crucible in a muffle furnace and use a programmed temperature rise method. Raise the temperature from room temperature to 400 °C and hold for 30 min, then gradually increase the temperature to 700 °C and hold for 60 min. After the sample has cooled, remove it and transfer the molten material to a 50 mL glass colorimetric tube using hot water. Incubate the tube in boiling water for 1 h, cool, and then dilute to the mark. Allow the solution to settle.

[0051] 2.2 Blank Test

[0052] At least two blank tests were conducted along with the sample.

[0053] 2.3 Sample Determination

[0054] Transfer 1.0-2.0 mL of supernatant to a beaker, add 9.0 mL or 8.0 mL of experimental water to a final volume of 10.0 mL, then add 5.0 mL of sodium carbonate (36 g / L) and 5.0 mL of acetic acid solution (8%), shake well, add 5.0 mL of tetraalkaloid (0.1 g / L) solution, shake well, then add 1.0 mL of chloramine T solution, mix well, and quickly transfer the solution to a 1 cm or 3 cm cuvette. Using water as a reference solution, record the maximum absorbance value at a spectrophotometer wavelength of 600 nm.

[0055] 2.4 Curve Determination

[0056] Accurately transfer 0.00 mL, 0.10 mL, 0.20 mL, 0.40 mL, 0.80 mL, and 1.00 mL of iodine working solution (ρ(I) = 100 μg / L) into beakers, add water to 10 mL, and plot the working curve according to step 2.3.

[0057] III. Results and Discussion

[0058] 3.1 Effect of sample sintering temperature and time

[0059] To reduce the rapid volatilization of iodine at high temperatures, this study followed the method described in this paper and adopted a stepwise heating approach. The temperature was raised from room temperature to 400°C and held for 30 minutes to allow the flux to fully react with the sample, avoiding local overheating or incomplete reaction. Then, the temperature was raised to the set endpoint temperature.

[0060] Standard substances with different content ranges were selected, and three parallel portions were weighed. Eska reagent was added, and the effects of four sintering temperatures (600℃, 650℃, 700℃, 750℃, 800℃, and 850℃) on the iodine determination results were investigated. The experimental results showed that an accurate and stable determination result could be obtained at a sintering temperature of 700℃; higher or lower sintering temperatures resulted in lower determination results. Therefore, a sintering temperature of 700℃ was selected for the experiment. The holding time experiment showed that holding the temperature for 40-80 minutes after reaching the set temperature yielded accurate and stable determination results. Therefore, the sintering time was determined by heating from a low temperature to 700℃ and then holding for 60 minutes.

[0061] 3.2 Effect of water bath extraction time

[0062] By varying the water bath time to four conditions (30, 40, 60, and 70 min), the effects of bathing 0.08 μg of iodide ions (I₂) on the concentration of iodide ions were investigated. ⁻ The absorbance of the solution was measured. The results showed that the iodine recovery rate reached 102% when heated in a boiling water bath for 1 h. Considering both analytical efficiency and measurement accuracy, 60 min was selected as the optimal water bath time for this method.

[0063] 3.3 Effects of Temperature and Acidity

[0064] Iodine has multiple variable valence states, is easily oxidized and reduced, and is highly volatile, requiring careful temperature control during determination. This experiment utilizes the catalytic effect of iodide ions to determine iodine; temperature affects the reaction rate. Higher temperatures accelerate the oxidation and loss of iodide ions, leading to lower experimental results. Experimental results show that for the same batch of samples, the temperature needs to be kept constant, controlled between 18-23℃, rather than adhering to a single temperature, which is more conducive to operation. For every 1℃ increase, the catalytic reaction rate increases by approximately 5%.

[0065] The catalytic process of iodide ions is rapid. From the addition of chloramine T to the formation of the unstable blue intermediate, the color fades quickly. Therefore, the measurement must be rapid. If the absorbance only decreases and does not increase, the measurement must be repeated.

[0066] An 8% acetic acid solution was prepared, and the absorbance of a solution containing 0.08 μg of iodide ions (I⁻) was measured under varying amounts. Using 3 g of sodium carbonate-zinc oxide flux for sample melting, the optimal amount of acetic acid was determined to be 5 mL. Although hydrogen ions (H⁺) participate in the oxidation reaction of hypochlorite ions (ClO⁻) on I⁻, the color depth does not monotonically increase with increasing H⁺ concentration. Actual results show that once the H⁺ concentration reaches a certain level, further increasing the acidity leads to a decrease in the color depth. Therefore, for the same batch of samples, the amount of flux and acetic acid added must be strictly controlled to ensure consistency of experimental conditions. Furthermore, after adding acetic acid, the mixture should be shaken rapidly to remove carbon dioxide bubbles and avoid prolonged standing, which could affect the measurement. Based on practical experience, the tetraalkali solution was added 10 min after adding the acetic acid solution.

[0067] 3.4 Effects of the four bases

[0068] The tetraalkali solution has poor stability and must be prepared and used immediately. The experiment also investigated the effect of the settling time after adding the tetraalkali on the absorbance: after adding the tetraalkali to a solution containing 0.08 μg of iodide ions (I⁻), the color depth remained basically consistent within 5-60 minutes of settling. Therefore, this experiment selected to complete the colorimetric determination within 60 minutes after adding the tetraalkali.

[0069] 3.5 Effect of chloramine T

[0070] An ammonium chloride T solution with a concentration of 1.0 g / L was prepared. While varying the amount of ammonium chloride T added, the absorbance of the solution containing 0.08 μg of iodide ions (I⁻) was measured. The experimental results showed that the optimal amount of ammonium chloride T added was 1 mL.

[0071] 3.6 Selection of Cuvettes

[0072] When the sample content is ≤0.5mg / kg, 1mL of sample is taken and the absorbance is measured using a 1cm cuvette. Due to the low content, the repeatability RD% is between -9.1% and 43%, indicating poor repeatability. To ensure accurate measurement, 1cm and 3cm cuvettes are used for sample determination. The results show that the results measured using the 3cm cuvette are consistent with the results determined by the ICP-MS method in "Determination of Iodine and Bromine Content in Soil and Aqueous Sediments - Semi-melting Inductively Coupled Plasma Mass Spectrometry" (GB / T 42248-2022).

[0073] When the sample content is between 0.5 mg / kg and 1.0 mg / kg, the repeatability of the colorimetric results from the 1 cm cuvette is less than 10%, and it is basically consistent with the results from the 3 cm cuvette and ICP-MS. To reduce errors, samples with iodine content ≤ 1.0 mg / kg are determined using a 3 cm cuvette.

[0074] When the sample content is >1.0 mg / kg, the results obtained by measuring with 1cm and 3cm cuvettes are consistent. To avoid dilution of high-content samples, reduce workload, and improve efficiency, 1cm cuvettes are used to measure samples with iodine content >1.0 mg / kg.

[0075] The results are shown in Table 1.

[0076] Table 1. Effect of trace iodine on cuvettes (unit: mg / kg)

[0077]

[0078] In Table 1, the experimental numbers represent the year of sample collection, project code, batch number, and sample serial number. For example, in 23DDS030-0015, 23 refers to the year of sample collection in 2023; DDS refers to the geological survey project code; 030 refers to the batch number; and 0015 refers to the sample serial number. Other experimental numbers are the same.

[0079] 3.7 Effect of the volume taken

[0080] When the iodine content in the sample is ≤1.0 mg / kg, 1 mL, 2 mL, and 3 mL of the preparation solution were aliquoted into a 1 cm cuvette for catalytic reaction to investigate the effect of the aliquot volume on the determination results. The results showed that when the aliquot volume was 1 mL, the measured values ​​of both the actual sample and the national primary standard reference material were 16%–54% lower; while when the aliquot volume was 2 mL or 3 mL, satisfactory analytical results were obtained for national primary standard reference materials such as GSD-16, GSD-19, GSD-20, GSS-56, GSD-7a, and GSS-58.

[0081] When the iodine content in the sample is >1.0 mg / kg, 1 mL of the preparation solution is aliquoted for the catalytic reaction, and the result is determined using a 1 cm cuvette. The analytical results of standard substances such as GSS-8a, GSS-14, and GSS-32 all indicate that the accuracy and precision of this method are satisfactory. The results are shown in Table 2.

[0082] Therefore, using a 1cm cuvette, when the sample content is ≤1.0mg / kg, 2mL of the preparation solution is taken for the catalytic reaction; when the sample content is >1.0mg / kg, 1mL of the solution is taken for the catalytic reaction.

[0083] Table 2 Effect of trace iodine separation volume (unit: mg / kg)

[0084]

[0085] In Table 2, the experimental numbers represent the year of sample collection, project code, batch number, and sample serial number. For example, in 23DDS058-0184, 23 refers to the year of sample collection in 2023; DDS refers to the geological survey project code; 058 refers to the batch number; and 0184 refers to the sample serial number. Other experimental numbers are the same. GSD-16 and other designations refer to national first-class certified reference materials; GSD refers to aquatic sediment reference materials; and GSS refers to soil reference materials.

[0086] 3.8 Method Detection Limit

[0087] Under the selected experimental conditions, 12 blank samples were measured in parallel, and the detection limit (3S) of the method was calculated to be 0.12 μg / g.

[0088] 3.9 Precision and Accuracy of the Method

[0089] Based on the experimental conditions determined in this study, 12 parallel determinations were performed on 12 national primary standard reference materials. The results are shown in Table 3.

[0090] Table 3. Precision and accuracy of the method

[0091]

[0092] Table 3 (continued) Precision and accuracy of the method

[0093]

[0094] As shown in Table 3, the analysis results are consistent with the reference values. The accuracy of this method is ΔlgC≤0.02, and the precision is 3.6-8.5%, which meets the requirements of the "Specification for Multi-Objective Regional Geochemical Survey (1:250000)" (DZ / T 0258-2014).

[0095] Based on the experimental results described above, this invention, building upon the optimized chloramine-T-tetraalkali system catalytic spectrophotometric method, proposes for the first time a segmented determination of iodine in regional geochemical samples according to its content. When the iodine content in the sample is ≤1.0 mg / kg, 2.0 mL of the preparation solution can be aliquoted for the catalytic reaction and measured using a 1 cm cuvette, or 1.0 mL of the preparation solution can be aliquoted and measured using a 3 cm cuvette. When the iodine content in the sample is >1.0 mg / kg, 1.0 mL of the preparation solution can be aliquoted and measured using a 1 cm cuvette. The solution used for catalytic colorimetry does not require precise volume determination, and stable and reliable results can be obtained by measuring within the range of 18–23℃. This method features a low detection limit, high accuracy, good precision, simple operation, low cost, and analytical efficiency superior to current national standard methods, making it suitable for the rapid determination of iodine in large batches of geochemical samples.

[0096] The above detailed description is a specific description of one of the feasible embodiments of the present invention. This embodiment is not intended to limit the patent scope of the present invention. All equivalent implementations or modifications that do not depart from the present invention should be included within the scope of the technical solution of the present invention.

Claims

1. A method for determining iodine in regional geochemical survey samples, characterized in that, Includes the following steps, S1 Sample preparation: Crush the sample, sieve it, and then dry it; S2 Sample Pretreatment: Mix the sample with Esc flux and stir evenly in a container, then cover with another layer of Esc flux; calcine using a programmed temperature rise method; cool after calcine; transfer the molten mass to a container with hot water, place in a boiling water bath, cool and bring to volume to the mark after the water bath, and let it stand to clarify; thus obtaining the sample solution. The Aiska mixed flux is a mixture of sodium carbonate and zinc oxide in a mass ratio of 3:2; Preparation of blank sample S3: Prepare at least two blank samples using the same sample pretreatment method as S2; obtain blank experimental solution; Determination of S4 sample: Take the supernatant from the sample solution and add water to obtain a fixed volume solution; Sodium carbonate solution and acetic acid solution were added sequentially and quantitatively until homogenized. Then, 4,4'-tetramethyldiaminodiphenylmethane solution was added and homogenized. Chloramine T solution was added and homogenized. The solution was then transferred to a cuvette, and the absorbance was measured at a wavelength of 600 nm using water as a reference solution. The same operation was performed on the blank experimental solution, and the blank absorbance was recorded. S5 curve determination: Take iodine standard solutions of different concentrations and follow the same steps as the sample determination in step S4 to determine the maximum absorbance value and plot the absorbance-iodine content standard curve. S6 Iodine content calculation: The iodine content of the sample is calculated based on the absorbance-iodine content standard curve after subtracting the blank absorbance. The formula for iodine content is: In the formula: A 样品 —Absorbance measured from uncalibrated samples; A 空白 —The average absorbance of the blank sample; a — The slope of the absorbance-iodine content standard curve; b — Intercept of the absorbance-iodine content standard curve; S — Dilution factor; m — Mass of the sample, in grams (g).

2. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, In the preparation of the S1 sample, the particle size of the sample should be <74μm; and it should be dried at (105±5)℃ for 2h.

3. The method for determining iodine in regional geochemical survey samples according to claim 1, wherein in step S2 sample pretreatment, the amount of EAST mixed flux used when mixing the sample with EAST mixed flux is three times the mass of the sample; and the amount of EAST mixed flux used when covering the sample is three times the mass of the sample.

4. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, The specific method for programmed temperature rise in the S2 sample pretreatment is as follows: Phase 1: Raise the room temperature to 400℃ and maintain it for 30 minutes; Second stage: Increase from 400℃ to 700℃ and maintain for 60 minutes.

5. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, In the S2 sample pretreatment, the boiling water bath time is 60 minutes.

6. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, In the sample determination in step S4, sodium carbonate solution, acetic acid solution, 4,4'-tetramethyldiaminodiphenylmethane solution, and chloramine T solution are added sequentially in quantitative amounts.

7. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, In the determination of the S4 sample, the absorbance was measured within 60 minutes after adding 4,4'-tetramethyldiaminodiphenylmethane solution, and the ambient temperature was between 18-23℃ during the measurement.

8. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, The concentration of the 4,4'-tetramethyldiaminodiphenylmethane is 0.1 g / L; the concentration of the sodium carbonate solution is 36 g / L; the concentration of the acetic acid solution is 8% (v / v); and the concentration of the chloramine T solution is 1.0 g / L.

9. The method for determining iodine in regional geochemical survey samples according to claim 1, characterized in that, When the iodine content in the sample is ≤1.0 mg / kg, in step S4, 2.0 mL of supernatant is taken and colorimetrically measured in a 1 cm cuvette, or 1.0 mL of supernatant is taken and colorimetrically measured in a 3 cm cuvette. When the iodine content in the sample is >1.0 mg / kg, take 1.0 mL of the supernatant and measure the color in a 1 cm cuvette; The total volume of the diluted solution is 10.0 mL.

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