CN104330522A - Measurement method of cation exchange capacity in soil - Google Patents

Abstract

The invention discloses a method for measuring cation exchange capacity in soil, which comprises five steps of preparing soil samples, using EDTA-ammonium acetate mixed solution and 95% ethanol for exchange and elution, using a full-automatic distiller to distill, measuring with instruments, and calculating. The present invention optimizes and improves soil sample extraction and rinsing conditions, selects a fully automatic FOSSKjeltec8400 distiller, and optimizes and screens relevant detection conditions, improves and optimizes the absorption liquid concentration and volume conditions, and makes the cation exchange in the soil Quantities are measured more precisely. The method has the advantages of simple operation, rapidity, accuracy and good repeatability.

Description

A kind of assay method of cation exchange capacity in soil

technical field

The invention relates to an analysis method of cation exchange capacity, in particular to a method for measuring the cation exchange capacity in soil.

Background technique

Soil colloids make soil adsorptive due to their huge specific surface area and chargeability. In the diffusion layer of the soil colloidal electric double layer, the compensation ions can exchange equivalently with the ions of the same charge in the solution based on the ion valence, which is called ion exchange. Soil cation exchange capacity refers to the total amount of various cations that can be adsorbed by soil colloid, expressed in milligram equivalents of cations adsorbed on 100 g of dry soil. The size of the cation exchange capacity can be used as an index to evaluate the soil fertility. Cation exchange capacity is the main source of soil buffer performance and an important basis for soil improvement and rational fertilization. Therefore, it is very important to measure the cation exchange capacity, which reflects the total amount of soil negative charge and characterizes the main index of soil properties.

Tobacco planting soil has many types of soil due to the different geographic locations where tobacco grows, and the soil properties vary greatly, which makes the determination process change greatly.

The determination of soil cation exchange capacity is affected by many factors, such as the nature of the exchanger, the concentration of the salt solution, the pH and the washing method. At present, the methods for determining soil cation exchange capacity mainly include neutral ammonium acetate method, sodium acetate method and calcium acetate method. The neutral ammonium acetate method is also a routine analysis method used in soil and agrochemical laboratories in my country, and is suitable for acidic and neutral soils. Because the pH value and ionic strength of the leachate taken in this method are too high, for acidic and slightly acidic soils, the measured results are much higher than the actual situation. Calcium acetate method is mainly applicable to the analysis of calcareous soil samples. The Ministry of Agriculture of my country adopted the calcium acetate method as the standard method for analyzing the cation exchange capacity of calcareous soil samples in 2006. The pretreatment process of these previous methods is complicated and time-consuming; and the repeated manual treatment process is very easy to cause the loss of the sample, which makes the measurement result low.

Contents of the invention

In view of this, the purpose of the present invention is to provide a method for more accurately measuring the cation exchange capacity in soil.

In order to solve the above technical problems, the technical solution provided by the present invention is to provide a method for measuring cation exchange capacity in soil, comprising the following steps:

Step 1) Soil sample preparation

Take soil samples, air-dry them, remove impurities, and crush the soil samples so that the particle size of all samples is less than 2mm;

Step 2) Exchange Elution

Accurately weigh the prepared soil sample, put it into a centrifuge tube, add part of the EDTA-ammonium acetate mixture, vortex until the slurry is uniform, then add part of the EDTA-ammonium acetate mixture, and continue to vortex;

Put the centrifuge tube into the centrifuge for centrifugation, discard the supernatant in the centrifuge tube after centrifugation; then add 95% ethanol to the centrifuge tube containing the sample, vortex and shake until it becomes a uniform slurry, then add 95% ethanol, and centrifuge again , so repeated centrifugation 3-6 times;

Step 3) Distillation

Add water to the centrifuge tube after centrifugation in step 2), vortex until it becomes a paste, wash it into the digestion tube without damage, and add magnesium oxide to the digestion tube before distillation;

Step 4) Instrumental measurement

Determination using a fully automatic distiller: add calibrated 0.02 mol/L hydrochloric acid solution to the titrant barrel, put the digestion tube into the fully automatic distiller, and add 2% boric acid-indicator receiving solution to the digestion tube ;

Step 5) Calculate

Calculate the cation exchange capacity in the tobacco planting soil according to the following calculation formula:

Cation exchange capacity=1000 c (VV ₀ )/(m 10) (cmol kg ^－1 )

In the formula: c—concentration of standard hydrochloric acid solution, mol L ^-1 ;

V—the amount of hydrochloric acid standard solution consumed for titrating the sample to be tested, mL;

V ₀ —Consumption of hydrochloric acid standard solution for blank titration, mL;

m—mass of air-dried sample, g;

10—convert mmol to cmol multiples;

1000—converted to cmol per kg.

Preferably, in step 1), the fine fibrous roots in the soil sample are removed by electrostatic adsorption.

Further, the step 2) is specifically: Weigh 2 g of the air-dried soil sample with an electronic balance, put it into a 100 mL plastic centrifuge tube with stopper, add 2mL LEDTA-ammonium acetate mixture, and put it in a vortex Stir the sample on the oscillator to make it into a uniform slurry; add 38mL of mixed solution to make the total volume reach 40mL, vortex for 1.5min, and the shaking speed is 3000r/min; put the centrifuge tubes in pairs on the two plates of the coarse balance, add Make the mixed solution balance, then put it into the centrifuge symmetrically, centrifuge at a speed of 4 min, discard the supernatant in the centrifuge tube; add 2mL of 95% ethanol to the centrifuge tube containing the sample, and stir on a vortex shaker For the sample, make the soil sample into a uniform slurry, add 48 mL of 95% ethanol, vortex for 1.5 min, and shake at a rate of 3000 r/min, put the centrifuge tubes in pairs on the two plates of the coarse balance, add ethanol to make it balanced, Put them in a centrifuge symmetrically and centrifuge for 4 minutes, discard the ethanol clear solution; repeat this process until there is no ammonium ion.

Preferably, the number of repetitions is 4 times, the centrifugation rate of the first two times is 3000r/min, and the centrifugation rate of the last two times is 4500r/min.

Further, step 3) is specifically: add 2 mL of water into the centrifuge tube, vortex to stir the ammonium ion-saturated soil into a paste, and wash it into a 500 mL digestion tube without damage. The volume of washing is controlled at 60 mL. Add 1.5g-3g magnesium oxide to the digestion tube.

Further, the fully automatic distiller is a FOSS Kjeltec 8400 distiller; a calibrated 0.02 mol/L hydrochloric acid solution is pre-added in the titrant barrel, the digestion tube is put into the distiller, and 2 % boric acid-indicator receiving solution 30 mL.

Preferably, the distillation parameters of the distiller are: steam volume 40%, temperature 100°C, distillation time 7'30".

Preferably, the EDTA-ammonium acetate mixture is a mixture of 0.005mol/LEDTA and 1mol/L ammonium acetate, adjusted to pH7.0 or pH8.5 with ammonia water and glacial acetic acid.

Preferably, the calibrated hydrochloric acid solution is a 0.02mol/L hydrochloric acid standard solution.

Preferably, the boric acid-indicator is a 2% boric acid-indicator receiving solution by mass volume fraction, and the indicator is bromocresol green and methyl red.

Compared with the prior art, one of the above technical solutions has the following advantages:

1. The present invention optimizes and improves soil sample extraction and leaching conditions, selects a fully automatic FOSS Kjeltec 8400 distiller, and optimizes and screens relevant detection conditions, improves and optimizes the concentration and volume conditions of the absorption liquid, so that The determination of cation exchange capacity in soil is more accurate.

2. Compared with traditional methods, the present invention simplifies the pretreatment and analysis steps.

3. The present invention uses a vortex oscillator. Compared with the traditional method using manual stirring, the stirring is more sufficient, uniform and repeatable; no stirring rod is required, which reduces sample loss. According to the experimental equipment, 4 samples can be shaken at the same time, which improves the pretreatment efficiency and reduces labor consumption.

4. The present invention selects the FOSS Kjeltec 8400 distiller. This distiller has a large nitrogen-fixing bottle. No need to add paraffin during detection, and no bumping phenomenon will occur. Since the addition of paraffin is reduced, the cleaning of the nitrogen-fixing bottle is relatively easy .

5. Use the automatic azotometer - FOSS Kjeltec 8400 distiller to save labor and do not need cumbersome collection and titration processes.

6. This method has the advantages of simple operation, rapidity, accuracy and good repeatability.

Detailed ways

The following will be described in conjunction with a specific embodiment.

1. Instruments and reagents: FOSS Kjeltec 8400 distiller (FOSS company), VtexMixer 230VeU oscillator (US Labnet company); Sigma 3K15 centrifuge (Germany Sigma company), PH meter.

EDTA, ammonium acetate, ammonia water, glacial acetic acid, 95% ethanol, boric acid, methyl red, bromocresol green, magnesium oxide (analytical grade).

2. Solution preparation

2.1 Prepare a mixture of 0.005mol L ^-1 EDTA and 1mol L ^-1 ammonium acetate: weigh 77.09g ammonium acetate (analytical pure) and 1.461g ethylenediaminetetraacetic acid (analytical pure), add water to dissolve and wash together into 2000ml capacity Add water to the bottle to about 900ml, adjust to pH7.0 or pH8.5 with ammonia water and glacial acetic acid, and then set the volume to the mark for later use. The EDTA-ammonium acetate mixture with a pH value of 7.0 is used for the extraction of acidic and neutral soils, and the EDTA-ammonium acetate mixture with a pH value of 8.5 is used for the extraction of calcareous and neutral soils.

2.2 Magnesium oxide: Burn the magnesium oxide at 500°C for 0.5h in a high-temperature electric furnace.

2.3 2% boric acid-indicator receiving solution:

2.3.1 2% (m/V) boric acid solution: Weigh 200.00g of boric acid, dissolve it in water, and dilute to 10L.

2.3.2 Add 100mL bromocresol green ethanol solution. 100mg bromocresol green was dissolved in 100mL 95% ethanol to prepare bromocresol green solution.

2.3.3 Add 70mL methyl red ethanol solution. Dissolve 100mg methyl red in 100mL 95% ethanol to make methyl red ethanol solution.

2.4 Preparation and calibration of hydrochloric acid solution

2.4.1 Prepare 0.1mol/L hydrochloric acid solution: Take 500ml of distilled water, add 8.4ml of concentrated hydrochloric acid with a mass volume fraction of 37%, and set the volume to 1000mL, which is 0.1mol/L hydrochloric acid solution.

2.4.2 Calibration of 0.1mol/L hydrochloric acid solution: Weigh 0.2000g of standard anhydrous sodium carbonate and burn it in a muffle furnace at 270-300°C for 1 hour to a constant amount, dissolve it in 50ml of water, add 10 drops of bromocresol green- Mix methyl red as an indicator agent, titrate with 0.1mol/L hydrochloric acid solution until the solution turns from green to dark red, boil for 2 minutes, continue to titrate after cooling until the solution turns dark red. At the same time do a blank test.

2.4.3 Dilute the calibrated 0.1mol/L hydrochloric acid solution 5 times to obtain a 0.02mol/L hydrochloric acid standard solution.

3 Determination steps:

The soil samples collected from the field are placed on the sample tray in time, spread out, and placed in a clean and tidy indoor ventilated place to dry naturally. Exposure to the sun is strictly prohibited, and attention should be paid to prevent pollution by acids, alkalis and other gases and dust. During the air-drying process, it should be turned frequently Crush the soil samples and crush large clods to speed up the drying, and at the same time remove the immersion bodies other than the soil.

Lay the air-dried sample on the sample board, roll it with a wooden stick or a plastic stick, and remove the plant residues, stones and other intrusive bodies and new bodies. The small and broken plant fibrous roots are electrostatically sucked clear. All the crushed soil samples must pass through the 2mm sieve, and the unsieved soil particles must be re-rolled and sieved until all the samples pass through the 2mm sieve. The soil samples passed through the 2mm sieve can be used for the determination of cation exchange capacity.

Weigh 2.00g of the air-dried Liangshan Tobacco Area soil sample that has passed through a 2mm aperture sieve, put it into a 100mL plastic centrifuge tube with a stopper, add 2mL of LEDTA-ammonium acetate mixture, and stir the sample on a vortex oscillator to make a uniform slurry shape. Add the mixed solution to make the total volume reach 40mL, and vortex for 1.5min at a shaking rate of 3000r/min.

Put the centrifuge tubes in pairs on the two plates of the coarse balance, add EDTA-ammonium acetate mixture to make it balanced, then put them symmetrically into the centrifuge, centrifuge for 4 min at the speed of 3000r/min for the first time, discard the centrifuge clear liquid in the tube. Add 2mL of 95% ethanol to the centrifuge tube containing the sample, place the sample on a vortex oscillator to stir the sample to make the soil sample into a uniform slurry, add 48mL of 95% ethanol, vortex for 1.5min, and the shaking speed is 3000r/min. Place the centrifuge tubes in pairs on the two plates of the coarse balance, add ethanol to balance them, and then put them symmetrically into the centrifuge for a second centrifugation at a speed of 3000r/min for a second time, and discard the ethanol clear liquid. So repeated, until no free state ammonium ions. Nessler's reagent was used to test whether the free ammonium ions were completely washed out. In this embodiment, the centrifugation was performed 4 times in total, the first and second centrifugation speeds were 3000 r/min, and the third and fourth centrifugation speeds were 4500 r/min.

Add 2 mL of water into the tube, vortex to stir the combined ammonium ion-saturated soil into a paste, and wash it into a 500 mL digestion tube without damage. The washing volume is controlled at 60 mL, and 1.5- 3.0 g magnesia, more specifically, 2.0 g magnesia was added to each digestion tube.

Put the digestion tube into the fully automatic FOSS Kjeltec 8400 distiller, set to add 2% boric acid-indicator receiving solution 30mL, steam volume 40%, distillation time 7′30″, measure, and do a blank experiment without soil samples at the same time .

The formula for calculating the cation exchange capacity of tobacco planting soil is as follows:

Cation exchange capacity=1000 c (VV ₀ )/(m 10) (cmol kg ^-1 )

In the formula: c—concentration of standard hydrochloric acid solution, mol L ⁻¹ ;

V—the amount of hydrochloric acid standard solution consumed for titrating the sample to be tested, mL;

V0—Consumption of standard hydrochloric acid solution for blank titration, mL;

m—mass of air-dried sample, g;

10—convert mmol to cmol multiples;

1000—converted to cmol per kg.

4. method validation

4.1 Detection limit

The blank solution was continuously measured 10 times, and the concentration corresponding to 3 times the standard deviation was calculated as the detection limit of the instrument ^. The limit of detection (LOD)=3*SD=0.12 cmolkg ^-1 was calculated according to the formula.

4.2 Repeatability and accuracy

The soil standard substances GBW07412A and GBW07414A provided by the Academy of Agricultural Sciences were measured, and the results of 5 consecutive determinations of GBW07412A were 20.3, 20.3, 20.8, 20.3, 20.2 mg/kg, and the RSD was 1.17%. The results of five consecutive determinations of GBW07414A were 16.8, 16.8, 16.9, 16.6, 17.1 mg/kg, and the RSD was 1.08%. The determination results are in line with the uncertainty range requirements of the standard material cations (GBW07412A: 20.2-23.0 mg/kg and GBW07414A 16.0-18.0 mg/kg), indicating that the accuracy and repeatability of the method are good.

Tobacco planting soil in Huili County, Liangshan Prefecture was taken to detect the cation exchange capacity, and the cation exchange capacity was calculated to be 12.7cmol kg-1.

The above are only preferred implementations of the present invention, and it should be noted that the above preferred implementations should not be regarded as limiting the present invention, and the scope of protection of the present invention should be based on the scope defined in the claims. For those skilled in the art, without departing from the spirit and scope of the present invention, some improvements and modifications can also be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims (10)

1. a method for assaying cation exchange capacity in soil, is characterized in that, comprises the following steps: Step 1) Soil sample preparation Take soil samples, air-dry them, remove impurities, and crush the soil samples so that the particle size of all samples is less than 2mm; Step 2) Exchange Elution Accurately weigh the prepared soil sample, put it into a centrifuge tube, add part of the EDTA-ammonium acetate mixture, vortex until the slurry is uniform, then add part of the EDTA-ammonium acetate mixture, and continue to vortex; Put the centrifuge tube into the centrifuge for centrifugation, discard the supernatant in the centrifuge tube after centrifugation; then add 95% ethanol to the centrifuge tube containing the sample, vortex and shake until it becomes a uniform slurry, then add 95% ethanol, and centrifuge again , so repeated centrifugation 3-6 times; Step 3) Distillation Add water to the centrifuge tube after centrifugation in step 2), vortex until it becomes a paste, wash it into the digestion tube without damage, and add magnesium oxide to the digestion tube before distillation; Step 4) Instrumental measurement  Determination using a fully automatic distiller: pre-add the calibrated hydrochloric acid solution into the titrant bucket, put the digestion tube into the fully automatic distiller, and add boric acid-indicator receiving solution to the digestion tube; Step 5) Calculate Calculate the cation exchange capacity in the tobacco planting soil according to the following calculation formula: Cation exchange capacity=1000 c (VV ₀ )/(m 10) (cmol kg ^－1 ) In the formula: c—concentration of standard hydrochloric acid solution, mol L ^-1 ; V—the amount of hydrochloric acid standard solution consumed for titrating the sample to be tested, mL; V ₀ —Consumption of hydrochloric acid standard solution for blank titration, mL; m—mass of air-dried sample, g; 10—convert mmol to cmol multiples; 1000—converted to cmol per kg. 2. The method for measuring cation exchange capacity in soil according to claim 1, characterized in that, in step 1), electrostatic adsorption is used to eliminate fine fibrous roots in the soil sample. 3. The method for measuring cation exchange capacity in soil according to claim 1, characterized in that, said step 2) is specifically: weigh 2 g of air-dried soil sample with an electronic balance of 1/10,000, and put it into 100 mL Add 2mL LEDTA-ammonium acetate mixture to a plastic centrifuge tube with a stopper, stir the sample on a vortex shaker to make a uniform slurry; add 38mL of the mixture to make the total volume reach 40mL, vortex for 1.5min, and the shaking speed 3000r/min; put the centrifuge tubes in pairs on the two plates of the coarse balance, add the mixed liquid to make it balanced, and then put them into the centrifuge symmetrically, centrifuge at a speed of 4 min, discard the clear liquid in the centrifuge tubes; Add 2 mL of 95% ethanol to the centrifuge tube of the sample, place it on a vortex shaker and stir the sample to make the soil sample into a uniform slurry, then add 48 mL of 95% ethanol, vortex for 1.5 min, and shake at a rate of 3000 r/min. Place the centrifuge tubes in pairs on the two plates of the coarse balance, add ethanol to balance them, then put them into the centrifuge symmetrically and centrifuge for 4 min, discard the ethanol clear solution; repeat this process until there is no ammonium ion. 4. The assay method of cation exchange capacity in soil according to claim 3, characterized in that, the number of repetitions is 4 times, the first two centrifugal speeds are 3000r/min, and the latter two centrifugal speeds are 4500r/min. 5. The method for measuring cation exchange capacity in soil according to claim 3 or 4, characterized in that step 3) is specifically: add 2 mL of water to the centrifuge tube, vortex and oscillate the ammonium ion-saturated soil into a paste, Wash it into a 500mL digestion tube without damage, and control the washing volume at 60 mL. Add 1.5g-3g magnesium oxide to the digestion tube before distillation. 6. the assay method of cation exchange capacity in soil according to claim 5 is characterized in that, described full-automatic distiller is FOSS Kjeltec 8400 distiller; In titrant barrel, pre-add 0.02 mol/L hydrochloric acid that has been calibrated solution, put the digestion tube into the distiller, and add 30 mL of boric acid-indicator receiving solution with a mass volume fraction of 2% into the digestion tube. 7. The assay method of cation exchange capacity in soil according to claim 6, characterized in that, the distillation parameters of the distiller are: steam volume 40%, temperature 100°C, distillation time 7′30″. 8. the assay method of cation exchange capacity in soil according to claim 1, is characterized in that, described EDTA-ammonium acetate mixed solution is the mixed solution of 0.005mol/LEDTA and 1mol/L ammonium acetate, with ammoniacal liquor and glacial acetic acid Adjust to pH7.0 or pH8.5. 9. the assay method of cation exchange capacity in soil according to claim 1, is characterized in that, described calibrated hydrochloric acid solution 0.02mol/L hydrochloric acid standard solution. 10. the assay method of cation exchange capacity in soil according to claim 1, is characterized in that, described boric acid-indicator is that mass volume fraction is 2% boric acid-indicator receiving solution, and described indicator is bromocresol Green and Methyl Red.