GB/T 18609-2011 Determination of acid number of crude oil — Potentiometric titration method

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.

Evidence note: original study compilation for the 2011 edition, not the full text of the standard; technical review is pending. Based on the 13-page original held, doubts are retained regarding the boundaries of the scope and of the weighing table, the wording 125 mL ± 0.01 mL, the equality at 162 mV and the description of the strong acid number variables. It cannot serve directly as an operating procedure, a conformity judgement or a product applicability claim.

1. Edition identity and evidence boundaries: Original PDF pages 1-3; official version status record. The cover and body are GB/T 18609-2011, Determination of acid number of crude oil — Potentiometric titration method, issued 2011-09-29, implemented 2012-01-01, replacing GB/T 18609-2001. The 13-page file comprises the cover, foreword, body, a nearly blank end page and the copyright page; the page count does not mean the technical review is complete. In this round both official services of the State Administration for Market Regulation list the 2011 edition as current; the standards information platform also lists revision plan 20252658-T-469 as under review. A plan is not an already published new edition, and two records of the same operator are not two independent verifications of technical originals. The sample, the contract version and the final applicability still require confirmation. The foreword records the modified adoption of ASTM D664-07, with differences in the original with respect to crude oil and the referenced documents. Only the historical adoption fact is recorded here; no claim is made that the different standards are today equivalent or interchangeable.

2. Scope: Original PDF page 3, clause 1. The scope in the original is a crude oil acid number of 0.1 to 150 mg/g with a water content below 0.5%. The 0.05 to <260 mg/g range of the weighing table must not be expanded into the scope of the method, and mgKOH/g must not be treated as a mass fraction in %. The method applies to acidic components soluble in the mixed toluene and isopropanol solvent; the original addresses components with a dissociation constant greater than 10⁻⁹, very weak acids below 10⁻⁹ not interfering, and salts with a hydrolysis constant greater than 10⁻⁹ being able to react. The original thresholds are retained here; no chemical suitability of a specific product is judged from them. The scope requires a water content < 0.5%, yet clause 7.2 uses > 0.5% as the trigger for dehydration; the treatment boundary at exactly 0.5% is not closed in the original [source unclear] and must not be altered on the card's own initiative to ≤ or ≥.

3. Terms and referenced documents: Original PDF page 3, clauses 2-3. The acid number is the amount of potassium hydroxide required to neutralize unit mass of sample under the specified conditions, expressed as mgKOH/g; the strong acid number has its own definition and calculation. Crude oils generally do not require the strong acid number, and it is determined by its own procedure only when explicitly requested. The two must not be treated as the same name, and the total acid number must not be used in place of the strong acid number. The original refers to GB/T 4756, GB/T 6682, GB/T 8929, SY/T 5317 and SY/T 6520, covering sampling, water quality, water content and related crude oil methods. This card does not guess years that are not listed, does not automatically adopt other versions, and adds no product relationships from the reference list.

4. Principle: Original PDF pages 3-4, clause 4. The sample is dissolved in the specified mixed solvent and titrated with alcoholic potassium hydroxide, recording the change of potential with titrant volume; the end point is determined from the break in the curve or from the potential of a suitable buffer solution, and the blank and the standardization of the concentration enter the calculation of the result. The strong acid number uses a different end point and a hydrochloric acid blank. The end-point method, solvent, sample mass and modifications are interrelated; a single potential or number does not establish conformity.

5. Reagents and titrants: Original PDF pages 4-5, clause 5. The original uses analytical-grade reagents and grade 3 water according to GB/T 6682; buffer solutions of pH 4, 7 and 11, and a 1 to 3 mol/L lithium chloride ethanol solution for the reference electrode. Quality, certificates and product specifications are not inferred from this method card. The titration solvent is 5 ± 0.2 mL water, 495 ± 5 mL isopropanol and 500 ± 5 mL toluene; a blank is run on the day of use, and the sum of the three volumes does not replace the original tolerances. The 0.1 mol/L alcoholic potassium hydroxide stock is 6 g in 1 L of isopropanol, refluxed in a 2 L flask for about 10 min, left to stand for 2 days and protected from carbon dioxide. Standardization uses 0.1 to 0.15 g of potassium hydrogen phthalate dried at 105 °C for 2 h, weighed to 0.0002 g, dissolved in about 100 mL of boiled, cooled water; a new value is adopted when the concentration changes by more than 0.0005 mol/L. The 0.2 mol/L potassium hydroxide uses 12 to 13 g in 1 L of isopropanol, and standardization uses 0.2 to 0.3 g of potassium hydrogen phthalate weighed to 0.0002 g. The masses for the two concentrations must not be interchanged. The 0.1 mol/L alcoholic hydrochloric acid stock is 9 mL of hydrochloric acid in 1 L of isopropanol; standardization uses about 8 mL of accurately standardized potassium hydroxide, with the volume to 0.01 mL, diluted with 125 mL of water, and a concentration change threshold of 0.0005 mol/L. The 125 mL of water in this passage and the 125 mL of mixed solvent in the sample titration serve different purposes.

6. Apparatus and metrological boundaries: Original PDF page 5, clause 6. The resistance between the two electrodes is 0.2 to 20 MΩ; the potentiometer has an accuracy of ±0.005 V, a sensitivity of ±0.002 V and a range of at least ±0.5 V; the original requires earthing. Resistance, accuracy and sensitivity must not be used interchangeably. The burettes are 10 mL/0.05 mL, 5 mL/0.02 mL and 2 mL/0.01 mL and are protected from carbon dioxide; beakers are 250 mL, and for dynamic automatic titration the recommended maximum volume increment is 0.5 mL and the recommended minimum 0.05 mL. The original oven is at 180 ± 10 °C, and the balance has a precision of 0.0001 g and a maximum capacity of 200 g. These are the requirements of the method and do not show that any particular equipment has been calibrated, complies or is usable.

7. Test sample and weighing table: Original PDF pages 5-7, clause 7 and Table 1 of 8.2.1. The sample is handled according to the original sampling and water-content methods and made homogeneous. A water content > 0.5% requires dehydration; the study steps must not be taken directly as on-site safety instructions. The table below is taken cell by cell from original Table 1; the weighing accuracy of the sample and the precision of the balance are kept separate. The first row, 0.05 to <1.0, and the last row, 100 to <260, extend beyond the 0.1 to 150 range of clause 1, and that range is not thereby presumed to be widened. | Acid number / (mg/g) | Sample mass / g | Weighing accuracy / g | | --- | --- | --- | | 0.05 to <1.0 | 20.0 ± 2.0 | 0.10 | | 1.0 to <5.0 | 5.0 ± 0.5 | 0.02 | | 5.0 to <20 | 1.0 ± 0.1 | 0.005 | | 20 to <100 | 0.25 ± 0.02 | 0.001 | | 100 to <260 | 0.1 ± 0.01 | 0.0005 |

8. Electrode preparation and checks: Original PDF pages 6-7, clause 8.1 and clauses 8.1.1 to 8.1.4. The original requires a new glass electrode to be pre-soaked for 24 h and cleaned at least weekly; the reference electrode solution is replaced weekly, and electrodes are not stored in the titration solvent. The glass electrode is wetted with water at pH 4.5 to 5.5 for at least 5 min and then rinsed in the original sequence. The pH 4 and pH 11 buffer solutions are stirred for about 5 min with the temperature within ±2 °C of the titration temperature; where there is no break, a suitable buffer potential is used, and all curves must not be forced to the same end point. The electrode check is at 20 to 25 °C with pH 4 and pH 7 each stirred for 1 min; a potential difference > 162 mV is acceptable. If it is < 162 mV, first make the reference sleeve flow and measure again; if it is still low, clean or replace the electrode, and re-measure the electrode pair after each replacement. The original does not state the case of exactly 162 mV [source unclear], so no pass or fail decision is made on the card's own initiative.

9. Titration procedure and end point: Original PDF pages 7-9, clause 8 and Figure 1. The original writes 125 mL ± 0.01 mL of titration solvent added; the scanned wording has been checked, but the reasonableness of the tolerance still awaits technical confirmation and it is not changed to ±0.1 or ±1 mL. Sparingly soluble samples may use a reduced sample mass and involve heating and stirring at 60 ± 5 °C; the order of 500 + 495 + 5 mL, toluene first and then isopropanol and water, and the treatment of waxy samples must be verified against the complete procedure. Where necessary the original permits replacement by chloroform, but the results may differ and the precision does not cover chloroform; the warning wording in the original about chloroform is not turned here into an independent chemical safety certification. Ventilation, protection and solvent safety are confirmed by the formal documents. The electrode is half immersed and the burette tip is 25 mm below the liquid surface; 0.1 mol/L potassium hydroxide is usual, with another choice for low acid numbers of 0.05 to 0.1. The manual stability condition is < 5 mV/min; when the change exceeds 30 mV per 0.1 mL, 0.05 mL increments are used, and below 30 mV the increment may be increased provided a single change does not exceed 30 mV. The termination conditions include a change of < 5 mV per 0.1 mL and exceeding the alkaline potential of the corresponding buffer solution. A pronounced break close to the potential of a suitable fresh buffer solution is used as the break end point; where there is no break the buffer end point is used. The 15 mV/30% criterion of the 0.05 mL step in the original should be read together with the curve conditions before and after it and cannot serve as a single criterion for all samples. The four example curves in Figure 1 represent blank, no break, weak acid, and strong acid plus weak acid; they are not actual measurement data for a fixed sample.

10. Blank and calculation variables: Original PDF pages 9-10, clauses 8.5 and 9.1-9.2. The acid number blank uses fresh 125 mL of mixed solvent, titrated with potassium hydroxide in increments of 0.01 to 0.05 mL. The strong acid number blank uses 0.1 mol/L hydrochloric acid; although both equations use V₀, the first is the potassium hydroxide blank and the second the hydrochloric acid blank, and they must not be substituted for each other. Equation (1): wTAN = (V₁−V₀)C×56.1/m. V₁ is the volume of potassium hydroxide at the acid number end point, V₀ the volume of potassium hydroxide of the acid number blank (mL); C is the potassium hydroxide concentration (mol/L), m the sample mass (g), and 56.1 the value of the molar mass of potassium hydroxide in the original equation; the result is in mgKOH/g. Equation (2): wSAN = (V₂C + V₀C₁)×56.1/m. V₂ is the volume of potassium hydroxide titrated to the potential of the pH 4 buffer (mL); in this equation V₀ is the hydrochloric acid blank volume and C₁ the hydrochloric acid concentration (mol/L); the plus sign is retained as in the source image and must not be written as a minus. The lead-in text of original equation (2) writes wTAN while the equation is labelled wSAN; the inconsistency in the original symbols is stated explicitly and the two are not mixed in calculation.

11. Precision: Original PDF page 10, clause 10. X is the mean of two acid number results in mg/g. The repeatability and reproducibility at about 95% probability in the original are calculated separately according to the end-point method used, and must not be treated as the precision of the strong acid number or as a general acceptance limit. Equation (3), repeatability for the break end point: r = 0.044(X+1). Equation (4), repeatability for the buffer end point: r = 0.117X. Equation (5), reproducibility for the break end point: R = 0.141(X+1). Equation (6), reproducibility for the buffer end point: R = 0.44X. Chloroform as replacement solvent is not covered by the original precision; non-recommended sample masses and other modifications require reporting and technical assessment, and a number obtained from the equations does not by itself demonstrate that the result is reliable.

12. Quality control and record chain: Original PDF pages 4-10, clauses 5, 7 and 8-11 (combined study record chain). The following is a combined study record chain compiled from the relevant clauses; it is not a claim that every field is a verbatim requirement of clause 11. Record the standardization date and actual concentration of the titrant, the blank type, the solvent batch, the electrode pair check and buffer potentials, the sample mass, the water-content treatment, the titration volume-potential curve, the end-point method and the calculation. Each variable is linked to the raw record; the final result alone must not be kept. After each titration the glass electrode is allowed to recover in water for at least 5 min and is then treated in the original order with isopropanol and mixed solvent. The daily blank and electrode check cannot be replaced by switching on the instrument or by the visible presence of bubbles; this card establishes no controlled record or release rule.

13. Reporting points: Original PDF pages 10-11, clause 12. The report records the sample identification, this standard number and year, each determination and the mean result, the values obtained with the specified blank and calculation, whether the break or the buffer end point was used, and any non-recommended sample mass, chloroform, deviations and anomalies, and the test date. Items a to f of the original reporting clause span the page break, and the continuation on page 11 must not be omitted. The original gives no unified rounding of results or detection limit, and none is guessed here. mgKOH/g, titrant volume in mL and concentration in mol/L are retained separately; the acid number is not called a percentage, and no claim is created that a certificate can be obtained or that a product complies.

14. Verified content and unresolved items: 13-page original held and independent segmented image verification in this round. This card sets out 14 sections, the 15 data cells of Table 1, two calculation equations and four precision equations; the four example curves of Figure 1 have not been reconstructed from actual data. Page 12 of the file is nearly blank and page 13 is the copyright page; the original held has no annexes, and missing annexes are not invented. The body study compilation is not the full text of the standard or the closure of a technical review. Still [to be verified]: the difference between the upper and lower bounds of the scope and of the weighing table, the pre-treatment boundary at exactly 0.5% water, the equality boundary at 162 mV, the 125 mL ± 0.01 mL tolerance, and the inconsistency between the wTAN wording and the wSAN formula in equation (2). Until these doubts are closed the method cannot be operated directly, results released by calculation, or conformity judged. The original is preserved read-only. The official status, referenced versions, sample/apparatus applicability, controlled operating procedure, technical acceptance and independent Hermes/Grok acceptance are each to be confirmed; no product master data or new method-product relationship is stated.

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