GB/T 258-2025 Determination of acidity of light petroleum products

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

• Standard number: GB/T 258—2025
• Chinese full title: Determination of acidity of light petroleum products
• English title: Determination of acidity of light petroleum products
• Replacement relationship: replaces GB/T 258—2016 "Determination of acidity of light petroleum products"
• First publication: first published in 1977, first revision in 2016, this is the second revision
• Publication/implementation: published on 2025-10-05, implemented on 2026-05-01
• Custodian: National Technical Committee for Standardization of Petroleum Products and Lubricants (SAC/TC 280)
• Main technical changes compared with the 2016 version (foreword): a) changed the scope (see Chapter 1, Chapter 1 of the 2016 version); b) added content related to automatic instruments (see Chapter 1, 6.1, 9.3, 11.2); c) added content related to quality control (see 5.7, Chapter 10); d) added precision for automatic titration (see 13.2); e) added a chapter "Test report" (see Chapter 14); f) added Annex A (normative) "Fully automatic acidity tester for light petroleum products" (see Annex A).

2. Scope of application and non-applicability

• This document describes a method for determining the acidity of light petroleum products using the color indicator method.
• Applicable to light petroleum products with obvious color change of the indicator at the titration endpoint, such as gasoline, naphtha, kerosene, jet fuel, diesel, etc.
• Automatic instruments are only applicable to the determination of acidity of diesel products; the determination range of automatic titration acidity (calculated as KOH) is 1.00 mg/100mL～12 mg/100mL.
• Samples outside this content range can also be determined using this document, but the precision has not been investigated.
• Not applicable: no explicit "not applicable" clause is provided in the available source.

3. Method principle

The acidic substances in light petroleum products are extracted with ethanol, and in the presence of a color indicator, titrated with potassium hydroxide ethanol standard titration solution. (See Chapter 4 Method summary)

4. Reagents and materials (purity / preparation)

• 5.1 95% Ethanol: analytical reagent.
• 5.2 Potassium hydroxide: analytical reagent.
• 5.3 Hydrochloric acid: analytical reagent.
• 5.4 Cresol red.
• 5.5 Alkali blue 6B.
• 5.6 Phenolphthalein.
• 5.7 Quality control (QC) sample: should be similar in composition to the test sample, and its average value and control limits have been determined.

Preparation and standardization (see Chapter 8 Test preparation):

• 8.1 0.05 mol/L Preparation and standardization of potassium hydroxide ethanol standard titration solution shall be carried out in accordance with 4.6 in SH/T 0079—1991.
• 8.2 0.05 mol/L Preparation and standardization of hydrochloric acid standard titration solution shall be carried out in accordance with 4.1 in SH/T 0079—1991.
• 8.3 Preparation of cresol red indicator solution: weigh 0.1 g cresol red, to the nearest 0.001 g. Grind fine, dissolve in 100 mL 95% ethanol, and boil under reflux in a water bath for 5 min; while hot, titrate with 0.05 mol/L potassium hydroxide ethanol standard titration solution until the cresol red indicator solution changes from orange-red to dark red, and upon cooling can return to orange-red.
• 8.4 Preparation of alkali blue 6B indicator solution: weigh 1 g alkali blue 6B, to the nearest 0.01 g. Then add it to 50 mL boiling 95% ethanol, and reflux in a water bath for 1 h, filter after cooling. If necessary, in order to make the indicator color change more sensitive, it is necessary to neutralize the boiled clear filtrate with 0.05 mol/L potassium hydroxide ethanol standard titration solution or 0.05 mol/L hydrochloric acid standard titration solution, until the addition of 1 drop～2 drops of 0.05 mol/L potassium hydroxide ethanol standard titration solution can make the alkali blue 6B indicator solution change from blue to light red and upon cooling can return to blue.
• 8.5 1% (mass fraction) phenolphthalein indicator solution preparation: weigh 1 g phenolphthalein, dissolve in 95% ethanol, and dilute to 100 mL with 95% ethanol.
• Note: The phenolphthalein indicator is suitable for the determination of colorless petroleum products or petroleum products in which a light rose-red color can be easily distinguished in the titration mixture.

5. Instruments and equipment

• 6.1 Fully automatic acidity tester for light petroleum products: main characteristics shall meet the requirements of Annex A.
• 6.2 Conical flask: 250 mL.
• 6.3 Spherical reflux condenser: about 300 mm long.
• 6.4 Pipette: 25 mL, 50 mL and 100 mL.
• 6.5 Microburette: 2 mL, graduation value 0.02 mL; 3 mL, graduation value 0.02 mL; 5 mL, graduation value 0.05 mL.
• 6.6 Electric heating plate or water bath.
• 6.7 Balance: graduation value 0.001 g.

Annex A (normative) Fully automatic acidity tester for light petroleum products:

• A.1 Overview: Uses industrial digital camera, machine vision technology and automatic control technology; after startup, automatically completes heating and boiling, condensation and reflux, addition of indicator, stirring, dropwise addition of standard titration solution, image acquisition and color judgment of titration endpoint. Machine vision technology is used to obtain the color image of the tested sample, and different color space models are used to describe the color, such as using two different color space models RGB and HSV, comparing the image color with known endpoint colors in the database to determine the titration endpoint. The instrument can automatically and continuously complete the acidity determination of one blank sample and two parallel test samples. The schematic diagram of the instrument structure is shown in Figure A.1.
• A.2 Main components of the instrument:
  • A.2.1 Digital camera: pixel count not less than 20 ten thousand, color, can acquire the color of the tested sample in real time, connected to the image processing system and automatic control system to realize real-time image transmission.
  • A.2.2 Image processing system: according to parameters such as pixel distribution and brightness, uses different color space models (such as RGB and HSV) to extract color information, and performs related operations such as separation, linearization, blurring, and color fitting, compares the image color with known endpoint colors in the database, and completes the titration endpoint judgment and feedback.
  • A.2.3 Automatic control system: through the electronic main controller, controls image acquisition, temperature adjustment, dropwise addition of standard titration solution, stirring, cleaning and other processes.
  • A.2.4 Illumination light source: LED light source, color temperature 6500 K±100 K.
• A.3 Instrument parameters (Table A.1 Main technical requirements of the instrument):
  • Acidity measurement range (calculated as KOH)/(mg/100mL): 0～20
  • Endpoint color RGB range:
    • Cresol red indicator solution: (161,111,140)～(194,96,83)
    • Alkali blue 6B indicator solution: (163,69,103)～(217,71,61)
  • Titration volume graduation/mL: 0.02
  • Indicator addition graduation/mL: 0.05
  • Titration syringe/mL: 25
  • Disposable indicator tip/mL: 1
  • Titration rate/(mL/s): 0.007～0.400
  • Stirring speed/(r/min): 260～480
  • Boiling temperature/℃: 70～85
  • Condensation temperature/℃: 0～20
  • Reflux time/min: 5
  • Determination time/min: <30
• A.4 Instrument inspection:
  • A.4.1 Titration syringe: regularly calibrate the syringe volume according to the instrument operating instructions.
  • A.4.2 Digital camera: regularly self-calibrate the white balance of the digital camera according to the instrument operating instructions.

6. Recommended operating conditions

• Sample amount: diesel sample amount is 20 mL, other sample amounts are 50 mL; measure the sample at 20 ℃±3 ℃ (9.1).
• Manual titration: inject 95% ethanol 50 mL into a conical flask, boil for 5 min; after adding indicator, neutralize while hot; after adding sample, boil for 5 min; titrate while hot (9.2).
• Automatic titration: before the test, the instrument should be preheated for more than 0.5 h (9.3.1); respectively measure 50 mL 95% ethanol into two determination bottles, measure the sample according to 9.1 and add to these two determination bottles, and separately measure 50 mL 95% ethanol into a third determination bottle as a blank sample (9.3.2); during titration, a suitable stirring rate should be used (9.3.4).
• Titration time: the time from stopping heating the conical flask to reaching the titration endpoint should not exceed 3 min (9.4).
• Instrument parameters (Annex A Table A.1): boiling temperature 70 ℃～85 ℃; condensation temperature 0 ℃～20 ℃; reflux time 5 min; determination time <30 min; titration rate 0.007 mL/s～0.400 mL/s; stirring speed 260 r/min～480 r/min.

7. Sampling and sample preparation

• Unless otherwise specified, sampling shall be carried out in accordance with GB/T 4756 or GB/T 27867. The sampling amount shall not exceed three-quarters of the sample container. (Chapter 7)
• Sample amount: the diesel sample amount is 20 mL, and the sample amounts for other samples are all 50 mL. Measure the sample under 20 ℃±3 ℃. (9.1)

8. Analysis Procedure

8.1 Sample Preparation (9.1)

The diesel sample amount is 20 mL, and the sample amounts for other samples are all 50 mL. Measure the sample under 20 ℃±3 ℃.

8.2 Manual Titration (9.2)

9.2.1 95% Preparation of the ethanol-indicator solution mixture:

• 9.2.1.1 Take 95% ethanol (5.1) 50 mL and pour it into a clean, anhydrous conical flask (6.2). After plugging the conical flask with a stopper equipped with a spherical reflux condenser (6.3), boil for 5 min. If cresol red (5.4) or alkali blue 6B (5.5) is used as the indicator, operate according to the procedure in 9.2.1.2; if phenolphthalein (5.6) is used as the indicator, operate according to the procedure in 9.2.1.3.
• 9.2.1.2 After adding 0.5 mL cresol red indicator solution or alkali blue 6B indicator solution to the boiling 95% ethanol, neutralize while hot with 0.05 mol/L potassium hydroxide ethanol standard titration solution under continuous shaking, until the mixture in the conical flask changes from yellow to purple-red with the cresol red indicator solution, or from blue to light red with the alkali blue 6B indicator solution.
• 9.2.1.3 Add several drops of 1% (mass fraction) phenolphthalein indicator solution to the boiled 95% ethanol, and neutralize while hot with 0.05 mol/L potassium hydroxide ethanol standard titration solution under continuous shaking, until the mixture in the conical flask shows a light rose-red color.

9.2.2 Determination of acidity:

• 9.2.2.1 According to the sample amount specified in 9.1, add the sample to a conical flask containing the treated 95% ethanol-indicator solution mixture. After plugging the conical flask with a stopper equipped with a spherical reflux condenser (6.3), boil the mixture in the conical flask for 5 min. If cresol red (5.4) or alkali blue 6B (5.5) is used as the indicator, operate according to the procedure in 9.2.2.2; if phenolphthalein (5.6) is used as the indicator, operate according to the procedure in 9.2.2.3.
• 9.2.2.2 For the sample mixture treated according to 9.2.2.1, at this time correspondingly add 0.5 mL of cresol red indicator solution or 0.5 mL of alkali blue 6B indicator solution, and titrate while hot with 0.05 mol/L potassium hydroxide ethanol standard titration solution under continuous shaking, until the cresol red indicator solution in the 95% ethanol layer changes from yellow to purple-red, or the alkali blue 6B indicator solution changes from blue to light red.
• 9.2.2.3 For the sample mixture treated according to 9.2.2.1, titrate while hot with 0.05 mol/L potassium hydroxide ethanol standard titration solution under continuous shaking, until the phenolphthalein solution in the 95% ethanol layer shows a light rose-red color.

8.3 Automatic Titration (9.3)

• 9.3.1 Carry out instrument preparation according to the instrument operating instructions. Before testing, the instrument should be preheated for more than 0.5 h.
• 9.3.2 Respectively measure 50 mL 95% ethanol (5.1) into two determination bottles, then measure the sample according to the provisions of 9.1, and add them respectively to these two determination bottles. Separately measure 50 mL 95% ethanol (5.1) into a third determination bottle as a blank sample.
• 9.3.3 Place the blank sample determination bottle and the two parallel sample determination bottles in sequence on the sample turntable. Correctly place the solution bottle containing 0.05 mol/L potassium hydroxide ethanol standard titration solution, and the indicator bottle containing the indicator, in the acidity analyzer. Input the volumes of the two samples and the actual concentration of the 0.05 mol/L potassium hydroxide ethanol standard titration solution.
• 9.3.4 After heating to boiling, the instrument will automatically add the indicator and begin titration. To avoid the sample splashing out due to stirring and affecting the determination result, an appropriate stirring rate should be used during the titration process.
• 9.3.5 After the titration process ends, the instrument automatically records the volumes of 0.05 mol/L potassium hydroxide ethanol standard titration solution consumed by the blank sample and the two parallel samples, and respectively calculates the acidity of the two parallel samples after deducting the titration volume consumed by the blank sample.

8.4 Titration Time (9.4)

During each titration process, the time elapsed from stopping heating the conical flask to reaching the titration endpoint shall not exceed 3 min.

8.5 Quality Control (Chapter 10)

Confirm the performance of the procedure and equipment by analyzing quality control (QC) samples. It is recommended to analyze QC samples at least monthly to ensure the reliability and accuracy of the results. Control chart techniques may be used to analyze QC samples. If the QC sample results show that the acidity test system is in a state of statistical out-of-control, the cause needs to be found and corresponding measures taken. Within the predetermined period of use, there should be a sufficient supply of QC samples, and the QC samples should be uniform and stable.

9. Qualitative and Quantitative and Calculation Formulas

9.1 Manual Titration Acidity Calculation (11.1)

The acidity X of the sample is calculated according to formula (1):

X = 56.1 × c × V / V1 × 100 ……………………(1)

Where:

• X —— the acidity of the sample (calculated as KOH), in milligrams per 100 milliliters [mg/100mL];
• 56.1 —— the molar mass of potassium hydroxide, in grams per mole (g/mol);
• 100 —— the constant for converting acidity to 100 mL;
• c —— the concentration of the potassium hydroxide ethanol standard titration solution, in moles per liter (mol/L);
• V —— the volume of 0.05 mol/L potassium hydroxide ethanol standard titration solution consumed when titrating the sample, in milliliters (mL);
• V1 —— the volume of the sample, in milliliters (mL).

9.2 Automatic Titration Acidity Calculation (11.2)

The acidity X of the sample is calculated according to formula (2):

X = 56.1 × c × (V2 − V0) / V1 × 100 ……………………(2)

Where:

• X —— the acidity of the sample (calculated as KOH), in milligrams per 100 milliliters [mg/100mL];
• 56.1 —— the molar mass of potassium hydroxide, in grams per mole (g/mol);
• 100 —— the constant for converting acidity to 100 mL;
• c —— the concentration of the potassium hydroxide ethanol standard titration solution, in moles per liter (mol/L);
• V0 —— the volume of 0.05 mol/L potassium hydroxide ethanol standard titration solution consumed when titrating the blank sample, in milliliters (mL);
• V1 —— the volume of the sample, in milliliters (mL);
• V2 —— the volume of 0.05 mol/L potassium hydroxide ethanol standard titration solution consumed when titrating the parallel sample, in milliliters (mL).

9.3 Expression of Results (Chapter 12)

Take the arithmetic mean of the results of two parallel determinations as the acidity of the sample (calculated as KOH), and the result is accurate to 0.01 mg/100mL.

10. Precision

10.1 Manual Titration Precision (13.1)

• 13.1.1 Overview: The manual titration precision is applicable to light petroleum products with obvious indicator color changes at the titration endpoint, such as gasoline, naphtha, kerosene, jet fuel, diesel, etc. Judge the reliability of the test results according to the provisions in 13.1.2 and 13.1.3 (confidence level of 95%).
• 13.1.2 Repeatability (r): In the same laboratory, by the same operator, using the same instrument, according to the same test method, for continuous determination of the same sample, the difference between the two results obtained by manual titration shall not exceed the requirements of Table 1.
• 13.1.3 Reproducibility (R): In different laboratories, by different operators, using different instruments, according to the same test method, for determination of the same sample, the difference between the two single, independent results obtained by manual titration shall not exceed the requirements of Table 1.

Table 1 Manual Titration Precision Unit: mg/100mL

10.2 Automatic Titration Precision (13.2)

• 13.2.1 Overview: The automatic titration precision data come from the test results of 9 laboratories on 12 diesel samples. The test results were determined through interlaboratory statistical analysis with reference to GB/T 6683.1, and the reliability of the test results is judged according to the provisions in 13.2.2 and 13.2.3 (confidence level of 95%). Note: The automatic titration interlaboratory study used cresol red as the indicator.
• 13.2.2 Repeatability (r): In the same laboratory, by the same operator, using the same instrument, according to the same test method, for continuous determination of the same sample, the difference between the two independent results obtained by automatic titration shall not exceed the calculated value of formula (3).

r = 0.1479 X1^0.3829 ……………………(3)

Where: X1 —— the average value of two consecutive test results of the same sample.

• 13.2.3 Reproducibility (R): The difference between two single, independent results obtained by automatic titration on the same test sample in different laboratories, by different operators, using different instruments, according to the same test method, shall not exceed the calculated value of formula (4).

R = 0.3197 X2^0.3829 ……………………(4)

Where: X2 — the average of two single, independent test results of the same test sample.

11. Report

Chapter 14 Test report: The report shall at least include the following contents:

• a) Sample name, number;
• b) Standard used;
• c) Method used;
• d) Results;
• e) Observed abnormal phenomena;
• f) Test date.

12. Normative references

• GB/T 4756 Petroleum liquids — Manual sampling
• GB/T 27867 Petroleum liquids — Automatic pipeline sampling
• SH/T 0079—1991 Preparation of reagent solutions for testing petroleum products
• Reference: [1] GB/T 6683.1 Petroleum and related products — Determination of precision of measurement methods and results — Part 1: Determination of precision data of test methods

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 Q&A