SH/T 0222-1992 Determination of total sulfur content in liquefied petroleum gas (coulometric method)

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

• Method number: SH/T 0222—1992
• Full name: Determination of total sulfur content in liquefied petroleum gas (coulometric method)
• English name: not provided in the available source
• Replacement relationship: not provided in the available source

2. Scope of application and non-applicability

• This standard specifies a method for the determination of total sulfur content in samples by the coulometric method.
• This standard applies to liquefied petroleum gas with sulfur content in the range of 10 mg/m³ to 10000 mg/m³.
• Not applicable: not provided in the available source.
• Note: In this clause of the source, “bowl content” and “mg/m’” are source-text extraction noise; according to the context, they should be “sulfur content” and “mg/m³”; requires confirmation against the source standard (source-text extraction).

3. Method principle

3.1 A certain amount of liquefied petroleum gas sample is carried by nitrogen into a nitrogen stream maintained at about 600 ℃, flows out through a quartz tube nozzle into an oxygen stream at 900 ℃ for combustion, so that sulfur compounds in the sample are converted into sulfur dioxide, and enter the titration cell with the gas stream to react with triiodide ions. Due to the consumption of triiodide ions, the indicating electrode pair generates a deviation signal that is input to the coulometer. The coulometer controls the electrolysis current according to the magnitude of its signal to replenish the consumed triiodide ions. The amount of sulfur dioxide entering the titration cell is determined by the total quantity of electricity consumed to generate triiodide ions. After calibration with a standard sample, the total sulfur content in the sample can be calculated.

3.2 Chemical reactions occurring in the titration cell:

• 3I⁻ + SO₂ + H₂O → SO₄²⁻ + 3I⁻ + 2H⁺ (as in the source, source-text extraction doubtful point, requires confirmation against the source standard (source-text extraction))
• Electrode reaction for electrolytic generation of triiodide ions: 3I⁻ → I₃⁻ + 2e (as in the source, source-text extraction doubtful point, requires confirmation against the source standard (source-text extraction))

3.3 The liquefied petroleum gas sample must flow out of the sampler in liquid form and be completely vaporized at 60 ℃ to 70 ℃, then enter the sampling valve quantitative tube at constant temperature, and be carried by nitrogen into the quartz tube.

3.4 A liquid standard sample with known sulfur content is injected into the quartz combustion tube under the same conditions, and the sulfur content in the sample is calibrated according to the quantity of electricity it consumes.

4. Reagents and materials (purity / preparation)

4.2 Materials

• 4.2.1 Nitrogen: purity higher than 99%.
• 4.2.2 Oxygen: purity higher than 99%.
• 4.2.3 Distilled water.

5 Reagents

• 5.1 Glacial acetic acid: analytical reagent.
• 5.2 Potassium iodide: guaranteed reagent.
• 5.3 Iodine: analytical reagent.
• 5.4 Cyclohexane, isooctane or refined oil: solvent used to prepare standard samples; the sulfur content must be less than 0.5 ppm. When preparing standard samples less than 10 ppm, the sulfur content must be less than 0.2 ppm. Refine and determine the sulfur content in the standard sample solvent according to the method specified in SH/T 0253.
• 5.5 n-Butyl sulfide (CH₃CH₂CH₂CH₂)₂S: used as sulfur standard substance; other standard substances such as octyl mercaptan may also be selected, but the purity must be higher than 99.5%.
• 5.6 Anhydrous glycerol: chemically pure.
• 5.7 Mannitol: chemically pure.

6.1 Preparation of grease insoluble in hydrocarbons

100 Mix g anhydrous glycerol, 29 g starch and 14 g mannitol, then slowly heat to boiling, quickly remove from the heat source, then stir until cooled for later use.

6.2 Preparation of electrolyte

0.5 A solution of g potassium iodide and 0.04% (V/V) glacial acetic acid. (In the source, “0.5g/ potassium iodide and .0.04%: (VV) glacial acetic acid” is source-text extraction noise; requires confirmation against the source standard (source-text extraction))

6.3 Preparation of standard solutions

• 6.3.1 Sulfur standard stock solution (sulfur content about 300 ppm): accurately weigh about 0.5000 g n-butyl sulfide and pour it into a weighed 500 mL volumetric flask, dilute to the mark with any one of the standard sample solvents described in clause 5.4, then weigh again.
  • The accurate sulfur content S₁ (ppm) of the sulfur standard stock solution is calculated according to formula (1): S₁ = m₁ × 0.2192 × 10⁶ / m₂ Where: m₁—mass of n-butyl sulfide, g; m₂—total mass (of n-butyl sulfide and solvent), g; 10⁶—conversion factor from one gram to micrograms, μg/g; 0.2192— ratio of the molecular weight of sulfur to that of n-butyl sulfide.
• 6.3.2 Sulfur standard solution: use a pipette to take 10 mL of the sulfur standard stock solution, place it in a 100 mL volumetric flask, and dilute to the mark with any one of the standard sample solvents described in clause 5.4.
  • The sulfur content S₂ (ppm) of this standard solution is calculated according to formula (2): S₂ = S₁ / 10

5. Instruments and equipment

• 4.1.1 Coulometer: capable of measuring the potential difference between the indicating-reference electrode pair, and having a bias voltage to offset this potential difference. It has an adjustable amplification control system to amplify this potential difference. When the output amplified voltage signal is applied to the electrolysis electrode pair, the voltage signal is proportional to the electrolysis current, and it has a variable range attenuation.
• 4.1.2 Recorder: full scale less than 10 mV.
• 4.1.3 Pyrolysis furnace and gas flow controller: the pyrolysis furnace can provide the following two-stage temperatures, and the temperature fluctuation is less than ±10 ℃:
  • Inlet section: 500 ℃ to 600 ℃
  • Pyrolysis section: 850 ℃ to 900 ℃
  • The gas flow controller can provide stable nitrogen and oxygen flows.
• 4.1.4 Quartz combustion tube: has a liquid sample vaporization section and a pyrolysis combustion section. The liquid sample can be vaporized in nitrogen (see Table 1) and enter the pyrolysis section to burn in oxygen. The quartz tube also has injection port devices for liquid samples and gas samples.
• 4.1.5 Titration cell: has an indicating and reference electrode pair capable of detecting changes in the concentration of triiodide ions in the electrolyte, and has an anode-cathode electrode pair for generating triiodide ions and an inlet for combustion gas. The indicating electrode is a platinum sheet electrode; the reference electrode is a half-cell composed of a platinum wire inserted into an electrolyte saturated with iodine; the anode and cathode are both composed of platinum sheets or platinum wires. The titration cell is filled with electrolyte and placed on a stirrer. For detailed dimensions of the titration cell and quartz tube, see SH/T 0253.
• 4.1.6 Stirrer: can make the stirring bar in the titration cell stir smoothly, and does not generate heat during continuous use.
• 4.1.7 Microsyringe: 10 μL.
• 4.1.8 Liquid sample injector: various liquid injectors capable of achieving the standard sample injection speed 0.1 μL/S to 0.25 μL/S specified in this standard.
• 4.1.9 Liquefied petroleum gas sampler: use the dual-valve type 100 mL sampler specified in SH 0233.
• 4.1.10 Sampling valve: used to obtain a certain amount of gas sample, and nitrogen can be used to carry the sample into the quartz combustion tube. Figure 1 is a recommended glass sampling valve.
• 4.1.11 Liquefied petroleum gas injection system, as shown in Figure 2. Place the regulating valve, vaporization chamber and sampling valve in an aluminum heating bath, and maintain them at a constant temperature of 60 ℃ to 70 ℃, with temperature indication. The system also includes a nitrogen flow control valve, a rotameter, and a liquefied petroleum gas flow indicator.
• 4.1.12 Arrangement of the apparatus: the entire apparatus is shown in Figure 3.

6. Recommended operating conditions

Table 1 Typical operating conditions (the original table source-text extraction is disordered; the following is the identifiable content; requires confirmation against the source standard (source-text extraction)):

• Fuel gas flow rate (oxygen), mL/min: no explicit value provided in the available source
• Nitrogen/carrier gas flow rate: 40
• Oxygen: 160
• About 20 to 40
• Inlet furnace temperature: 500 to 600 ℃
• Central furnace temperature: 850 to 900 ℃
• Vaporization chamber temperature: 60 to 70 ℃
• Sample gas flow rate, mL/min: 20 to 40
• Coulometer bias voltage, mV: about -110 to -150
• Liquid standard sample injection speed, μL/S: 0.1 to 0.25

7. Sampling and sample preparation

• 8 Sampling: carry out sampling according to SH 0233, but only stainless steel samplers and stainless steel tubes may be used.
• 6.4 Calibration of the volume of the quantitative tube in the sampling valve:
  • 6.4.1 The quantitative tube in the sampling valve must be volume-calibrated by the manufacturer, and a calibration table must accompany it.
  • 6.4.2 When contamination of the quantitative tube is found, it should be cleaned promptly.
  • 6.4.3 Apply grease insoluble in hydrocarbons to the sampling valve, and rotate repeatedly to make it uniform.
  • 6.4.4 Install the sampling valve into the liquefied petroleum gas injection system, and connect it to the vaporization chamber, nitrogen line and quartz line.

8. Analytical procedure

6.5 Preparation of the instrument

• 6.5.1 Install the instrument according to the instrument manual.
• 6.5.2 Add electrolyte to the titration cell, and rinse several times, each time releasing part of the electrolyte from the side arm, maintaining the electrolyte about 6 mm above the platinum sheet of the electrode.
• 6.5.3 If necessary, wrap the quartz tube outlet and titration cell inlet with insulation tape to keep the quartz tube outlet temperature above 100 ℃.
• 6.5.4 Raise the furnace temperature and injection system to the required temperature and keep them constant. Adjust the gas flow to the required value, connect the titration cell and quartz tube, start the stirrer, so that the electrolyte in the titration cell can produce an obvious vortex but no solution splashes from the liquid surface.
• 6.5.5 Adjust the operating parameters of the coulometer so that the sample peak shows a symmetrical peak shape on the recorder, the recorded peak returns to baseline within 2 min, and the peak shape does not overshoot. Table 1 lists typical operating conditions.

7 Calibration

• 7.1 Take a standard sample with a sulfur content similar to that of the test sample, about 5 μL, inject it into the quartz combustion tube through the liquid sample silicone rubber injection plug, and check whether the recorded peak shape is normal and whether all parameters are appropriate.
  • Note: If the baseline noise is very large, the indicator electrode may be heated red-hot on a glassblower's lamp 5 min, and after cooling, inserted into the electrolyte and soaked for 2 h before use.
• 7.2 The mass of the injected liquid standard sample is obtained by accurately weighing the difference in mass of the syringe and standard sample before and after injection. Weighing to 0.1 mg is sufficient.
• 7.3 The mass of the injected liquid standard sample can also be calculated by measuring the volume of the injected standard sample. Use a 10 μL standard sample syringe to draw up 5 μL of the standard sample, pull the syringe plunger backward so that the lower meniscus falls on the 1 μL mark, and record the volume of the standard sample in the syringe. After injecting the standard sample, pull the syringe plunger again so that the lower meniscus still falls on the 1 μL mark. Record the volume of the standard sample remaining in the syringe. The difference between the two volume readings is the actual volume of the injected standard sample. Read to 0.1 μL. Then, based on the density of the standard sample, the mass of the standard sample entering the quartz tube can be calculated.
• 7.4 The injection speed of the standard sample is controlled at 0.1 μL/S～0.25 μL/S.
• 7.5 Since only part of the sulfur entering the quartz combustion tube is converted to sulfur dioxide, and another part is converted to sulfur trioxide, the percentage of sulfur in the generated sulfur dioxide relative to the total sulfur in the standard sample is defined as the sulfur recovery rate. Under the above operating conditions, the recovery rate should be above 75%. Otherwise, the operator should check all operating parameters and whether the system leaks. If it is caused by contamination of the quartz tube, pure oxygen should be passed through to burn the quartz tube at 900 ℃, and if necessary, the quartz tube may be cleaned with hydrofluoric acid.
• 7.6 The recovery rate of sulfur in the standard sample Ss is calculated according to formula (3): Ss = (2×16 / 96500) × (Q / (m₄ × S₂ × 10³)) × 100 Where: Q— the measured total electric quantity, μC; 16—1/2 the mass of the atomic weight of sulfur, g; 96500— Faraday's electrolysis constant, C/N; m₄—the mass of the injected standard sample, mg; S₂—the sulfur content in the standard sample, ppm; 10³—the coefficient for converting milligrams to grams, g/mg.
• When two consecutive measurement results comply with the provisions of 11.1, calculate the correction factor F (ng/μC) according to formula (4), and take its arithmetic mean: F = Q₁ / S₂ (the original formula source-text extraction is incomplete, requires confirmation against the source standard (source-text extraction)) Where: Q₁—the electric quantity displayed by the coulometer, μC.

9 Test Procedure

• 9.1 Connect the sampler to the regulating valve on the liquefied petroleum gas injection system, and keep the injection system constant at any temperature from 60 ℃～70 ℃.
• 9.2 Open the sampler outlet valve. Rotate the sampling valve to the vent position, adjust the regulating valve, and control the outflow speed of the liquid test sample in the sampler so that the liquefied petroleum gas flow rate is 20 mL/min～40 mL/min.
• 9.3 Rotate the sampling valve to the sampling position and allow the liquefied petroleum gas to flow through the quantitative tube for 30 s～60 s.
• 9.4 When the baseline displayed on the recorder is stable, rotate the sampling valve to the injection position and allow nitrogen to carry the test sample in the quantitative tube into the quartz tube.
• 9.5 When the recorder pen returns to the baseline and the coulometer displays a result, record the electric quantity displayed by the coulometer.
• 9.6 Repeat the operations in clauses 9.3～9.5 until two results comply with the provisions of clause 11.1.
• 9.7 Record the temperature in the vaporization chamber during injection and the laboratory atmospheric pressure.

9. Qualitative and Quantitative Determination and Calculation Formulas

The sulfur content of the test sample under standard conditions Sa (mg/m³) is calculated according to formula (5) or formula (6):

• Sa = Q₂ · F · (273 + t) · P₁ / (V · P × 273) (formula 5, the original text source-text extraction is garbled, requires confirmation against the source standard (source-text extraction))
• Sa = Q₂ · F · (273 + t) · P / (V · B × 273) (formula 6, the original text source-text extraction is garbled, requires confirmation against the source standard (source-text extraction))

Where: V— test sample volume, mL; P₁—atmospheric pressure, kPa; P— atmospheric pressure, mmHg; t— constant temperature of the gaseous test sample, ℃; Q₂—the electric quantity displayed by the coulometer when analyzing the test sample, μC; F— correction factor, ng/μC.

10. Precision

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

11.1 Repeatability: The difference between two results of repeated measurements by the same operator shall not exceed the following values:

Where X (mg/m³) is the arithmetic mean of the sulfur content.

11.2 Reproducibility: The difference between two results submitted by different laboratories shall not exceed the following values: R = 0.27X Where X (mg/m³) is the arithmetic mean of the results submitted by the two laboratories.

11. Report

Not provided in the available source.

12. Reference Documents

• SH 0233 Liquefied petroleum gas sampling method
• SH/T 0253 Determination of total sulfur content in light petroleum products (coulometric method)

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 Q&A