CN115166020A - Method for determining total silicon in soil through hydrofluoric acid wet digestion - Google Patents

Abstract

The invention belongs to the technical field of analytical chemistry, and discloses a method for determining total silicon in soil through hydrofluoric acid wet digestion. The detection method of the invention is used for measuring under the conditions of solving the problems of silicon fluoride which is easy to generate volatility after hydrofluoric acid is added and silicon dioxide precipitation which is easy to generate after buffer solution is added, is suitable for various soil properties, uses a small amount of reagents, reduces the experiment cost and has simple and convenient operation on environmental pollution; the measurement range is wide, samples from low content to high content can be analyzed at the same time, and unnecessary repeated experiments are reduced; the measurement precision is high, and the accuracy is greatly improved by using an inductively coupled plasma emission spectrometer for analysis; the invention has the advantages of low detection limit, good stability, short analysis time and low cost, can realize batch detection, and meets the requirements of modern analysis on high efficiency and convenience.

Description

Method for determining total silicon in soil through hydrofluoric acid wet digestion

Technical Field

The invention belongs to the technical field of analytical chemistry, and particularly relates to a method for determining total silicon in soil through hydrofluoric acid wet digestion.

Background

Silicon is a basic component of soil and rock, and has the effects of promoting plant growth, enhancing plant resistance, participating in the process of biogeochemical cycle, regulating global carbon cycle, and relieving global warming tendency. The main analytical methods for determining the silicon element in the solid in China are gravimetric method and volumetric method, and the standard methods mainly comprise methods for determining the silicon amount by gravimetric method (GB 6730.10-1986) and molybdenum blue spectrophotometry (GB 11064.8-89). At present, the domestic standard method for analyzing silicon in soil and sediments by adopting alkali fusion-inductive coupling plasma atomic emission spectrometry is more, and the analysis of metal elements in soil is mostly carried out by adopting an acid dissolution method for pretreatment, but the common acid dissolution method cannot completely dissolve silicon elements in solids or is easy to cause silicon volatilization loss.

The existing method for measuring silicon dioxide is more in the field of rock and ore, and the analysis method comprises a gravimetric method, a volatilization method, a volumetric method, a colorimetric method, an inductively coupled plasma emission spectrometry and the like. However, the prior art has the following disadvantages: (1) The gravimetric method has complex operation process and long time consumption, is easy to cause quality loss and pollute the environment, and other precipitates are easy to generate and are included when a sample with higher titanium content is dissolved by acid to form silicate precipitates, thereby influencing the determination result; (2) The volumetric method needs to strictly control the precipitation condition of the fluosilicic acid, has higher dependence on the experience of operators and poorer operability; (3) The volatilization method is only suitable for quartzite samples with higher purity, is not suitable for common silicate rocks, and has narrow application range; (4) The method has the advantages that the sample is processed by adopting a platinum crucible high-temperature open melting mode at 1000 ℃, the method is easy to cause sample cross contamination, poor in accuracy and high in cost, and has certain danger due to the adoption of muffle furnace high-temperature melting and aqua regia extraction.

Aiming at the defects in the prior art, a detection method which is suitable for various soil properties, has the advantages of low detection limit, short analysis time, realization of batch detection and the like needs to be researched to meet the requirements of modern analysis on high efficiency and convenience.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention firstly aims to provide a method for determining total silicon in soil by hydrofluoric acid wet digestion, silicon in a soil sample reacts with excessive hydrofluoric acid to generate fluosilicic acid, ammonium bicarbonate is used for neutralization to generate ammonium fluosilicate, an inductively coupled plasma emission spectrometer is used for testing silicon elements in the ammonium fluosilicate, and the intensity of a generated characteristic spectrum is in direct proportion to the content of the silicon elements in the sample within a certain range.

The purpose of the invention is realized by the following scheme:

a method for determining total silicon in soil through hydrofluoric acid wet digestion comprises the following steps:

(1) Preparing a sample solution to be detected and a blank sample solution: weighing soil, adding a digestion acid into the soil, and heating and digesting to obtain a digestion solution; adding a buffer solution into the hydrolysate, and performing constant volume filtration to obtain a sample solution to be detected; digesting the blank sample by adopting the same processing mode as the sample to obtain a blank sample solution;

(2) Preparation of standard working solution: preparing a series of silicon standard concentration working solutions with the certified standard substances of 0mg/L, 10mg/L, 20mg/L, 40mg/L, 60mg/L, 80mg/L, 100mg/L, 300mg/L and 500 mg/L;

(3) Analysis and determination: before analysis is carried out by using an inductively coupled plasma emission spectrometer, washing the sample injection system by using an acid solution until a blank intensity value is within 100, after an analysis signal is stable, carrying out linear range test on the standard working solution with different concentrations in the step (2), and establishing a standard working curve by using the mass concentration of silicon element in the standard series solution as a horizontal coordinate, using the emission intensity of the silicon element as a vertical coordinate and adopting a radial observation mode; detecting the sample liquid to be detected and the blank sample liquid prepared in the step (1) under the same condition;

(4) And (3) calculating: and substituting the measured emission intensities of the sample liquid to be measured and the blank sample liquid into a standard working curve to obtain the mass concentration of the silicon element in the sample liquid to be measured, and then calculating to obtain the total silicon content in the sample to be measured.

The pH value of the soil in the step (1) is acidic, neutral or alkaline;

in the step (1), the digestion acid is any one or more of hydrofluoric acid, nitric acid and hydrochloric acid;

correspondingly adding 3-9 mL of digestion acid into every 0.1g of soil in the step (1);

the digestion acid is a mixed acid of hydrofluoric acid and nitric acid with the volume ratio of preferably 3:1; adding hydrofluoric acid with the volume more than or equal to 3mL correspondingly to each 0.1g of soil;

the heating digestion conditions in the step (1) are that the heating temperature is 30-120 ℃, and the digestion time is 30-120 min;

preferably, the heating digestion condition is that the heating temperature is 90 ℃ and the digestion time is 120min;

the buffer solution in the step (1) is at least one of ammonium bicarbonate, sodium hydroxide and sodium bicarbonate; the concentration of the buffer solution is 0.5-2.0 mol/L, preferably 1mol/L ammonium bicarbonate;

adding 1mL of buffer solution correspondingly to 1mL of digestion solution in each step (1), and fixing the volume to 10mL;

the silicon standard solution with the certified standard substance of 1000mg/L obtained in the step (2) is purchased from national analysis and test center for nonferrous metals and electronic materials;

the acid solution in the step (3) is a 2-5% nitric acid solution, preferably a 2% nitric acid solution;

the measurement conditions of the emission spectrum of the inductively coupled plasma in the step (3) are preferably as follows:

TABLE 1 measurement conditions for inductively coupled plasma emission Spectroscopy

The calculation in step (4) calculates the total silicon (in terms of SiO) in the sample according to the following formula ₂ Meter) content ω (%):

in the formula: omega-the content of total silicon in the soil sample,%;

ρ ₁ the concentration of silicon element in the sample, mg/L, is found from the calibration curve;

ρ ₀ -the concentration of elemental silicon in the blank, mg/L;

v, the volume of the digested sample is determined to be mL;

m is the sample weighing of the soil sample, g;

M ₂ -silica molar mass;

M ₁ silicon molar mass.

Compared with the prior art, the invention has the following advantages and beneficial effects:

1. in the field of soil detection, the platinum crucible is commonly used for alkali fusion digestion and then testing for testing the total silicon, and digestion research by an acid wet method is less and the method is not suitable for testing the total silicon. The detection method disclosed by the invention is used for detecting under the conditions of solving the problems that volatile silicon fluoride is easily generated after hydrofluoric acid is added and silicon dioxide precipitates are easily generated after buffer solution is added, is suitable for various soil properties, is simple and convenient to operate, low in detection limit, good in stability, short in analysis time and low in cost, can realize batch detection, and meets the requirements of modern analysis on high efficiency and convenience.

2. The reagent is less, so that the experiment cost and the pollution to the environment are reduced;

3. the pretreatment step only needs wet digestion, so that the requirement on the operation of personnel is low, and the error risk of the experiment is reduced;

4. the experiment can be completed only in 2-3 hours from the beginning of the pretreatment to the end of the analysis, and the time consumption is short;

5. the measurement range is wide, samples with the silicon dioxide content of 1-99% can be measured, and unnecessary repeated experiments are reduced;

6. the measurement precision is high, and the accuracy is greatly improved by using an inductively coupled plasma emission spectrometer for analysis;

7. the on-machine analysis of the previous batch of samples can be operated simultaneously with the pretreatment of the next batch of samples, which is beneficial to realizing the streamlined operation and is suitable for the detection of a large batch of samples.

8. The used experimental consumables are low in cost, and precious consumables of a platinum crucible do not need to be purchased.

Drawings

FIG. 1 is a graph of different digestion times versus silica concentration.

FIG. 2 is a graph of different digestion temperatures versus silica concentration.

FIG. 3 is a standard operating curve with a linear range of 0-100 mg/L.

Detailed Description

The present invention will be described in further detail with reference to examples and drawings, but the embodiments of the present invention are not limited thereto. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are conventional products which are not indicated by manufacturers and are commercially available.

The reagents used in the examples are commercially available without specific reference.

The method firstly researches the reaction time, temperature, the type and volume of consumed acid and the like between hydrofluoric acid and soil with different properties, and explores the optimal pretreatment experimental conditions.

(1) Optimization of reaction time conditions

Selecting soil standard substance GBW07390 (GSS-34, siO) ₂ The content of 56.47 +/-0.32 percent), respectively weighing 0.1g, adding digestion acid (3 mL of hydrofluoric acid and 1mL of nitric acid) into a polypropylene digestion tank, placing the polypropylene digestion tank on a graphite digestion instrument at 25℃,Heating and digesting at 90 deg.C for 30min, 60min, 90min and 120min, cooling to constant volume, and filtering.

The results are shown in FIG. 1, where the silica content increases with the digestion time, and at 30 minutes the silica content has reached substantially more than 70% at different temperatures, and at 90 minutes the digestion has been substantially completed at a temperature of 9 ℃ at 0 ℃. In order to ensure the digestion quality, 120min is selected as the digestion time of the method.

(2) Digestion-consuming acid species selection

Soil standard substance GBW07388 (GSS-32 ₂ The content is 67.33 +/-0.36 percent), 0.1g of the solution is weighed respectively, different acids (the types of the added acids are shown in table 2) are added into a polypropylene digestion tank, the polypropylene digestion tank is placed on a graphite digestion instrument for heating and digestion at 100 ℃ for 120min, and the solution is cooled, fixed in volume and filtered to be tested. The experimental results are shown in Table 2:

table 2 statistics table for acid types added

According to the digestion results of different concentrations and different types of acids, the measured content of the silicon dioxide is increased along with the addition volume of the acid, the measured value is slightly increased, and the main reaction is as follows:

regarding the reaction of hydrofluoric acid and silicon dioxide: siO 2 ₂ +4HF＝SiF ₄ ↑+2H ₂ O

In case of excess hydrofluoric acid: siO 2 ₂ +6HF＝H ₂ SiF ₆ +2H ₂ O

Silicon dioxide reacts with hydrofluoric acid to easily generate silicon fluoride volatilization, so that the measured value is reduced, but the silicon dioxide and the hydrofluoric acid can generate fluosilicic acid under the condition of excessive hydrofluoric acid; meanwhile, excessive acid is added to bring silica impurities into interference, so that the measurement result is higher. Through experimental determination, the volume of hydrofluoric acid is 3mL, and the volume of nitric acid is 1mL, so that the determination requirement of the accuracy of the soil standard substance can be met.

(3) Optimization of digestion temperature conditions

Selecting three kinds of soil with pH values of acid, medium and alkali, respectively weighing 0.1g, adding a digestion acid (3 mL of hydrofluoric acid and 1mL of nitric acid) into a polypropylene digestion tank, respectively placing the digestion acid and the graphite digestion tank on a low-temperature refrigerating chamber at 10 ℃ and a graphite digestion instrument, respectively performing heating digestion at 30 ℃, 50 ℃, 90 ℃, 100 ℃, 110 ℃ and 120 ℃ for 120min, cooling, fixing the volume, and filtering to be tested.

The experimental result is shown in figure 2, silicon in soil reacts with hydrofluoric acid under different temperature conditions, and the reaction is exothermic, so the measured data change is not obvious along with the rise of temperature, the deviation is within 10 percent, but according to the digestion time condition test, the reaction rate can be accelerated by heating, the digestion time is saved, and the reaction temperature is selected to be 90 ℃.

(4) Selection of buffer

Hydrofluoric acid has a serious corrosion effect on experimental instruments, and the corrosion of hydrofluoric acid and the like on quartz vessels is reduced by diluting the digestion solution and adding a proper buffer solution.

Selecting soil standard substance GBW07390 (GSS-34, siO) ₂ The content is 56.47 +/-0.32%), 0.1g of the digestion solution is weighed respectively, digestion acid (3 mL of hydrofluoric acid and 1mL of nitric acid) is added into a polypropylene digestion tank, and the polypropylene digestion tank is placed on a graphite digestion instrument to be heated and digested for 120min at 90 ℃ to obtain the digestion solution.

The digestion solution was divided into three groups, 1mL of digestion solution was sucked into 10mL of cuvettes containing the solutions in Table 3, and the test was performed after shaking to a constant volume.

TABLE 3 buffer selection

TABLE 4 SiO measurement under different buffer conditions ₂ Content (%)

According to the test results, the test concentration of silica is slightly reduced with the addition of buffer solution, and the main reaction is as follows:

reacting fluosilicic acid with ammonium bicarbonate:

H ₂ SiF ₆ +2NH ₄ HCO ₃ ＝(NH ₄ ) ₂ SiF ₆ +2H ₂ O+2CO ₂ ↑

in case of excess ammonium bicarbonate:

(NH ₄ ) ₂ SiF ₆ +4NH ₄ HCO ₃ ＝SiO ₂ ↓+6NH ₄ F+2H ₂ O+4CO ₂ ↑

because excessive ammonium bicarbonate can generate trace silicon dioxide precipitation in the reaction, the measurement data becomes low, sodium hydroxide, sodium bicarbonate and fluosilicic acid are easy to generate sodium fluosilicate precipitation, and certain influence is caused on the test accuracy, so that 1mol/L ammonium bicarbonate is selected and added as a buffer solution.

(5) Detection limit measurement

Selecting soil standard substance GBW07390 (GSS-34, siO) ₂ The content is 56.47 +/-0.32%), 0.1g of the solution is weighed respectively, digestion acid (3 mL of hydrofluoric acid and 1mL of nitric acid) is added into a polypropylene digestion tank, the polypropylene digestion tank is placed on a graphite digestion instrument to be heated and digested for 120min at 90 ℃, after digestion solution is obtained, 1mL of the digestion solution is sucked into a 10mL colorimetric tube containing 1mL of ammonium bicarbonate (1 mol/L) solution, and the test is carried out after constant volume shaking is carried out.

Digesting 7 blank samples by adopting the same processing mode as the samples, respectively carrying out blank measurement in two modes of adding 1mL of ammonium bicarbonate (1 mol/L) and not adding ammonium bicarbonate, and calculating the standard deviation and the method detection limit for 7 times.

TABLE 5 comparison of detection limits by method

The experiment result shows that the blank concentration and the detection limit of the test are better than those of the test without adding the ammonium bicarbonate after adding the ammonium bicarbonate, so the test by adding the ammonium bicarbonate is carried out.

Example 1 determination of total silicon in soil by hydrofluoric acid Wet digestion

1. Preparing a sample solution to be detected and a standard working solution:

(1) Preparing a sample solution to be detected and a blank sample solution:

soil with different properties and different contents is selected as a detection sample, the soil sample is air-dried and ground according to the requirements of relevant standards, and then the soil sample is sieved by a 0.15mm nylon sieve for later use.

Sample solution to be tested: weighing 0.1g of sieved soil sample, placing the sieved soil sample in a 50mL polypropylene digestion tank with a reticle, adding digestion acid (3 mL hydrofluoric acid and 1mL nitric acid), covering, placing on a graphite digestion instrument (or a constant-temperature water bath kettle) at 90 ℃ for closed heating digestion for 120min, cooling, adding primary water to fix the volume to the reticle, and shaking up.

Sucking 1mL of digestion solution into a 10mL colorimetric tube containing 1mL of ammonium bicarbonate (1 mol/L) solution, fixing the volume to a scribed line with primary water, and shaking up to obtain a sample solution to be detected;

blank sample liquid: adding a digestion acid (3 mL of hydrofluoric acid and 1mL of nitric acid) into a polypropylene digestion tank by adopting the same treatment mode as the sample, covering, placing on a graphite digestion instrument (or a constant-temperature water bath kettle) at 90 ℃ for closed heating digestion for 120min, cooling, then using primary water to fix the volume to a scribed line, and shaking up.

Sucking 1mL of digestion solution into a 10mL colorimetric tube containing 1mL of ammonium bicarbonate (1 mol/L) solution, fixing the volume to a scribed line by using primary water, and shaking up to obtain a blank sample solution;

(2) Preparation of standard working solution: according to the total content of the silicon dioxide in the soil of cultivated land of China being 50-70%, the average content is about 60%, and the concentration range of the silicon dioxide in the soil is selected from 0-100 mg/L for quantitative determination. A series of working solutions with silicon standard concentrations of 0mg/L, 10mg/L, 20mg/L, 40mg/L, 60mg/L, 80mg/L and 100mg/L are prepared by using a certified standard substance (1000 mg/L, national nonferrous metal and electronic material analysis and test center).

2. Inductively coupled plasma emission spectrum measurement conditions

The instrument conditions were as follows: the optimal test conditions of different models of instruments are different, and the test conditions are optimized according to the requirements of instrument specifications. After the plasma is ignited, setting is carried out according to working parameters provided by a manufacturer, and measurement is carried out after the instrument is preheated until various indexes are stable.

The measurement conditions of the emission spectrum of the inductively coupled plasma in example 1 are shown in table 1 below.

TABLE 1 measurement conditions for inductively coupled plasma emission Spectroscopy

3. Analysis and determination:

before analysis is carried out by using an inductively coupled plasma emission spectrometer, flushing the sample injection system by using a 2% nitric acid solution until the blank intensity value is within 100, after the analysis signal is stable, carrying out linear range test on the standard working solution with different concentrations in the step (2), and establishing a standard working curve by using the mass concentration of silicon element in the standard series solution as a horizontal coordinate, using the emission intensity of the silicon element as a vertical coordinate and adopting a radial observation mode, wherein the standard working curve is shown in figure 3; and (3) detecting the sample liquid to be detected and the blank sample liquid prepared in the step (1) by an inductively coupled plasma emission spectrometer under the same condition. During the sample measurement process, if the element concentration of the sample exceeds the range of the calibration curve, the sample needs to be diluted and then measured again.

As can be seen from FIG. 3, the correlation coefficient is >0.999 with good linearity when the linear range is 0-100 mg/L by ICP-OES radial mode observation.

4. And (4) calculating a result:

and after the emission intensity of the sample to be detected is obtained, substituting the emission intensity into a standard working curve of a concentration range of 0-100 mg/L to obtain the concentration of the silicon element in the sample liquid to be detected, and then calculating to obtain the total silicon content in the sample to be detected.

All-silicon (as SiO) in soil samples ₂ Meter) content ω (%):

in the formula: omega-the content of total silicon in the soil sample,%;

ρ ₁ the concentration of silicon element in the sample, mg/L, is found from the calibration curve;

ρ ₀ -the concentration of elemental silicon in the blank, mg/L;

v, the volume of the digested sample is determined to be mL;

m is the sample weighing of the soil sample, g;

M ₂ -silica molar mass;

M ₁ silicon molar mass.

5. Method comparison test

Selecting soil standard substance GBW07402 (GSS-2 ₂ 73.35 + -0.18%), (GBW 07447 (GSS-18, siO ₂ The content of 60.40 +/-0.26 percent), GBW07390 (GSS-34 ₂ The content is 56.47 +/-0.32%), three different soil standard substance samples are simultaneously compared and analyzed according to the HJ 974 alkali fusion method and the method, and the test results are as follows:

TABLE 6 comparison of Experimental methods

Experimental results show that the hydrofluoric acid wet digestion is adopted for digestion determination, the results of the digestion determination are basically consistent with the determination results of alkali fusion-inductively coupled plasma emission spectrometry for determination of HJ 974-2018 elements, the results are all within the uncertainty requirement range of the first-level standard substance, and the determination precision and the accuracy can also meet the test requirements.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.

Claims (10)

1. A method for determining total silicon in soil through hydrofluoric acid wet digestion is characterized by comprising the following steps:

(1) Preparing a sample solution to be detected and a blank sample solution: weighing soil, adding digestion acid, heating and digesting to obtain digestion solution; adding a buffer solution into the hydrolysate, and performing constant volume filtration to obtain a sample solution to be detected; digesting the blank sample by adopting the same processing mode as the sample to obtain a blank sample solution;

(2) Preparation of standard working solution: preparing a series of silicon standard concentration working solutions with the certified standard substances of 0mg/L, 10mg/L, 20mg/L, 40mg/L, 60mg/L, 80mg/L, 100mg/L, 300mg/L and 500 mg/L;

(3) Analysis and determination: before analysis is carried out by using an inductively coupled plasma emission spectrometer, washing the sample injection system by using an acid solution until a blank intensity value is within 100, after an analysis signal is stable, carrying out linear range test on the standard working solution with different concentrations in the step (2), and establishing a standard working curve by using the mass concentration of silicon element in the standard series solution as a horizontal coordinate, using the emission intensity of the silicon element as a vertical coordinate and adopting a radial observation mode; detecting the sample liquid to be detected and the blank sample liquid prepared in the step (1) under the same condition;

(4) And (3) calculating: substituting the emission intensity of the sample liquid to be measured and the emission intensity of the blank sample liquid into a standard working curve to obtain the concentration of silicon element in the sample liquid to be measured, then calculating to obtain the total silicon content in the sample to be measured, and using SiO to calculate the total silicon content ₂ And (6) counting.

2. The method of claim 1, wherein: the pH value of the soil in the step (1) is acidic, neutral or alkaline.

3. The method of claim 1, wherein: in the step (1), the digestion acid is any one or more of hydrofluoric acid, nitric acid and hydrochloric acid;

in the step (1), 3-9 mL of the digestion acid is correspondingly added into every 0.1g of soil.

4. A method as claimed in claim 3, characterized in that: the digestion acid is a mixed acid of hydrofluoric acid and nitric acid with the volume ratio of preferably 3:1; adding hydrofluoric acid with the volume more than or equal to 3mL for each 0.1g of soil.

5. The method of claim 1, wherein: the heating digestion conditions in the step (1) are that the heating temperature is 30-120 ℃, and the digestion time is 30-120 min;

the heating digestion condition is preferably that the heating temperature is 90 ℃, and the digestion time is preferably 120min.

6. The method of claim 1, wherein: the buffer solution in the step (1) is at least one of ammonium bicarbonate, sodium hydroxide and sodium bicarbonate;

the concentration of the buffer solution is 0.5-2.0 mol/L, and preferably 1mol/L ammonium bicarbonate.

7. The method of claim 1, wherein: in the step (1), 1mL of buffer solution is correspondingly added into 1mL of digestion solution every time, and the volume is determined to be 10mL.

8. The method of claim 1, wherein: the silicon standard solution with the certified standard substance of 1000mg/L obtained in the step (2) is purchased from national analysis and test centers for nonferrous metals and electronic materials.

9. The method of claim 1, wherein: the acid solution in the step (3) is a 2-5% nitric acid solution, preferably a 2% nitric acid solution.

10. The method of claim 1, wherein:

the measurement conditions of the emission spectrum of the inductively coupled plasma in the step (3) are preferably as follows:

TABLE 1 measurement conditions for inductively coupled plasma emission Spectroscopy

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