GB/T 258-2016 Light petroleum products — Determination of acidity

Summary page. This page is a compiled excerpt/summary of the standard — it is not the full standard text and does not replace it. Verify every value against the official standard and the product certificate before use.

GB/T 258-2016 specifies the determination of the acidity of light petroleum products; this page covers the scope and definition of acidity, reagents and indicators, the determination procedure, calculation and precision of the 2016 edition.

Evidence note: this page is a non-official method study card compiled from the original of the 2016 edition and is not the full text of the standard; technical review is pending. Check the controlled text of the standard and the product certificate before use.

Version and reading boundaries: Cover, foreword. This card covers GB/T 258-2016, "Light petroleum products — Determination of acidity". The cover states an issue date of 2016-06-14 and an implementation date of 2017-01-01, and notes replacement of GB/T 258-1977; these are historical records of that cover, not proof of current status. An earlier summary had mixed in the automatic-titration range and conditions of the 2025 edition; this card has been reorganised from the 2016 original. The two years cannot substitute for each other, and this page does not give the measuring range, blank deduction or instrument calibration requirements of the automatic titration method.

Scope and definition of acidity: Printed page 1, clauses 1 and 3. This edition addresses the acidity of light petroleum products, including gasoline, naphtha, kerosene, diesel and jet fuel. The precision applies on the premise that the colour change of the indicator end point can be identified accurately; whether a sample meets that condition must still be checked individually. Acidity is expressed as the mass of potassium hydroxide required to neutralise 100 mL of sample, in mg/100 mL. This is an acidity by volume and cannot be taken directly as an acid number by mass.

Principle: Printed page 1, clause 4. Ethanol is used to extract the acidic components from the oil sample; with the aid of a colour indicator, the extract is neutralised by titration with a standard ethanolic potassium hydroxide solution. The acidity is calculated from the titration consumption, the concentration and the sample volume.

Apparatus and consumable specifications: Printed page 1, clause 5. The specifications are the consumable requirements of the original method, not proof of the suitability of any particular product on this site. Item | Specification listed in this edition Conical flask | 250 mL Spherical reflux condenser | length about 300 mm Pipettes | 25 mL, 50 mL, 100 mL Microburette | 2 mL and 3 mL capacities with a division of 0.02 mL; 5 mL capacity with a division of 0.05 mL Heating device | hot plate or water bath Balance | weighing accurate to 0.001 g

Reagents and reference materials: Printed page 2, clauses 6 and 8. The basic reagents are analytical-grade 95 % ethanol, potassium hydroxide and hydrochloric acid; the hydrochloric acid also refers to the requirements of GB/T 622. The standard ethanolic potassium hydroxide solution and the standard hydrochloric acid solution used for titration are both 0.05 mol/L. Alkali blue 6B indicator: weigh 1 g to 0.01 g, add 50 mL of boiled 95 % ethanol, reflux in a water bath for 1 h, cool and filter. The original also requires a hot neutralisation adjustment with the standard potassium hydroxide solution or a hydrochloric acid solution of the same concentration where necessary; the aim is that 1–2 drops of the potassium hydroxide solution turn the blue to a pale red, returning to blue on cooling. This check of the colour response must not be omitted. Cresol red indicator: weigh 0.1 g to 0.001 g, grind fine and dissolve in 100 mL of 95 % ethanol, boiling under reflux in a water bath for 5 min. While hot, adjust with the potassium hydroxide solution to a deep red, which should return to orange-red on cooling. The phenolphthalein indicator is a 1 % solution of phenolphthalein in ethanol. It is used for colourless oils, or for oils in which a pale rose-red in the titration mixture is easy to recognise; the background colour of the sample must not be ignored.

Sampling and sample preparation: Printed page 2, clauses 7 and 9.1. Unless otherwise specified, sampling follows GB/T 4756 or GB/T 27867; the container is filled to not more than three quarters of its capacity. The sample is measured at 20 °C ± 3 °C: 20 mL for diesel and 50 mL for the other samples within the scope. The sample volume must be distinguished from the volume of ethanol used afterwards.

Standard solutions and ethanol treatment: Printed page 2, clauses 8 and 9.2; printed page 3, clause 9.2.3. The preparation and standardisation of the 0.05 mol/L standard ethanolic potassium hydroxide titrant refers to clause 4.6 of SH/T 0079-1991; the standard hydrochloric acid titrant of the same concentration refers to clause 4.1. The retained 2016 original does not reproduce the full text of these two preparation and standardisation clauses, and this card does not guess a formulation or standardisation factor. Place 50 mL of 95 % ethanol in a clean, dry conical flask, attach the spherical reflux condenser and boil for 5 min. When alkali blue 6B or cresol red is used, add 0.5 mL of the corresponding indicator; while shaking, neutralise hot with the standard ethanolic potassium hydroxide solution, observing blue to pale red and yellow to purple-red respectively. When phenolphthalein is chosen, add a few drops of the indicator to the boiled ethanol and neutralise to rose-red while shaking and hot. This is the ethanol treatment stage and must not be merged with the later addition of indicator in the oil determination stage.

Determination procedure and time control: Printed page 3, clauses 9.3 and 9.4. Add the oil sample in the specified volume, mix with the treated ethanol and indicator, attach the spherical reflux condenser and boil again for 5 min. The alkali blue 6B or cresol red route requires a further 0.5 mL of the corresponding indicator, then shaking and hot titration; the end points are the ethanol layer changing from blue to pale red and from yellow to purple-red respectively. The phenolphthalein route is titrated hot until a rose-red appears in the ethanol layer. Each titration, from the moment heating of the conical flask stops to the end point, must not exceed 3 min. The complete safety measures, apparatus operation and specific end-point judgement must be specified in the laboratory's controlled working documents; this card cannot replace an operating procedure.

Calculation formula and symbols: Printed page 3, clause 10, equation (1): X = (56.1 × c × V / V₁) × 100. X is the acidity in mg/100 mL; 56.1 is the molar mass of potassium hydroxide in g/mol; c is the concentration of the standard ethanolic potassium hydroxide titrant in mol/L; V is the volume of that solution consumed in the titration in mL; V₁ is the sample volume in mL; 100 is the constant converting to 100 mL of sample. The denominator is V₁ and the titrant consumption is V. Equation (1) on that page has no (V−V₀) term for an automatic method; a blank-deduction formula must not be imported from another year. The formula in this card was checked visually against the scanned page, and independent technical review is still pending.

Expression of results and reporting: Printed page 3, clause 11. The arithmetic mean of two duplicate determinations is the test result, accurate to 0.01 mg/100 mL. This clause gives the requirement for expressing the result; this card does not invent a set of report fields or acceptance limits from it.

Precision and interpretation boundaries: Printed pages 3–4, clauses 12.1–12.3 and Table 1. The original uses the 95 % confidence level, and the precision applies only to light petroleum products whose indicator end-point colour change can be judged accurately. The difference between two results obtained consecutively by the same laboratory, operator, apparatus and method is compared with the repeatability requirement; the difference between two single independent results obtained by different laboratories, operators and apparatus is compared with the reproducibility requirement. The original table is rearranged below by acidity interval, all values in mg/100 mL. The middle interval keeps the original page's notation "≥0.5–1.0"; no numerical precision is derived and the scope is not extended. Acidity interval (original notation) | Repeatability r | Reproducibility R <0.5 | 0.08 | 0.20 ≥0.5–1.0 | 0.10 | 0.25 >1.0 | 0.20 | — (not given in the original table)

Normative references and dependency gaps: Printed page 1, clause 2; printed page 2, clause 8. This edition lists GB/T 622, "Chemical reagent — Hydrochloric acid", GB/T 4756, "Petroleum liquids — Manual sampling", GB/T 27867, "Petroleum liquids — Automatic pipeline sampling", and SH/T 0079-1991, "Preparation method of reagent solutions for petroleum product tests". Only the retained original of GB/T 258-2016 was checked this time; the referenced documents and their current status were not re-reviewed. The specific preparation and standardisation of the standard solutions depend on the referenced documents, and this card cannot claim that all operating material is complete.

Verification notes and remaining discrepancies: Verification record of this compilation. This card is a non-official study compilation, not the full text of the standard. The retained file contains the cover, copyright page, foreword and printed body pages 1–4, the body ending at clause 12.3, with no annex or later figure/table references found; "no sign of missing pages" does not mean that collation against the official controlled copy is complete. The version identity, consumable specifications, key volumes and temperatures, times, equation (1) and the precision table have been checked visually. Technical review is pending, and the current status of the standard, final applicability and product certificates must still be checked separately. The scanned original is not published on this site. In the original table the reproducibility for acidity > 1.0 is shown by a dash, which must not be read as zero, as acceptable, or as needing no evaluation; requirements that are not given must not be invented. The automatic titration, monthly QC or instrument calibration clauses of the 2025 edition have not been moved into this card.

Quick answers: Basis: the original pages above. Object of measurement: acidity of light petroleum products; unit of result: mg/100 mL. The diesel sample volume differs from that of other samples, 20 mL and 50 mL respectively. Can this page prove that a product on this site can be used for this edition? No. The products below only carry the base-number association of the original public catalogue; an unmarked year, a joint catalogue listing or a simultaneous enquiry cannot replace the product certificate and technical check for this edition. Can this page be used to fix automatic titration parameters or the current status of the standard? No; this card only compiles the retained original of the 2016 edition. Publicly associated products, by method number in the public catalogue (an unmarked year does not mean applicability to this edition has been confirmed; the specific applicability must be checked against the product certificate); 4 items, the first 4 shown: diesel acidity reference oil, RIPP, SINOPEC Research Institute of Petroleum Processing · RIPP-1904-258-CKY · 250 mL/bottle; diesel acidity reference sample (6 mg/100 mL), RIPP · RS000104; diesel acidity reference sample (3 mg/100 mL), RIPP · RS000105; diesel acidity reference sample (1 mg/100 mL), RIPP · RS000106. [View publicly associated products; where no record exists, "no associated item" is shown.]

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