Applications

Determination of Available Copper in Soil (Flame Atomic Absorption Spectrometry)

August 1, 2025

1 Method Overview

A buffer solution of diethylenetriaminepentaacetic acid-calcium chloride-triethanolamine (DTPA-CaCl₂-TEA) with pH=7.3 is used as the extractant to chelate and extract available copper from the soil. Among them, DTPA acts as a chelating agent; calcium chloride can prevent the dissolution of free calcium carbonate in calcareous soils and avoid the impact caused by the release of elements such as zinc and iron encapsulated by calcium carbonate; triethanolamine, as a buffering agent, can maintain the solution pH at around 7.3 and also inhibit the dissolution of calcium carbonate. The content of copper in the leachate is determined using an atomic absorption spectrophotometer with an acetylene-air flame.

2 Instruments and Reagents

2.1 Instruments and Equipment

2.1.1 Detection Instruments

Serial Number	Name	Quantity	Technical Requirements	Accessories
1	Flame Atomic Absorption Spectrophotometer	1 set	/	Copper hollow cathode lamp
2	Air Compressor	1 set	Rated exhaust pressure: 0.3 MPa
3	Acetylene	1 cylinder	Purity ≥ 99.99%

2.1.2 Pretreatment Equipment

Serial Number	Name	Quantity	Technical Requirements
1	Electronic Balance	1 unit	Sensitivity of 1 mg
2	Ordinary Oscillator	1 unit	Temperature: 25℃ ± 2℃, oscillation frequency: (180 ± 20) r/min
3	Stoppered Erlenmeyer Flask	Several	150 mL
4	Micropipette	1 each	100 μL ~ 1000 μL; 1000 μL ~ 5000 μL
5	Volumetric Flask	Several	100 mL, 1000 mL

2.2 Reagents

2.2.1 Reagents

Serial Number	Name	Technical Requirements
1	Water	Grade II
2	Calcium Chloride	Analytical grade
3	Triethanolamine	Analytical grade
4	Diethylenetriaminepentaacetic Acid (DTPA)	Analytical grade
5	Ammonia Water	Guaranteed reagent
6	Hydrochloric Acid	Guaranteed reagent

2.2.2 Prepared Reagents

Serial Number	Name	Preparation Method
1	DTPA Extractant	Weigh 1.967 g of DTPA and dissolve it in 14.92 g (13.3 mL) of TEA and a small amount of water. Then dissolve 1.47 g of calcium chloride in water, transfer all to a 1 L volumetric flask, add water to approximately 950 mL. Adjust the pH of the DTPA solution to 7.3 using hydrochloric acid (1+1) or ammonia solution (1+1) on a pH meter, and dilute to the mark with water. This solution will remain stable for several months, but the pH should be checked and calibrated before use.
2	Hydrochloric Acid (1+1)	Measure 50 mL of hydrochloric acid, slowly add it to 40 mL of water, cool, then dilute to 100 mL with water and mix well.
3	Ammonia Solution (1+1)	Measure 50 mL of ammonia water, slowly add it to 40 mL of water, cool, then dilute to 100 mL with water and mix well.

2.3 Reference Standards

2.3.1 Stock Solutions

Serial Number	Number	Name	Technical Requirements	Remarks
1	GSB 07-1257-2000	Copper Standard Stock Solution	1000 mg/L	Institute for Environmental Reference Materials, Ministry of Environmental Protection

3 Operational Procedures

3.1 Sample Processing

3.1.1 Preparation of Test Solution

Sample Preparation
1. Removal of Impurities and Air-Drying (Only for Fresh Undried Soil Samples) First, remove foreign materials other than soil, such as plant roots, insect bodies, bricks, and stones. Then spread the sample on clean paper in a thin layer and air-dry it in a cool, well-ventilated room, avoiding direct sunlight. During the air-drying process, the sample should be frequently turned to accelerate drying. The air-drying area should be protected from contamination by acid, alkali gases, and dust. When the soil reaches a semi-dry state, large clods must be crushed in time to prevent them from hardening after drying, which makes them difficult to crush.
2. Grinding and Sieving Take an appropriate amount of air-dried sample using the quartering method, grind it with a mortar until all passes through a 2 mm aperture nylon sieve. The sieved soil sample should be thoroughly mixed and stored in a glass wide-mouth bottle, plastic bottle, or clean soil sample bag for later use.
Extraction of Available Copper from Soil Accurately weigh 10.00 g of the sample, place it in a dry 150 mL stoppered Erlenmeyer flask or plastic bottle, add 20.0 mL of DTPA extractant at 25℃ ± 2℃, tightly cap the bottle, shake at a frequency of 180 r/min ± 20 r/min for 2 hours at 25℃ ± 2℃, then filter immediately. Retain the filtrate and complete the determination within 48 hours.

If a larger amount of test solution is required for determination, 15.00 g or 20.00 g of the sample can be weighed, but the sample-to-solution ratio must be maintained at 1:2, and the container used for extraction should be large enough to ensure sufficient oscillation of the sample.
Preparation of Blank Test Solution Except for not adding the sample, the reagent dosage and operational steps are the same as in step 2.

3.1.2 Preparation of Standard Solution

Preparation of Copper Standard Working Solution (0.10 mg/mL) Accurately pipette 10 mL of the copper standard stock solution (1000 mg/L) into a 100 mL volumetric flask, dilute to the mark with pure water, and shake well.
Preparation of Copper Standard Series Solutions Pipette 0.00 mL, 0.25 mL, 0.50 mL, 1.00 mL, 2.00 mL, and 3.00 mL of the copper standard working solution (0.10 mg/mL) into a set of 100 mL volumetric flasks respectively, dilute to the mark with DTPA extractant, and mix well. The mass concentrations of this copper standard series solution are 0.00 μg/mL, 0.25 μg/mL, 0.50 μg/mL, 1.00 μg/mL, 2.00 μg/mL, and 3.00 μg/mL respectively.

3.2 Sample Testing

Test Conditions Reference Conditions for Flame Atomic Absorption Spectrophotometer Detection

Before determination, according to the properties of the element to be measured and with reference to the instrument manual, select and adjust the instrument working conditions such as wavelength, lamp current, slit, energy, air-acetylene flow ratio, and burner height to optimize the instrument performance.
Sample Testing Before determination, according to the properties of the element to be measured and with reference to the instrument manual, select the instrument working conditions such as wavelength, lamp current, slit, energy, air-acetylene flow ratio, and burner height, and adjust the instrument to the best working state.

Calibrate the instrument zero with DTPA extractant, use an acetylene-air flame, and measure the absorbance of copper in the standard solution on the atomic absorption spectrophotometer. Plot a standard working curve for copper with concentration as the abscissa and absorbance as the ordinate.

Determine the concentration of copper in the blank test solution and the sample solution following the same steps as for drawing the standard working curve.

If the concentration of the element to be determined in the sample solution is high, it can be appropriately diluted with DTPA extractant before measurement. Sometimes, according to the instrument manual, a less sensitive resonance line can be selected or the burner angle can be rotated for determination without dilution.