GB/T 4945-2002 Petroleum products and lubricants — Determination of acid number and base number — Color indicator method

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Card type: Test method

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

• Standard number: GB/T 4945-2002
• Chinese full title: Petroleum products and lubricants — Determination of acid number and base number — Color indicator method
• English title: Standard test method for acid and base number of petroleum products and lubricants by Colour-indicator titration
• Replacement relationship: replaces GB/T 4945-1985(1991) "Petroleum products and lubricants — Determination of neutralization number — Color indicator method"
• Adoption relationship: equivalent adoption of ASTM D974-1997 "Standard test method for acid and base number by color-indicator titration"
• Issued/implemented: issued on 2002-01-14, implemented on 2002-08-01
• Issuing body: General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China
• Technical committee: Sinopec Research Institute of Petroleum Processing
• Drafting organization: Lanzhou Lubricating Oil Research and Development Center, Lubricant Branch, PetroChina Company Limited
• Main drafters: Li Suping, Zhou Yabin, Gao Jun
• First issued: 1985

2. Scope of application and non-applicability

• 1.1 is applicable to the determination of acidic or basic components of petroleum products and lubricants that are completely or almost completely soluble in a mixed solvent of toluene and isopropanol; it is applicable to the determination of acids or bases whose dissociation constant in water is greater than 10⁻⁹ (the source text at source-text extraction reads "10-,", requires confirmation against the source standard (source-text extraction)); extremely weak acids or bases whose dissociation constant is less than 10⁻⁹ (the source text at source-text extraction reads "10"", requires confirmation against the source standard (source-text extraction)) do not interfere with the determination; if the hydrolysis constant of salts is greater than 10⁻⁹ (the source text at source-text extraction reads "10 s", requires confirmation against the source standard (source-text extraction)), then there is interference.
• Note 1: In new oils and used oils, components considered acidic include organic acids, inorganic acids, esters, phenolic compounds, lactones, resins, heavy metal salts, and additives such as antioxidants and detergents; components considered basic include organic bases, inorganic bases, amine compounds, salts of weak acids (soaps), basic salts of polybasic acids, heavy metal salts, and additives such as antioxidants and detergents.
• Note 2: This method is not applicable to the determination of the base number of lubricating oils containing multiple types of alkaline additives; the base number of such oils can be determined by the test method of SH/T 0688.
• 1.2 can be used to indicate the relative change of oils in service under oxidizing conditions; it cannot determine absolute acidity and basicity to predict the performance of oils under service conditions; there is no necessary connection between acid number or base number and bearing corrosion.
• Note: For certain oils, such as cutting oils, rust-preventive oils and similar compounded oils, or dark-colored oils, because the endpoint shown by the indicator is not obvious, the acid number cannot be determined by this method and can be determined by the test method of GB/T 7304. The acid number determined by the color indicator method is not necessarily numerically equal to the result of GB/T 7304; the base number determined by the color indicator method is also not necessarily numerically equal to that of SH/T 0688, but the data have a certain correlation.

3. Method principle

• Chapter 4 Method summary: When determining acid number or base number, the sample is dissolved in a mixed solvent of toluene and isopropanol containing a small amount of water to make it a homogeneous system, and at room temperature it is titrated with a standard alcoholic solution of base or acid, respectively. The endpoint is indicated by the color change of added p-naphtholbenzein solution (orange in acidic solution, dark green in basic solution). When determining strong acid number, the sample is extracted with hot water, and the aqueous extract is titrated with standard alcoholic potassium hydroxide solution using methyl orange as indicator.

4. Reagents and materials (purity / preparation)

• 6.1 Hydrochloric acid: analytical reagent, relative density 1.19.
• 6.2 Potassium hydroxide: analytical reagent.
• 6.3 Isopropanol: analytical reagent.
• 6.4 Distilled water: meets the grade 3 water specification of GB/T 6682.
• 6.5 Potassium hydrogen phthalate: primary standard reagent.
• 6.6 Barium hydroxide: analytical reagent.
• 6.7 Toluene: analytical reagent.
• 8.1.1 Standard hydrochloric acid isopropanol solution (0.1 mol/L): mix 9 mL hydrochloric acid with 1000 mL isopropanol. Accurately measure 0.1 mol/L standard potassium hydroxide isopropanol solution 8 mL, dilute with 125 mL carbon dioxide-free water, use this solution as titrant, and calibrate by potentiometric titration, calibrating frequently to ensure the calibration error is not greater than 0.0005 mol/L.
  • Note 1: Hydrochloric acid is corrosive, and its fumes are irritating.
  • Note 2: To simplify calculation, both the potassium hydroxide solution and the hydrochloric acid solution can be corrected, that is, 1.00 mL hydrochloric acid is equivalent to 5.00 mg potassium hydroxide. Sodium hydroxide solution and sulfuric acid solution can be used to replace potassium hydroxide solution and hydrochloric acid solution, respectively.
• 8.1.2 Methyl orange indicator solution: dissolve 0.1 g methyl orange in 100 mL water.
• 8.1.3 p-Naphtholbenzein indicator solution: p-naphtholbenzein must comply with the provisions of Appendix A. Prepare a 10 g/L±0.01 g/L p-naphtholbenzein solution in the titration solvent.
• 8.1.4 Standard potassium hydroxide isopropanol solution (0.1 mol/L): add 6 g solid potassium hydroxide to a 2 L flask containing nearly 1 L isopropanol solution. Allow the mixture to boil slowly for 10 min～15 min, and stir to prevent the solid from caking at the bottom of the flask. Add at least 2 g barium hydroxide, then allow it to boil slowly for 5 min～10 min, cool to room temperature and let stand for several hours, filter the supernatant through a fine sintered fritted glass funnel or porcelain funnel, avoiding prolonged exposure to carbon dioxide during filtration. Store the solution in a chemically resistant reagent bottle without a cork stopper, without a rubber stopper, or without a saponifiable grease stopper, and protect it with a tube containing ascarite or alkaline non-fibrous silicate desiccant.
  • Calibration of potassium hydroxide solution: calibrate frequently to detect changes in 0.0005 mol/L. The method is as follows: weigh about 0.2 g (weighed to 0.1 mg) of potassium hydrogen phthalate dried in 110℃±1℃ for at least 1 h, dissolve it in 40 mL±1 mL carbon dioxide-free water, and titrate with potassium hydroxide isopropanol solution to any one of the following endpoints: a) when calibrating by potentiometric titration, titrate to a well-defined inflection point whose potential value matches that of a basic buffer solution; b) when titrating by the color indicator method, add 6 drops of phenolphthalein indicator solution and titrate until a persistent pink color appears. Perform a blank titration on the water used to dissolve the potassium hydrogen phthalate.
  • The concentration M(mol/L) of the standard potassium hydroxide isopropanol solution is calculated according to formula (1): M = (W × 1000) / [(V − Vb) × 204.23] Where: W—— mass of potassium hydrogen phthalate, g; 204.23—— molecular weight of potassium hydrogen phthalate; V—— amount of titrant required to titrate the salt to the specified endpoint, mL; Vb—— amount of titrant required to titrate the blank, mL.
  • Note 1: Potassium hydroxide is highly corrosive to human tissue.
  • Note 2: Barium hydroxide is strongly alkaline, can irritate the skin and cause inflammation, and is toxic if inhaled.
  • Note 3: Because organic solutions such as isopropanol have a relatively large volume expansion coefficient, standard alcoholic solutions must be calibrated at a temperature close to that used when titrating the sample.
• 8.1.5 Phenolphthalein indicator solution: dissolve 0.1 g±0.01 g pure solid phenolphthalein in 50 mL ethanol and 50 mL carbon dioxide-free water.
• 8.1.6 Titration solvent: add 500 mL toluene and 5 mL water to 495 mL anhydrous isopropanol.
  • Note: Toluene vapor is toxic and flammable; keep away from heat sources, sparks and flames.

5. Apparatus and equipment

• 6.1 Burette: 50 mL, with a graduation of 0.1 mL; or 10 mL, with a graduation of 0.05 mL.
  • Note: An automatic burette whose delivered titrant amount can be controlled to 0.05 mL or lower may be used, but the precision data listed in this standard are only applicable to manual burettes.
• 6.2 Erlenmeyer flask: 250 mL or 500 mL.
• 6.3 Separatory funnel: 250 mL.
• 6.4 Graduated cylinder: 10 mL, 100 mL, 1000 mL.
• 6.5 Sintered fritted glass funnel or porcelain funnel.
• Appendix A2.5 Potentiometer, glass electrode, calomel electrode, stirrer, beaker and workbench shall comply with the provisions of method GB/T 7304.

6. Recommended operating conditions

• Titration temperature: acid number determination 9.2 specifies titration below 30℃; base number determination 10.1 specifies titration below 30℃.
• Heating temperature for preparation of used oil samples: 8.2.2 specifies heating to 60℃±5℃.
• Drying conditions for potassium hydrogen phthalate: dry in 110℃±1℃ for at least 1 h.
• Appendix A2.8.1 Potentiometer calibration temperature: 25℃±2℃.
• Appendix A2.8.2 Alkaline non-aqueous buffer solution calibration temperature: 25℃±2℃.

7. Sampling and sample preparation

• 8.2.1 Sampling shall be carried out strictly as described in 8.2.2, because used oils often contain sediments, some of which are acidic or basic, and some of which have adsorbed acidic or basic components from the sample. A sample that is not representative will cause relatively large errors.
• 8.2.2 Heat the used oil sample in the original container to 60℃±5℃, and stir until all sediment is uniformly distributed in the oil. If the original container is not transparent, or the sample amount exceeds three-quarters of its volume, transfer all the sample to a transparent glass bottle whose volume is at least one-third larger than the sample volume, and vigorously stir the sample in the original container so that all sediment in the original container is transferred to the bottle. After all sediment is completely suspended, filter the sample or a portion of the sample through a sieve of 154 μm(100 mesh to remove large contaminating particles.
  • Note 1: When there is no obvious sediment in the sample, the heating step and the filtration step may be omitted.
  • Note 2: Because used oil is prone to change during storage, after sampling from the lubrication system it should be analyzed as soon as possible, and the sampling date and test date should be noted.
• Table 1 Sample amount:
  • New oil or light-colored oil:
    • Acid number or base number 0.0～3.0 mgKOH/g: sample amount 20.0 g±2.0 g, weighing accuracy 0.05 g
    • Acid number or base number >3.0～25.0 mgKOH/g: sample amount 2.0 g±0.2 g, weighing accuracy 0.01 g
    • Acid number or base number >25.0～250.0 mgKOH/g: sample amount 0.2 g±0.02 g, weighing accuracy 0.001 g
  • Used or dark-colored oil products:
    • Acid number or base number 0.0～25.0 mgKOH/g: sample amount 2.0 g±0.2 g, weighing accuracy 0.01 g
    • Acid number or base number >25.0～250.0 mgKOH/g: sample amount 0.2 g±0.02 g, weighing accuracy 0.001 g
  • Note: Light-colored oil products have a low acid number, so weighing 20 g of sample is permitted to obtain more accurate results; the sample amount of dark-colored oil products must be limited to the specified weighing range to reduce interference from dark color.
• 11.1 Strong acid number determination: In a 250 mL separatory funnel, add 25 g of a representative sample, accurate to 0.1 g.

8. Analytical procedure

• 9 Procedure for determining acid number:
  • 9.1 In a 250 mL conical flask, weigh the sample amount according to the provisions of Table 1, add 100 mL of titration solvent and 0.5 mL of p-naphtholbenzein indicator solution, and shake continuously until the sample is completely dissolved by the titration solvent. If the mixture is orange-yellow, proceed according to step 9.2; if it turns green or dark green, proceed according to Chapter 10.
  • 9.2 Immediately below 30℃, gradually add 0.1 mol/L of potassium hydroxide isopropanol standard solution for titration. Near the endpoint, shake vigorously, but avoid carbon dioxide entering the solution (in the case of acidic oil, the orange color turning dark green indicates approach to the endpoint). If the color change persists for 15 s or two drops of 0.1 mol/L of hydrochloric acid isopropanol standard solution cause the color to return, the endpoint is considered to be reached.
    • Note: To observe the titration endpoint of dark-colored oil products, when the color changes after adding the last few drops of standard solution, shake the bottle vigorously to produce a small amount of foam, and observe the titrated liquid under a white fluorescent lamp on the laboratory bench.
  • 9.3 Blank test: In 100 mL of titration solvent and 0.5 mL of p-naphtholbenzein indicator solution, gradually add 0.1 mol/L of potassium hydroxide isopropanol standard solution in an amount of 0.1 mL or less. Record the volume of standard solution required to reach the endpoint.
    • Note: The titration solvent usually contains weak acid impurities, which react with the strong base components in the sample. To correct the base number of the sample, the blank acid number in the titration solvent should be determined.
• 10 Procedure for determining base number:
  • 10.1 If the titration solution containing the sample appears green or dark green after adding p-naphtholbenzein indicator solution, immediately below 30℃ titrate with 0.1 mol/L of hydrochloric acid isopropanol standard solution until the dark green changes to orange.
  • 10.2 Blank test: Carry out the blank titration as described in 9.3.
• 11 Procedure for determining strong acid number:
  • 11.1 In a 250 mL separatory funnel, add 25 g of a representative sample, accurate to 0.1 g, and add 100 mL of boiling water, and shake vigorously. After separation, put the aqueous phase into a 500 mL conical flask, extract the sample with boiling water two or more times, using 50 mL each time, and add all extracts to the conical flask. Add 0.1 mL of methyl orange indicator solution to the extracted solution. If the solution turns pink or red, titrate with 0.1 mol/L of potassium hydroxide isopropanol standard solution until the solution turns yellow. If the initial color of the solution is not pink or red, report the strong acid number as zero.
  • 11.2 Blank test: In a 250 mL conical flask, after adding the same amount of boiling water 200 mL and 0.1 mL of methyl orange indicator solution as used when titrating the sample, if the solution color is orange-yellow, titrate with 0.1 mol/L of hydrochloric acid isopropanol standard solution to the same color as the sample when the indicator was added. If the color is pink or red, titrate with 0.1 mol/L of potassium hydroxide isopropanol standard solution to the same color as the sample at the titration endpoint.
• 12 Quality control check: By analyzing a quality control (QC) sample, ensure that the equipment or operation used each day is normal. For example, when appropriate, after a batch of samples has been tested or after a fixed number of samples have been tested, analyze an additional QC sample to ensure the accuracy of the analytical results. The analytical results obtained from the QC sample can be processed using control chart techniques. When the test result obtained from a QC sample exceeds the limit value of the test method, the instrument and equipment need to be recalibrated. The QC sample must correspond to the purpose of use and must be homogeneous and stable under storage conditions. If possible, the QC sample should preferably be representative of the samples being analyzed, and before checking the equipment, its average value and control limit values should be determined. The precision of the QC sample must be similar to the precision and bias given in this method in order to prove that the instrument is operating normally.
  • Note: Because the acid number or base number of the QC sample changes during storage, when an out-of-control state occurs, the stability of the QC sample may be a source of error.

9. Qualitative and quantitative determination and calculation formulas

• 13.1 The acid number of the sample (mgKOH/g) is calculated according to formula (2): Acid number = [(A − B)M × 56.1] / W Where: A—— the amount of potassium hydroxide isopropanol standard solution required to titrate the sample (9.2), mL; B—— the amount of potassium hydroxide isopropanol standard solution required to titrate the blank (9.3), mL; M—— the concentration of potassium hydroxide isopropanol standard solution, mol/L; W—— sample amount, g; 56.1—— the molar mass of potassium hydroxide, g/mol.
• 13.2 The strong acid number of the sample (mgKOH/g) is calculated according to formula (3) or formula (4):
  • 13.2.1 If acid is used to titrate the blank: Strong acid number = [(CM + Dm) × 56.1] / W (3) Where: C—— the amount of potassium hydroxide isopropanol standard solution required to titrate the water extract of the sample (11.1), mL; M—— the concentration of potassium hydroxide isopropanol standard solution, mol/L; D—— the amount of hydrochloric acid isopropanol standard solution required to titrate the blank solution (11.2), mL; m—— the concentration of hydrochloric acid isopropanol standard solution, mol/L; W—— sample amount, g; 56.1—— the molar mass of potassium hydroxide, g/mol.
  • 13.2.2 If base is used to titrate the blank: Strong acid number = [(C − D)M × 56.1] / W (4) Where: C—— the amount of potassium hydroxide isopropanol standard solution required to titrate the water extract of the sample (11.1), mL; D—— the amount of potassium hydroxide isopropanol standard solution required to titrate the blank (11.2), mL; M—— the concentration of potassium hydroxide isopropanol standard solution, mol/L; W—— sample amount, g; 56.1—— the molar mass of potassium hydroxide, g/mol.
• 13.3 The base number of the sample (mgKOH/g) is calculated according to formula (5): Base number = [(Em + FM) × 56.1] / W (5) Where: E—— the amount of hydrochloric acid isopropanol standard solution required to titrate the sample (10), mL; m—— the concentration of hydrochloric acid isopropanol standard solution, mol/L; F—— the amount of potassium hydroxide isopropanol standard solution required to titrate the blank acid number, mL; M—— the concentration of potassium hydroxide isopropanol standard solution, mol/L; W—— sample amount, g; 56.1—— the molar mass of potassium hydroxide, g/mol.
• Definitions:
  • 3.1 Acid number acid number: the amount of base required to titrate 1 g of sample to the specified endpoint, expressed as mgKOH/g.
  • 3.2 Base number base number: the amount of acid required to titrate 1 g of sample to the specified endpoint, expressed as mgKOH/g.
  • 3.3 Strong acid number strong acid number: with methyl orange solution as indicator, the amount of base required to titrate the hot water extract of 1 g of sample to the yellow endpoint, expressed as mgKOH/g.
  • 3.4 Used oil used oil: oil in operating or non-operating equipment components (for example, oil in engines, gearboxes, transformers, or turbines).

10. Precision

• 14.1 Precision: The reliability of test results shall be judged according to the following provisions (95% confidence level).
• 14.1.1 Repeatability: The difference between two results obtained by the same operator in the same laboratory using the same instrument on the same test sample in repeated determinations shall not exceed the following values:
  • Acid (strong acid) value or base value 0.00～0.1 mgKOH/g: Repeatability 0.03 mgKOH/g
  • >0.1～0.5 mgKOH/g: Repeatability 0.05 mgKOH/g
  • >0.5～1.0 mgKOH/g: Repeatability 0.08 mgKOH/g
  • >1.0～2.0 mgKOH/g: Repeatability 0.12 mgKOH/g
• 14.1.2 Reproducibility: The difference between two individual results obtained by different operators in different laboratories on the same test sample shall not exceed the following values:
  • Acid (strong acid) value or base value 0.00～0.1 mgKOH/g: Reproducibility 0.04 mgKOH/g
  • >0.1～0.5 mgKOH/g: Reproducibility 0.08 mgKOH/g
  • >0.5～2.0 mgKOH/g: Reproducibility 15% of the average value (the original source-text extraction is "1500", requires confirmation against the source standard (source-text extraction))
• Note 1: These precision values do not apply to those dark-colored samples in which the color change at the endpoint cannot be observed (the source term for the samples is unclear and requires verification against the original standard).
• Note 2: The precision of this method applies only to new, light-colored mineral oils without additives and new or used turbine oils with inhibitors added.
• Note 3: The precision listed is obtained only with a manual burette and may or may not be the same as results obtained using an automatic burette.
• 14.2 Bias: This method has no bias, because acid value and base value can only be determined by this method.

11. Report

• 15 Report: Take the arithmetic mean of two results from repeated determinations as the test result.

12. Normative references

• GB/T 6682 Water for analytical laboratory use—Specification and test methods
• GB/T 7304 Determination of acid number of petroleum products and lubricants (potentiometric titration method)
• SH/T 0688 Determination of base number of petroleum products and lubricants (potentiometric titration method)
• ASTM D974-1997 (equivalent adoption source)

13. Source and verification

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14. Quick Q&A reference