GB/T 8025-1987 Determination of trace sulfur in petroleum waxes and petroleum fats (Microcoulometric method)

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Card type: Test method

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

• Standard number: GB 8025-87
• Chinese full name: Determination of trace sulfur in petroleum waxes and petroleum fats (Microcoulometric method)
• English name: Petroleum waxes and petrolatums—Determination of trace sulfur—Microcoulometric method
• Replacement relationship: not provided in the available source
• Approval/implementation: approved by China Petrochemical Corporation 1987-05-12, implemented on 1988-05-01
• UDC number: 85.521.8; $548.06.546.22 (requires confirmation against the source standard (source-text extraction), original text as such)

2. Scope of application and non-applicability

• Applicable to the determination of trace sulfur content in petroleum waxes and petroleum fats, including white oil, liquid paraffin, petrolatum, paraffin wax, and (wax, requires confirmation against the source standard (source-text extraction)).
• Determination range is 0.5~500 ppm.
• The halogen content in the analytical sample shall not be greater than 10 times the sulfur content, the silver (original text “silver [source character uncertain]”, requires confirmation against the source standard (source-text extraction)) content shall not be greater than 10%, and heavy metals shall not exceed 500 ppm.

3. Method principle

The analytical sample is vaporized in the vaporization section of the pyrolysis tube and mixed with carrier gas (nitrogen, original text “[gas name garbled in several forms]” requires confirmation against the source standard (source-text extraction)) to enter the combustion section, where it is mixed with oxygen and undergoes pyrolysis and oxidation. Sulfur is converted to sulfur dioxide, which enters the titration cell together with the carrier gas and reacts with triiodide ions in the electrolyte according to the following equation: I₃⁻ + SO₂ + H₂O → SO₄²⁻ + 3I⁻ + 2H⁺ (original equation source-text extraction is “Is+SOz+H0-S0+31-+2H”, requires confirmation against the source standard (source-text extraction)) The concentration of triiodide ions in the titration cell decreases, the measuring electrode senses the bias voltage and the potential change between the “reference-measuring” electrode pair, this signal is input to the microcoulometer amplifier, and after amplification the output voltage is applied to the electrolysis electrode, where the following reaction occurs at the electrolysis anode: 3I⁻ → I₃⁻ + 2e (original source-text extraction is “31-—-1+2e”, requires confirmation against the source standard (source-text extraction)) The consumed triiodide ions are replenished, and the quantity of electricity consumed is the integral of the electrolysis current over time. According to Faraday's law of electrolysis, the total sulfur amount equivalent to sulfur dioxide is obtained, and the sulfur content of the sample is calculated according to the recovery rate.

4. Reagents and materials (purity / preparation)

All reagents used in the test are of chemical purity.

• 3.1 Glacial acetic acid
• 3.2 Potassium iodide
• 3.3 Sodium azide (NaN₃) (original text “NaNg”, requires confirmation against the source standard (source-text extraction))
• 3.4 Iodine: for the reference electrode, use iodine particles with a particle size of 20 mesh or smaller than 20 mesh
• 3.5 Distilled water: use once-distilled water treated with activated carbon and mixed ion exchange resin, or use water redistilled in a quartz still
• 3.6 Oxygen
• 3.7 Nitrogen (original text “[garbled gas name]”, requires confirmation against the source standard (source-text extraction))
• 3.8 Desulfurized white oil or desulfurized liquid paraffin: sulfur content less than 0.1 ppm, used to prepare standard samples
• 3.9 Reference reagents for sulfur compounds: sulfur compounds such as dibenzyl disulfide and benzothiophene may be selected
• 4.1 Electrolyte preparation: dissolve 0.5 grams of potassium iodide and 0.6 grams of sodium azide in 500 milliliters of distilled water, add 5 milliliters of glacial acetic acid, and dilute to 1000 milliliters. Store in a brown bottle.
• 4.2 Preparation of sulfur standard samples: when preparing, the concentration range of the standard samples should be compatible with the sulfur content of the analytical sample, and a series of standard samples with different concentrations should be prepared accordingly. Using the method of weighing a certain amount of desulfurized white oil and reference sulfur compound, the sulfur content of the standard sample (S ppm) is calculated according to formula (1): S ppm = (g·S × 10⁶)/(g+G) (1) Where: g— weight of reference sulfur compound, grams; S— percentage sulfur content in the reference sulfur compound; G— weight of solvent, grams.

5. Instruments and equipment

• 2.1 Microcoulometer: capable of measuring the potential between the “measuring-reference” electrode pair. It has a bias voltage that cancels this potential, an adjustable amplification control, and the amplified voltage signal is applied to the electrolysis electrode pair. The voltage signal is proportional to the electrolysis current and has variable range attenuation.
• 2.2 Recorder: sensitivity 1~10 millivolts/25 centimeters, or equipped with a coulometric integrator.
• 2.3 Pyrolysis furnace: has two or three furnace temperature control sections.
• 2.4 Pyrolysis tube: made of quartz glass, shape and dimensions as shown in Figure 1. Note: Other types of pyrolysis tubes may be used instead. However, the combustion section of the pyrolysis tube must ensure complete combustion of the sample to achieve the required minimum detection limit.
• 2.5 Titration cell: shape and dimensions as shown in Figure 2, or a titration cell with equivalent performance.
• 2.6 Electromagnetic stirrer: adjustable speed, ensuring smooth stirring of the stirring bar in the titration cell.
• 2.7 Gas flow meter: float-type flow meter of 0~200 milliliters/minute.
• 2.8 Oxygen and nitrogen should be stabilized in pressure and flow to ensure stable gas volume.
• 2.9 Platinum boat: made of platinum, dimensions are length 10 millimeters, width 3 millimeters, height 2 millimeters.
• 2.10 Microsyringe: 10 microliters.
• 2.11 Balance: accuracy one part in one hundred thousand (original text “one hundred-thousandth [incomplete expression]”, requires confirmation against the source standard (source-text extraction)).

6. Recommended operating conditions

4.5 Adjust gas flow and furnace temperature, operating conditions are as follows:

• Pyrolysis furnace temperature:
  • Inlet section: 550~700 ℃
  • Center section: 900 ℃
  • Outlet section: 800~900 ℃
• Gas flow (milliliters/minute):
  • Reaction gas oxygen (O₂): 90~100
  • Carrier gas nitrogen (N₂): 100~110
• 5.1 The operating conditions of the microcoulometer should be adjusted according to the sulfur content, with the bias voltage adjusted between 120~160 millivolts, and an appropriate amplification factor and range selection resistance value selected. Within the recording range, obtain a peak shape that does not tail or overshoot.
• 4.8 A heating tape should be wound around the outside of the inlet conduit of the titration cell to prevent condensation of water vapor.
• 4.10 It is very important to control a suitable stirring speed; the most suitable speed is one that causes slight swirling of the electrolyte.

7. Sampling and sample preparation

• 4.3 Replace the 2 sample inlet of the quartz pyrolysis tube with a new heat-resistant silicone rubber septum.
• 4.4 Carefully place the quartz pyrolysis tube in the pyrolysis furnace and connect oxygen and nitrogen as required in Figure 1.
• 4.6 Pretreatment of the platinum boat used for weighing samples: from the 2 sample inlet, place the empty platinum boat into the sample boat, push the sample boat into the furnace 5~8 centimeters, and let it stay for about 20 minutes. Use the push rod to move the sample boat out of the furnace and leave it in the pyrolysis tube for later use.
• 4.7 Rinse the titration cell several times with electrolyte. If there are bubbles in the side arms of the reference electrode and electrolysis electrode, carefully remove them. Keep the electrolyte level 3~6 millimeters above the platinum electrode.
• 4.9 Place the titration cell on the stirrer. Note that the bottom of the cell should be placed at the center of the stirrer, the spherical ground joint at the inlet of the titration cell should be connected to the outlet of the pyrolysis tube, and adjust the position of the titration cell cover so that the electrolysis anode platinum sheet faces the electrolysis cathode side.
• 4.11 Turn on the power to the heating tape and adjust the voltage to reach the required temperature.
• 5.5 There are two methods for taking the sample: the volume method and the weight method:
  • 5.5.1 Volume method: for white oil and liquid paraffin, use a 10 microliter syringe to take the sample. First rinse the syringe several times with the analytical sample, accurately draw a certain amount of sample, inject the sample into the platinum boat through the 1 sample inlet, and then push the push rod to introduce the sample.
  • 5.5.2 Weight method: from the 2 sample inlet, take out the empty platinum boat that has been treated in the furnace and place it in a weighing bottle with a lid. Then accurately weigh a certain amount of paraffin wax or petrolatum sample. Place the platinum boat containing the sample into the sample boat through the 2 sample inlet, cover the ground stopper, and then push the push rod to introduce the sample.
• 5.6 Sample amount: depending on the sulfur content of the sample, the sampling amount range is as follows; weigh accurately to 0.01 milligrams.
  • Sample sulfur content, ppm: <1 | 1–5 | 5–10 | (subsequent columns missing in the original text, requires confirmation against the source standard (source-text extraction))
  • Sampling amount, milligrams or microliters: 10–5 | 10–3 | 5–1 | 4–1 | about 1 (original arrangement is disordered, requires confirmation against the source standard (source-text extraction))

8. Analytical procedure

• 5.2 Check whether the platinum boat has a blank peak. Push the sample boat into the furnace at the same boat introduction speed as the sample introduction speed for the determination. If no peak appears on the recorder, the platinum boat can be taken out and placed in a weighing bottle for later use; otherwise, treat it again.
• 5.3 Select a standard sample with a sulfur content close to the estimated sulfur content in the sample, carry out a calibration test, and take the average of three determinations. The recovery rate C is calculated according to formula (2): C = S₁/S₂ × 100 (2) Where: C— recovery rate, %; S₁—average measured sulfur content of the standard sample, ppm; S₂—prepared sulfur content value of the standard sample, ppm.
• 5.4 The recovery rate must be greater than 75% before the test can be carried out.
• 5.7 The sample introduction speed is closely related to whether combustion is complete, and is controlled in three stages. From the 1 or 2 sample inlet, push the sample boat to the furnace mouth at a speed of 10–12 millimeters/second. After the peak appears, the sample boat remains stationary until the peak is about at its maximum. Continue pushing the sample boat into the pyrolysis furnace at a speed of 1 millimeters/second until it reaches 10–40 centimeters, and the pushing depth increases as the melting point of the sample increases.
• 5.8 After each sample peak is complete and returns to baseline, pull the sample boat out of the furnace and place it between the furnace mouth and the 2 sample inlet. After cooling, perform the next determination.
• 5.9 The test procedure for unknown samples is the same as for standard samples.

9. Qualitative and quantitative determination and calculation formulas

Calculate the test results according to one of the two formulas (3) and (4). Formula (3) is suitable for instruments equipped with coulometric integration; formula (4) is suitable for instruments not equipped with coulometric integration or for determining samples with lower sulfur content.

• Formula (3): S ppm = (0.166 × Q)/(G × C)
• Formula (4): S ppm = (9948 × A × V' × S')/(R × G × C) (the source source-text extraction is “9948×A• '•5' / R•G•C”, requires confirmation against the source standard (source-text extraction)) Where:
• S— total sulfur content of the sample, ppm;
• Q— measured electric quantity, microcoulombs;
• G— sample weight, milligrams;
• C— recovery rate, %;
• A— measured peak area, square centimeters;
• V'—recorder sensitivity, millivolts per centimeter;
• S'—recorder paper speed, minutes per centimeter;
• R— resistance value selected for the microcoulometer range, ohms;
• 9948— constant;
• 0.166— coefficient, micrograms per microcoulomb.

Appendix A Derivation of the formula (reference item):

• According to Faraday's law of electrolysis, the total sulfur amount in the sample is calculated according to formula (A1): S grams = Q/F × 16 grams/gram equivalent (the source source-text extraction is “S grams = - unclear in the source text gram equivalent 16 grams”, requires confirmation against the source standard (source-text extraction))
• Because ppm is micrograms/gram, the total sulfur content in the sample can be according to formula (A2) or formula (A3):
  • S ppm = (Q × 16 × 10⁶ × 10⁻⁶)/(F × V × d × C × 10⁻³) = (0.166 × Q)/(G × C) (A2/A3)
  • S ppm = (50 × 16 × 10⁻⁸ × 10 × 60)/(F × R × V' × d × C × 10⁻³) = (9948 × A × V' × S')/(R × G × C) (the source source-text extraction is garbled, requires confirmation against the source standard (source-text extraction))
• Where: 16— gram equivalent of sulfur, grams/equivalent; 10⁶—coefficient for converting grams to micrograms, micrograms/gram; 10⁻⁶—coefficient for converting microcoulombs to coulombs, coulombs/microcoulomb; F— Faraday electrolysis constant is 96500, coulombs/gram equivalent; V— sample volume, microliters; d— sample density, grams/milliliter; 10⁻³—coefficient for converting microliters to milliliters, milliliters/microliter; coefficient for converting milligrams to grams, grams/milligram; 60— coefficient for converting minutes to seconds, seconds/minute.

10. Precision

• 1.1 (source numbering, requires confirmation against the source standard (source-text extraction)) Repeatability: the same operator in the same laboratory, measuring the same sample, the difference between two parallel determination results shall not exceed the maximum allowable difference of the repeatability line in Figure 3.
• 1.2 (source numbering, requires confirmation against the source standard (source-text extraction)) Reproducibility: different operators in different laboratories, measuring the same sample, the difference between determination results shall not exceed the maximum allowable difference of the reproducibility line in Figure 3.
• Figure 3 is the precision curve; the figure is labeled: repeatability, R— reproducibility, average value (the source source-text extraction is garbled, requires confirmation against the source standard (source-text extraction)).

11. Report

For each sample, conduct two parallel determination tests, and take the average value as the determination value of the sample. (The source “for each sample, carry out [garbled repeat count] 4 [garbled parallel-test wording], and take the mean as the sample result”, requires confirmation against the source standard (source-text extraction))

12. Normative references

not provided in the available source.

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