A method for simultaneously separating and detecting potassium ions, sodium ions, nitrate ions and nitrite ions in molten salts

Abstract

The present invention relates to a method for simultaneously separating and detecting potassium ions, sodium ions, nitrate ions and nitrite ions in solar molten salt. By the method of the present invention, it is possible to simultaneously separate and quantitatively detect the contents of potassium ions, sodium ions, nitrate ions and nitrite ions in solar molten salt in a single sample injection. The method has high resolution, good stability, high accuracy, and is simple to operate, time-saving and highly efficient.

Description

Technical Field

[0001] The present invention relates to the technical field of molten salt detection. Specifically, it relates to a method for simultaneously separating and detecting potassium ions, sodium ions, nitrate ions, and nitrite ions in molten salt. Background Art

[0002] The existing detection standard for solar molten salt (nitro type) is GB / T 36376-2018. Among them, the main component potassium nitrate is titrated with sodium tetraphenylborate to estimate its content, and sodium nitrite is titrated with potassium permanganate to estimate the nitrite content, and the content of sodium nitrate is indirectly calculated by deducting other known impurities at the same time. This standard does not directly measure the main components, and the sodium tetraphenylborate precipitation titration method has cumbersome steps and is easily interfered by other impurity ions such as ammonium; the potassium permanganate method for determining nitrite is also easily interfered by other reducing substances, which will cause the results to deviate from the true value and have a large error.

[0003] Currently, inductively coupled plasma emission spectrometry ICP is widely used for the testing of cations (sodium ions, potassium ions), and ion chromatography IC is widely used for the testing of anions (nitrate ions, nitrite ions). However, for the testing of the main components in molten salt, it needs to be diluted about 10,000 times. The dilution factor is too large and it cannot be detected simultaneously, resulting in a large systematic error, making the ratio of nitrate ions to potassium ions and the ratio of nitrite ions to sodium ions deviate greatly from the theoretical ratio of 1:1.

[0004] At present, there is no reliable method to simultaneously and rapidly separate and accurately detect the main components potassium ions, sodium ions, nitrate ions, and nitrite ions in molten salt.

[0005] Therefore, the present invention has developed a method for simultaneously separating and quantitatively detecting the main components potassium ions, sodium ions, nitrate ions, and nitrite ions in solar molten salt by high performance liquid chromatography in a single sample injection. This method has strong specificity and little interference, simple pretreatment, small dilution factor, and only needs one-step water dilution to complete the detection by injection; in addition, the standard product of the salt to be measured has high purity and is easily obtained, and the calibration calculation is accurate, avoiding the errors caused by separate detections. Summary of the Invention

[0006] The present invention provides a method for simultaneously separating and quantitatively detecting potassium ions, sodium ions, nitrate ions, and nitrite ions in solar molten salt in a single sample injection. The method includes the following steps:

[0007] Prepare an aqueous solution of solar molten salt containing potassium nitrate and sodium nitrite;

[0008] Inject the solution into a high performance liquid chromatograph (HPLC) for separation and detection. The stationary phase used for separation is a porous silica gel particle packing coated with sulfonated nanopolymer beads. The inner pore region of the porous silica gel particles is modified with a covalently bonded hydrophilic layer, and the outer pore region is modified with sulfonated nanopolymer beads. The mobile phase consists of 10 - 100 mM acetate buffer solution with a pH of 4 - 5 as mobile phase A, acetonitrile as mobile phase B, and formic acid solution as mobile phase C. Based on the total volume of the entire mobile phase, the volume ratio of mobile phase A to mobile phase B is controlled to be 40:45 to 40:50.

[0009] The HPLC is successively connected in series with an ultraviolet detector and a charged aerosol detector (CAD). The ultraviolet detector is used for the detection of nitrate and nitrite, and the charged aerosol detector is used for the detection of sodium ion and potassium ion.

[0010] In the method of the present invention, an aqueous solution of a solar molten salt containing potassium nitrate and sodium nitrite is prepared. That is, the step of preparing the sample solution usually includes weighing a certain amount, usually about 0.1 g, of the solar molten salt and diluting it with water by about 1000 times to obtain the sample solution.

[0011] Before or after the step of preparing the sample solution, the method of the present invention further includes the step of preparing a standard solution. This step includes dissolving potassium nitrate standard and sodium nitrite standard in water to obtain a mixed standard solution of potassium nitrate and sodium nitrite. The concentration range of the potassium nitrate and sodium nitrite standard solutions is usually 10 μg / mL - 1000 μg / mL, and those skilled in the art can determine the appropriate concentration of the standard solution according to specific circumstances.

[0012] In the method of the present invention, high performance liquid chromatography is used to separate and detect the prepared sample solution. Specifically, the prepared sample solution is injected into an HPLC. The stationary phase of the chromatographic column of the HPLC used is a porous silica gel particle packing coated with sulfonated nanopolymer beads. The inner pore region of the porous silica gel particles is modified with a covalently bonded hydrophilic layer, and the outer pore region is modified with sulfonated nanopolymer beads.

[0013] The packing is based on high purity spherical porous silica gel particles with a particle size usually of 3 μm. The inner pore region is modified with an organic layer providing reversed-phase and anion-exchange properties by using a nanopolymer silica hybridization technique, and the outer pore region is modified with a sulfonated nanopolymer having cation-exchange functionality, ensuring spatial separation of the anion-exchange region (inner pore region) and the cation-exchange region (outer pore region), so that the two retention mechanisms can function simultaneously and allows independent control of them. Therefore, the stationary phase packing has three retention mechanisms: anion exchange, cation exchange, and reversed phase.

[0014] The mobile phase used includes acetate buffer solution as mobile phase A, acetonitrile as mobile phase B, and formic acid solution as mobile phase C. The acetate buffer solution is usually adjusted to a pH of 4 - 5 with acetic acid and formic acid in mobile phase C, and its concentration is usually 10 - 100 mM, preferably 15 - 50 mM, more preferably 20 - 30 mM. Based on the total volume of the mobile phase, the volume ratio of mobile phase A to mobile phase B is controlled to be 40:45 to 40:50, preferably 40:45.

[0015] In a specific embodiment, the chromatographic column used is an Acclaim Trinity P1 liquid chromatography column. In the high-performance liquid chromatograph, isocratic elution is preferably used. The flow rate of the mobile phase is 0.7 - 0.9 mL / min, preferably 0.8 mL / min. The column temperature is 30 - 40 °C, preferably 40 °C.

[0016] In the method of the present invention, the detector used in the high-performance liquid chromatograph includes an ultraviolet detector and an electrospray detector connected in series in sequence. The sample separated after elution from the chromatographic column first enters the ultraviolet detector to detect nitrate and nitrite, and then enters the electrospray detector to detect sodium ions and potassium ions. The ultraviolet detector is usually preferably a diode array detector (DAD). In the ultraviolet detector, the detection wavelength for detecting nitrate and nitrite is usually in the range of 205 nm - 230 nm, preferably 210 nm.

[0017] The contents of sodium ions, potassium ions, nitrate, and nitrite in the sample solution can be calculated by the external standard method. Specifically, using the chromatographic conditions described above, the prepared mixed standard solution is injected into the high-performance liquid chromatograph to obtain the chromatograms of standard solutions with different concentrations, and the corresponding peak areas are measured. Taking the mass concentrations of sodium ions, potassium ions, nitrate, and nitrite ions as the abscissa and the peak area as the ordinate, the standard curves of nitrate and nitrite ions are plotted. According to the standard curves, the contents of sodium ions, potassium ions, nitrate, and nitrite in the sample solution to be measured are calculated by the external standard method.

[0018] The present invention adopts liquid chromatography, uses a chromatographic column containing the above-mentioned specific stationary phase packing material, especially an Acclaim Trinity P1 liquid chromatography column combined with an acetate buffer / acetonitrile mobile phase. Based on the combined action of the hydrophobic and electrostatic effects of the stationary phase, multiple retention mechanisms such as reverse phase, anion exchange, and cation exchange are generated. By strictly controlling the proportion of mobile phase components to adjust the hydrophobicity of nitrate and nitrite, setting reasonable elution conditions, and connecting an ultraviolet detector and an electrospray detector in series in sequence, it is possible to simultaneously separate and quantitatively detect the contents of potassium ions, sodium ions, nitrate, and nitrite in solar molten salt in a single sample injection. This method has high resolution, good stability, high accuracy, and is simple to operate, time-saving and efficient. Description of Drawings

[0019] Figure 1 : Standard curve graphs of sodium ions, potassium ions, nitrate ions and nitrite ions in the standard solution

[0020] Figure 2 : Chromatogram obtained by separating the solar molten salt sample by liquid chromatography according to Example 1 through a DAD detector

[0021] Figure 3 : Chromatogram obtained by separating the solar molten salt sample by liquid chromatography according to Example 1 through a CAD detector

[0022] Figure 4 : Chromatogram obtained by separating the solar molten salt sample by liquid chromatography according to Comparative Example 1

[0023] Figure 5 : Chromatogram obtained by separating the solar molten salt sample by liquid chromatography according to Comparative Example 2

[0024] Figure 6 : Chromatogram obtained by separating the solar molten salt sample by liquid chromatography according to Comparative Example 3 Detailed Description of the Invention

[0025] Next, the technical solutions of the present invention will be described more clearly and completely in conjunction with the embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments, and do not limit the protection scope of the present invention in any way.

[0026] Experimental materials:

[0027] Potassium nitrate standard and sodium nitrite standard: Purchased from Sinopharm Chemical Reagent Co., Ltd.

[0028] Solar molten salt sample: Purchased from Weifang Changcheng Nitrate Co., Ltd.

[0029] Instruments and equipment:

[0030] Liquid chromatograph: Vanquish high-performance liquid chromatograph, purchased from Thermo Fisher Scientific, USA

[0031] DAD detector: Model Vanquish Diode Array Detector HL, purchased from Thermo Fisher

[0032] CAD detector: Model Vanquish Charged Aerosol Detector F, purchased from Thermo Fisher

[0033] Example 1:

[0034] Use the Vanquish high performance liquid chromatograph of Thermo Fisher Company in the United States, and set the chromatographic conditions as follows:

[0035] Chromatographic column: Thermo Acclaim Trinity P1 (specification 3.0mm * 150mm * 3μm);

[0036] Elution mode: isocratic elution;

[0037] Mobile phase: Mobile phase A is 20 mM NH4OAc / 0.1% HOAc buffered saline solution (pH 5), mobile phase B is acetonitrile, mobile phase C is 0.1% formic acid, and the volume ratio of mobile phases A, B, and C is 40:45:15;

[0038] Flow rate: 0.8 mL / min;

[0039] Column temperature: 40 °C;

[0040] Injection volume: 1.0 μL;

[0041] Detector: A DAD detector (detection wavelength: 210 nm) and a CAD detector are connected in series.

[0042] Preparation of standard solution:

[0043] Weigh 250 mg of potassium nitrate standard and sodium nitrite standard respectively, dissolve and quantify with water, and prepare a series of mixed standard solutions of KNO3 and NaNO2.

[0044] Preparation of sample solution:

[0045] Weigh 100 mg of the sample and dilute it to 100 mL with water.

[0046] According to the above chromatographic conditions, inject 1.0 μL of the sample solution into the high performance liquid chromatograph, record the chromatogram, and the sample chromatogram is shown in Figure 2 and Figure 3 .

[0047] The chromatogram shows that the retention times of sodium ions, potassium ions, nitrite ions, and nitrate ions are approximately 3.8 min, 4.5 min, 12.6 min, and 15.2 min respectively, and their peak emergence times differ greatly, with good resolution.

[0048] The inventor found that nitrite ions could not be detected by connecting a CAD detector to a liquid chromatograph. Based on this discovery, the inventor connected a DAD detector and a CAD detector in series to the liquid chromatograph. First, nitrate ions and nitrite ions were detected by the DAD detector, and then sodium ions and potassium ions were detected by the CAD detector, thus successfully separating and detecting sodium ions, potassium ions, nitrate ions, and nitrite ions, as Figure 2 and Figure 3 shown.

[0049] According to the above chromatographic conditions, 1.0 μL of a mixed standard solution of KNO3 and NaNO2 was injected into the high-performance liquid chromatograph for liquid chromatographic separation and detection, obtaining a quadratic linear fitting curve graph of sodium ions and potassium ions and a linear equation curve graph of nitrate ions and nitrite ions, as Figure 1 shown, where the linear equation for nitrite ions is: y = 0.0901x + 0.0947, and the correlation coefficient R 2 = 0.9992; the linear equation for nitrate ions is: y = 0.1016x - 0.0778, and the correlation coefficient R 2 = 0.9991; the quadratic fitting linear equation for sodium ions is: y = -0.000019x 2 + 0.0319x + 0.1420, and the correlation coefficient R 2 = 0.9987; the quadratic fitting linear equation for potassium ions is: y = -0.000014x 2 + 0.0250x + 0.0096, and the correlation coefficient R 2 = 0.9991, where x represents the concentration of each ion; y represents the response peak area of the corresponding ion in the detection spectrum. The linear relationships of each linear equation are good, so they can be well used for the determination of the sodium ions, potassium ions, nitrate ions, and nitrite ions.

[0050] Using the above linear relationship equations, the contents of sodium ions, potassium ions, nitrate ions, and nitrite ions in the above samples were calculated by the external standard method. Each of the three batches of samples was measured in parallel 3 times, and the average value of each batch of samples was calculated. The results are shown in Table 1 below:

[0051] Table 1: Contents of sodium ions, potassium ions, nitrate ions, and nitrite ions in solar molten salt samples

[0052]

[0053] From the above results, it can be seen that by the method of the present invention, the contents of sodium ions, potassium ions, nitrate ions, and nitrite ions can be simultaneously separated and detected, and the obtained results have good repeatability and high accuracy. Moreover, the ratios of nitrate ions to potassium ions and nitrite ions to sodium ions deviate very little from the theoretical ratio of 1:1, both within 3%.

[0054] Accuracy test:

[0055] The spike recovery rate of the method of the present invention was determined as follows, and the results are shown in Table 2 below:

[0056] 1. Preparation of calibration curve series solutions: Weigh 500 mg each of potassium nitrate and sodium nitrite standard products into a 50 mL volumetric flask, record the mass accurately to 0.1 mg, dissolve with water and make up to the mark. This solution is the standard stock solution with a concentration of about 10000 mg / L. Then, prepare the calibration curve series solutions by successive dilution and test them.

[0057] 2. Sample preparation: Considering that the surface of solar molten salt is extremely easy to absorb water, and it is in large granular form with uneven distribution, there will be concentration deviations even with the same sampling method. However, in terms of its chemical composition, it is potassium nitrate and sodium nitrite. Therefore, simulate samples with similar concentrations were prepared using potassium nitrate and sodium nitrite standard products and then tested. This eliminates the problem of sample non-uniformity and still allows testing the accuracy of the method. The specific steps are as follows: Weigh about 0.05 g of potassium nitrate and 0.045 g of sodium nitrite into a 100 mL volumetric flask, record the mass accurately to 0.1 mg, and prepare 5 portions in parallel by the same steps. Two of them were directly dissolved with water and made up to the mark for testing, and the average value of the results was taken to obtain the ion concentrations of the simulated samples; for the other three, 2.5 mL of the 10000 mg / L standard stock solution was added, then dissolved with water and made up to the mark for testing. The recovery rate was calculated based on the ion concentration values of the spiked sample solutions and the average ion concentrations of the simulated samples.

[0058] Table 2: Spike recovery rate

[0059]

[0060] As can be seen from the above table, the recovery rate ranges of the four ions in the molten salt samples are all between 95% and 100%, indicating that the method of the present invention has a relatively high recovery rate and good accuracy. Therefore, the measurement results of the detection method of the present invention are accurate and reliable.

[0061] Comparative Example 1:

[0062] The same high-performance liquid chromatograph as in Example 1 and the same chromatographic condition settings were used, except that the volume ratio of mobile phase A to B was 40:60, and there was no mobile phase C at this time. The sample solution was separated and detected, and the obtained chromatogram is as Figure 4 shown.

[0063] Comparative Example 2:

[0064] The same high-performance liquid chromatograph as in Example 1 and the same chromatographic condition settings were used, except that the volume ratio of mobile phase A to B was 40:20. The sample solution was separated and detected, and the obtained chromatogram is as Figure 5 shown.

[0065] It can be seen from Figure 4 and Figure 5 that when the acetonitrile ratio in the mobile phase is increased to 60%, nitrite and nitrate are co-eluted; when the acetonitrile ratio in the mobile phase is decreased to 20%, nitrite and nitrate can be baseline separated, but the nitrate peak becomes wider and shows tailing, which is not conducive to its quantification. When the acetonitrile ratio in the mobile phase is set to 45%, nitrite and nitrate can not only be baseline separated, but also the two ion peaks are sharp and symmetrical. This makes the quantification more accurate and the repeatability better. Therefore, nitrite is greatly affected by the acetonitrile ratio in the mobile phase, and it is particularly important to appropriately adjust the acetonitrile ratio for the separation of nitrite.

[0066] Comparative Example 3:

[0067] Using the same high-performance liquid chromatograph and the same chromatographic condition settings as in Example 1, except that the chromatographic column used is replaced with a C18 chromatographic column: Acclaim carbonyl C18 (3.0 * 150 mm, 3 μm), the sample solution is separated and detected, and the obtained chromatogram is as shown in Figure 6 shown.

[0068] It can be seen from Figure 6 that on the C18 reversed-phase chromatographic column of the same specification, according to the response signals of CAD and DAD, none of the four substances to be analyzed are retained and are directly co-eluted.

Claims

1. A method for simultaneously separating and quantitatively detecting potassium ions, sodium ions, nitrates and nitrites in solar molten salts in a single sample injection, the method comprising the following steps: preparing an aqueous solution of solar molten salt comprising potassium nitrate and sodium nitrite; The solution is injected into a high performance liquid chromatograph for separation and detection, wherein the stationary phase used for separation is a porous silica particle filler coated with sulfonated nano polymer beads, the inner pore region of the porous silica particle is modified with a covalently bonded hydrophilic layer, and the outer pore region is modified with sulfonated nano polymer beads, The high performance liquid chromatograph is sequentially connected in series with a diode array detector and a charged aerosol detector, wherein the diode array detector is used for detecting nitrate and nitrite, and the charged aerosol detector is used for detecting sodium and potassium ions. Wherein, the chromatographic conditions of the high performance liquid chromatograph are specifically set as follows: Chromatographic column: Thermo Acclaim Trinity P1, specification 3.0mm*150mm*3μm; Elution mode: isocratic elution; Mobile phase: Mobile phase A was 20 mM pH 5 NH4OAc / 0.1% HOAc buffered saline solution, mobile phase B was acetonitrile, and mobile phase C was 0.1% formic acid aqueous solution. The volume ratio of mobile phases A, B, and C was 40:45:

15. The detection wavelength of the diode array detector is 210 nm.

2. The method according to claim 1, wherein in the high performance liquid chromatograph, the flow rate of the mobile phase is 0.7-0.9 mL / min.

3. The method according to claim 1 or 2, wherein the column temperature used in the high performance liquid chromatograph is 30-40°C.

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