GB/T 6532-2012 Determination of salt content in crude oil - Potentiometric titration method

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Card type: Test method

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

• Standard number: GB/T 6532-2012
• Chinese title: Determination of salt content in crude oil - Potentiometric titration method
• English title: Test method for salt in crude oils (Potentiometric method)
• Replacement relationship: replaces GB/T 6532-1986 "Determination of salt content in crude oil and its products"
• Adoption relationship: modified adoption using the redrafting method of ASTM D6470-99 (2010)
• Publication/implementation: published on 2012-12-31, implemented on 2013-07-01
• Custodian: National Technical Committee for Standardization of Petroleum and Natural Gas (SAC/TC 355)
• Drafting organizations: Sinopec Research Institute of Petroleum Processing, Daqing Oilfield Engineering Co., Ltd., PetroChina Tarim Oilfield Quality Inspection Center, PetroChina Pipeline R&D Center
• Drafters: He Pei, Fan Dengli, Yao Shuhua, Qiao Jinling, Zhang Shuangfeng
• Previous versions: GB/T 6532-1986

2. Scope of application and non-applicability

• This standard specifies a method for determining salt content in crude oil by potentiometric titration.
• Applicable to crude oils with salt content (mass fraction) in the range 0.0005%～0.15%, as well as heavy petroleum products such as residual fuel oil and fuel oil.
• Also applicable for judging seawater contamination of used turbine oil and marine fuel oil.
• Not applicable: no explicit non-applicability clause is not provided in the available source.

3. Method principle

After the sample is mixed uniformly, a certain amount of test portion is weighed and dissolved in 65°C xylene. Extraction is carried out with ethanol, acetone and water in the specified extraction apparatus. After removal of sulfides from the extract, the total halide content is determined by potentiometric titration and then converted to sodium chloride content.

4. Reagents and materials (purity / preparation)

• 4.2.1 Acetone, analytical reagent. Warning: flammable.
• 4.2.2 Anhydrous ethanol (or isopropanol), analytical reagent. Warning: flammable.
• 4.2.3 Nitric acid, analytical reagent. Warning: strongly corrosive.
• 4.2.4 Hydrochloric acid, analytical reagent. Warning: strongly corrosive.
• 4.2.5 Silver nitrate, analytical reagent. Warning: harmful to health, avoid contact.
• 4.2.6 Sodium chloride, primary standard reagent.
• 4.2.7 Xylene, analytical reagent. Warning: toxic, avoid inhalation.
• 4.2.8 Lead acetate test paper, preparation method see 5.1.8.
• 4.2.9 Glycerol, analytical reagent.
• 4.2.10 Fluorescein (fluorescein yellow and fluorescein red), analytical reagent.
• 4.2.11 Starch, analytical reagent.
• 4.2.12 Barium nitrate, analytical reagent. Warning: harmful to health, avoid contact.
• 4.2.13 Water, meeting the technical requirements for grade three water of GB/T 6682.

Preparation:

• 5.1.1 0.1 mol/L Hydrochloric acid solution: add 9 mL concentrated hydrochloric acid to 1000 mL water.
• 5.1.2 5 mol/L Nitric acid solution: carefully pour 325 mL concentrated nitric acid into 1000 mL water while stirring.
• 5.1.3 0.5% Starch indicator: weigh 0.5 g soluble starch in a beaker, add 10 mL water and stir, then add 90 mL water while stirring. Place the beaker on an electric furnace and heat to slight boiling for 2 min, then cool and let stand; take the supernatant liquid for use. During testing, freshly prepared 0.5% starch indicator must be used.
• 5.1.4 0.5% Fluorescein indicator: weigh 0.5 g fluorescein (fluorescein yellow and fluorescein red), dissolve in anhydrous ethanol, and dilute to 100 mL with anhydrous ethanol.
• 5.1.5 0.1 mol/L Silver nitrate standard solution: weigh 17.5 g silver nitrate, dissolve in 1000 mL water, and shake well. Store the solution in a brown ground-glass stoppered volumetric flask. For standardization, weigh primary standard sodium chloride that has been ignited at 500°C～600°C to constant mass, 0.1 g (accurate to 0.0002 g), dissolve in 70 mL water, then add 0.5% starch indicator 5 mL, titrate with 0.1 mol/L silver nitrate solution, add 3 drops of 0.5% fluorescein indicator near the endpoint, and continue titration until the solution changes from yellow to pink.
  • Concentration calculation formula (1): c = m×900 / (V·M) (original text source-text extraction doubtful point: the coefficient "900" in the formula requires confirmation against the source standard (source-text extraction); meanings of symbols in the formula: m— mass of sodium chloride (g); V— volume of silver nitrate solution consumed (mL); M— molar mass of sodium chloride (g/mol))
  • Note 1: Other standard methods may be used to standardize the silver nitrate standard solution, see GB/T 601.
  • Note 2: Certified reference materials with certificates may be used.
• 5.1.6 0.01 mol/L Silver nitrate standard solution: pipette 0.1 mol/L silver nitrate standard solution 100 mL, dilute with water to 1000 mL. Prepare before use.
• 5.1.7 1 mmol/L Sodium chloride solution: weigh 59 mg±1 mg sodium chloride, dissolve in water, and dilute to 1000 mL.
• 5.1.8 Preparation of lead acetate test paper: add 12.5 mL glacial acetic acid, 40 mL glycerol (glycerin) and 100 mL deionized water to a beaker, mix uniformly, then add 25 g lead acetate. After all lead acetate has dissolved, dilute to 250 mL with deionized water. Cut quantitative filter paper into strips of 1 cm×5 cm, soak in the above solution for 20 min, then remove, absorb the water on the strips with dry filter paper until nearly dry, store in a stoppered ground-glass bottle, and seal for use.
  • Note: Commercially available lead acetate test paper may be used.

5. Apparatus and equipment

• 4.1.1 Extraction apparatus see Figure 1. It mainly consists of the following parts:
  • a) Distillation flask, volume 500 mL;
  • b) Reflux condenser;
  • c) Dropping funnel;
  • d) Heating tube, with internal convection tube;
  • e) Heating wire;
  • f) Variable resistor;
  • g) Conical flask.
  • Note: The condenser and cylindrical dropping funnel may be connected with ground-glass joints meeting 19/26 group or equivalent ground-glass joints.
• 4.1.2 Potentiometric titrator mainly consists of the following parts:
  • a) Silver indicator electrode;
  • b) Glass reference electrode;
  • c) Burette, 10 mL;
  • d) Magnetic stirrer.
• 4.1.3 Constant-temperature water bath: controllable to 65°C±5°C.
• 4.1.4 Pipette.
• 4.1.5 Beaker.
• 4.1.6 Sampling tube, glass tube about 600 mm long and about 5 mm in diameter, with a 100 mL bulb in the middle.
• 4.1.7 High-speed stirrer, with speed not less than 3 000 r/min.
• 4.1.8 Electronic balance, readability 0.0001 g.
• 4.1.9 Electronic balance, readability 0.01 g.
• 4.1.10 Electronic balance, readability 0.1 g.

Apparatus preparation:

• 5.2.1 Preparation of extraction apparatus: place the extraction apparatus in a fume hood and install it according to the product installation requirements.
• 5.2.2 Preparation of potentiometric titrator:
  • 5.2.2.1 Preparation of glass reference electrode: before use, first rinse the electrode with water, then soak in 0.1 mol/L hydrochloric acid for 10 min, and finally rinse with water. After use, soak the electrode in water.
  • 5.2.2.2 Preparation of silver indicator electrode: before use, polish the silver electrode with polishing sandpaper until the electrode surface is clean and bright, and finally rinse with water.

6. Recommended operating conditions

• Sample dissolution temperature: 65°C±5°C (see 7.1.1)
• Constant-temperature water bath temperature: 65°C±5°C (see 4.1.3)
• High-speed stirrer speed: not less than 3 000 r/min (see 4.1.7); during homogenization use a closed high-speed shear stirrer, speed 3 000 r/min (see 6.2.3)
• Sample mixing temperature: room temperature (15°C～25°C) or lower (see 6.2.1)
• Temperature difference before and after sample mixing: must not be greater than 10°C (see 6.2.4)
• Total volume of solution in titration cell: about 150 mL (see 7.3.1); total volume of blank solution 150 mL～170 mL (see 7.4)
• The tip of the dropper is about 25 mm below the liquid surface (see 7.3.3)

7. Sampling and test portion preparation

• 6.1 Sampling method: sample according to GB/T 4756 or SY/T 5317.
• 6.2 Homogenization of sample:
  • 6.2.1 At room temperature (15°C～25°C) or lower, mix the sample; record the sample temperature before mixing. For crude oil samples that are solid at room temperature, heat the sample, and the heating temperature should be higher than the pour point of the sample by 3°C.
  • 6.2.2 Select the type of stirrer according to the volume of crude oil in the container. At the same time, investigate its mixing efficiency according to Annex A. When there is any change in the type, volume, container shape, or mixing conditions (such as mixing speed and mixing time) of the crude oil sample, the mixing efficiency must be re-investigated according to the method in Annex A.
  • 6.2.3 For containers with a volume of 50 mL to 500 mL holding the sample, when mixing the sample, a closed high-speed shear mixer (rotation speed of 3 000 r/min) shall be used for stirring. The mixing time, mixing speed, and the height of the stirring head from the bottom shall all meet the requirements of Annex A.
  • 6.2.4 After the sample is uniformly mixed, immediately record the temperature of the sample. The temperature difference before and after mixing shall not be greater than 10°C.
  • 6.2.5 After the sample is uniformly mixed, immediately weigh the test portion. For crude oil containing rapidly settling impurities, after homogenization, use a sampling tube to take a sample; the tip of the sampling tube shall be close to the bottom of the container. After drawing the sample, immediately transfer the test portion into the extraction apparatus.
• Test portion mass: 40 g±0.1 g (see 7.1.1)

8. Analysis steps

• 7.1 Extraction of inorganic salts:
  • 7.1.1 Place the extraction apparatus and the constant-temperature water bath in a fume hood, and turn on the exhaust switch. Accurately weigh 40 g±0.1 g of the test portion into a 250 mL beaker. In the constant-temperature water bath, heat both the test portion and xylene to 65°C±5°C. Then slowly pour 40 mL±1 mL of hot xylene into the sample beaker, and stir continuously to completely dissolve the crude oil test portion with xylene. Quantitatively transfer this mixed solution through a feed funnel into the extraction flask, then use 30 mL±1 mL and about 65°C of hot xylene to wash the beaker and feed funnel in two portions, and also transfer them into the extraction flask.
  • 7.1.2 Immediately measure 25 mL±1 mL of anhydrous ethanol (or isopropanol) and 15 mL±1 mL of acetone to rinse the beaker, and transfer the solution through the feed funnel into the extraction flask.
  • 7.1.3 Turn on the heating switch of the extraction apparatus, heat at full speed until the solution begins to boil; then reduce the heating power so that the solution boils moderately, but ensure that the solution does not rush out of the flask and enter the condenser. To prevent danger from bumping of the solution and bursting of the glass flask, the fume hood sash shall be lowered throughout the heating process.
  • 7.1.4 After boiling the mixture vigorously for 2 min, stop heating. Wait until boiling stops, then immediately add 125 mL±1 mL of distilled water. Resume heating to make the solution boil and reflux for 15 min.
  • 7.1.5 Turn off the heater and allow the mixture to cool and separate into layers for 5 min to 10 min. Discharge the lower layer solution into a 250 mL conical flask, while filtering with qualitative filter paper.
• 7.2 Removal of hydrogen sulfide: In a fume hood, use a pipette to transfer 50 mL of the extract into a beaker, then add 5 mol/L of nitric acid solution 5 mL, and cover the beaker with a watch glass. Heat the beaker in the fume hood to make the solution in the beaker boil. Use lead acetate test paper to test for hydrogen sulfide in the vapor. After hydrogen sulfide is removed (the test paper does not change color), continue boiling for 5 min. After the solution cools, transfer it to the titration cell, and rinse the beaker with water; at the same time, also transfer the rinse liquid to the titration cell.
• 7.3 Determination of salt content:
  • 7.3.1 Accurately pipette 1 mmol/L of sodium chloride solution 10 mL into the titration cell, add an appropriate amount of acetone so that the total volume of the solution reaches about 150 mL. Then add 0.5 g of barium nitrate crystals.
    • Note: Adding equal amounts of sodium chloride solution to the blank and sample solutions can ensure that samples with low salt content can also obtain a titration endpoint.
  • 7.3.2 Place the titration cell on the stirrer platform of the titrator, turn on the stirrer, and stir to dissolve the barium nitrate crystals and mix the solution uniformly.
  • 7.3.3 Fill the burette with 0.01 mol/L of silver nitrate solution. Insert the electrode into the solution, with the tip of the dropper about 25 mm below the liquid surface.
  • 7.3.4 Perform titration using an automatic potentiometric titrator. After titration is completed, the instrument will display the volume of silver nitrate standard solution consumed by chloride ions in 50 mL of extract.
• 7.4 Determination of blank solution: Use a pipette to transfer 1 mmol/L of sodium chloride solution 10 mL into the titration cell, then add 50 mL of water, 5 mol/L of nitric acid 5 mL, and 0.5 g of barium nitrate crystals, and add acetone so that the total volume of the solution reaches 150 mL to 170 mL. Stir to dissolve the barium nitrate crystals, then titrate the blank solution.

9. Qualitative and quantitative determination and calculation formulas

• 8.1 Calculation: The salt content determination result is expressed as the mass fraction of sodium chloride w, with the value expressed in %, calculated according to formula (2):
  • w = [c×(V−V₀)×58.44×100] / (m×P×10³) (original text source-text extraction doubt: source-text extraction is shown as “w=9(V-YoX58.44×100 / mP X103”, formula symbols and structure require confirmation against the source standard (source-text extraction))
  • Where:
    • c— concentration of the standard silver nitrate solution, in moles per liter (mol/L);
    • V— volume of silver nitrate standard solution consumed in titrating the test portion, in milliliters (mL);
    • V₀—volume of silver nitrate standard solution consumed in titrating the blank test portion, in milliliters (mL);
    • m— mass of the test portion, in grams (g);
    • P— extraction ratio; if anhydrous ethanol is used, then P=50/158; if isopropanol is used, then P=50/152;
    • 58.44— molar mass of sodium chloride, in grams per mole (g/mol).
  • Note: If the water content in the test portion (determined according to method GB/T 8929) is less than 5%, and ethanol is used for extraction, the extract volume can be considered as 158 mL; if isopropanol is used, the extract volume can be considered as 152 mL. Therefore, the value of P is 50/158 or 50/152. However, if the water content in the test portion is greater than 5%, then the water content shall also be added to the calculation of the value of P.
• 8.2 Expression of results: Take the arithmetic mean of two duplicate determination results as the salt content of the test portion, and retain two significant figures in the result.

10. Precision

• 9.1 Repeatability: For determinations on the same test portion by the same operator, in the same laboratory, using the same instrument, the difference between two duplicate determination results shall not exceed the value calculated according to formula (3) (95% confidence level):
  • r = 0.024 3 × X^0.612 (original text source-text extraction doubt: source-text extraction is shown as “T=0.024 3 X0.612”, symbols and superscript require confirmation against the source standard (source-text extraction))
  • Where: X— arithmetic mean of two test results, %.
• 9.2 Reproducibility: For determinations on the same test portion by different operators, in different laboratories, using different instruments, the difference between two single and independent results shall not exceed the value calculated according to formula (4) (95% confidence level):
  • R = 0.0477 × X^0.612 (original text source-text extraction doubt: source-text extraction is shown as “R=0.0477 X0.612”, superscript requires confirmation against the source standard (source-text extraction))
  • Where: X— arithmetic mean of two test results, %.

11. Report

• 11 Test report: The test report shall at least give the following aspects:
  • a) All information required to identify the sample tested;
  • b) The standard used (including the year of publication);
  • c) Test results;
  • d) Differences from the specified analysis steps;
  • e) Abnormal phenomena observed during the test;
  • f) Test date.

12. Normative references

• GB/T 601 Chemical reagents — Preparation of standard titration solutions
• GB/T 4756 Petroleum liquids — Manual sampling (GB/T 4756—1998, eqv ISO 3170:1988)
• GB/T 6682 Water for analytical laboratory use — Specification and test methods (GB/T 6682-2008, ISO 3696:1987, MOD)
• GB/T 8929 Determination of water content in crude petroleum — Distillation method (GB/T 8929-2006, ISO 9029:1990, MOD)
• GB/T 11146 Determination of water content in crude petroleum — Karl Fischer coulometric titration method (GB/T 11146—2009, ISO 10337:1997, MOD)
• SY/T 5317 Petroleum liquids — Automatic pipeline sampling (SY/T 5317-2006, ISO 3171:1988 IDT)
• SY/T 7552 Crude petroleum — Determination of water — Karl Fischer potentiometric titration method (SY/T 7552-2005, ISO 10336:1997, IDT)

Technical differences adjusted from ASTM D6470-99 (2010):

• Replaced ASTM E200 with GB/T 601 (see Note 1 of 5.1.5);
• Replaced ASTM D4057 with GB/T 4756, which equivalently adopts an international standard (see 6.1);
• Replaced ASTM D1193 with GB/T 6682, which modifies and adopts an international standard (see 4.2);
• Replaced ASTM D4006 with GB/T 8929, which modifies and adopts an international standard (see Note of 8.1);
• Replaced ASTM D4928 with GB/T 11146, which modifies and adopts an international standard (see A.1);
• Replaced ASTM D4177 with SY/T 5317, which identically adopts an international standard (see 6.1);
• Replaced ASTM D4377 with SY/T 7552, which is identical to the international standard (see A.1).

13. Source and verification

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