A graphene electrode for detecting chromium (VI) ions in an aquatic environment and its application.

Abstract

This invention discloses a graphene electrode for detecting chromium (VI) ions in aquatic environments and its applications, belonging to the field of water quality testing technology. This invention utilizes a laser-induced graphene method to fabricate working, counter, and reference electrode regions on a polyimide (PI) film substrate, integrating the three-electrode system onto the same substrate. Simultaneously, the working electrode region is modified with a Fe3O4@PANI / Ce-MOF composite material exhibiting excellent electrochemical performance for the detection of hexavalent chromium in aquatic environments. The modified laser-induced graphene electrode can replace commercially available working (glassy carbon), counter (platinum wire), and reference (Ag / AgCl) electrodes, enabling mass production of the electrode. This not only reduces detection costs but also provides the integrated electrode with a degree of flexibility, making it simple and convenient to use and possessing higher practical value.

Description

Technical Field

[0001] This invention relates to the field of water body detection technology, and in particular to a graphene electrode for detecting chromium (VI) ions in aquatic environments and its application. Background Technology

[0002] Laser-induced graphene (LIG) is a novel method for preparing carbon-based electrodes in recent years. Polymer-based materials are commonly used induction substrates due to their high strength, flexibility, ability to withstand high temperatures, corrosion resistance, and low cost. These include poly(p-polyimide) (PI), polyethylene terephthalate (PET), and polydimethylsiloxane (PDMS). PI is a common laser-induced graphene electrode substrate material because its imine structure contains abundant hexagonal carbon, making it an excellent precursor for graphene synthesis. Furthermore, PI has high absorption rates in the infrared and near-ultraviolet regions, allowing various types of lasers to directly convert PI films into graphene structures. The principle of laser-induced graphene involves using a laser to etch the PI film, causing a photothermal decomposition reaction on its surface. The thermal energy from the laser radiation causes the CN, CO, and C=O bonds on the imine rings of the main chain to saturate and break, leaving only six-membered carbon rings that cross-link and polymerize, ultimately synthesizing a six-membered ring-like graphene structure. Therefore, mass production of graphene electrodes can be achieved using PI thin films as substrates and laser-induced technology. Laser-induced graphene fabrication of carbon-based electrodes is simple and time-efficient, avoiding any wet chemical reactions or steps in a one-step laser-induced process, and the resulting graphene exhibits excellent electrical conductivity. Furthermore, compared to other conductive components, the pattern shape of LIG can be designed on demand via computer.

[0003] Based on this, the present invention proposes a graphene electrode for the detection of chromium (VI) ions in aquatic environments and its application. The working electrode, counter electrode and reference electrode regions are prepared on a PI substrate by laser-induced graphene, and the three-electrode system is integrated on the same substrate. At the same time, the working electrode region is modified with Fe3O4@PANI / Ce-MOF composite material with excellent electrochemical performance for the detection of hexavalent chromium in aquatic environments. Summary of the Invention

[0004] The purpose of this invention is to provide a low-cost graphene electrode for detecting chromium (VI) ions in aquatic environments based on composite material modification, and its application, which can perform real-time on-site detection. The Fe-P-Ce / LIG sensor prepared using this electrode has a wide linear range, as well as better reproducibility and stability, and can be more conveniently and accurately applied to the detection of hexavalent chromium in aquatic environments.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A graphene electrode for detecting chromium (VI) ions in an aquatic environment is formed by integrating a working electrode, a counter electrode, and a reference electrode system on a flexible substrate using a laser-induced graphene method. The working electrode surface is modified with Fe3O4@PANI / Ce-MOF.

[0007] Preferably, the method of forming a three-electrode system on a flexible substrate by laser-induced graphene specifically includes the following:

[0008] A1. Draw the patterns of the working electrode, reference electrode, and counter electrode in the computer program, and set the power and depth of laser etching in the program;

[0009] A2. The PI film is flattened and tightly attached to the treated carbon paper, and then placed in a laser etching machine for laser etching to obtain the working electrode, counter electrode and reference electrode.

[0010] A3. Apply a uniform layer of silver paste to the etched reference electrode area and allow it to air dry naturally.

[0011] A4. Cut the three electrodes on the PI film and attach a blue insulating layer.

[0012] Preferably, the working electrode is prepared as follows:

[0013] A1. Weigh 10-20 mg of Fe3O4@PANI nanoparticles and add them to 10-20 mL of 75% ethanol, and sonicate for 30-60 min; then weigh 5-10 mg of Ce-MOF powder and add it to the above mixture and continue to sonicate for 2-3 h. After mixing evenly, Fe3O4@PANI / Ce-MOF suspension is obtained.

[0014] A2. The Fe3O4@PANI / Ce-MOF suspension obtained in A1 is drop-coated onto the working electrode area and dried to prepare the modified working electrode, denoted as Fe-P-Ce / LIG.

[0015] Preferably, the mass ratio of Fe3O4@PANI nanoparticles to Ce-MOF powder used in A1 is 2:1.

[0016] Application of graphene electrodes in the detection of chromium (VI) ions in aquatic environments.

[0017] Preferably, the specific detection method includes the following steps:

[0018] S1. A Fe-P-Ce / LIG sensor was fabricated based on a three-electrode system containing Fe-P-Ce / LIG.

[0019] S2. The Fe-P-Ce / LIG sensor prepared in S1 is used to determine the chromium (VI) ions in the water body to be tested.

[0020] Compared with the prior art, the present invention provides a graphene electrode for the detection of chromium (VI) ions in an aquatic environment and its application, which has the following beneficial effects:

[0021] This invention utilizes laser-induced graphene to fabricate working, counter, and reference electrode regions on a PI substrate, integrating a three-electrode system onto a single substrate. Simultaneously, the working electrode region is modified with a Fe3O4@PANI / Ce-MOF composite material exhibiting excellent electrochemical performance for the detection of hexavalent chromium in aquatic environments. The modified laser-induced graphene can replace commercially available working (glassy carbon), counter (platinum wire), and reference (Ag / AgCl) electrodes, enabling mass production of the electrodes. This not only reduces detection costs but also provides the integrated electrode with a degree of flexibility, ease of use, and higher practical application value. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the laser-induced graphene preparation of three electrodes and modification process in Example 1 of the present invention;

[0023] Figure 2 (A) CV images of LIG and Fe-P-Ce / LIG in K3[Fe(CN)6] (5 mmol / L) solution; Figure 2 (B) EIS images of LIG and Fe-P-Ce / LIG in K3[Fe(CN)6] (5 mmol / L) solution;

[0024] Figure 3 (A) DPASV images of LIG and Fe-P-Ce / LIG in 0.1 mg / L Cr(VI) solution; Figure 3 (B) DPASV images of Cr(VI) solution detected when Fe3O4@PANI / Ce-MOF composite material was modified with LIG and GCE respectively;

[0025] Figure 4 (A) is a photograph of the actual product at power levels of 30%, 60%, and 90%. Figure 4 (B) is the CV image of LIG in K3[Fe(CN)6] solution with power of 60% to 90%; Figure 4 (C) shows the LIG physical images at depths of 10, 20, 30, and 40. Figure 4 (D) is the CV image of LIG in K3[Fe(CN)6] solution at depths of 10–30;

[0026] Figure 5(A) is a schematic diagram of the DPSV response of different concentrations of Cr(VI); Figure 5 (B) is a schematic diagram showing the linear relationship between concentration and peak current value;

[0027] Figure 6 (A) is a schematic diagram of the peak current values ​​corresponding to four consecutive detections of hexavalent chromium solution by Fe-P-Ce / LIG. Figure 6 (B) is a schematic diagram showing the current values ​​of multiple Fe-P-Ce / LIG detectors for the same hexavalent chromium solution. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0029] It should be emphasized that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While any methods, apparatus, and materials similar to or equivalent to those herein may be used in the practice or testing of this invention, preferred methods, apparatus, and materials are now described.

[0030] This invention proposes a graphene electrode for detecting chromium (VI) ions in aquatic environments and its application. The Fe-P-Ce / LIG sensor prepared using this electrode has a wide linear range and good reproducibility, and can be applied more conveniently and accurately to the detection of hexavalent chromium in aquatic environments. Specific embodiments are as follows.

[0031] Example 1:

[0032] (1) Experimental reagents and materials

[0033] Table 1. Reagents, specifications, and suppliers used in the experiment.

[0034]

[0035]

[0036] (2) Experimental apparatus

[0037] The main experimental instruments used in this invention are an electrochemical workstation (CHI660E, Shanghai Chenhua) and a laser etching machine (NanoPro-Ⅲ, Tianjin Jiayin Nanotechnology Co., Ltd.). All electrochemical experiments were conducted on the electrochemical workstation. Instead of commercially available glassy carbon electrodes, platinum wire, and Ag / AgCl, the three electrodes were prepared by inducing the formation of graphene in a PI film using a computer-controlled laser etching machine, followed by the application of silver paste.

[0038] (3) Preparation of solutions required for the experiment

[0039] 3.1) Preparation of electrolytes supported by 0.1M hydrochloric acid

[0040] Use a pipette to transfer 4.17 mL of concentrated hydrochloric acid into a 50 mL volumetric flask, dilute to volume, shake well, and pour into a reagent bottle to obtain a 1.0 M hydrochloric acid solution as a stock solution. Continue using a pipette to transfer 10 mL of the stock solution into a 100 mL volumetric flask, dilute to volume, and obtain a 0.1 M hydrochloric acid supporting electrolyte solution.

[0041] 3.2) Preparation of 50 mg / L hexavalent chromium standard solution

[0042] Weigh 0.1415±0.0001 g of potassium dichromate (K2Cr2O7, analytical grade) dried at 110℃ for 2 h using an analytical balance. Dissolve the potassium dichromate in water, transfer the solution to a 1000 ml volumetric flask, dilute to the mark with water, and mix well. This solution is a 50 mg / L hexavalent chromium standard solution and should be stored as a stock solution.

[0043] 3.3) Preparation of 1 mg / L hexavalent chromium standard solution

[0044] Prepare a clean 100mL volumetric flask. Use a pipette to transfer 5mL of 50mg / L lead solution into a 250mL volumetric flask. Dilute to volume with deionized water, shake well, and dilute to 1mg / L to obtain a 1mg / L hexavalent chromium standard solution.

[0045] (4) Preparation of Modification Materials

[0046] 4.1) Synthesis of Ce-MOF

[0047] First, an aqueous solution of Ce(NO3)3 was obtained by dissolving Ce(NO3)3·6H2O (0.5 mmol, 0.217 g) in 1 mL of deionized water. This Ce(NO3)3 aqueous solution was then added to a vigorously stirred 1,3,5-H3BTC (0.5 mmol) aqueous-ethanol solution (40 mL, v / v = 1:1) and continuously stirred with a magnetic stirrer for 5 h. The resulting product was centrifuged at 4000 pm for 8 min, washed several times with deionized water, and vacuum dried at 80 °C to obtain a white solid powder of Ce-MOF.

[0048] 4.2) Synthesis of Fe3O4@PANI

[0049] Fe3O4 polyaniline composites were prepared by oxidative polymerization of aniline using ammonium persulfate (APS) in an acidic medium. First, 0.4 g of Fe3O4 nanoparticles were weighed and dispersed in 50 mL of 1 M hydrochloric acid using an electronic balance, and sonicated for 45 min. After sonication, another 50 mL of 1 M hydrochloric acid was added to the solution, followed by 98 μL of aniline. The mixture was stirred mechanically for 30 min at room temperature. Then, 0.24 g of ammonium persulfate was completely dissolved in 50 mL of 1 M hydrochloric acid and added to the reaction mixture, with stirring continued for 12 h. Finally, a black precipitate was obtained by filtration, washed repeatedly with distilled water, and vacuum dried to obtain the Fe3O4@PANI nanocomposite material.

[0050] 4.3) Preparation of Fe3O4@PANI / Ce-MOF suspension

[0051] Weigh 10 mg of Fe3O4@PANI nanoparticles and add them to 10 mL of 75% ethanol, then sonicate for 30 min. Next, weigh 5 mg of the prepared Ce-MOF powder and add it to the above mixture, then continue sonicating for 2 h to ensure that the two are mixed evenly.

[0052] (5) Laser-induced fabrication of graphene electrodes

[0053] The process of fabricating working electrodes, counter electrodes, and reference electrodes using laser etching to induce graphene is as follows (e.g.) Figure 1 As shown in the diagram: First, the patterns of the working electrode, reference electrode, and counter electrode are drawn in the computer program, and the power and depth of laser etching are set in the program; then, the PI film is flattened and tightly attached to the treated carbon paper, and placed in the instrument for laser etching. Multiple electrodes can be prepared at once using this method; a uniform silver paste layer is brushed onto the etched reference electrode area and allowed to air dry naturally; next, the electrode on the PI film is cut, and a blue insulating layer is attached to control the reaction area of ​​the working electrode and increase the waterproof performance of the electrode; finally, the Fe3O4@PANI / Ce-MOF composite material suspension is drop-coated onto the working electrode area and dried, and the laser-induced graphene electrode (LIG) preparation is completed.

[0054] (6) Electrochemical detection process

[0055] CV and EIS were performed using K3[Fe(CN)6] (5 mmol / L) as a probe. The CV voltage range was set to -0.5 V to 0.7 V, and the scan rate was 100 mV / s. EIS measurements were performed at open-circuit voltage with a frequency of 0.1–1.0 x 10⁻⁶. 6The reaction was performed in the Hz range. DPASV was used to detect 100 μg / L Cr(VI) in a solution, with a deposition potential of -0.8 V, a deposition time of 100 s, and a dissolution voltage range of 0.0 V to 1.3 V.

[0056] (7) Results and Discussion

[0057] 7.1) Characterization of electrochemical behavior

[0058] Please see Figure 2 , Figure 2 (A) shows CV images of the unmodified LIG and the Fe3O4@PANI / Ce-MOF modified electrode (Fe-P-Ce / LIG) in K3[Fe(CN)6] solution. According to Figure 2 (A) It can be seen that LIG can still produce relatively symmetrical Fe in K3[Fe(CN)6] solution. 2+ / 3+ The presence of redox peak pairs indicates the successful fabrication of the three-electrode system. After modifying the LIG working electrode region with Fe3O4@PANI / Ce-MOF composite material, the redox peak (Ig) was significantly increased. pa and I pc The significant increase indicates that the modified material effectively improved the conductivity of LIG. Simultaneously, the redox peak potential difference (ΔEp) decreased significantly; a smaller ΔEp indicates a significantly enhanced electrocatalytic ability of the modified LIG.

[0059] Figure 2 (B) shows the resistance values ​​of LIG and Fe-P-Ce / LIG after fitting. Unmodified LIG has a lower resistance because graphene itself has good conductivity. However, the resistance further decreases after modification with the Fe3O4@PANI / Ce-MOF composite material, indicating that [Fe3CN)6 3- / 4- Electron transfer kinetics are faster at the electrode surface. The improved conductivity is due to the excellent conductivity properties of Fe3O4NPs and polyaniline.

[0060] 7.2) Electrochemical detection of hexavalent chromium

[0061] The 0.1 mg / L Cr(VI) was also detected using pulsed differential stripping voltammetry (DPASV). Please refer to [link to relevant documentation]. Figure 3 ,from Figure 3 (A) It can be seen that the unmodified LIG three-electrode detection did not show a Cr(VI) dissolution peak at a specific potential. However, after LIG was modified with composite nanomaterials, a Cr(VI) dissolution peak appeared at around 0.9 V. Figure 3In (B), a comparison of DPASV images detected on two different substrates (LIG and GCE) reveals that the peak position shifts to the left, which may be caused by the difference between the electrode substrate and the reference electrode.

[0062] 7.3) Optimization of laser etching conditions

[0063] The power and depth of laser etching of PI films determine the formation of graphene on their surface, thus affecting the electrochemical performance of LIG electrodes. Therefore, the power and depth of laser etching were explored and optimized. Cyclic voltammetry (CV) images in a K3[Fe(CN)6] probe solution were used to determine the conductivity and electrocatalytic performance of LIG electrodes prepared at different powers and depths. First, the etching depth was fixed at 20 μm, and three different powers (30%, 60%, and 90%) were selected. The results are as follows... Figure 4 As shown in (A), when the power is 30%, the carbonization degree of the PI film is low and the graphene formation is not uniform, therefore the LIG prepared at this power cannot be used normally. Increasing the power to 60% shows a significant deepening of the graphene color, indicating a substantial increase in the carbonization degree of the PI film and a more uniform distribution. At a power of 90%, the carbonization degree further increases, and the graphene formation is even better. Therefore, LIG ​​prepared at power levels between 60% and 90% was further selected for CV experiments in K3[Fe(CN)6] solution. Figure 4 (B) As can be seen, the redox peak current of Fe increases with increasing power, indicating better conductivity. Therefore, 90% was chosen as the optimal power for etching. After determining the power (90%), comparative studies were conducted at different depths from 1 to 40.

[0064] like Figure 4 As shown in (B), at a constant power, different etching depths have a significant impact on the formation of graphene on the PI film surface. First, electrodes were etched to depths between 10 and 40 μm, and the results are as follows... Figure 4 As shown in (C), as the etching depth increases from 10, the resulting graphene not only becomes darker in color but also thicker. However, when the depth reaches 40, over-carbonization is evident, and graphene detaches from the PI film surface. Therefore, the etching depth was further increased in the range of 10 to 30, and LIGs of different depths were placed in K3[Fe(CN)6] solution for CV experiments. Figure 4 (D) It can be seen that LIG at a depth of 10 did not show any redox peaks in the probe solution, indicating that its electrochemical performance was extremely poor and it could not be used. However, as the depth continued to increase, LIG ​​clearly began to show redox peaks and the peak values ​​began to increase. Therefore, an etching depth of 25 was selected, which had the highest redox peak and the smallest potential difference, indicating better conductivity.

[0065] 7.4) Detection linear range

[0066] LIG three-electrode fabricated using the optimal power and depth of laser etching were employed to detect Cr(VI) solutions of different concentrations. The results are as follows: Figure 5 (A) and Figure 5 As shown in (B), within the concentration range of 0.01–1.0 mg / L, the current signal generated by LIG at 0.9 V increases with increasing Cr(VI) concentration, exhibiting a good linear relationship. The linear regression equation is as follows:

[0067] y = 2.87x + 0.81, R 2 =0.960

[0068] 7.5) Stability and reproducibility of Fe-P-Ce / LIG

[0069] Multiple Fe-P-Ce / LIG electrodes were prepared using the same power, depth, and composite modification materials to detect the same hexavalent chromium solution. Peak values ​​were compared to determine the reproducibility of the electrodes. Figure 6 (A) It can be seen that the current values ​​of the same batch of electrodes do not change much, so Fe-P-Ce / LIG still has good reproducibility.

[0070] To investigate the stability of Fe-P-Ce / LIG in practical applications, a three-electrode system was used to continuously measure hexavalent chromium multiple times. The results are as follows: Figure 6 As shown in (B), the electrode exhibits the highest electrical signal and best performance during the first detection. However, the electrical signal begins to decline significantly with increasing detection frequency. This is because LIG is a paper-based electrode with poor waterproofing. Even with an insulating and waterproof blue film covering its surface, prolonged immersion in aqueous solution will still affect its electrode performance. Although Fe-P-Ce / LIG has poor stability, its reproducibility is good. Therefore, this electrode is more suitable as a disposable electrode for 1 to 2 hexavalent chromium detections in practical applications.

[0071] 7.6) Actual water sample detection results from the Fe-P-Ce / LIG sensor

[0072] The collected water samples were spiked using an Fe-P-Ce / LIG sensor. Cr(VI) solutions with concentrations of 50, 100, and 500 μg / L were prepared from tap water and groundwater after simple filtration (V1:V2 = 5:1, where V1 is hexavalent chromium solution and V2 is 0.1 M hydrochloric acid supporting electrolyte solution). The results were analyzed using the DPASV method. The results are shown in Table 2.

[0073] Table 4-2. Measurement of actual water samples by Fe-P-Ce / LIG sensor

[0074]

[0075] As can be seen from Table 2, the recovery rate of hexavalent chromium in actual water samples is between 91.2% and 104.64%, therefore, this electrode can be used for the detection of actual water samples.

[0076] In summary, this invention uses laser-induced graphene technology to prepare a three-electrode system integrating a graphene carbon-based working electrode, a reference electrode, and a counter electrode. Fe3O4@PANI / Ce-MOF composite material is modified onto the working electrode region of the three electrodes, thereby preparing Fe-P-Ce / LIG and fabricating a Fe-P-Ce / LIG sensor for the detection of Cr(VI) solution. Specifically, the invention includes: (1) The invention utilizes the symmetrical redox peaks observed by the K3[Fe(CN)6] probe, which has sensitive electrochemical activity, to verify the successful preparation of the three-electrode system. Furthermore, after modification with Fe3O4@PANI / Ce-MOF composite material, the conductivity and catalytic performance of the LIG are improved; (2) This invention optimizes the power and depth of laser etching of the PI film, determining the optimal power and depth to be 90% and 25%, respectively. Under these conditions, the LIG prepared exhibits the best electrochemical performance; (3) This invention investigates the stability and reproducibility of the electrodes. Because the LIG is based on carbon paper, prolonged immersion in aqueous solution will decrease its stability. However, the reproducibility of the electrode is within an acceptable range; (4) The Fe-P-Ce / LIG sensor prepared based on Fe-P-Ce / LIG of this invention has a wide linear range and can meet the standard limit detection (50 μg / L) of Class III water in the surface water and groundwater standards; (5) Compared with the commonly used three-electrode system (glassy carbon electrode, platinum wire counter electrode and Ag / AgCl reference electrode) in the laboratory, the Fe-P-Ce / LIG sensor of this invention, which uses polyimide PI film as substrate and integrates three electrodes, has a lower cost, can be mass-produced, and has a certain degree of flexibility, and can be bent to a certain extent during use. Therefore, the electrode is more convenient and simple to use in practice.

[0077] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A graphene electrode for detecting chromium (VI) ions in an aquatic environment, characterized in that, A working electrode, a counter electrode, and a reference electrode system are integrated on a flexible substrate using a laser-induced graphene method, wherein the surface of the working electrode is modified with Fe3O4@PANI / Ce-MOF. The method for preparing the working electrode is as follows: A1. Weigh 10 mg of Fe3O4@PANI nanoparticles and add them to 10 mL of 75% ethanol, and sonicate for 30 min; then weigh 5 mg of Ce-MOF powder and add it to the above mixture and continue to sonicate for 2 h. After mixing evenly, a Fe3O4@PANI / Ce-MOF suspension is obtained; the mass ratio of Fe3O4@PANI nanoparticles to Ce-MOF powder used is 2:

1. A2. The Fe3O4@PANI / Ce-MOF suspension obtained in A1 is drop-coated onto the working electrode area and dried to prepare the modified working electrode, denoted as Fe-P-Ce / LIG.

2. The graphene electrode for detecting chromium (VI) ions in an aquatic environment according to claim 1, characterized in that, A three-electrode system is formed on a flexible substrate using laser-induced graphene, specifically including the following: B1. Draw the patterns of the working electrode, reference electrode, and counter electrode in the computer program, and set the power and depth of laser etching in the program; B2. The PI film is flattened and tightly attached to the treated carbon paper, and then placed in a laser etching machine for laser etching to obtain the working electrode, counter electrode and reference electrode. B3. Apply a uniform layer of silver paste to the etched reference electrode area and allow it to air dry naturally. B4. Cut the three electrodes on the PI film and attach a blue insulating layer.

3. The application of the graphene electrode as described in any one of claims 1-2 in the detection of chromium (VI) ions in an aqueous environment.

4. The application according to claim 3, characterized in that, The specific testing method includes the following steps: S1. A Fe-P-Ce / LIG sensor was fabricated based on a three-electrode system containing Fe-P-Ce / LIG. S2. The Fe-P-Ce / LIG sensor prepared in S1 is used to determine the chromium (VI) ions in the water body to be tested.

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