Overview of GB/T 17136-1997 Standard
GB/T 17136-1997 "Soil Quality - Determination of Total Mercury - Cold Vapor Atomic Absorption Spectrophotometry" is an important standard method for determining total mercury content in soil in China's environmental monitoring field. This standard is applicable to the determination of total mercury in various soil samples and features high sensitivity, simple operation, and minimal interference.
The cold vapor atomic absorption spectrophotometry method is based on the principle of mercury atoms absorbing the characteristic spectral line at 253.7 nm, and mercury content is quantitatively analyzed by measuring the absorbance value. This method has a low detection limit and can meet the needs of environmental soil background value surveys and pollution assessments.
Method Principle and Scope of Application
Basic Principle
After acid digestion of soil samples, the mercury therein is converted to divalent mercury ions. In an acidic medium, stannous chloride is used to reduce divalent mercury to metallic mercury, and then a carrier gas is used to bring mercury vapor into the absorption cell of a cold vapor atomic absorption spectrophotometer.
Mercury atoms produce characteristic absorption of 253.7 nm ultraviolet light, and the absorption intensity is proportional to the mercury concentration, based on which quantitative determination is carried out. This method does not require a heated atomizer, hence it is called "cold vapor" technology.
Scope of Application and Detection Limit
- Applicable to the determination of total mercury in soil, sediments, and similar solid samples.
- The method detection limit is typically 0.005 mg/kg, and the lower limit of determination is 0.02 mg/kg.
- The measurement range is generally 0.02–2.0 mg/kg; high-concentration samples can be diluted before determination.
Instruments and Reagents
Main Instruments
- Cold vapor atomic absorption spectrophotometer: equipped with a 253.7 nm light source, absorption cell, and recording system.
- Mercury reduction apparatus: including reaction bottle, gas path, and carrier gas control system.
- Digestion equipment: electric heating plate, water bath, or microwave digestion instrument.
- Analytical balance: precision 0.0001 g.
Main Reagents
- Nitric acid (guaranteed reagent), hydrochloric acid (guaranteed reagent), sulfuric acid (guaranteed reagent).
- Stannous chloride solution (10%): weigh 10 g of stannous chloride and dissolve in 100 mL of hydrochloric acid; prepare fresh before use.
- Mercury standard solution: prepared with mercuric chloride, diluted stepwise to the required concentration.
- Potassium permanganate solution, hydroxylamine hydrochloride solution, etc.
Analysis Procedure
Sample Pretreatment
- Air-dry and grind the soil sample, pass through a 100-mesh nylon sieve, and mix well.
- Weigh 0.5–2.0 g of sample (accurate to 0.0001 g) into a digestion tube.
- Add an appropriate amount of nitric acid-hydrochloric acid mixed acid, soak overnight or heat for digestion.
- After complete digestion, cool, make up to volume, let stand or centrifuge to obtain the supernatant.
Determination Operation
- Connect the cold vapor atomic absorption spectrophotometer and preheat for 30 minutes.
- Set instrument parameters: wavelength 253.7 nm, carrier gas flow rate 1.0–1.5 L/min.
- Take an appropriate amount of digestion solution into the reaction bottle, add stannous chloride solution, and immediately cap tightly.
- Record the absorption peak and measure peak height or peak area.
- Perform blank tests and standard curve simultaneously.
Standard Curve Preparation
Prepare a mercury standard series of 0, 0.01, 0.02, 0.05, 0.10, and 0.20 mg/L, and measure absorbance according to the sample determination procedure. Plot the standard curve with absorbance against mercury concentration and calculate the regression equation.
Key Parameters and Condition Optimization
| Parameter | Recommended Value | Description |
|---|---|---|
| Wavelength | 253.7 nm | Characteristic absorption line of mercury |
| Carrier gas flow rate | 1.0–1.5 L/min | Nitrogen or argon |
| Stannous chloride concentration | 10% | Reducing agent, prepare fresh before use |
| Acidity | 5%–10% HNO3 | Ensure reduction efficiency |
| Reaction temperature | Room temperature | Avoid high temperature causing mercury volatilization |
Interference Elimination and Precautions
Common Interferences
- Volatile organic compounds: may produce background absorption, requiring blank correction or pretreatment removal.
- Sulfides: form precipitates with mercury, affecting reduction; potassium permanganate can be added for oxidation.
- Halogens: interfere with absorption; hydroxylamine hydrochloride can be used to eliminate.
Key Operating Points
- All vessels must be soaked in nitric acid to avoid mercury contamination.
- Avoid excessively high temperatures during digestion to prevent mercury loss.
- Stannous chloride solution must be freshly prepared; it will lose effectiveness if left too long.
- Keep the gas path sealed during measurement to prevent mercury vapor leakage.
Result Calculation and Expression
The total mercury content in soil (mg/kg) is calculated according to the following formula:
ω(Hg) = (c × V) / (m × 1000)
- c: mercury concentration obtained from the standard curve (mg/L)
- V: constant volume of digestion solution (mL)
- m: mass of sample weighed (g)
Results are retained to two significant figures and noted as calculated on a dry weight basis.
Quality Control and Quality Assurance
- At least two blanks should be performed for each batch of samples, and blank values should be below the detection limit.
- Use certified reference materials for accuracy control; recovery should be 90%–110%.
- Insert one parallel sample for every 10 samples; relative deviation should be less than 10%.
- The correlation coefficient of the standard curve should be ≥0.999.