GB/T 11146-2009 Determination of water content in crude petroleum—Karl Fischer coulometric titration method

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

• Standard number: GB/T 11146-2009
• Chinese title: Determination of water content in crude petroleum—Karl Fischer coulometric titration method
• English title: Crude petroleum—Determination of water—Coulometric Karl Fischer titration method
• Adoption relationship: Modified adoption of ISO 10337:1997 (MOD)
• Replacement relationship: Replaces GB/T 11146-1999 "Determination of water content in crude petroleum (Karl Fischer method)"
• Previous versions: GB/T 11146-1989, GB/T 11146-1999
• Publication/implementation: Published on 2009-03-16, implemented on 2009-10-01
• Custodian: National Technical Committee for Standardization of Petroleum and Natural Gas SAC/TC 355
• Proposed by: China National Petroleum Corporation
• Responsible drafting organization: Sinopec Research Institute of Petroleum Processing
• Participating drafting organizations: Daqing Oilfield Engineering Co., Ltd., Sinopec Zhenhai Refining & Chemical Company
• Main drafters: Wang Shuqing, Kang Wei, Yao Shuhua, Xiao Xuexi
• Main differences from ISO 10337:1997: In the formula note ρ of Chapter 8, according to the density standard metering temperature specified in China, the temperature was changed from "15 °C" to "20 °C"; this modification has no effect on the precision of the results
• Main revisions to GB/T 11146-1999: Coulometric titration method replaces volumetric titration method; detailed explanation of sulfide interference; added description of the mixer for sample treatment and the steps for mixing samples; modified measurement range, sample amount and precision according to ISO 10337:1997
• Nature of appendices: Appendix A is a normative appendix, Appendix B is an informative appendix

2. Scope of application and non-applicability

• This standard specifies a method for the direct determination of water content in crude petroleum by Karl Fischer coulometric titration (1.1).
• For crude petroleum with a content of thiol sulfur or divalent sulfide ion sulfur, or the total of both (mass fraction), in the range 0.005% to 0.05%, the determination range of this method for water (mass fraction) is 0.050% to 5.00% (1.2).
• For crude petroleum with a content of thiol sulfur or divalent sulfide ion sulfur, or the total of both (mass fraction), less than 0.005%, the determination range of this method for water (mass fraction) is 0.020% to 5.00% (1.2).
• Note 1: Large amounts of substances and various compounds that coagulate or undergo redox reactions can interfere with the determination of water content by Karl Fischer coulometric titration. However, the interfering substances in crude petroleum can only be thiols and ionic sulfides, and when the determination range of water (mass fraction) is 0.050% to 5.00%, sulfur interference of less than 0.05% (mass fraction) is very small.
• Note 2: For crude petroleum with a content of thiol sulfur or divalent sulfide ion sulfur, or the total of both (volume fraction), less than 0.05%, and when the water (volume fraction) is not within the range 0.050% to 5.00%, Appendix B provides an alternative test method using a volumetric measurement sample. For the limitations of using this optional volumetric measurement method, see Appendix B. If the content of the above interfering substances, thiol sulfur or divalent sulfide ion sulfur, or the total of both (volume fraction), is less than 0.005%, this volumetric measurement method can also be used to determine water content (volume fraction) in the range 0.020% to 5.00%.
• 1.3: The application of this standard may involve hazardous substances, operations and equipment, but this standard does not address all safety issues associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.
• Non-applicability: The source text does not explicitly list "non-applicability" clauses (except for the limitations of the volumetric measurement method described in Appendix B).

3. Method principle

After the crude petroleum is uniformly mixed by a mixer, a certain amount of sample is weighed and injected into the titration cell of a Karl Fischer coulometer. Iodine for the Karl Fischer coulometric reaction is generated by electrolysis at the anode. When all the water has been titrated, excess iodine is detected by the coulometric endpoint detector, and the titration ends. Based on the stoichiometric relationship of the reaction, 1 mole of iodine reacts with 1 mole of water. According to Faraday's law, the water content of the sample can be calculated. (Chapter 3)

4. Reagents and materials (purity / preparation)

• 4.1 Xylene, analytical reagent grade
• 4.2 Karl Fischer coulometric reagent: Use commercially available reagents that meet the performance requirements described in 7.3.
  • 4.2.1 Anolyte: Mix commercially available Karl Fischer coulometric anolyte and xylene uniformly in a volume ratio of 6:4. If the performance requirements of 7.3 are met, other ratios of Karl Fischer coulometric anolyte and xylene mixture may also be used.
  • 4.2.2 Catholyte: Use commercially available Karl Fischer coulometric catholyte.
• 4.3 Water, meeting the 3 grade requirements of ISO 3696:1987.

5. Apparatus and equipment

• 5.1 Automatic Karl Fischer coulometric titrator. Note: There are many commercially available instruments. The operating instructions for these devices are provided by the manufacturers and are not described here.
• 5.2 Closed mixer, meeting the homogenization efficiency requirements given in Appendix A. Note: Insertion-type mixers and circulation-type external mixers, such as those used with automatic crude petroleum sampling systems, may be used as long as they meet the requirements given in Appendix A.
• 5.3 Syringe: The sample is injected into the titration cell by a glass syringe; the length of the syringe needle should be such that after insertion through the septum, the tip can reach below the liquid surface of the anolyte. The needle bore should be as fine as possible, but not too fine, so as to avoid back pressure or blockage problems during sample injection.
  • Note 1: A suitable needle size is 0.5 mm to 0.8 mm.
  • Note 2: Recommended syringe sizes are: a) 10 μL, with a fixed needle, for injection of water during periodic performance testing of the titrator; b) 250 μL and 1 mL, for sampling of crude petroleum samples.
• 5.4 Balance: capable of weighing to 0.1 mg.
• 5.5 Thermometer: capable of measuring sample temperature to 1 °C.

6. Recommended operating conditions

• 7.1 Preparation of the instrument: Prepare and operate the instrument as required. Add anolyte to the outer chamber of the titration cell, with the amount added based on the height recommended by the manufacturer; add catholyte to the inner chamber, and the height of the catholyte should be lower than the anolyte by 2 mm to 3 mm. Seal all connections to the titration cell to prevent moisture from the atmosphere from entering. Turn on the titrator and stirrer, and titrate the residual moisture in the titration cell until the endpoint. Subsequent operations can only be carried out after the background current is stable and less than the maximum background current recommended by the instrument manufacturer.
• Note: If a high background current persists for too long, it may be caused by moisture on the inner wall of the titration cell. Gently shake the titration cell (or increase the stirring speed) to rinse the inner wall with the electrolyte. At the same time, check all devices to ensure that moisture from the atmosphere does not enter the titration cell. It is recommended to keep the titrator on to stabilize to a low background current.
• 7.3 Calibration test: With use, reagent performance will decrease, and routine monitoring should be carried out by accurately injecting 10 μL water. The recommended interval is from initially fresh reagent to after injection of 10 samples. The anolyte and catholyte should be replaced when any of the following occurs:
  • The titration result of injecting 10 μL water exceeds 10 000 μg±200 μg;
  • The background current remains high or unstable;
  • Phase separation occurs in the titration cell or the sample covers the electrode;
  • The total amount of sample in the titration cell exceeds one third of the anolyte volume;
  • The titrator displays an error message recommending replacement of the electrolyte.
• If the titration cell is contaminated by crude petroleum, thoroughly clean the anode chamber and cathode chamber with xylene. Acetone or other ketones must not be used for cleaning.
• Appendix A A.2.2: A high-shear mechanical mixer is inserted into the sample container, with the tip of its shaft positioned within 30 mm of the bottom of the container. Operation of the mixer at a shear rate of 15 000 min⁻¹ is usually suitable. Usually a mixing time of 3 min each time is sufficient. During mixing, an increase in temperature exceeding 10 °C should be avoided.
• Appendix A A.2.3: The circulation flow rate should be such that the sample in the container is circulated at least once per minute. The usual mixing time is 15 min.
• Appendix A A.4.4: Completion of sample transfer must not exceed 20 min.

7. Sampling and sample preparation

• 6.1 Summary: Sample according to all required steps specified, so as to obtain representative samples from pipelines, storage tanks or other systems, and place these samples in laboratory test containers.
• 6.2 Laboratory sample: Only representative samples obtained in accordance with GB/T 4756 or SY/T 5317 may be used for the tests of this standard.
• 6.3 Preparation of laboratory sample: In addition to 6.2, the following sample handling steps shall also be followed.
  • 6.3.1 Immediately record the temperature of the sample before mixing.
  • 6.3.2 Immediately mix the laboratory sample before analysis to ensure it is completely homogeneous. Mix the laboratory sample in the original container. Its mixing time, mixing power (rotational speed) and the position of the mixer relative to the bottom of the container shall ensure the execution of A.3.3 in Appendix A. The volume of crude petroleum and water content shall not exceed the maximum permitted value of A.3.3 in Appendix A.
  • 6.3.3 Immediately record the temperature of the laboratory sample after mixing. The temperature rise between this reading and the reading in 6.3.1 shall not exceed 10 °C, otherwise water loss may occur or the emulsion may become unstable.
• Appendix A (normative appendix) Sample handling:
  • A.1 Summary: A.1.1 The sample handling method between the sampling point and the laboratory bench or sample storage point shall ensure that the sample maintains its properties and integrity. A.1.2 The sample handling method depends on the purpose of the sample. The analytical procedure used in the laboratory often requires a dedicated handling step related to it. Therefore, reference shall be made to the appropriate test method to provide people with the necessary instructions on sample handling when sampling. If there are inconsistent requirements among several analytical procedures used, sample separately and apply the appropriate handling procedure to each sample. A.1.3 Special attention shall be given to the following situations: a) Liquids containing volatile substances may suffer evaporation loss; b) Liquids containing water and sediment or either of the two will separate in the sample container; c) Liquids in which wax deposition may occur will deposit if a sufficient temperature cannot be maintained. A.1.4 When preparing composite samples, special care shall be taken not to allow loss of light components in volatile liquids and not to change the content of water and sediment. This operation is very difficult and shall be avoided as far as possible. A.1.5 At the sampling site, do not transfer volatile liquid samples to other containers; instead, transport them to the laboratory in the original sample container, cooling and inverting if necessary. If a sample container contains both volatile components and free water, handle it with even greater care.
  • A.2 Homogenization of the sample: A.2.1 Before transferring the sample from the sampling container to a smaller container, or into the laboratory test instrument, a sample homogenization step is specified because the sample may contain water and sediment or be in another non-uniform state. For the procedure to check whether the sample has been mixed uniformly before transferring the sample, see Chapter A.3. For liquid samples containing water and sediment, manual mixing cannot sufficiently disperse the water and sediment in the sample. Before sample transfer and subsampling, vigorous mechanical or hydraulic mixing is required to make the sample uniform. Sample homogenization can be accomplished by various methods. Regardless of the method, the recommended homogenization system produces droplets smaller than 50 μm, but not smaller than 1 μm. Droplets smaller than 1 μm will form a stable emulsion, and the water content cannot be determined by centrifugation. A.2.2 Homogenize with a high-shear closed mechanical mixer (see Chapter 6 for recommended operating conditions). A.2.3 Circulate with a closed-loop mixing system (see Chapter 6 for recommended operating conditions).
  • A.3 Determination of mixing time: A.3.1 If the sample remains uniform and stable after mixing (for example, completely miscible components, such as blended lubricating oil additives), continue mixing until samples taken successively give the same mixing result. This is the minimum mixing time. Note: After this mixing, the sample is uniform, and after a portion is removed and transferred, it can still remain uniform without further mixing. A.3.2 If the mixed sample no longer remains uniform after a relatively short period of time (for example, water and sediment as part of the mixture), determine the mixing time according to the special method described in A.3.3. Note: Due to the characteristics of crude oil, it is necessary to obtain subsamples while mixing is in progress. A.3.3 Ensure that the extracted sample is injected into a pre-weighed container to about three-quarters full, and homogenize the sample, recording the mixing time, mixer speed, and the position of the mixer in the container relative to the bottom of the container. After mixing, draw duplicate test portions and immediately determine their water content according to a standard method (see A.3.4). If the test results are very consistent, within the repeatability of the method, record the average value obtained as the blank water content. If the test cannot obtain consistent duplicate results, repeat the operation with a longer mixing time or a faster mixing speed. Weigh the sample container again and calculate the crude oil mass. Add an accurately measured amount of water to raise the water content to about 2% (mass fraction). Homogenize the sample as a blank using the same mixing time, mixer speed, and relative position of the mixer to the bottom of the container. After mixing, take duplicate test portions and immediately determine their water content according to the standard method. If the results conform to the repeatability of the method, and the measured water content is consistent with the known total amount of water (the added water plus the known amount of water in the blank), record the appropriate mixing conditions. From the end of mixing to the taking of the second test portion, the sample has been uniform and stable. If the results do not conform to the repeatability range of the method, these results shall be discarded. Repeat the mixing step of A.3.3, using a longer mixing time and a faster mixing speed, or changing one of them. A.3.4 The water content cannot be determined by centrifugation (ISO 3734) or distillation (ISO 3733) to determine this mixing system, because these methods cannot determine the total water content.
  • A.4 Transfer of the sample: A.4.1 If the sample container is not portable, or it is not convenient to extract the sample directly from the sample container into the laboratory test instrument, a representative test portion may be transferred to the laboratory by means of a portable sample container. A.4.2 At each stage of sample transfer, each step used for the sample shall be carried out in accordance with A.2. The sample in the sample container must be uniform. A.4.3 According to the method specified in A.3, determine the mixing time for each set of container and mixer combination. A.4.4 During the completion of sample transfer, the mixture shall already be uniform and stable. This period is very short, and the completion of sample transfer shall not exceed 20 min.

8. Analytical procedure

• 7.1 Preparation of the instrument: see Chapter 6.
• 7.2 Test portion:
  • 7.2.1 Mix the sample as described in 6.3.1～6.3.3. Take a clean, dry syringe of suitable capacity (see Table 1), and rinse it at least 3 times with the mixed sample to be tested. Immediately draw an appropriate amount of sample, wipe the needle clean with filter paper, and accurately weigh the syringe and its contents to within 0.1 mg. Insert the syringe into the titration cell, start the titration, and inject the entire test portion with the needle below the liquid surface. Remove the syringe, wipe the needle clean, reweigh the syringe to within 0.1 mg, and record the sample amount. After reaching the titration endpoint, record the water content titration value displayed by the titrator.
  • Table 1 Sample amount depending on expected water content:

• Note: If the water content of the laboratory sample is completely unknown, it is best to start the test with a small test portion to avoid excessively long titration time and excessive reagent consumption. Further adjustment of the test portion may be made if necessary.
• 7.2.2 When the background current drops to the stable reading displayed by 7.1, take another test portion for duplicate determination. During the re-extraction and injection of the sample, ensure that the sample is uniform and stable in accordance with A.3.3 in Appendix A. The repeatability of the determination shall conform to the repeatability requirements specified in 10.1.1.
• 7.2.3 Fill the 10 μL syringe with water and expel air bubbles, wipe the needle clean with filter paper, insert the syringe needle below the surface of the electrolyte that has reached the endpoint, quickly inject the water, and titrate the water until a stable endpoint is reached and maintained for at least 30 s. Do not shake the titration cell after adding water. Record the water content determination result displayed when the endpoint is reached.
• 7.3 Calibration test: see Chapter 6.
• Appendix B (Informative appendix) Optional test method for measuring the test portion by volume:
  • B.1 Summary: This optional test method contains the method for measuring the water content of crude oil described in this standard, except that the portion of the crude oil test portion injected into the titration cell is measured by volume. The limitations of this optional test method are as follows: prior agreement of both parties is obtained; the vapor pressure and viscosity of the crude oil will not affect the accurate measurement of the test portion volume.
  • B.2 Principle: The principle is the same as that described in Chapter 3, except that the test portion is measured by volume rather than by mass. The operating procedure of this standard is also followed, and the differences are given below.
  • B.3 Other interferences: The appearance of air bubbles in the syringe will become a source of interference, resulting in low results. The tendency of crude oil to form bubbles is related to the type, condition, and corresponding vapor pressure of the crude oil. Crude oil that can be confirmed to be viscous is difficult to measure with a precision syringe.
  • B.4 Apparatus: Syringes: capacities of 250 μL, 500 μL, and 1 mL, accurate to 2 μL, 2 μL, and 10 μL, respectively.
  • B.5 Operating procedure: Immediately after mixing, add the crude oil test portion to the titration cell. The operating procedure is as shown in 6.3.1～6.3.3, and the method of adding the test portion is as follows. Use a clean, dry syringe of suitable capacity (see Table B.1) to draw at least three test portions and discard them as waste. Immediately draw one test portion, wipe the needle clean with filter paper, and record the volume of the test portion taken, accurate to 2 μL or 10 μL (see B.4). Insert the syringe into the titration cell, start the titration, and keep the needle below the liquid surface to inject the test portion completely. After reaching the titration endpoint, record the water content titration value displayed by the titrator.
  • Table B.1 Sample amount depending on expected water content:

• Note: If the water content of the laboratory sample is completely unknown, it is best to start the test with a small test portion to avoid excessively long titration time and excessive reagent consumption.

9. Qualitative and quantitative determination and calculation formulas

• Chapter 8 Calculation: Calculate the water content w according to formula (1), expressed as mass fraction:
  • w = 100 × m₂ / (m × 10⁶) = 10⁻⁴ × m₂ / m …… (1)
  • Where: w— water content (mass fraction), %; m— test portion mass, in grams (g); m₂—the mass of water displayed by the titrator, in micrograms (μg).
  • Note: The original formula (1) source-text extraction appears as "W= 100xml / mX100 / m2 / -i0'xm", which contains noise; the above formula has been organized according to context and dimensions, requires confirmation against the source standard (source-text extraction).
• If the result is additionally required to be expressed as a volume fraction, calculate using formula (2):
  • φ = w × ρ …… (2)
  • Where: φ— water content (volume fraction), %; w— water content (mass fraction), %; ρ— density of the crude oil sample at 20 °C, in kilograms per cubic meter (kg/m³).
  • Note: The original formula (2) source-text extraction appears as "¢=WXP", requires confirmation against the source standard (source-text extraction).
• Chapter 9 Expression of results:
  • If the water content (mass fraction) is less than 1.00%, report the water content of the sample as a mass fraction, to the nearest 0.001%.
  • If the water content (mass fraction) is between 1.00% and 5.00%, report the water content of the sample as a mass fraction, to the nearest 0.01%.
• Annex B B.6 Calculation: Calculate the water content φ according to formula (B.1), expressed as a volume fraction:
  • φ = 100 × V₂ / (V₁ × 10⁶) = 10⁻⁴ × V₂ / V₁ …… (B.1)
  • Where: φ— water content of the test sample (volume fraction), %; V₁—volume of the test sample, in milliliters (mL); V₂—volume of titrated water, in nanoliters (nL) (equivalent to the mass of water displayed in micrograms on the titrator, i.e., the mass of water in 1 nL is 1 μg).
  • Note: The original formula (B.1) source-text extraction appears as "9=i00xV, X100 / V2 / 9= io'Xv", which contains noise; requires confirmation against the source standard (source-text extraction).
• Annex B B.7 Expression of results: Report the water content of the sample as a volume fraction, to the nearest 0.01%.

10. Precision

• Chapter 10 Precision: The precision of this test method is determined by statistical examination of test results from multiple laboratories, and the reliability of test results is judged according to the following provisions (95% confidence level).
• 10.1 Repeatability, r: For the same operator using the same instrument, under unchanged operating conditions, and following the test method, the difference between successive test results obtained shall not exceed the value obtained from formula (3) (see Table 2).
  • r = 0.040 X^(2/3) …… (3)
  • X is the average of results with mass fractions in the range 0.020% to 5.00%.
• 10.2 Reproducibility, R: For different operators in different laboratories, using the same test material, and following the test method, the difference between two separate and independent results obtained shall not exceed the value obtained from formula (4) (see Table 2).
  • R = 0.105 X^(2/3) …… (4)
  • Note: The original formula (4) source-text extraction appears as "R=0.105X2/8", the exponent is suspected to be 2/3, requires confirmation against the source standard (source-text extraction).
  • X is the average of results with mass fractions in the range 0.020% to 5.00%.
• Table 2 Precision intervals:

• Annex B B.8 Precision: The precision of this test method is determined by statistical examination of test results from multiple laboratories, and the reliability of test results is judged according to the following provisions (95% confidence level).
  • B.8.1 Repeatability, r: For the same operator using the same instrument, under unchanged operating conditions, and following normal and correct test methods, the difference between successive test results obtained shall not exceed the value obtained from formula (B.2) (see Table B.2).
    • r = 0.056 X^(2/3) …… (B.2)
    • Note: The original formula (B.2) source-text extraction appears as "r= 0.056X23", the exponent is suspected to be 2/3, requires confirmation against the source standard (source-text extraction).
    • X is the average of results with volume fractions in the range 0.020% to 5.00%.
  • B.8.2 Reproducibility, R: For different operators in different laboratories, using the same test material, and following normal and correct test methods, the difference between two separate and independent results obtained shall not exceed the value obtained from formula (B.3) (see Table B.2).
    • R = 0.112 X^(2/3) …… (B.3)
    • Note: The original formula (B.3) source-text extraction appears as "R= 0.112X2/3", the exponent is suspected to be 2/3, requires confirmation against the source standard (source-text extraction).
    • X is the average of results with volume fractions in the range 0.020% to 5.00%.
• Table B.2 Precision intervals (the original source-text extraction table data is misaligned; the following is organized according to the original numbers, requires confirmation against the source standard (source-text extraction)):

• Note: The original source-text extraction of Table B.2 appears as two columns of stacked numbers ("0.004 / 0.008 / 0.008 / 0.01 / 0.03 …"), which cannot be reliably matched to each row; the above table is filled in row order, requires confirmation against the source standard (source-text extraction).

11. Report

• Chapter 11 Test report: The test report shall include at least the following information:
  • a) Reference to this standard;
  • b) Type and complete identification of the test product;
  • c) Test results (see Chapter 9);
  • d) Mixer model, mixer speed, mixing time, and mixing position relative to the bottom of the sample container;
  • e) Temperature of the sample before and after mixing;
  • f) Any differences from the specified procedure as agreed or otherwise;
  • g) Date of test.
• Annex B B.9 Test report: When the test sample is measured by volume, Annex B needs to be added to the reference of this standard. If this is not done, any data used for legal and contractual purposes shall be invalid.

12. Normative references

• GB/T 4756—1998 Petroleum liquids—Manual sampling (eqv ISO 3170:1988)
• SY/T 5317—2006 Petroleum liquids—Automatic pipeline sampling (ISO 3171:1988, IDT)
• ISO 3696:1987 Water for analytical laboratory use—Specification and test methods
• ISO 3733:1999 Petroleum products and bituminous materials—Determination of water—Distillation method
• ISO 3734:1997 Petroleum products—Determination of water and sediment in residual fuel oils—Centrifuge method
• ISO 10337:1997 Determination of water in crude oil—Karl Fischer coulometric titration method (English version) (this standard is modified adoption)

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14. Quick reference for questions and answers