NB/SH/T 0915-2015 Determination of total active sulfur value in light petroleum distillates — Potentiometric titration method

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Card type: Test method

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

• Standard number: NB/SH/T 0915—2015
• Chinese title: Determination of total active sulfur value in light petroleum distillates — Potentiometric titration method
• English title: Determination of total active sulfur number in light petroleum fractions by potentiometric titration method
• Replacement relationship: This standard is first released (the statement of replacing other standards is not provided in the available source)
• Release/implementation: released on 2015-10-27, implemented on 2016-03-01
• Issuing authority: National Energy Administration
• Drafting organization: Sinopec Research Institute of Petroleum Processing
• Main drafters: Wei Zitong, Tian Songbai, Wang Xiaowei, Wang Jing (the characters of the names require confirmation against the source standard (source-text extraction))
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants, Subcommittee on Petroleum Fuels and Lubricants (SAC/TC280/SC1)
• Drafting rules: drafted according to the rules given in GB/T 1.1—2009

2. Scope of application and non-applicability

• This standard specifies a method for determining the total active sulfur value in light petroleum distillates by potentiometric titration.
• Applicable to light petroleum distillates with a total active sulfur value range of 3.0 mg/kg～400.0 mg/kg and a final boiling point not greater than 350 ℃.
• Not applicable: no explicit non-applicability clause is provided in the available source.

3. Method principle

• Disulfides and elemental sulfur in the sample are reduced with zinc powder, then using a mixed solution of ammonia water and alcohol as the titration medium, a glass electrode as the reference electrode, and a silver-silver sulfide electrode as the indicator electrode, potentiometric titration is carried out with an isopropanol standard solution of silver nitrate (4.1).
• Under certain conditions, zinc powder can reduce disulfides in petroleum distillates to mercaptans, see formula (1):
  • Zn + 2H⁺ → (under the action of Zn) 2RSH + Zn²⁺ (original formula (1) source-text extraction has noise, requires confirmation against the source standard (source-text extraction))
• Elemental sulfur and hydrogen sulfide also react with zinc powder, see formula (2) and formula (3):
  • Formula (2): the original source-text extraction is “-ZnS•”, requires confirmation against the source standard (source-text extraction)
  • Formula (3): Zn + H₂S → ZnS + H₂
• The generated ZnS will dissociate free S²⁻ in an acidic medium, thus the active sulfur in the sample is converted into S²⁻ and RSH. Then the sample is filtered, and with an aqueous isopropanol solution as the titration medium, potentiometric titration is carried out with an isopropanol standard solution of AgNO₃. The titration reaction formulas are shown in formula (4) and formula (5):
  • Formula (4): 2Ag⁺ + S²⁻ → Ag₂S
  • Formula (5): Ag⁺ + RSH → RSAg + H⁺
• From formula (4) and formula (5), it can be seen that 1 mol of S²⁻ needs to react with 2 mol of silver nitrate, while 1 mol of SH needs to react with 1 mol of silver nitrate. Since the jump potentials of silver sulfide and silver mercaptide are very close, there is only one jump point in the titration curve. This method uses the titration endpoint of silver mercaptide as the titration endpoint of the test, and calculates the test results according to the reaction of formula (5). Therefore, under the test conditions of this method, the total active sulfur value obtained is different from the total active sulfur content.
• Relationship formula (6): total active sulfur value = 2×(W_S + W_H₂S) + W_RSH + W_RSSR
  • Wherein W_S, W_H₂S, W_RSH and W_RSSR respectively represent the contents of elemental sulfur, hydrogen sulfide, mercaptans and disulfides in the sample (mg/kg).
• Significance (5.1, 5.2): Under certain conditions, the total active sulfur value in light petroleum distillates can reflect the magnitude of their potential corrosiveness. By determining the total active sulfur value, the level of corrosiveness can be predicted, and it helps predict the service life of equipment materials. This method can obtain the result of total active sulfur value without separately determining various types of active sulfur. The method is rapid and simple, and the determination is not affected by halogen anions and naphthenic acids.

4. Reagents and materials (purity / preparation)

• General: Unless otherwise specified, all reagents used in the test are analytical grade reagents. If other grades of reagents are used, their purity shall not reduce the measurement precision. The water used in the test complies with the grade three water provisions in GB/T 6682—2008.
• 7.1 Glacial acetic acid
• 7.2 Anhydrous methanol
• 7.3 Silver nitrate
• 7.4 Isopropanol
• 7.5 Ammonia water: concentration 28% (mass fraction) (the original source-text extraction has noise, requires confirmation against the source standard (source-text extraction))
• 7.6 Zinc powder: high-purity zinc powder in air (easily oxidized), should be stored sealed (the original source-text extraction has noise, requires confirmation against the source standard (source-text extraction))
• 7.7 Potassium chloride: before use (should be dried at a certain temperature) dry 2 h, cool in a desiccator (the drying temperature original source-text extraction is missing, requires confirmation against the source standard (source-text extraction))
• 7.8 Concentrated sulfuric acid: concentration 98% (mass fraction) (the original source-text extraction has noise, requires confirmation against the source standard (source-text extraction))
• 7.9 Stirring bar: small polytetrafluoroethylene-coated (the original wording requires confirmation against the source standard (source-text extraction))
• 7.10 Quantitative filter paper: fast (the original source-text extraction is incomplete, requires confirmation against the source standard (source-text extraction))
• Solution preparation:
  • 9.1 Titration medium: prepare isopropanol and ammonia water in a volume ratio of 75∶1.
  • 9.2 Methanol-glacial acetic acid mixed solution: prepare anhydrous methanol and glacial acetic acid in a volume ratio of 25∶1.
  • 9.3 Dilute sulfuric acid: prepare concentrated sulfuric acid and water in a volume ratio of 1∶1.
  • 9.4 Preparation of 0.1 mol/L silver nitrate-isopropanol standard solution: accurately weigh 8.4940 g silver nitrate into a 200 mL beaker, dissolve with 40 mL (water, the original source-text extraction is missing, requires confirmation against the source standard (source-text extraction)), transfer to a 500 mL volumetric flask, rinse the beaker with isopropanol, pour back into the volumetric flask, and dilute to the mark with isopropanol. Store in a (brown) bottle, standardize once a week.
  • Lower concentration silver nitrate-isopropanol standard solution can be prepared by dilution with isopropanol.
  • 9.5 Standardization of silver nitrate-isopropanol standard solution: accurately weigh 0.0900 g～0.1000 g of dried potassium chloride into a dry (beaker), add 100 mL of distilled water and a few drops of dilute sulfuric acid, with stirring, titrate with 0.1 mol/L of silver nitrate-isopropanol standard solution. Calculate the accurate concentration of the silver nitrate-isopropanol standard solution according to formula (7):
    • C = m / (0.07455 × V) (the original formula (7) source-text extraction is “m (0.074551)”, requires confirmation against the source standard (source-text extraction)))
    • Where: C— concentration of the silver nitrate-isopropanol standard solution, mol/L; m— mass of dried potassium chloride weighed in the beaker, g; V— volume of silver nitrate-isopropanol standard solution used for titration, mL; 0.07455— molar mass of potassium chloride multiplied by 0.001 (conversion factor from milliliters to liters).

5. Instruments and equipment

• 6.1 Automatic potentiometric titrator: equipped with a composite silver electrode, can be used to determine mercaptans and standardize silver nitrate solution; the burette has optional specifications such as 5 mL, 10 mL or 20 mL, with a dosing precision of less than or equal to 0.2% of the titration volume.
• 6.2 Electromagnetic stirrer
• 6.3 Balance: accurate to 0.0001 g
• 6.4 Conical flask: 100 mL, with a ground glass stopper
• 6.5 Beaker
• 6.6 Graduated cylinder: 50 mL, 100 mL
• 6.7 Volumetric flask: 500 mL
• 6.8 Pipette: (specification original source-text extraction is missing, requires confirmation against the source standard (source-text extraction)) mL
• 6.9 Funnel

6. Recommended operating conditions

• Titration mode (10.3.2): calibrate the instrument according to the instrument manual, and perform titration in a potential equilibrium manner, or titrate at a variable titration speed, that is: during the addition, add at a speed of less than 0.2 mL/min, and near the equivalence point, preferably add at a speed of 0.05 mL/min or lower.
• Titration endpoint determination (10.3.3): continue titration until after the potential jump it again becomes relatively constant (potential change less than 5 mV/0.1 mL).
• Electrode maintenance (10.3.3): remove the burette, raise the electrode holder, first rinse the electrode with the titration medium, then wash with water, and then wipe with lens paper. Between consecutive titrations on the same day, immerse the electrode in the titration solvent of 100 mL containing 0.5 mL of 0.1 mol/L silver nitrate standard solution for at least 5 min.
• Reaction conditions (10.1): stir and react at room temperature for 15 min.
• Other: no more recommended operating conditions are provided in the available source.

7. Sampling and sample preparation

• 8.1 Sample according to the method of GB/T 4756.
• For light petroleum distillates less than 200 ℃, because hydrogen sulfide and low-molecular-weight mercaptans in the sample are volatile, they should be stored at low temperature (and) determined as soon as possible (the original source-text extraction has noise, requires confirmation against the source standard (source-text extraction)).
• Sample preparation (10.1): In a 100 mL conical flask, accurately weigh (sample, mass range in the source text source-text extraction is "0000 g~5.0000", requires confirmation against the source standard (source-text extraction)), add (zinc powder and stirring bar) in a mass ratio of sample to zinc powder of 5:1, add 15 mL methanol-glacial acetic acid mixed solution, then add (high) purity zinc powder and stirring bar according to (the above ratio), cover the conical flask, and stir at room temperature for reaction 15 min.

8. Analytical procedure

• 10.1 Reaction of sample with zinc powder: In a 100 mL conical flask, accurately weigh the sample (mass range requires confirmation against the source standard (source-text extraction)), add zinc powder and stirring bar at a sample-to-zinc-powder mass ratio of 5:1, add 15 mL methanol-glacial acetic acid mixed solution, cover the conical flask, and stir at room temperature for reaction 15 min.
• 10.2 Sample filtration: Filter the reacted sample quickly through quantitative filter paper into a 200 mL beaker, rinse the reaction flask and filter paper with 100 mL titration medium, and add the rinse liquid to the filtrate.
• 10.3 Sample titration:
  • 10.3.1 Add a stirring bar to the filtrate, insert the electrode and stir, and titrate with standardized silver nitrate-isopropanol standard solution. Because thiols are easily oxidized, the contact time between the sample and air should be minimized.
  • 10.3.2 Calibrate the instrument according to the instrument manual, and perform titration in potentiometric equilibrium mode, or titrate at a variable titration rate, that is: during the addition, add at a rate less than 0.2 mL/min, and near the equivalence point, preferably add at 0.05 mL/min or lower.
  • 10.3.3 Continue titration until after the potential jump it again becomes relatively constant (potential change less than 5 mV/0.1 mL). Remove the burette, raise the electrode holder, first rinse the electrode with titration medium, then wash with water, and wipe with lens paper. Between consecutive titrations on the same day, immerse the electrode in 100 mL titration solvent containing 0.5 mL of 0.1 mol/L silver nitrate standard solution for at least 5 min.
  • 10.3.4 After the titration process is completed, draw the titration curve as shown in Figure 1.
  • Note 1: Some potentiometric titrators show a small potential jump within a few seconds after the start of titration. This jump is caused by the electrode, not by the titration endpoint of the sample.
  • Note (source-text extraction severe noise, requires confirmation against the source standard (source-text extraction)): If this jump is (caused) by the titration of active sulfur in the sample, the corresponding consumed volume of silver nitrate standard solution should also be doubled.

9. Qualitative and quantitative determination and calculation formulas

• The total active sulfur value obtained by sample titration is calculated according to formula (8):
  • S = (32 × V × C) / m₁ × 1000
  • Where: S— total active sulfur value in the sample, mg/kg; V— volume of silver nitrate-isopropanol solution consumed to reach the titration endpoint, mL; C— concentration of silver nitrate-isopropanol standard solution, mol/L; m₁—sample mass, g; 32— relative atomic mass of sulfur.
• Note: From formula (4) and formula (5), it can be seen that because 1 mol of S²⁻ needs to react with 2 mol of silver nitrate, while 1 mol of SH needs to react with 1 mol of silver nitrate. However, during the titration process, the potential jumps of silver sulfide and silver thiolate are very close, and only one jump point can be obtained. This method uses the titration endpoint of silver thiolate as the titration endpoint of the test, and calculates the test results according to the reaction of formula (5). Therefore, under the conditions of this test, the result calculated by formula (8) is the total active sulfur value, which is higher than the actual total active sulfur content obtained by adding elemental sulfur, hydrogen sulfide, thiols, and disulfides.
• Qualitative: Determine the titration endpoint by the potential jump (10.3.3).

10. Precision

• 13.1 Precision: Precision was obtained by testing a total of 15 samples of light petroleum distillates with different total active sulfur values in 5 laboratories using different instruments. According to the provisions of GB/T 6683, the test results were statistically calculated. Judge the reliability of the test results according to the following provisions (95% confidence level):
  • 13.1.1 Repeatability (r): For the same operator, using the same instrument, under the same operating conditions, the difference between two results obtained by continuous determination of the same sample shall not exceed the following value:
    • r = 0.274 X₁^0.3030 (original formula (9) source-text extraction is "7=0.274X,0.3030.", requires confirmation against the source standard (source-text extraction))
    • Where: X₁—average of two repeat determination results.
  • 13.1.2 Reproducibility (R): For different operators working in different laboratories, using different instruments, the difference between two single, independent test results obtained from the same sample shall not exceed the following value:
    • R = 0.406 X₂^0.3333 (original formula (10) source-text extraction is "R=0.406X,"333", requires confirmation against the source standard (source-text extraction))
    • Where: X₂—average of two single, independent results.
• 13.2 Bias: Because the total active sulfur value is defined only by this method and there is no suitable reference material, the bias of this method has not yet been determined.

11. Report

• Report the total active sulfur value in the sample, mg/kg, with the result retained to one decimal place.

12. Referenced documents

• GB/T 4756 Manual sampling method for petroleum liquids
• GB/T 6682—2008 Specifications and test methods for water used in analytical laboratories
• GB/T 6683 Determination method for precision data of petroleum product test methods (reference document)

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 Q&A reference