Soil quality - Determination of total chromium - Flame atomic absorption spectrophotometric method
First, the subject content and scope of application element Cr Measuring wavelength ( nm ) 357.9 Usually width (mm) 0.7 Flame nature Reducing Secondary sensitive line (nm) 359.0; 360.5; 4254 Burner height 0mm ( make the hollow cathode light spot through the bright blue part of the flame ) Five, sample Where: c - the absorbance of the test solution minus the absorbance of the blank solution, and then find the chromium content (mg / L) on the calibration curve; Soil standard Number of laboratories Guaranteed value mg / kg Total mean mg/kg Indoor relative standard deviation % Relative standard deviation between rooms % Relative error % ESS-1 5 57. 2±4.2 56.1 2 9.8 1.9 ESS-3 6 99.0 ± 7.4 93.2 2.3 8.3 -4.9 Nine, soil moisture content was measured by A1 weighed 100 mesh sieve air-dried soil samples 5 ~ 10g (accurate to 0.01g), placed in an aluminum weighing box or bottle, 4 ~ 5h drying at 105 ℃ oven drying To constant weight. Where: ƒ - soil moisture content, %; Tianchi Changzhou Textile Machinery Co., Ltd , https://www.tcloom.com
1. This standard specifies the flame atomic absorption spectrophotometry for the determination of total chromium in soil.
2. The detection limit of this standard (calculated by weighing 0.5g sample to 50ml) is 5mg/kg.
3. Interference 1) Chromium is an element that easily forms high temperature resistant oxides. Its atomization efficiency is greatly affected by the flame state and the burner. It is necessary to have a flammable (reducing) flame, and the observation height is best at 0 mm. .
Second, the principle <br> The use of hydrochloric acid, nitric acid, hydrofluoric acid, perchloric acid total decomposition method, destroy the mineral lattice of the soil, so that all the elements in the sample to enter the test solution, and, in the digestion process, All chromium is oxidized to Cr 2 O 7 2- . The digestion solution is then sprayed into a rich air-acetylene flame. At the high temperature of the flame, a chromium ground state atom is formed, and a selective absorption of a characteristic line of 357.9 nm emitted from the chromium hollow cathode lamp is generated. The absorbance of chromium was determined under the optimal measurement conditions selected.
III. Test reagents <br> The reagents used in this standard use analytical reagents and deionized water or water of equivalent purity in accordance with national standards unless otherwise stated.
1. Hydrochloric acid (HCl), Ï = 1.19 g / ml, excellent grade pure.
2. Hydrochloric acid solution, prepared with (3),
3. Nitric acid (HNO 3 ), Ï = 1.42 g / ml, excellent grade pure.
4. Hydrofluoric acid (HF), Ï = 1.49 g/ml.
5. Sulfuric acid (H 2 SO 4 ), Ï = 1.84 g / ml, excellent grade pure.
6. Sulfuric acid solution, 1+1: formulated with (3.5).
7. Ammonium chloride aqueous solution, mass fraction of 10%
8. Chromium standard stock solution, 1.000mg/ml: Accurately weigh 0.2829g of reference potassium dichromate (K 2 Cr 2 O 2 ), dissolve it in a small amount of water, transfer it into a 100ml volumetric flask, and dilute to the mark with water. Shake well.
9. Chromium standard use, 50mg / L: Transfer 5.00ml of chromium standard stock solution in a 100ml volumetric flask, add water to the mark, and shake.
IV. Instrument <br> 1. General laboratory equipment and the following instruments.
2. Atomic absorption spectrophotometer.
3. Chrome hollow cathode lamp.
4. Acetylene cylinders.
5. Air compressors should be equipped with water removal, degreasing and dust removal devices.
6. Instrument parameters The optimal measurement conditions of different types of instruments are different, and can be selected according to the instruction manual of the instrument. Generally, this standard uses the measurement conditions in the table below.
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Mix the collected soil samples (generally not less than 500g) and dilute to about 100g by quartering. After the shrinking soil samples are air-dried (naturally dried or freeze-dried), remove the stones and plants and animals in the soil samples. Foreign matter such as a residue is ground with a wooden stick (or agate stick), and passed through a 2 mm nylon sieve (removing grit of 2 mm or more) and mixed. The soil sample passed through a 2 mm nylon sieve was ground to a total of 100 mesh (pore size 0.149 mm) nylon mesh using an agate mortar, and mixed for use.
Sixth, analysis steps
1. Preparation of test solution Prepare 0.2~0.5g (accurate to 0.0002g) sample in 50ml PTFE crucible, wet with a small amount of water, add sulfuric acid solution (5ml, 10ml nitric acid. Stand still. After the violent reaction is stopped, it is capped and transferred to a hot plate for heating and decomposing for about 1 hour. When the lid is opened, when the soil decomposition product is viscous, 5 ml of hydrofluoric acid is added and the silicon is removed by heating at a medium temperature, in order to achieve a good flying silicon effect, Shake it frequently. When heated to SO 2 white smoke, cover it to fully decompose the black organic carbide. Then, remove the crucible, slightly cold, rinse the lid and the inner wall with a small amount of water, and then heat the SO 3 . The white smoke was exhausted and steamed until the contents were not flowing. Remove the sputum and let it cool slightly. Add 3 ml of hydrochloric acid solution, dissolve the soluble residue warmly, transfer the whole amount to a 50 ml volumetric flask, add 5 ml of NHCl solution, and cool to volume. Shake well.
Due to the variety of soils, the organic matter contained in the machine is quite different. When dissolving, it should be observed that the amount of various acids can be increased or decreased as appropriate.
Note: The temperature of the hot plate should not be too high, otherwise the Teflon will be deformed.
2. Measurement Adjust the instrument to the best working condition according to the instruction manual of the instrument, and measure the absorbance of the test solution.
3. Blank test The deionized water was used instead of the sample, and the whole procedure white solution was prepared by the same procedure and reagents, and the measurement was carried out according to the step (2). Prepare at least 2 blank solutions for each batch of samples.
4. Accurately transfer the calibration curve to the standard chromium solution of 0.00, 0.50, 1.00, 2.00, 3.00, 4.00ml in a 50ml volumetric flask, then add 5ml NH2Cl solution, 3ml hydrochloric acid solution, dilute to the mark with water, shake, The chromium content is 0.05, 1.0, 2.0, 3.0, 4.0 mg/L. This concentration range should include the concentration of chromium in the test solution. The absorbance of the standard solution was sequentially determined from low to high concentration according to the conditions in (2).
A calibration curve is drawn by subtracting the absorbance of the blank from the corresponding element content (mg/L).
7. The result indicates that the content of chromium in the soil sample W (mg/kg) is calculated by the following formula: 
V——the volume of the sample to be volume (ml)
M——weigh the weight of the sample (g)
ƒ——The content of moisture in the sample (%)
VIII. Precision and Accuracy The accuracy and accuracy of chromium in the ESS series of soil standards using this method in several laboratories are shown in the table below.
A2 The moisture content of the air-dried soil sample expressed as a percentage is calculated by the following formula:
W1——the weight of the soil sample before drying, g;
W2——the weight of the soil after drying, g.