Standard Overview

GB/T 17135-1997 is a Chinese national standard that specifies the determination method for total arsenic in soil quality—the potassium borohydride-silver nitrate spectrophotometric method. This standard is applicable to the determination of total arsenic content in various types of soil. It features high sensitivity and minimal interference, making it one of the commonly used methods for environmental monitoring and soil pollution surveys.

This standard was issued by the former State Bureau of Technical Supervision. Since its implementation in 1998, it has played an important role in the field of soil environmental monitoring. The following provides a detailed interpretation from the aspects of method principle, reagents and instruments, analytical procedures, etc.

Method Principle

After acid digestion of soil samples, the arsenic therein is converted to pentavalent arsenic. In an acidic medium, potassium borohydride is used to reduce pentavalent arsenic to arsine (AsH₃) gas. The generated arsine is absorbed by a silver nitrate-polyvinyl alcohol-ethanol absorption solution and undergoes a reduction reaction to form a silver sol, causing the solution to appear yellow.

The absorbance is measured at a wavelength of 400 nm. The absorbance is proportional to the arsenic content, thereby allowing quantitative determination of total arsenic in soil. This method has high sensitivity, with a detection limit as low as 0.1 mg/kg.

Reagents and Instruments

Main Reagents

  • Nitric acid (guaranteed reagent grade)
  • Sulfuric acid (guaranteed reagent grade)
  • Perchloric acid (guaranteed reagent grade)
  • Potassium borohydride solution: Weigh an appropriate amount of potassium borohydride and dissolve it in 0.5% sodium hydroxide solution. Prepare fresh before use.
  • Silver nitrate-polyvinyl alcohol-ethanol absorption solution: Store in the dark. Valid for approximately one week.
  • Arsenic standard solution: Prepare using arsenic trioxide and dilute stepwise.

Main Instruments

  • Spectrophotometer: Wavelength range covering 400 nm, equipped with 1 cm cuvettes.
  • Arsine generation apparatus: Including reaction flask, gas delivery tube, and absorption tube.
  • Constant temperature water bath: Temperature control accuracy ±1°C.
  • Analytical balance: Sensitivity 0.0001 g.

Analytical Procedure

Sample Pretreatment

  1. Air-dry and grind the soil sample, then pass through a 100-mesh nylon sieve.
  2. Weigh 0.5 g (accurate to 0.0001 g) of sample into a digestion flask.
  3. Add 10 mL of nitric acid-perchloric acid mixed acid (volume ratio 4:1) and soak overnight.
  4. Heat for digestion until the solution is clear, evaporate the acid to near dryness, dissolve the residue with 5% sulfuric acid, and make up to volume for testing.

Preparation of Standard Curve

  1. Pipette 0, 0.5, 1.0, 2.0, 3.0, 4.0, 5.0 mL of arsenic standard solution (1 μg/mL) into reaction flasks respectively.
  2. Add 5% sulfuric acid to a total volume of 10 mL.
  3. Add 2 mL of potassium borohydride solution, immediately connect the absorption tube, and react for 10 minutes.
  4. Using the reagent blank as reference, measure the absorbance at 400 nm and plot the standard curve.

Sample Determination

  1. Pipette an appropriate amount of digestion solution (containing 0.5~5 μg arsenic) into a reaction flask, and add 5% sulfuric acid to 10 mL.
  2. Follow the same steps as for the standard curve preparation.
  3. Calculate the arsenic content based on the standard curve.

Result Calculation

The total arsenic content in soil (mg/kg) is calculated according to the following formula:

ω(As) = (m₁ - m₀) × V / (V₁ × m)

  • m₁: Mass of arsenic in the sample found from the standard curve (μg)
  • m₀: Mass of arsenic in the blank (μg)
  • V: Final volume of the digestion solution (mL)
  • V₁: Volume taken for measurement (mL)
  • m: Mass of soil sample weighed (g)

Precautions

  • Potassium borohydride solution must be prepared fresh before use to avoid deterioration.
  • Arsine gas is toxic; operations should be conducted in a fume hood.
  • During digestion, avoid evaporation to dryness to prevent arsenic loss.
  • The silver nitrate absorption solution decomposes upon exposure to light and should be stored in the dark.
  • If the soil sample contains interfering substances such as sulfides or organic matter, appropriately increase the digestion time or use other pretreatment methods.

Method Characteristics and Applicability

Item Description
Detection Limit 0.1 mg/kg
Measurement Range 0.5~5 μg arsenic
Precision Relative standard deviation ≤5%
Interference Sulfides, antimony, bismuth, etc. interfere and must be eliminated
Applicability Various types of soil, especially suitable for large-scale sample analysis

This method is simple to operate and relatively low in cost, making it suitable for routine laboratory testing. Compared with the atomic fluorescence method, its sensitivity is slightly lower, but it does not require expensive instruments and is easy to promote.

To view the original standard text, please download the PDF file: