Determination of Copper and Zinc Content in Soil by Atomic Absorption Spectrophotometry

I. Objectives and Requirements

1.1 Understand the principles of atomic absorption spectrophotometry;

1.2 Master soil sample digestion methods and the operation of atomic absorption spectrophotometers.

II. Principle

Flame atomic absorption spectrophotometry is an analytical method based on the selective absorption of characteristic spectral lines by the ground-state atoms of a target element. When a sample is introduced into the flame, compounds of the target element dissociate into atomic vapor. As radiation from the characteristic spectral lines of that element (emitted by a sharp-line light source, such as a hollow cathode lamp) passes through the atomic vapor layer, the ground-state atoms of that element selectively absorb the characteristic spectral lines. Under specific conditions, the change in intensity of the characteristic spectral line correlates proportionally with the concentration of the target element in the sample. By measuring the absorbance of the selected absorption line by the ground-state atoms, the concentration of that element in the sample is determined.

Wet digestion employs strong oxidizing acids such as HNO₃, H₂SO₄, or HClO₄ to co-boil with organic compound solutions, decomposing and removing organic matter. Dry ashing involves high-temperature calcination or incineration, where organic substances are oxidized and destroyed by atmospheric oxygen. This experiment employs wet digestion to process organic matter in soil samples.

III. Instruments and Reagents

3.1 AA-1800H atomic absorption spectrophotometer, copper and zinc hollow cathode lamps.

3.2 Zinc Standard Solution. Accurately weigh 0.1000 g of metallic zinc (99.9%). Dissolve in 20 mL of 1:1 hydrochloric acid. Transfer to a 1000 mL volumetric flask and dilute to the mark with deionized water. This solution contains 100 mg/L zinc.

3.3 Copper Standard Solution. Accurately weigh 0.1000 g of metallic copper (99.8%) and dissolve in 15 mL of 1:1 nitric acid. Transfer to a 1000 mL volumetric flask and dilute to the mark with deionized water. This solution contains 100 mg/L copper.

IV. Experimental Procedure

4.1  Plotting the Standard Curve

Take six 25 mL volumetric flasks. Add sequentially 0.0, 1.00, 2.00, 3.00, 4.00, and 5.00 mL of the 100 mg/L copper standard solution, and 0.00, 0.10, 0.20, 0.40, 0.60, 0.80 mL of 100 mg/L zinc standard solution. Dilute each to the mark with 1% nitric acid solution, mix thoroughly, and prepare a series of solutions containing 0.00, 0.40, 0.80, 1.20, 1.60, 2.00 mg/L copper standard series and 0.00, 0.40, 0.80, 1.20, 1.60, 2.40, 3.20 mg/L zinc standard series. Measure the absorbance at 324.7 nm and 213.9 nm respectively, then plot the standard curve.

4.2 Sample Analysis

4.2.1 Sample Digestion

Accurately weigh 1.000 g of soil sample into a 100 mL beaker (2 replicates). Moisten with a small amount of deionized water, then slowly add 5 mL aqua regia (nitric acid: hydrochloric acid = 1:3). Cover with a watch glass. Concurrently prepare one reagent blank. Place the beakers on an electric hotplate within a fume hood. Begin heating at low temperature, gradually increasing heat while maintaining a gentle boil to ensure thorough decomposition. Avoid excessive temperatures to prevent sample splashing. After vigorous reaction ceases and organic matter decomposes, remove beakers to cool. Add 2–4 mL perchloric acid along the beaker wall, then continue heating until white fumes appear. until the sample turns grayish-white. Remove the cover dish, expel excess perchloric acid, and evaporate the sample to near dryness. Remove for cooling. Add 5 mL of 1% nitric acid solution and heat. After cooling, filter through medium-speed quantitative filter paper into a 25 mL volumetric flask. Wash the residue with 1% nitric acid solution, then dilute to volume and mix thoroughly for analysis.

4.2.2  Determination

Directly inject the digested solution into an air-acetylene flame under identical conditions as the standard series to measure the absorbance value.

V. Data Processing

The measured absorbance value (if the reagent blank exhibits absorbance, subtract the blank absorbance value) corresponds to the concentration M (mg/mL) on the standard curve. Thus, in the sample:

Where:  — Corresponding concentration from the standard curve, mg/mL;

           — Volume to final volume, mL;

           — Sample mass, g.

VI. Precautions

6.1  Carefully control the temperature; excessive heating may cause overflow or carbonization of the reactants.

6.2  If the soil digest appears insufficient and grayish-white, add a small amount of perchloric acid and continue digestion. Due to perchloric acid's significant impact on blanks, strictly control its dosage.

6.3  Perchloric acid is highly oxidizing. Add it only after most organic matter in the soil has been digested and the reaction mixture has cooled, or add it at room temperature in the presence of ample nitric acid. Failure to do so may cause sample splashing or explosion in the cup. Exercise extreme caution during use.

6.4  If perchloric acid oxidation proceeds too rapidly, posing an explosion risk, rapidly cool the mixture or dilute with cold water to halt oxidation.

Atomic Absorption Measurement Conditions: