SH/T 0629-1996 Determination of arsenic content in naphtha (potassium borohydride-silver nitrate spectrophotometric method)

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Card type: Test method

1. Method number / full name / English name / replacement relationship

• Standard number: SH/T 0629-1996
• Full name: Determination of arsenic content in naphtha (potassium borohydride-silver nitrate spectrophotometric method)
• English name: not provided in the available source
• Replacement relationship: not provided in the available source
• Other: confirmed in 2004; approved by China Petrochemical Corporation 1996-05-24; implemented in 1992-12-01 (original text as such, the approval year and implementation year are suspected to be erroneous, requires confirmation against the source standard (source-text extraction))

2. Scope of application and non-applicability

• This standard specifies a method for determining the arsenic content in samples by potassium borohydride-silver nitrate spectrophotometry.
• It is applicable to naphtha and also to reforming feedstock oil.
• The applicable arsenic content range is 1～5501 µg/kg (the original text is “I[unreadable separator]5501 Ag/kg”, the value is suspected to be source-text extraction noise, requires confirmation against the source standard (source-text extraction)).
• Non-applicable scope: not provided in the available source.

3. Method principle

Arsenic in the sample is extracted with sulfuric acid and hydrogen peroxide. The extract is heated to destroy organic matter. In a tartaric acid medium, potassium borohydride is used to generate arsine. Impurities in the arsine are removed by purification. The arsine is absorbed with a silver nitrate-polyvinyl alcohol-ethanol solution to form a yellow silver sol. The absorbance is measured at 410nm.

4. Reagents and materials (purity / preparation)

• 4.1 Sulfuric acid: guaranteed reagent, prepared as a sulfuric acid solution with a volume ratio of 1:1.
• 4.2 30% Hydrogen peroxide: analytical reagent.
• 4.3 Tartaric acid: analytical reagent, prepared as a tartaric acid solution of 200g/L. Note: if the solubility of commercially available tartaric acid is poor, another one should be selected.
• 4.4 Potassium borohydride (or sodium borohydride): analytical reagent.
• 4.5 Sodium chloride: analytical reagent.
• 4.6 Lead acetate: analytical reagent, prepared as a lead acetate solution of 100g/L.
• 4.7 Methyl orange: indicator, prepared as a methyl orange indicator solution of 2g/L (the original text “2[unreadable unit]L” is suspected to be source-text extraction noise, requires confirmation against the source standard (source-text extraction)).
• 4.8 Ethanolamine: analytical reagent.
• 4.9 Dimethylformamide: analytical reagent.
• 4.10 Sodium sulfate (anhydrous): analytical reagent.
• 4.11 Potassium bisulfate: analytical reagent.
• 4.12 Silver nitrate: analytical reagent.
• 4.13 Nitric acid: guaranteed reagent.
• 4.14 Polyvinyl alcohol: chemically pure, average molecular weight is 1750±500.
• 4.15 Anhydrous ethanol: analytical reagent.
• 4.16 Arsenic trioxide: analytical reagent.
• 4.17 Sodium hydroxide: analytical reagent, prepared as a sodium hydroxide solution of 50g/L (the original text “50[unreadable unit]L” is suspected to be source-text extraction noise, requires confirmation against the source standard (source-text extraction)).
• 4.18 Perchloric acid: analytical reagent.
• 4.19 Hydrochloric acid: guaranteed reagent, prepared as a hydrochloric acid solution with a volume ratio of 1:24.
• 4.20 Ammonia water: analytical reagent, prepared as an ammonia water solution with a volume ratio of 1:1.
• 4.21 Distilled water and double-distilled water.

5. Instruments and equipment

• 5.1 Spectrophotometer: with 1cm cuvettes.
• 5.2 Arsine generation and absorption apparatus (see Figure 1).
  • Composition: 1— reaction tube; 2—3 rubber stopper; 3— gas guide tube; 4— vent stopper; 5— absorption tube; 6— deamine tube; 7—U shaped tube; a— lead acetate cotton, 5～7cm; b— absorbent cotton absorbing 2mL dimethylformamide mixed solution 10～12cm.
  • Note: The gas guide tube, vent stopper, deamine tube and U shaped tube should be cleaned and dried before use. A small amount of absorbent cotton is placed at both ends of the deamine tube.
  • 5.2.1 Reaction tube: outer diameter 23～25mm, volume 100mL, 19 ground joint.
  • 5.2.2 Gas guide tube: outer diameter 8mm, a high-pressure polyethylene tube that can be heated and drawn thin.
  • 5.2.3 Vent stopper: 14 ground joint.
  • 5.2.4 Absorption tube: 14 ground joint.
  • 5.2.5 Deamine tube: outer diameter 10～11mm, a high-pressure polyethylene tube containing deamine agent 9～10g. The deamine agent placed in the deamine tube can generally be used for 200 or more tests. If white smoke frequently appears in the absorption tube or if the gas path is blocked due to solid moisture caking, it should be replaced.
  • 5.2.6 U shaped tube: outer diameter 10～11mm, containing lead acetate cotton and absorbent cotton adsorbing dimethylformamide mixed solution. The absorbent cotton placed in the U shaped tube should be appropriately tight and uniform. After adding the dimethylformamide mixed solution to the absorbent cotton, slowly blow air with a rubber bulb for about 1min to make it evenly adsorbed on the absorbent cotton, preventing it from flowing to the bottom of the U shaped tube; when the cotton turns yellow, it should be replaced. When the lead acetate cotton has 1/4 blackened, it should be replaced. When newly adding the dimethylformamide mixed solution, use 5mL arsenic (V) standard solution to react once or twice according to the procedure of this standard to equilibrate the apparatus. Silver ions should not be allowed to mix into the dimethylformamide mixed solution.
  • 1. The arsine generation and absorption apparatus can be produced and provided by Fushun Petroleum Institute, supervised by the petroleum product standardization technical centralized unit.
• 5.3 Separatory funnel: 125, 250, 500 and 1000mL.
• 5.4 Erlenmeyer flask: 100mL.
• 5.5 Graduated cylinder: 10, 25, 100 and 500mL.
• 5.6 Pipette: 5mL.
• 5.7 Volumetric flask: 500 and 1000mL.
• 5.8 Hard glass sampling bottle: 2000mL.
• 5.9 Electric furnace: 2kW (adjustable).

6. Recommended operating conditions

• Color-developing absorption solution: silver nitrate solution, polyvinyl alcohol solution and anhydrous ethanol mixed in a volume ratio of 1:1:2, stable for 3h (see 6.7).
• Measurement wavelength: 410nm.
• Cuvette: 1cm.
• Reference: absorption solution (6.7).
• Arsine generation: place one potassium borohydride tablet in the reaction tube, immediately cover tightly with the rubber stopper, the reaction is complete in about 3min (no continuous bubbles in the absorption tube).
• Add tartaric acid solution 6mL to the reaction tube (see 6.11, 8.4).

7. Sampling and sample preparation

• When sampling at a process unit, first open the sampling port, discharge three to five times the amount corresponding to the “dead” corner oil, then directly sample into a clean, dry sampling bottle, and immediately cap it.
• When sampling from other containers, proceed in accordance with GB/T 4756.
• The sample taken should be analyzed and measured within 24h.
• Sample amount: accurately determine the sample density at the test temperature in accordance with GB/T 1884. Measure a certain volume of sample into a separatory funnel according to Table 1.

Table 1 Sample amount table

Note: If the arsenic content is greater than 200 µg/kg, when taking a sample less than or equal to 10mL, extraction may be omitted; directly take the sample into a 100mL Erlenmeyer flask, add 30% hydrogen peroxide 10mL and sulfuric acid solution 30mL, mix thoroughly with the sample, place on an electric furnace and heat, then proceed as in 8.3.

(In Table 1, “[unreadable value]- 21[unreadable characters]”, “2([unreadable characters])”, “,10[unreadable character]” etc. are source-text extraction noise, inferred from context, requires confirmation against the source standard (source-text extraction))

8. Analysis steps

• 8.1 Sample amount: measure the sample according to Table 1.
• 8.2 Extraction: add 30% hydrogen peroxide 5mL and sulfuric acid solution 15mL to the separatory funnel, shake vigorously for 5min, let stand to separate layers, and draw off the acid liquid into a 100mL Erlenmeyer flask. Then add 30% hydrogen peroxide 5mL and sulfuric acid solution 15mL, shake vigorously for 5min, let stand to separate layers, draw off the acid liquid and combine with the acid liquid from the previous extraction. Wash the residual acid liquid in the oil phase with 15mL water, draw off and combine with the acid liquids from the first two extractions, and mix thoroughly and uniformly.
• 8.3 Digestion: place the Erlenmeyer flask containing the acid liquid on an electric furnace and heat until the remaining liquid is 2～3mL (white fumes of sulfur trioxide evolve for about 10min. Do not evaporate to dryness, to avoid loss of arsenic by volatilization), remove the Erlenmeyer flask (if too much black residual carbon precipitates in the solution, add 30% hydrogen peroxide dropwise to make the residual carbon disappear, then heat until the remaining liquid is 2～3mL), cool to room temperature. Add distilled water 2～3mL along the wall of the Erlenmeyer flask, cool to room temperature. Add one to two drops of methyl orange indicator solution, add ammonia water solution dropwise to neutralize until it just turns yellow. Transfer the solution in the Erlenmeyer flask to the reaction tube, rinse the Erlenmeyer flask three times with 5～10mL distilled water, and also transfer the rinse liquid to the reaction tube. Note: The test is carried out in a fume hood.
• 8.4 Measurement: add tartaric acid solution 6mL to the reaction tube, then proceed as in 6.11, measure the absorbance A₁. Repeat the sample operation steps without adding sample, and measure the sample blank absorbance A₀. Calculate the net absorbance A (A=A₁-A₀). From A, find the corresponding arsenic content X (µg) on the working curve.
• 6.11 Preparation of the working curve: Into six reaction tubes, respectively add 0.0, 1.0, 2.0, 3.0, 4.0 and 5.0mL arsenic (V) standard solution, to each add tartaric acid solution 6mL, and dilute to 50mL with distilled water. Into six clean and dry absorption tubes, respectively add 5.0mL absorption liquid. Connect the experimental apparatus according to Figure 1. Place one potassium borohydride tablet into the reaction tube and immediately close tightly with a rubber stopper. After the reaction is complete (about 3min, with no continuous bubbles in the absorption tube), inject the developed absorption liquid into a 1cm clean and dry cuvette, use the absorption liquid (6.7) as reference, and measure the absorbance at 410nm. Plot the working curve with the absorbance measured for each arsenic standard solution minus the absorbance of the blank solution (zero concentration) as the ordinate and the corresponding arsenic content (µg) as the abscissa. After each absorption reaction is completed, the gas delivery tube inserted into the absorption liquid shall be immersed and rinsed in an absorption tube containing 1:1 nitric acid solution, and shall remain in this liquid when not in use. After the test is completed, the rubber stopper shall be removed from the reaction tube to prevent water vapor in the reaction tube from condensing and the solution in the absorption tube from being sucked back into the U-shaped tube. Calibrate the working curve while analyzing the sample. When analyzing samples with arsenic content lower than 5 µg/kg, the working curve may instead be prepared using 0.0, 0.5, 1.0, 1.5, 2.0 and 2.5mL arsenic (V) standard solution, and the volume of absorption liquid is 3.0mL. Note: The arsine gas generated in the reaction tube is a highly toxic substance; the entire reaction process shall be carried out in a fume hood or in a well-ventilated place.

9. Qualitative and Quantitative and Calculation Formula

The arsenic content in the sample As (µg/kg) is calculated according to formula (1):

As = X / (V × ρ) ………………(1)

Where:

• X—— the arsenic content corresponding on the working curve to the net absorbance of the sample, µg;
• V—— sample volume, mL;
• ρ—— density of the sample at the test temperature, g/mL.

(The original formula layout is “[garbled symbols] X [unreadable symbol] X - Vxp”, suspected to be source-text extraction noise, arranged according to context, requires confirmation against the source standard (source-text extraction))

10. Precision

Judge the reliability of the results according to the following provisions (95% confidence level).

• 10.1 Repeatability: The difference between two results obtained by repeated measurement by the same operator shall not be greater than the repeatability specified in Table 2.
• 10.2 Reproducibility: The difference between two results submitted by different operators in different laboratories shall not be greater than the reproducibility specified in Table 2.

Table 2 Precision (µg/kg)

Note: X is the arithmetic mean of the two results.

(In Table 2, “O.IX” is arranged according to context as 0.1X, requires confirmation against the source standard (source-text extraction))

11. Report

Take the arithmetic mean of two repeated measurement results as the determination result, retaining three significant figures.

12. Reference Documents

• GB/T 1884 Standard Test Method for Density of Crude Oil and Liquid Petroleum Products by Laboratory Determination (Hydrometer Method)
• GB/T 4756 Standard Practice for Manual Sampling of Petroleum Liquids
• Note: Unless otherwise expressly specified in the standard, the above referenced standards shall all be currently effective standards.

13. Source and verification

This page is an AI translation of the available Chinese method card. Human verification is pending. It is not an official translation of the standard. Check the source standard and product certificate before use.

14. Quick Reference for Questions and Answers