A method for rapid determination of silicon by potassium fluosilicate precipitation-acid-base titration

Abstract

This invention discloses a rapid method for determining silicon based on potassium fluorosilicate precipitation-acid-base titration, belonging to the field of analytical detection technology. The method of this invention omits the filtration, transfer, and washing steps of the traditional potassium fluorosilicate precipitation method, and includes: mixing and melting a silicon-containing sample with an alkaline flux, leaching with hot water, adjusting the pH of the solution to 12.8-13.2 with hydrochloric acid, and making up to volume; filtering the resulting sample solution and collecting the filtrate as the analytical solution; separating the analytical solution, adding phenolphthalein indicator, neutralizing with hydrochloric acid until colorless and in excess, adding solid potassium thiocyanate and potassium fluoride solution to generate potassium fluorosilicate precipitate, neutralizing the residual acid with potassium hydroxide solution until just showing color; adding boiling water to decompose the precipitate, immediately titrating with potassium hydroxide standard titration solution to the endpoint, recording the titration volume, and calculating the silicon dioxide content in the sample based on the blank test results. This invention, while ensuring accuracy and precision, shortens the single-sample determination time from 1.5-2 hours in traditional methods to 8-10 minutes, with a relative standard deviation of no more than 0.47%. It is suitable for the rapid determination of silicon in solid samples such as quartz stone, quartz powder, and high-silica quartz minerals with a silicon dioxide content ≥90% and an aluminum oxide content ≤2%.

Description

Technical Field

[0001] This invention belongs to the field of analytical testing technology, specifically relating to a method for determining the silicon content in a sample based on the principle of potassium fluorosilicate precipitation-acid-base titration, and particularly to a rapid method for determining silicon dioxide in solid samples with high silicon content (such as quartz, quartz powder, high-silicon quartz minerals, etc.). Background Technology

[0002] Silica is one of the most important analytical indicators in many fields, including geology, metallurgy, building materials, and refractory materials. Traditionally, the hydrofluoric acid gravimetric method is used to determine SiO2 in samples with high silica content. This method involves reacting SiO2 with hydrofluoric acid under strong acid heating conditions to generate SiF4 gas, which then volatilizes. The SiO2 content is obtained by calculating the mass loss before and after the reaction. While theoretically accurate, this method is complex, time-consuming, and the volatilized hydrofluoric acid and other harmful substances pose serious risks to human health and the environment. Furthermore, this method has high requirements for sample particle size; uneven particle size can lead to incomplete reactions, and uneven heating can cause sample splashing, affecting the results. Therefore, establishing a simple, rapid, and safe method for silica determination has always been a pursuit of analytical researchers.

[0003] The potassium fluorosilicate precipitation-acid-base titration method (also known as the potassium fluorosilicate volumetric method) is one of the classic analytical methods for determining silicon dioxide. Its basic principle is as follows: After a silicon-containing sample is treated with alkali fusion or acid dissolution, in a strongly acidic solution containing excess potassium and fluoride ions, silicic acid reacts with fluoride ions to form fluorosilicate ions, which in turn react with potassium ions to form a sparingly soluble potassium fluorosilicate precipitate. The resulting precipitate is hydrolyzed in hot water, releasing hydrofluoric acid. The acid produced is titrated with a standard sodium hydroxide or potassium hydroxide solution, allowing for the indirect calculation of the silicon dioxide content in the sample. Compared to the gravimetric method, the potassium fluorosilicate titration method does not require expensive platinum crucibles, does not involve hydrofluoric acid volatilization, and has lower instrument requirements, thus it is widely used in industrial settings and laboratories.

[0004] However, the traditional potassium fluorosilicate precipitation-acid-base titration method has a long-standing and unresolved problem in operation—the washing of the potassium fluorosilicate precipitate and the neutralization of residual acid.

[0005] After potassium fluorosilicate precipitate forms, a large amount of free acid remains on the precipitate surface and in the solution. If this residual acid is not sufficiently removed, it will consume the alkali during subsequent titration of the hydrofluoric acid generated by hydrolysis, leading to a significantly higher measurement result. To ensure accurate determination, traditional methods require filtering, transferring, and repeated washing of the potassium fluorosilicate precipitate to remove as much residual acid as possible. For example, the method for determining silica in siliceous refractory materials using potassium fluorosilicate precipitation-acid-base titration established by Li Huanting et al. requires filtration and washing after precipitate formation before it can be dissolved in boiling water for titration. While the improved method by Qiao Jinhua et al. reduces the number of washing steps, it still requires as many as three.

[0006] This washing process has many drawbacks: First, the operation is cumbersome and time-consuming. The filtration, transfer, and washing of the precipitate need to be repeated, and the determination time for a single sample is usually as long as 1.5 to 2 hours. This not only severely restricts the detection efficiency, but also makes it difficult for this method to meet the daily analysis needs of batch samples.

[0007] Secondly, the washing effect is unstable and reproducible. The influence of residual acid in the precipitate is the main reason for the poor reproducibility of the potassium fluorosilicate titration method. The commonly used method is to wash with potassium chloride-ethanol followed by neutralization of residual acid with sodium hydroxide. However, the neutralization point is greatly affected by various factors such as the degree of washing, washing time, and ambient temperature, resulting in unstable effects. There is also a method of washing directly to neutral with potassium fluoride, but for samples with high aluminum content, the effect is also unsatisfactory because of the high amount of aluminum adsorbed in the precipitate and residual liquid. This inherent instability often forces operators to perform parallel determinations using two samples to prevent errors.

[0008] Third, it requires highly skilled operators and is prone to errors. The entire washing and neutralization process involves multiple manual steps, and even slight differences in any step can affect the final result. For example, in determining the titration endpoint, if over-titration makes the solution too red, silicic acid will be titrated along with it, leading to a higher result; if the neutralization time is too long, especially at high room temperature, potassium fluorosilicate precipitate is prone to hydrolysis, resulting in a lower result.

[0009] To address the aforementioned issues, researchers have attempted improvements from various angles. For example, some methods employ a self-made vacuum filtration device instead of conventional filtration to accelerate the filtration process and reduce residual acid adsorption; others use solid potassium nitrate instead of saturated potassium nitrate solution and K-alcohol solution instead of tartaric acid-urea mixture to promote complete precipitation. However, these improvements do not fundamentally eliminate the core operational steps of filtering, transferring, and washing the potassium fluorosilicate precipitate, resulting in limited improvements in measurement efficiency.

[0010] In summary, existing techniques for determining silicon using potassium fluorosilicate precipitation-acid-base titration require filtration, transfer, and repeated washing of the potassium fluorosilicate precipitate to remove residual acid, resulting in cumbersome operation, time-consuming processes, and poor reproducibility, making it difficult to meet the practical needs of rapid batch analysis. How to effectively eliminate the interference of residual acid on titration results while ensuring accuracy and precision, without omitting the precipitation filtration and washing steps, is a pressing technical problem to be solved in this field. Summary of the Invention

[0011] To address the aforementioned technical problems, this invention provides a method for the rapid determination of silicon based on potassium fluorosilicate precipitation and acid-base titration. By precisely controlling the pH of the sample solution, rationally selecting potassium salt supplements, and optimizing neutralization and titration conditions, this invention achieves rapid and accurate determination of silicon dioxide in high-silicon solid samples without completely omitting the potassium fluorosilicate precipitation, filtration, and washing steps. The determination time for a single sample can be shortened to 8-10 minutes.

[0012] This invention provides a rapid method for determining silicon based on the principle of potassium fluorosilicate precipitation-acid-base titration, which omits the transfer and washing steps of potassium fluorosilicate precipitate, and includes the following steps: (1) Sample digestion and acidification: The silicon-containing sample is mixed with an alkaline flux and melted. The melt is extracted with hot water, cooled, and acid is added to adjust the pH of the solution to 12.8~13.2. The solution is then brought to a final volume to obtain the sample solution. As a preferred embodiment, the alkaline flux in step (1) is sodium hydroxide, with a dosage of 2.030~2.050g sodium hydroxide per 0.2g sample; or the alkaline flux is potassium hydroxide, with a dosage of 2.840~2.870g potassium hydroxide per 0.2g sample. The acid solution is a 6.0mol / L hydrochloric acid solution, with a dosage of 3.5mL per 200mL fixed volume. The melting temperature is 700℃, and the melting time is 15 minutes.

[0013] (2) Filtration to remove impurities: Filter the sample solution obtained in step (1) and collect the filtrate as the analytical solution; This filtration step can remove Al and other substances from the sample solution. 3+ It interferes with most metal ions other than Fe (residual amount in the ng range). After filtration, the Al in the test solution is analyzed. 3+ When the content is controlled below 2%, the phenomenon of repeated rapid fading at the endpoint disappears, the endpoint is determined, and it will not affect the determination.

[0014] (3) Precipitation formation and neutralization: Take the analytical solution from step (2), add an indicator, and neutralize with acid until the indicator changes color and is in excess; add solid potassium thiocyanate to the system as a potassium salt supplement, and add potassium fluoride solution; then neutralize the residual acid with alkali until the indicator just shows color. As a preferred embodiment, the volume of the analytical solution aliquoted in step (3) is 15.00 mL. The amount of solid potassium thiocyanate added is 3-4 g; the mass concentration of the potassium fluoride solution is 500 g / L, and the amount added is 1 mL; the alkaline solution is a 0.15 mol / L potassium hydroxide solution. The step of neutralizing with acid until the indicator changes color and is in excess is specifically: neutralizing with 6.0 mol / L hydrochloric acid solution until the phenolphthalein indicator is colorless and in excess by 6-7 drops; the step of neutralizing with alkaline solution until the indicator just shows color is specifically: neutralizing with 0.15 mol / L potassium hydroxide solution until the solution just turns red and does not fade after shaking for 30 seconds.

[0015] This invention uses solid potassium thiocyanate as a potassium salt supplement. By utilizing its high solubility and the large amount of heat absorbed during dissolution, the system temperature is lowered, effectively inhibiting the redissolution of potassium fluorosilicate precipitate. At the same time, potassium thiocyanate does not react with non-oxidizing acids such as hydrochloric acid, thus avoiding interference of redox reactions with the titration results.

[0016] (4) Hydrolysis titration: Add hot water to the system after neutralization in step (3), and immediately titrate with alkaline standard titration solution until the indicator just shows the color endpoint, and record the titration volume V1; (5) Blank test: Without adding the test sample, perform the same operation as steps (1) to (4) and record the blank titration volume V0; (5) Calculation: Calculate the mass percentage ω of silica in the sample to be tested according to the following formula: Where: ω is the percentage content of silica in the sample, in %; C0 is the concentration of the alkaline standard titration solution, in mol / L; V0 is the blank titration volume, in L; V1 is the sample titration volume, in L; m0 is the sample weight, in g; 60.08 is the molar mass of silica, dimensionless; 4 means that 1 mol of silica is equivalent to 4 mol of hydrogen ions.

[0017] Compared with existing technologies, this invention omits the core operation steps of potassium fluorosilicate precipitation, filtration, and washing, which significantly reduces the single-sample determination time from 1.5 to 2 hours in traditional methods to 8 to 10 minutes. It completely solves the problems of cumbersome operation and time-consuming process caused by precipitation, filtration, transfer, and repeated washing in traditional methods, making the potassium fluorosilicate precipitation-acid-base titration method truly suitable for the daily rapid analysis of batch samples. Meanwhile, by eliminating the error-prone steps of precipitation, filtration, and washing, and employing precise pH control (sample solution pH = 12.8~13.2), solid potassium thiocyanate as a potassium salt supplement (utilizing its high solubility and endothermic properties during dissolution to inhibit the redissolution of potassium fluorosilicate precipitate), and an optimized endpoint determination method (using phenolphthalein as an indicator, determining the abrupt change from colorless to red on a white substrate), this invention effectively eliminates systematic errors caused by residual acid and inaccurate titration endpoint determination, significantly improving the precision and reproducibility of analytical results. Experiments show that when using this invention to determine different quartz samples, the maximum relative standard deviation (RSD) is only 0.47%, and the difference in titration volume is within 0.1 mL, fully meeting the requirements of quantitative analysis. This invention, by controlling the sample solution pH at 12.8~13.2 and using medium-speed quantitative filter paper for dry filtration, can effectively remove Al. 3+ The interference from most metal ions other than Fe (residual amount in the ng range) clarified the mechanism and quantitative basis for filtration and impurity removal; for Al 3+ This amphoteric hydroxide interfering element, through experiments, has had its upper limit of content determined to be 2% (i.e., 20 mg / L). When Al in the sample... 3+ When the content does not exceed this limit, the repeated rapid fading phenomenon at the endpoint disappears, and the determination is unaffected, providing a clear basis for judging the suitability of the sample. This invention is particularly suitable for high-silicon solid samples with a silica content of over 90% and an alumina content of less than 2%. It can be used for the rapid determination of silicon content in quartz (powder) samples and high-silicon quartz minerals in mines and smelting plants, and has good practical value and prospects for widespread application. Detailed Implementation

[0018] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to the following embodiments. Experimental methods in the embodiments that do not specify specific conditions are generally performed under conventional conditions or according to the manufacturer's recommendations. Reagents or instruments used that do not specify the manufacturer are all commercially available conventional products.

[0019] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0020] I. Preparation of Reagents Before conducting sample analysis, the required reagents should be prepared as follows: Hydrochloric acid solution (6.0 mol / L): Measure 100.0 mL of analytical grade concentrated hydrochloric acid, dilute to 200.0 mL with deionized distilled water, bring to volume, mix well, and let stand for 24 hours before use. Potassium fluoride solution (500 g / L): Weigh 500 g of potassium fluoride and dissolve in 1000 mL of deionized distilled water, storing in a plastic bottle. Phenolphthalein ethanol solution (10 g / L): Weigh 1.0 g of phenolphthalein, dissolve in 100 mL of anhydrous ethanol, and mix well. Potassium hydroxide neutralization solution (0.15 mol / L): Weigh 4.25 g of potassium hydroxide, dissolve in water, dilute to 500 mL, and mix well.

[0021] The preparation of potassium hydroxide standard titration solution (approximately 0.05 mol / L) is as follows: Weigh approximately 2.8 g of potassium hydroxide, dissolve and dilute with water to 1000 mL, mix well, and let stand for later use. The standardization method for this solution is as follows: Take four parallel aliquots of 1 mg / mL silica standard solution, 15.00 mL each, and place them in a 400 mL PTFE beaker. Add 1 mL of phenolphthalein (10 g / L) to each aliquot, and neutralize with 6.0 mol / L hydrochloric acid solution until colorless, with an excess of 6-7 drops. Add 4 g of solid potassium thiocyanate and 1 mL of potassium fluoride solution (500 g / L). Neutralize the residual acid with 0.15 mol / L potassium hydroxide solution, shaking continuously until the solution just turns red (with an excess of 1 drop), the solution is slightly red, and there is no significant fading after shaking for 30 seconds; this process can be disregarded. If the color fades after standing for more than 1 minute, no additional drops are needed. Add approximately 150 mL of boiling water to the neutralized PTFE beaker, and quickly titrate with the potassium hydroxide standard solution to be standardized until a color change occurs. Stir rapidly with a plastic rod for 30 seconds until the color does not fade. Add one drop of water and record the titration volume V1. For the blank determination, perform the above analytical steps without adding any sample and record the blank titration volume V0. Substitute the titration volume V1 and the blank titration volume V0 into the following formula to calculate the concentration of the potassium hydroxide standard titration solution: Where, m s The value represents the mass (g) of silicon dioxide contained in 15.00 mL of silicon dioxide standard solution, 60.08 represents the molar mass of silicon dioxide, and 4 represents the equivalent of 4 mol of hydrogen ions in 1 mol of silicon dioxide. The difference between four parallel determinations should be within 0.1 mL.

[0022] The preparation method of the silica standard solution is as follows: Weigh 2-4 g of anhydrous sodium carbonate; accurately weigh 1.0000 g (±0.0001 g) of standard silica that has been ignited at 1200℃ for 1 hour and dried at 105℃ for 4 hours, and mix it with sodium carbonate in a platinum crucible. Melt it in a muffle furnace at 950℃ for 15-20 minutes; remove it and let it cool, wash it with hot water, and dilute it to volume in a 1000 mL volumetric flask. The concentration of this standard solution is 1 mg / mL. Alternatively, commercially available finished standard substances can be purchased.

[0023] II. Sample Pretreatment Solid samples should be reduced by the conical quartering method, processed into sample sieves, and the material passing through an 80-mesh sieve should be collected.

[0024] The sample digestion and acidification process was as follows: Accurately weigh 2.0300–2.0500 g of flux sodium hydroxide; accurately weigh 0.2000 ± 0.0001 g of sample, and mix them in a nickel crucible with the flux sodium hydroxide. Melt in a muffle furnace at 700℃ for 15 minutes; remove and allow to cool slightly, wash with hot water, and place in a 200 mL volumetric flask at the same temperature as water. Add 3.5 mL of 6.0 mol / L hydrochloric acid solution, make up to volume, and mix well to obtain the sample solution. The pH of this sample solution is between 12.8 and 13.2. Filter the sample solution dry using medium-speed quantitative filter paper (taking care to prevent filter breakage), discard the initial filtrate, and collect the subsequent filtrate as the analytical solution. This operation can filter out all impurities in the sample solution except for Al. 3+ It interferes with most metal ions other than Fe (residual amount in the ng level).

[0025] III. Sample Determination Transfer 15.00 mL of the analytical solution to a 400 mL PTFE beaker. Add 1 mL of phenolphthalein (10 g / L) to the analytical solution and neutralize with 6.0 mol / L hydrochloric acid solution until colorless and with an excess of 6-7 drops. Add 3-4 g of solid potassium thiocyanate (solid potassium chloride can also be used, but the subsequent titration procedure is different; see precautions for details), and add 1 mL of potassium fluoride solution (500 g / L). Neutralize the residual acid with 0.15 mol / L potassium hydroxide solution, shaking continuously until the solution just turns red with an excess of 1 drop, the solution is slightly red, and there is no significant fading after shaking for 30 seconds. This process can be disregarded; if the color fades after more than 1 minute, no additional drops are needed.

[0026] Add about 150 mL of boiling water to the neutralized polytetrafluoroethylene beaker, and quickly titrate with 0.05 mol / L potassium hydroxide standard titration solution until the solution just changes color. Stir rapidly with a plastic rod for 30 s until the color does not fade. Add 1 drop (if solid potassium chloride is used as a potassium salt supplement, it is not necessary to add a drop). Record the titration volume V1.

[0027] The blank test involves performing the exact same procedures as described above without adding any sample, and recording the blank titration volume V0.

[0028] The results are calculated using the following formula: Where: ω is the percentage content of silica in the sample, in %; C0 is the concentration of the alkaline standard titration solution, in mol / L; V0 is the blank titration volume, in L; V1 is the sample titration volume, in L; m0 is the sample weight, in g; 60.08 is the molar mass of silica, dimensionless; 4 means that 1 mol of silica is equivalent to 4 mol of hydrogen ions.

[0029] Example 1 Determination of silica in quartz (powder) samples: Accurately weigh 2.0300–2.0500 g of flux sodium hydroxide; accurately weigh 0.2000 ± 0.0001 g of quartz (powder) sample, and mix them with the flux sodium hydroxide in a nickel crucible. Melt in a muffle furnace at 700 °C for 15 minutes; remove and allow to cool slightly, wash with hot water, and place in a 200 mL volumetric flask at the same temperature as water. Add 3.5 mL of 6.0 mol / L hydrochloric acid solution, and dilute to volume and mix well. Filter dry through medium-speed quantitative filter paper, and retain the filtrate as the analytical solution.

[0030] Aliquot 15.00 mL of the analytical solution into a 400 mL PTFE beaker. Add 1 mL of phenolphthalein (10 g / L) to the analytical solution, and neutralize with 6.0 mol / L hydrochloric acid solution until colorless, with an excess of 6-7 drops. Add 4 g of solid potassium thiocyanate and 1 mL of potassium fluoride solution (500 g / L). Neutralize the residual acid with 0.15 mol / L potassium hydroxide solution, shaking continuously until the solution just turns red, with an excess of 1 drop. The solution should be slightly red, and the color should not fade significantly after shaking for 30 seconds. Add approximately 150 mL of boiling water to the neutralized PTFE beaker, and quickly titrate with 0.05 mol / L potassium hydroxide standard titrant until the color just changes. Stir rapidly with a plastic rod for 30 seconds until the color does not fade. Add 1 drop of water and record the titration volume V1. For the blank determination, perform the above analytical steps without adding any sample, and record the blank titration volume V0. Substitute the titration volumes V1 and V0 into the above formula to calculate the content.

[0031] To verify the precision of this invention, the same actual sample was weighed eight times, melted separately, and measured according to the above operating procedure. The measurement results are shown in Table 1 below.

[0032] Table 1 Precision Experimental Data As can be seen from the data in Table 1, when potassium thiocyanate is used as a potassium supplement, the maximum relative standard deviation (RSD) is only 0.47%, which does not exceed 0.5%, meaning that the difference in titration volume is within 0.1 mL, which fully meets the requirements for operational error.

[0033] Secondly, to verify the accuracy of this invention, the same actual sample was weighed twice, melted separately, and measured according to the above operating procedures. The measurement results were compared with those obtained by the silicomolybdenum yellow spectrophotometric method, and the results are shown in Table 2 below.

[0034] Table 2 Accuracy Experimental Data The results show that the determination results of this method are basically consistent with those of the spectrophotometric method of molybdenum yellow siliceous metal, and the difference between the two methods is within an acceptable range, indicating that this method has good accuracy.

[0035] In addition, the following points should be noted in actual operation. First, averaging twice is a requirement of the traditional potassium fluorosilicate precipitation method. When using phenolphthalein as an indicator, the process is lengthy and involves too many points to note, resulting in unstable results. For safety, two parallel measurements should be performed. Once this method is used and the operating procedure is fixed, only one determination is needed. Second, it is impossible to determine the standard by distinguishing light red, pink, red, and dark red on a red background, but it is easy to distinguish whether it changes from colorless to colored on a white background. The "just turning red" that occurs in the first neutralization and the second titration in this method refers to this point, and it is important to be precise. Due to the different volumes of the test liquid, the color at this point during the first neutralization will be darker than that during the second titration. Third, since the concentration of the potassium hydroxide standard titration solution is only 0.05 mol / L, which is relatively low, the standard potassium hydrogen phthalate or other standard substance used for standardization should be matched in equivalent volume with the potassium hydroxide standard titration solution. It is recommended to use a 0.0X mL bottle-top titrator and establish a silica control sample system. Fourth, when the system temperature is too high, potassium thiocyanate should be used as a potassium supplement. Dissolving approximately 4 g of potassium thiocyanate will lower the system temperature to below 15°C. To ensure titration accuracy, one drop can be added after the second titration endpoint. Fifth, if cost is not a concern, using potassium hydroxide as a flux will yield more stable results. The amount of potassium hydroxide used as flux is 2.840~2.870 g. Sixth, Al in the sample... 3+ The content should be controlled below 2% (20 mg / L), when Al 3+ When the content exceeds this limit, the endpoint fades rapidly and repeatedly, making it impossible to determine the endpoint and resulting in a lower measurement result.

Claims

1. A method for rapid determination of silicon based on the principle of potassium fluorosilicate precipitation-acid-base titration, characterized in that, The steps of transferring and washing potassium fluorosilicate precipitate are omitted, and the following steps are included: (1) Sample digestion and acidification: The silicon-containing sample is mixed with an alkaline flux and melted. The melt is extracted with hot water, cooled, and acid is added to adjust the pH of the solution to 12.8~13.

2. The solution is then brought to a final volume to obtain the sample solution. (2) Filtration to remove impurities: Filter the sample solution obtained in step (1) and collect the filtrate as the analytical solution; (3) Precipitation formation and neutralization: Take the analytical solution from step (2), add an indicator, and neutralize with acid until the indicator changes color and is in excess; add solid potassium thiocyanate to the system as a potassium salt supplement, and add potassium fluoride solution; then neutralize the residual acid with alkali until the indicator just shows color. (4) Hydrolysis titration: Add hot water to the system after neutralization in step (3), and immediately titrate with alkaline standard titration solution until the indicator just shows the color endpoint, and record the titration volume V1; (5) Blank test: Without adding the test sample, perform the same operation as steps (1) to (4) and record the blank titration volume V0; (5) Calculation: Calculate the mass percentage ω of silica in the sample to be tested according to the following formula: Where: ω is the percentage content of silica in the sample, in %; C0 is the concentration of the alkaline standard titration solution, in mol / L; V0 is the blank titration volume, in L; V1 is the sample titration volume, in L; m0 is the sample weight, in g; 60.08 is the molar mass of silica, dimensionless; 4 means that 1 mol of silica is equivalent to 4 mol of hydrogen ions.

2. The method according to claim 1, characterized in that, The alkaline flux in step (1) is sodium hydroxide, and the amount used is 2.030~2.050g sodium hydroxide per 0.2g sample; or the alkaline flux is potassium hydroxide, and the amount used is 2.840~2.870g potassium hydroxide per 0.2g sample; the acid solution is a 6.0mol / L hydrochloric acid solution, and the amount used is 3.5mL per 200mL fixed volume.

3. The method according to claim 1, characterized in that, The melting temperature in step (1) is 700°C and the melting time is 15 minutes.

4. The method according to claim 1, characterized in that, In step (3), the amount of solid potassium thiocyanate added is 3-4g; the mass concentration of the potassium fluoride solution is 500g / L and the amount added is 1mL; the alkali solution is a 0.15mol / L potassium hydroxide solution.

5. The method according to claim 1, characterized in that, The step (3) of neutralizing the indicator with acid until it changes color and is in excess specifically involves neutralizing the phenolphthalein indicator with 6.0 mol / L hydrochloric acid solution until it becomes colorless and then adding 6-7 drops in excess. The step of neutralizing the residual acid with alkali solution until the indicator just shows color specifically involves neutralizing the solution with 0.15 mol / L potassium hydroxide solution until it just turns red and does not fade after shaking for 30 seconds.

6. The method according to claim 1, characterized in that, The hot water mentioned in step (4) is boiling water, and the amount added is 150 mL; the alkaline standard titration solution is a 0.05 mol / L potassium hydroxide standard titration solution; the titration endpoint is when the solution just turns red and does not fade when stirred rapidly with a plastic rod for 30 seconds.

7. The method according to claim 1, characterized in that, The volume of the analytical solution aliquoted in step (3) is 15.00 mL.

8. The method according to claim 1, characterized in that, The aluminum ion content in the sample was controlled to be below 2%.

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