GB/T 7304-2014 Acid number of petroleum products by potentiometric titration

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Card type: Test method

1. Method number / Full name / English name / Replacement relationship

• Standard number: GB/T 7304—2014
• Chinese name: Determination of acid number of petroleum products - Potentiometric titration method
• English name: Standard test method for acid number of petroleum products by potentiometric titration
• Replacement relationship: Replaces GB/T 7304—2000 "Determination of acid number of petroleum products and lubricants (Potentiometric titration method)"
• Previous versions: GB/T 7304—1987, GB/T 7304—2000
• Adoption relationship: Modified adoption using the redrafting method of ASTM D664-11a "Determination of acid number of petroleum products (Potentiometric titration method)"
• This method is divided into Method A and Method B, Method B is newly added content

2. Scope of application and non-applicability

• This standard specifies two test methods for the determination of acid number of petroleum products, lubricants, biodiesel and biodiesel blended fuels by potentiometric titration.
• Method A: Applicable to the determination of acid number of petroleum products and lubricants that can be dissolved and basically dissolved in a mixed solvent of toluene and anhydrous isopropanol. Method A can determine those acidic components in the sample whose dissociation constant in water is greater than 1×10⁻⁹, extremely weak acids with a dissociation constant less than 1×10⁻⁹ do not interfere, but salts with a hydrolysis constant greater than 1×10⁻⁷ will participate in the reaction. The precision of Method A is established within the range of acid number (calculated as KOH) from 0.1 mg/g~150 mg/g.
• Method B: Applicable to the determination of acid number of biodiesel and biodiesel blended fuels with relatively low acidity and large differences in solubility.
• Note: Acidic components in new oil or used oil include organic acids, inorganic acids, esters, phenolic compounds, lactones, resins, and heavy metal salts, ammonium salts and salts of other weak bases, acid salts of polybasic acids, and certain antioxidants and detergent additives.
• This standard can reflect the relative changes occurring under oxidizing conditions during oil use without considering color and other performance changes. Although the titration is carried out under specific conditions, this standard cannot be used to determine the absolute acidity of oil to predict its performance under service conditions; no necessary connection has been found between acid number and bearing corrosion.
• Note 1: The acid number results measured by this standard and those measured by GB/T 4945 and SH/T 0163 may be the same or different. No attempt has been made in this standard to correlate the determination results with those obtained by other non-titration methods.
• Note 2: A few laboratories have observed that when aqueous or non-aqueous buffer solutions are used in this standard, there is a certain difference between the results obtained.
• 5.2 Since different oxidation products contribute differently to the acid number, and different organic acids have different corrosion characteristics, this standard cannot be used to predict the corrosion performance of oil, biodiesel and biodiesel blended fuels during use. No necessary connection has been found between the acid number of biodiesel and biodiesel blended fuels and lubricating oils and their corrosion tendency toward metals.

3. Method principle

Dissolve the sample in the titration solvent, use potassium hydroxide isopropanol standard solution as titrant for potentiometric titration, the electrode pair used is a glass indicating electrode and a reference electrode or a composite electrode. Manually or automatically plot the potentiometric titration curve of potential mV value versus titration volume, and take the obvious jump point as the endpoint; if there is no obvious jump point, take the potential value of the corresponding freshly prepared aqueous acid and base buffer solutions as the titration endpoint.

4. Reagents and materials (purity / preparation)

• 7.1 Purity of reagents: The chemical reagents used in this standard are all analytical grade. Other grades of reagents may also be used if it is determined that they have sufficient purity and will not affect the precision of the determination results.
• 7.1.1 Commercially available solutions may be used to replace solutions prepared in the laboratory, which has been proven feasible.
• 7.1.2 The volume of the prepared solution may differ from this standard, but its final concentration should be consistent.
• 7.2 Water: Unless otherwise specified, the water used in this standard shall comply with the provisions for grade three water in GB/T 6682—2008.
• 7.3 Primary standard substance: Primary standard substance or commercially available primary standard substance solution, used to standardize the titrant.
• 7.4 Ethanol. Warning: Flammable, toxic after denaturation, can cause poisoning.
• 7.5 Lithium chloride, LiCl.
• 7.6 Lithium chloride electrolyte: 1 mol/L~3 mol/L lithium chloride ethanol solution.
• 7.7 Potassium hydroxide. Warning: Can cause severe corrosion.
• 7.8 Anhydrous isopropanol: Water content less than 0.1%. If sufficiently dry reagent cannot be obtained, the reagent can be purified and dried by distillation through a multi-plate column still, discarding the first 5% of distillate and using the remaining 95%. Drying can also be achieved with molecular sieves, for example, Linde type 4A molecular sieves can be used, allowing the solvent to pass through the molecular sieve column from top to bottom, with a molecular sieve to solvent ratio of 1:10. Warning: Flammable. Note: It has been reported that if there is no restriction prohibiting addition, isopropanol may contain peroxides. If so, when the container or reagent bottle containing isopropanol is nearly empty or dry, an explosion may occur.
• 7.9 Commercially available aqueous buffer solutions with pH values of 4, 7 and 11 respectively. The above buffer solutions should be replaced periodically according to their stability, or when they are considered contaminated. Information on the stability of these substances can be obtained from their manufacturers. The above aqueous buffer solutions can also be prepared in the laboratory.
• 7.9.1 Buffer solution with pH value of 4: Weigh (115.0±5.0) ℃ dried 2 h~3 h potassium hydrogen phthalate primary standard reagent 10.12 g, dissolve in carbon dioxide-free distilled water, dilute to 1000 mL at 25 ℃.
• 7.9.2 Buffer solution with pH value of 7: Weigh (115.0±5.0) ℃ dried 2 h~3 h potassium dihydrogen phosphate primary standard reagent 6.81 g, add 0.1 mol/L sodium hydroxide solution 291 mL, dilute to 1000 mL with carbon dioxide-free distilled water at 25 ℃.
• 7.9.3 Buffer solution with pH value of 11: Weigh sodium bicarbonate primary standard reagent 2.10 g, add 0.1 mol/L sodium hydroxide solution 227 mL, dilute to 1000 mL with carbon dioxide-free distilled water at 25 ℃.
• 11.1 Method A Reagents
  • 11.1.1 See Chapter 7.
  • 11.1.2 Hydrochloric acid (HCl): Relative density 1.19. Warning: Corrosive, can cause burns.
  • 11.1.3 Toluene. Warning: Flammable.
  • 11.1.4 Hydrochloric acid isopropanol standard solution (0.1 mol/L): Mix 9 mL hydrochloric acid with 1 L anhydrous isopropanol; when standardizing, dilute about 8 mL (accurately measured) 0.1 mol/L potassium hydroxide isopropanol standard solution with 125 mL carbon dioxide-free water, and use this diluted solution as titrant to standardize the above hydrochloric acid isopropanol solution by potentiometric titration; the standardization frequency should be such that the standardization error is not greater than 0.0005 mol/L.
  • 11.1.5 Potassium hydroxide isopropanol standard solution (0.1 mol/L): Weigh 6 g potassium hydroxide and add to 1 L anhydrous isopropanol, bring to a slight boil for 10 min, let the solution stand for 2 d, then filter the supernatant through a sintered glass funnel, store the filtered solution in a chemically resistant reagent bottle; to avoid interference from carbon dioxide in the air, a drying tube filled with soda asbestos or soda lime desiccant can be attached to the reagent bottle, which also avoids contact of the solution with cork, rubber and saponifiable stopcock grease. When standardizing, dissolve potassium hydrogen phthalate in carbon dioxide-free water, use this as titrant, and standardize by potentiometric titration; the standardization frequency should be such that the standardization error is not greater than 0.0005 mol/L.
  • 11.1.6 Titration solvent: Add (5±0.2) mL water to (495±5) mL anhydrous isopropanol and mix evenly, then add (500±5) mL toluene; this titration solvent should be prepared in large quantities, and its blank value should be determined before use every day.
  • 11.1.7 Trichloromethane: Analytical grade. Warning: Flammable, toxic substance, fatal if swallowed. Harmful if inhaled. Toxic vapors are produced if burned.
• 12.1 Method B Reagents
  • 12.1.1 See Chapter 7.
  • 12.1.2 Potassium hydroxide isopropanol standard solution (0.01 mol/L): Weigh 0.56 g potassium hydroxide and add to about 1 L anhydrous isopropanol, bring to a slight boil for 10 min, let the solution stand for 2 d while avoiding contact with carbon dioxide, then filter the supernatant through a sintered glass funnel, store the filtered solution in a chemically resistant reagent bottle; to avoid interference from carbon dioxide in the air, a drying tube filled with soda asbestos or soda lime desiccant can be attached to the reagent bottle, which also avoids contact of the solution with cork, rubber and saponifiable stopcock grease. When standardizing, take a certain amount of carbon dioxide-free potassium hydrogen phthalate solution and standardize by potentiometric titration; the standardization frequency should be such that the standardization error is not greater than 0.0005 mol/L.
  • 12.1.3 Drying of potassium hydrogen phthalate primary standard reagent: Weigh about 10 g~20 g 150 μm of potassium hydrogen phthalate primary standard reagent of fine particle size in a weighing bottle, dry at 120 ℃ for 2 h, then cover the weighing bottle, and cool to room temperature in a desiccator.
  • 12.1.4 Potassium hydrogen phthalate solution: If the unit is to be mass volume fraction, weigh about 1 g of dried potassium hydrogen phthalate reference reagent into a 500 mL volumetric flask, accurate to 0.0001 g and record the weighed amount, then add water to the volumetric flask up to the mark. If the unit is to be mass fraction, weigh about 1 g of dried potassium hydrogen phthalate reference reagent into a 500 mL volumetric flask, accurate to 0.0001 g and record the weighed amount, then add water to near the mark, record the total mass of water and potassium hydrogen phthalate; at this time the concentration of the potassium hydrogen phthalate solution is expressed as KHP, in units of mg/g.
• Appendix B: When determining the acid value of synthetic oil, a mixed solvent of petroleum ether (90 ℃~120 ℃, chemically pure): anhydrous ethanol (analytical reagent) at 1:1 may be used as the titration solvent; the titration solvent may be 0.05 mol/L potassium hydroxide isopropanol standard solution, and the preparation and standardization of the solution are given in SH/T 0079; the internal liquid of the reference electrode is a saturated potassium chloride aqueous solution.

5. Instruments and Equipment

• 6.1 Manual Titration Equipment
  • 6.1.1 Potentiometer or voltmeter: When the electrodes comply with the provisions in 6.1.2 and 6.1.3, and the resistance between the two electrodes is between 0.2 MΩ~20 MΩ, the potentiometer or voltmeter shall have an accuracy of ±0.005 V, a sensitivity of ±0.002 V, and a range of at least ±0.5 V. The potentiometer should be grounded to prevent electrostatic fields generated throughout the entire operation by touching the ground wire, exposed parts of the glass electrode surface, glass electrode wires, titration stand, etc., from affecting the potentiometer reading. Note: The instrument may also consist of an electronic voltmeter designed for an input of less than 5×10⁻¹ A with continuous reading capability and an electrode system connected to the voltmeter input terminal with a resistance greater than 1000 MΩ and grounded through a metal shield, while the voltmeter is connected to the shielding wire of the glass electrode to prevent external electrostatic interference.
  • 6.1.2 Detection electrode: A standard pH electrode suitable for non-aqueous titration.
  • 6.1.3 Reference electrode: Calomel electrode or silver/silver chloride reference electrode, filled with 1 mol/L~3 mol/L lithium chloride ethanol solution.
  • 6.1.4 Combination electrode: A silver/silver chloride reference electrode is built into the detection electrode; this structure provides convenience for the use and maintenance of the electrode. This combination electrode has a sleeve-type diaphragm in the reference part and uses an inert ethanol electrolyte, for example, a lithium chloride ethanol solution with a concentration of 1 mol/L~3 mol/L. These combination electrodes have the same or even better response than the two-electrode system. It has a detachable sleeve-type diaphragm to facilitate cleaning and adding electrolyte. Note: A third electrode, such as a platinum electrode, can increase the stability of the electrode in some systems.
  • 6.1.5 Adjustable-speed mechanical stirrer: Any type is acceptable, but it should be equipped with a spiral stirring paddle. The stirring rate should provide as vigorous agitation as possible without splashing liquid or introducing air bubbles into the solution. A stirring paddle with a radius of 6 mm and a blade inclination of 30°~45° should be satisfactory. Magnetic stirring can also be satisfactory. Note: If electronic stirring equipment is used, the stirring equipment should be powered correctly and well grounded to prevent the potentiometer reading from being affected by turning the stirrer motor power on or off during titration.
  • 6.1.6 Burette: Capacity 10 mL or other specifications, minimum scale 0.05 mL or smaller, accuracy ±0.02 mL. The tip of the burette should extend 100 mm~130 mm beyond the stopcock so that the titrant does not contact the surrounding air and other vapors before entering the titration flask. The potassium hydroxide burette should be equipped with a protective tube containing soda lime or other carbon dioxide adsorbing material.
  • 6.1.7 Beaker: Capacity 250 mL, made of borosilicate glass or other suitable material.
  • 6.1.8 Titration stand: A stand capable of holding electrodes, stirrer, burette, etc. Note: Arrange reasonably so that moving the cup does not interfere with the electrodes and stirring.
• 6.2 Automatic Titration Equipment
  • 6.2.1 The automatic titration system should ensure that the analysis process specified in the standard can be carried out. At a minimum: the automatic potentiometric titrator should ensure compliance with the performance and requirements in 6.1.
  • 6.2.2 The addition of titrant adopts a dynamic mode. During titration, the addition rate and addition amount of titrant depend on the rate of change of the system. The recommended maximum increment is 0.5 mL, and the minimum increment is 0.05 mL.
  • 6.2.3 Graduated cylinder: 50 mL, or other measuring instrument capable of measuring (50±0.5) mL of liquid.
  • 6.2.4 Pipette: 2 mL, A grade.
  • 6.2.5 Beaker: Capacity 125 mL, 250 mL or other suitably sized beaker, made of borosilicate glass or other suitable material.

6. Recommended Operating Conditions

• Electrode system detection: Soak the electrode in an aqueous buffer solution with a pH value of 4, stir for 1 min, then read the mV value of the potentiometer; remove the electrode, rinse with water, soak the electrode in an aqueous buffer solution with a pH value of 7, stir for 1 min, then read the mV value of the potentiometer. A good electrode system should have a potential difference of at least 162 mV (20 ℃~25 ℃).
• Electrode maintenance: The glass electrode should be cleaned periodically (no less than once a week during continuous use) according to usage conditions and the type of sample analyzed by immersion in a chromium-free strong oxidizing cleaning solution. The reference electrode should be cleaned periodically during use or when installing a new electrode. The electrolyte of the reference electrode should be replaced once a week, filled with new lithium chloride ethanol electrolyte, and each time it should be filled to the filling port. During titration, the liquid level of the electrolyte in the reference electrode should always be kept higher than the liquid level in the titration cup.
• Before each titration, the prepared electrode should be soaked in distilled water (pH value 4.5~5.5) for at least 5 min, and rinsed with anhydrous isopropanol and titration solvent before use.
• Electrode life: Generally, depending on usage, the service life of the electrode is from 3 months to 6 months. If the electrode exceeds its service period, it should be tested before use (see 8.2).
• 9.1 Determination of voltmeter readings of aqueous buffer solutions: When an exact inflection point cannot be obtained in the titration curve, to ensure comparability of endpoint selection, voltmeter readings should be obtained daily for each electrode using acidic or alkaline aqueous buffer solutions.
• 9.2 Immerse the electrode in buffer solutions with pH value 4 and pH value 11 respectively, stir for about 5 min, and maintain the temperature of the buffer solution within the range of the titration temperature ±2 ℃. Read the potential values respectively. When there is no inflection point in the titration curve, this potential can be used as the titration endpoint.
• 11.2.4.3 Titrate with 0.1 mol/L potassium hydroxide isopropanol solution. When there is an obvious inflection point, adjust or program the instrument appropriately to facilitate calculation of results; the addition rate and volume of titrant should be based on the change in slope of the titration curve. The amount of titrant added should preferably produce a potential difference of 5 mV–15 mV after each addition, and the volume added each time should be between 0.05 mL–0.5 mL. If the potential change does not exceed 10 mV within 10 s after adding titrant, then the volume added in the next addition will be increased. The maximum waiting time between two titrant additions must not exceed 60 s.
• 11.2.4.4 The titration can be ended when the potential value exceeds the potential value of the buffer solution with pH value 11 by 200 mV.
• 12.2.2 Determination of solvent blank: Adjust the instrument to dynamic titration mode according to the manufacturer's instructions. The amount of titrant added each time should not exceed 0.2 mL. Measure 50 mL of anhydrous isopropanol using a graduated cylinder, pipette, or other equipment capable of accurately measuring (50±0.5) mL of liquid into a 125 mL beaker or other suitably sized beaker, turn on stirring and start titration, record the volume of potassium hydroxide isopropanol solution used up to the inflection point, accurate to 0.01 mL.

7. Sampling and Sample Preparation

• 10.1 Since sediments in used oil samples are acidic or alkaline, or have adsorbed acidic or alkaline substances from the oil sample, sampling procedures should be strictly followed during sampling. Otherwise, the sample taken is not representative, thereby causing large measurement deviations.
• 10.1.1 Sample according to the provisions of GB/T 4756 or SY/T 5317.
• 10.1.2 When the sample is a used lubricant product, the sampling process should make the sample representative. And there should be no external contamination. Note: Since used lubricant samples may undergo significant changes during storage, after sampling from the lubrication system, the determination should be carried out immediately, and the sampling and determination dates should be noted.
• 10.2 To uniformly disperse the sediment in the sample, the sample may be heated and stirred in the original container at 60 ℃±5 ℃. If the amount of sample in the container exceeds three-quarters of the container volume, the sample should be transferred to a clean glass bottle whose volume is at least one-third larger than the sample volume. During sample transfer, shake vigorously so that all sediment in the container is transferred out. Note: When there is no obvious sediment in the sample, the above heating step may be omitted.
• 10.3 After the sediment in the sample is uniformly dispersed, filter part or all of the sample through a sieve with a pore size of about 150 μm to remove large particulate matter in the sample. Note: When there is no obvious sediment in the sample, the above filtration step may be omitted.
• 11.2.1 In a 250 mL beaker or other suitable container, weigh the sample according to the recommended sample amount in Table 1 (see Note 1) and add 125 mL titration solvent (see Note 2).

Table 1 Sample weighing amount

Note 1: If it is suspected that the recommended sample amount will contaminate the electrode, a relatively smaller sample amount may be used. The results obtained using a relatively smaller sample amount and the results obtained using the recommended sample amount may not necessarily be the same. The precision does not include the results when a relatively smaller sample amount is used. Note 2: In the titration solvent, chloroform may be used instead of toluene to completely dissolve asphaltenic heavy residue components. The measurement results obtained using chloroform and the results obtained using toluene may not necessarily be the same. The precision results do not include the case where chloroform is used.

8. Analysis Procedure

8.1 Preparation of Electrodes

Before using the silver/silver chloride reference electrode, if the electrolyte contained therein is not a 1 mol/L~3 mol/L lithium chloride ethanol solution, it should be replaced. Drain the electrolyte from the electrode, first wash off all salt-like substances with distilled water, then rinse with ethanol. Then rinse multiple times with lithium chloride ethanol electrolyte. Finally, put the sleeve-type diaphragm on the electrode back in place and add lithium chloride ethanol electrolyte into the electrode through the filling hole. After disassembling and reassembling the sleeve-type diaphragm, ensure that the electrolyte inside the electrode can freely enter the titration system. The composite electrode is treated in the same way. The electrolyte in the composite electrode can be removed by vacuum suction.

8.2 Electrode Inspection

When the electrode is used for the first time, or when new electrolyte is added, or after being left standing for a long time, the potentiometer-electrode combination should be tested. First rinse the electrode with solvent and then with water. Then soak the electrode in an aqueous buffer solution with a pH value of 4. After stirring for 1 min, read the mV value of the potentiometer. Remove the electrode and rinse with water. Soak the electrode in an aqueous buffer solution with a pH value of 7. After stirring for 1 min, read the mV value of the potentiometer. Calculate the difference between the two measured potentiometer mV values. A good electrode system should have a potential difference of at least 162 mV (20 ℃~25 ℃). If the potential difference is less than 162 mV, gently lift the sleeve-type diaphragm of the electrode to allow a small amount of internal filling electrolyte to overflow, then reset the sleeve-type diaphragm and retest. If the potential difference is still less than 162 mV, clean or replace the electrode. If the indicating electrode and the reference electrode are independent of each other, a pair of electrodes can be regarded as one unit. If one of them is replaced, it can be considered that a different electrode pair is formed, and the test should be performed again.

8.3 Maintenance and Storage of Electrodes

Thoroughly clean the electrode to ensure that there is no foreign matter on the surface of the ground glass joint. Regularly performing potential tests on the electrode is very important for obtaining a stable potential. Electrode contamination may cause unstable, irregular, and difficult-to-observe liquid junction potentials. This may not be very important for titration processes that select the inflection point on the titration curve as the titration endpoint, but it is very important for titration processes that select a fixed potential as the titration endpoint. Note: Appendix A provides procedures for checking electrode performance.

• 8.3.1 Glass electrodes need to be cleaned by immersion in a chromium-free strongly oxidizing cleaning solution periodically (no less than once a week during continuous use) according to the usage conditions and the type of sample being analyzed. Reference electrodes should be cleaned periodically during use or when installing a new electrode. The electrolyte of the reference electrode should be replaced once a week, filled with new lithium chloride ethanol electrolyte, and each time it should be filled to the filling port. Ensure that no bubbles are generated in the electrode liquid. If bubbles are observed, place the electrode vertically and tap it gently to drive out the bubbles. During titration, always keep the liquid level of the electrolyte in the reference electrode higher than the liquid level in the titration beaker.
• 8.3.2 Before each titration, the prepared electrode should be soaked in distilled water (with a pH value of 4.5~5.5) for at least 5 min, and rinsed with anhydrous isopropanol and titration solvent before use.
• 8.3.3 When not in use, soak the lower half of the reference electrode in lithium chloride ethanol electrolyte. When using a glass electrode, store it in water acidified with HCl and with a pH value of 4.5~5.5. When there is a long interval between two titrations, never allow both electrodes to be inserted in the titration solvent. Although the electrodes are not very fragile, they should be handled with great care during use.
• 8.3.4 Electrode life: Generally speaking, depending on the usage conditions, the service life of the electrode is from 3 months to 6 months. If the electrode exceeds its service period, it should be tested before use (see 8.2).

9 Calibration of the Instrument

• 9.1 Determination of the voltmeter reading for aqueous buffer solutions: When an exact inflection point cannot be obtained in the titration curve, to ensure comparability of endpoint selection, the voltmeter reading should be obtained daily for each electrode using an acidic or alkaline aqueous buffer solution. Note: The hydrogen ion activity of different glass electrodes is different. Therefore, for each electrode system, the potential value of the acid or base buffer solution selected as the titration endpoint should be measured periodically.
• 9.2 Immerse the electrode respectively in buffer solutions with a pH value of 4 and a pH value of 11, stir for about 5 min, and maintain the temperature of the buffer solutions within the range of the titration temperature ±2 ℃. Read the potential values respectively. When there is no inflection point in the titration curve, this potential can be used as the titration endpoint.

11 Method A

11.2 Procedure for Determination of Acid Value and Strong Acid Value

• 11.2.1 In a 250 mL beaker or other suitable container, weigh the sample according to the sample amount recommended in Table 1 (see Note 1) and add 125 mL titration solvent (see Note 2). Prepare the electrode according to 8.2. Place the beaker or titration container on the titration stand and adjust its position so that the lower half of the electrode is immersed below the liquid surface. Start stirring; the stirring speed should be as high as possible without causing splashing of the solution or generation of bubbles.
• 11.2.2 Select a suitable burette, fill it with 0.1 mol/L potassium hydroxide isopropanol solution, install the burette on the titration apparatus, and ensure that the tip of the burette is inserted 25 mm below the liquid surface of the titration solution. Record the initial reading of the burette at this time and read the potential value reading of the voltmeter at this time.
• 11.2.3 Manual Titration Method
  • 11.2.3.1 Add a small amount of 0.1 mol/L potassium hydroxide isopropanol solution, wait until the potential stabilizes, record the amount of titrant, and read the potential value at this time.
  • 11.2.3.2 At the beginning of the titration and in any subsequent range (such as the inflection point), if for each increment of 0.1 mL of titrant the potential value changes by more than 30 mV, the amount added each time should be reduced to 0.05 mL.
  • 11.2.3.3 In the middle section (the flat portion of the curve), if for each increment of 0.1 mL of titrant the potential change is less than 30 mV, the amount added each time may be appropriately increased so that the resulting potential change is approximately equal to 30 mV, but should not be greater than 30 mV.
  • 11.2.3.4 Titrate in this manner until for each addition of 0.1 mL potassium hydroxide isopropanol solution the potential change is less than 5 mV. At this time, the alkalinity indicated by the electrode potential is stronger than that of the aqueous alkaline buffer solution.
  • 11.2.3.5 Remove the titration solution, insert the electrode and the burette tip into the titration solvent to rinse, then thoroughly rinse with anhydrous isopropanol and water respectively. Before performing the potentiometric titration of another sample, soak the electrode in water for at least 5 min to restore the liquid gel film of the glass electrode; after soaking in water for 5 min, rinse the electrode with anhydrous isopropanol and titration solvent before the next titration. If the electrode is found to be dirty or contaminated, treat it as described in clause 8.1. Store the electrode as described in 8.3.3.
• 11.2.4 Automatic Potentiometric Titration Method
  • 11.2.4.1 Adjust the instrument to dynamic titration mode according to the manufacturer's instructions.
  • 11.2.4.2 Determine the initial potential value of the sample (mV). By comparing this potential value with the potential value of an aqueous acidic buffer solution, determine whether a strong acid is present in the sample and confirm that the instrument will be used to determine the strong acid value. Record the volume of potassium hydroxide isopropanol solution consumed to titrate the sample solution from the initial potential value to the potential value indicated by an aqueous buffer solution with a pH value of 4. This value will be used to calculate the strong acid value of the sample. Continue the automatic titration, and record the potential curve or first derivative curve as appropriate.
  • 11.2.4.3 Titrate with 0.1 mol/L potassium hydroxide isopropanol solution. When there is an obvious inflection point, adjust or program the instrument appropriately to facilitate calculation of the results. The rate and volume of titrant addition should be based on the change in the slope of the titration curve. The amount of titrant added each time should preferably produce a potential difference of 5 mV–15 mV, and the volume added each time should be between 0.05 mL–0.5 mL. If after adding titrant the potential change does not exceed 10 mV within 10 s, then the volume added in the next titrant addition will be increased. The maximum waiting time between two titrant additions must not exceed 60 s.
  • 11.2.4.4 The titration may be ended when the potential value exceeds the potential value of a buffer solution with a pH value of 11 by 200 mV. If a maximum value appears on the first derivative curve of the titration curve that is significantly higher than the electrode noise signal, this point is the equivalence point (see 11.3.1.1).
  • 11.2.4.5 After titration is complete, rinse the electrode and burette tip with titration solvent, then thoroughly rinse with anhydrous isopropanol and water respectively. Before performing potentiometric titration of another sample, soak the electrode in water for at least 5 min to restore the liquid gel film of the glass electrode; before performing the next titration, rinse the electrode with anhydrous isopropanol and titration solvent. When the electrode is found to be dirty or contaminated, handle it as described in 8.1. Store the electrode as described in 8.3.3. Note: When the acid value (expressed as KOH) is expected to be less than 0.1 mg/g, the method can be improved in various ways to obtain better precision, for example, using 0.01 mol/L or 0.05 mol/L potassium hydroxide in isopropanol solution; or increasing the sample weighing amount to 20 g; if the sample is analyzed by manual titration, the manual burette (0.05 mL graduations) can be converted to an automatic burette, allowing smaller additions of potassium hydroxide in isopropanol solution.
• 11.2.5 Blank
  • 11.2.5.1 For a set of samples and a batch of new titration solvent, a blank titration of 125 mL solvent should be performed. For manual titration, add 0.01 mL–0.05 mL of 0.1 mol/L potassium hydroxide in isopropanol solution until the potential no longer changes after two consecutive additions, and read the potential value and titrant addition amount before the potential value becomes constant. For automatic titration, use the same titration mode as for determining acidic substances in the sample, but use a smaller titrant addition amount, such as 0.01 mL–0.05 mL; when the sample amount is large, attention should be paid to regularly checking the blank value.
  • 11.2.5.2 When strong acids are present in the sample and the strong acid value has been determined, a blank titration of 125 mL titration solvent should be performed, adding 0.1 mol/L hydrochloric acid in isopropanol solution in the manner specified in 11.2.5.1 at an addition amount of 0.01 mL~0.05 mL.

12 Method B

This method should use an automatic potentiometric titrator equipped with an automatic endpoint determination function.

12.2 Test Procedure

• 12.2.1 0.01 mol/L Standardization of potassium hydroxide in isopropanol titrant
  • 12.2.1.1 If the potassium hydrogen phthalate solution uses mass fraction as the unit, weigh about 2 g of potassium hydrogen phthalate solution into a 125 mL or other suitably sized beaker, accurate to 0.0001 g; if the potassium hydrogen phthalate solution uses mass volume fraction as the unit, pipette 2 mL of potassium hydrogen phthalate solution into a beaker; then add 50 mL of carbon dioxide-free water to the beaker, titrate this solution with potassium hydroxide in isopropanol solution to determine the titration concentration of the potassium hydroxide in isopropanol solution; this process consumes approximately 2 mL of 0.01 mol/L potassium hydroxide in isopropanol solution.
  • 12.2.1.2 Prepare two additional portions of potassium hydrogen phthalate solution and standardize the titrant according to the procedure in 12.2.1.1.
  • 12.2.1.3 Calculate the average concentration of the potassium hydroxide in isopropanol titrant using the results of three titrations, and the difference between the molar concentration results of the three determinations and the average value should be within 0.0005 mol/L.
• 12.2.2 Determination of solvent blank: adjust the instrument to dynamic titration mode according to the manufacturer's instructions. The amount of titrant added each time should not exceed 0.2 mL. Measure 50 mL of anhydrous isopropanol using a graduated cylinder, pipette, or other equipment that can accurately measure (50±0.5) mL of liquid into a 125 mL beaker or other suitably sized beaker, turn on stirring and start titration, record the volume of potassium hydroxide in isopropanol solution used up to the inflection point, accurate to 0.01 mL.
• 12.2.3 Sample Analysis
  • 12.2.3.1 Adjust the instrument to dynamic titration mode according to the manufacturer's instructions.
  • 12.2.3.2 On an analytical balance, weigh about 5 g of sample into a 125 mL beaker or other suitably sized beaker, accurate to 0.0001 g, use a pipette or other liquid measuring equipment to measure (50±0.5) mL of anhydrous isopropanol into a suitable beaker, prepare the electrode according to the procedure in 8.1, place the beaker or other container on the titration stand, and adjust the position so that the lower half of the electrode is immersed in the solution, turn on stirring, and the stirring speed should be as high as possible without causing splashing of the solution or generating bubbles.
  • 12.2.3.3 When performing sample titration and blank titration, the difference in volume of titration solvent used should not exceed 0.5 mL, otherwise errors will occur.
  • 12.2.3.4 Select a suitable burette, fill it with 0.01 mol/L potassium hydroxide in isopropanol solution, install the burette on the titration apparatus, and ensure that the burette tip is inserted 25 mm below the liquid surface of the titration solution.
  • 12.2.3.5 After titration is complete, rinse the electrode and burette tip with anhydrous isopropanol, then rinse with water. Before performing potentiometric titration of the next sample, soak the electrode in water for at least 5 min to restore the liquid gel film of the glass electrode; before the next titration, rinse the electrode again with anhydrous isopropanol. When the electrode is found to be dirty or contaminated, handle it as described in 8.1. Store the electrode as described in 8.3.3.
  • 12.2.3.6 Biodiesel may, during long-term storage, undergo oxidation to produce organic acids, resulting in multiple titration inflection points during potentiometric titration (see Figure 2); the volume of titrant used at the last clear inflection point should be used to calculate the total acid value.

9. Qualitative and Quantitative and Calculation Formulas

11.3 Calculation (Method A)

• 11.3.1 Manual titration: plot the curve of 0.1 mol/L potassium hydroxide in isopropanol solution addition amount versus the corresponding potential change (see Figure 1). Only when the inflection point is very obvious (see Note) and very close to the potential value obtained with an aqueous acid or base buffer solution may this inflection point be taken as the titration endpoint. If the inflection point is uncertain or does not appear at all (see curve B in Figure 1), then take the potential of the corresponding aqueous buffer solution as the titration endpoint. Note: Usually, during several consecutive additions of 0.05 mL, if the potential change caused by each addition is greater than 15 mV or the potential change produced is at least 30% greater than that produced by the same amount of addition before or after, then a clear inflection point is considered to have appeared; generally speaking, a clear inflection point can only be obtained with equal-amount additions.
• 11.3.1.1 Some additive components may produce inflection points beyond the potential endpoint of the buffer solution. For additives, use the last inflection point for calculation. If automatic titration is used, instrument parameters need to be adjusted to determine such endpoints.
• 11.3.1.2 For titration of acids in all used oils, use the potential of the corresponding aqueous base buffer solution (pH=11) as the endpoint during titration; for strong acid value, use the potential of the corresponding aqueous acid buffer solution (pH=4) as the endpoint during titration. Note: Studies have shown that for

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