GB/T 5530-2005Animal and vegetable fats and oils -- Determination of acid value and acidity (English PDF)

4.1 General

This method is a reference method for the determination of the acid value of fats (see

Chapter 1).

Note 3: Under the conditions specified in this method, short-chain fatty acids are volatile.

4.2 Principle

The specimen is dissolved in hot ethanol and titrated with sodium hydroxide or potassium hydroxide aqueous solution.

4.3 Reagents

Unless otherwise specified, only reagents confirmed as analytically pure are used. The water used shall meet the requirements of grade-3 water in GB/T 6682-1992.

4.3.1 Ethanol: The minimum concentration is 95%.

4.3.2 Sodium hydroxide or potassium hydroxide: The concentration of the standard

solution is c(NaOH or KOH) = 0.1 mol/L.

4.3.3 Sodium hydroxide or potassium hydroxide: The concentration of the standard

solution is c(NaOH or KOH) = 0.5 mol/L.

4.3.4 Phenolphthalein indicator: 10 g/L; 10 g of phenolphthalein is dissolved in 1 L of

95% ethanol solution.

Note 4: When measuring darker samples, add 1 mL of 0.1% methylene blue solution to every

100 mL of phenolphthalein indicator solution, to observe the titration endpoint.

4.3.5 Basic blue 6B or thymolphthalein (for dark oils and fats): 20 g/L; 20 g of basic (4.3.2), until the solution changes color. Keep the solution unchanged for 15 s, which is the end point.

Note 6: When the color of the oil is dark, more ethanol and indicator need to be added.

Transfer the neutralized ethanol to the conical flask containing the test sample; mix thoroughly; boil. Titrate with sodium hydroxide or potassium hydroxide standard solution (4.3.2 or 4.3.3 depends on the estimated acid value of the sample); shake thoroughly during the titration process. When the solution changes color and remains unfaded for 15 seconds, the titration end point is reached.

5.1 General

This method is applicable to light-colored oils and fats.

5.2 Principle

The sample is dissolved in a mixed solvent and titrated with potassium hydroxide ethanol solution.

5.3 Reagents

Unless otherwise specified, only reagents confirmed to be analytically pure are used.

The water used shall meet the requirements of grade-3 water in GB/T 6682-1992.

5.3.1 Ether and 95% ethanol: 1 + 1 volume mixture.

Warning: Ether is highly flammable and can generate explosive peroxides. It must be used with extreme caution.

Before use, add 0.3 mL of phenolphthalein solution (5.3.3) to every 100 mL of mixed solvent; accurately neutralize with potassium hydroxide ethanol solution (5.3.2) (see note in 4.7.2).

If it is not possible to use ether, the following mixed solvents can be used:

- Toluene and 95% ethanol, 1+1 volume mixture;

- Toluene and 99% isopropanol, 1+1 volume mixture;

- When determining crude oil and refined vegetable fats, 99% isopropanol can be used instead of the mixed solvent.

ethanol, 8 g of potassium hydroxide, 0.5 g of aluminum flakes; boil and reflux for 1 h;

then immediately distill; dissolve the required amount of potassium hydroxide in the distillate; let it stand for a few days; slowly pour out the supernatant; discard the potassium carbonate precipitate.

5.3.2.3 The solution can also be prepared by non-distillation method: Add 4 mL of aluminum butyrate to 1000 mL ethanol; let it stand for a few days; slowly pour out the supernatant; dissolve the required potassium hydroxide. The prepared solution needs to be calibrated.

Note: The preparation and calibration of potassium hydroxide or sodium hydroxide standard solution can also be carried out, in accordance with the national standard GB/T 601-2002.

5.3.3 Phenolphthalein indicator, see 4.3.4.

5.3.4 Basic blue 6B or thymolphthalein, see 4.3.5.

5.4 Instruments

Conventional laboratory instruments, as well as the microburette described in 4.4.1.

5.5 Sampling

See 4.5.

5.6 Specimen preparation

See 4.6.

5.7 Procedure

5.7.1 Weighing the sample

According to the estimated acid value, use a sufficient amount of sample (5.6) as shown in Table 1. Weigh the sample into a 250 mL conical flask.

5.7.2 Determination

5.7.2.1 Dissolve the sample (5.7.1) in 50 mL ~ 150 mL of the pre-neutralized mixed solvent (5.3.1).

Titrate with potassium hydroxide solution (5.3.2) (see 5.7.2.3) while shaking, until the indicator shows the endpoint described in 4.7.2.

5.7.2.2 When the acid value is < 1, a nitrogen flow needs to be slowly introduced into the solution.

5.7.2.3 When the volume of 0.1 mol/L potassium hydroxide solution (solution A)

required for titration exceeds 10 mL, use 0.5 mol/L potassium hydroxide solution (solution B) instead.

5.7.2.4 If the solution becomes turbid during titration, add an appropriate amount of mixed solvent (5.3.1) until it becomes clear.

6.1 Principle

In anhydrous medium, potassium hydroxide isopropanol solution is used to titrate the free fatty acids in the specimen by potentiometric titration.

6.2 Reagents

All reagents used are analytically pure; the water used shall meet the requirements of grade-3 water in GB/T 6682-1992.

6.2.1 4-Methyl-2-pentanone (methyl isobutyl ketone): Neutralize with potassium hydroxide solution (6.2.2) before use and measure with a pH meter (6.3.1).

6.2.2 Potassium hydroxide standard solution C: 0.1 mol/L potassium hydroxide

isopropanol solution; the preparation method is as shown in the method for preparing solution A (5.3.2).

6.2.3 Potassium hydroxide standard solution D: 0.5 mol/L potassium hydroxide

isopropanol solution; the preparation method is as shown in the method for preparing solution B (5.3.2).

6.3 Instruments

Routine laboratory instruments, as well as the following instruments:

6.3.1 pH meter: Equipped with glass electrode and calomel electrode.

The saturated potassium chloride solution and the experimental solution are kept in contact with a sintered glass or porcelain disk, which has a thickness of at least 3 mm.

Note 9: Soak the glass electrode in methyl isobutyl ketone for 12 hours before titration. Gently wipe it dry with filter paper before use. Rinse it with methyl isobutyl ketone, isopropanol, distilled water immediately after the measurement.

Note 10: If the electrode effect is poor, soak it in 1 mol/L hydrochloric acid isopropanol solution for 14 hours to revive the electrode. The soaked electrode shall be rinsed with distilled water, isopropanol, methyl isobutyl ketone in turn.

Note 11: A coarse sintered glass or porcelain disk is used to maintain contact between the saturated potassium chloride solution and the experimental solution, to prevent the generation of diffusion current and additional voltage.

6.3.2 Magnetic stirrer.

6.3.3 Analytical balance, same as 4.4.2.

6.4 Sampling

See 4.5.

6.5 Specimen preparation

See 4.6.

6.6 Steps

6.6.1 Weighing

Weigh 5 g ~ 10 g of sample (6.5) to the nearest 0.01 g. Place in a 150 mL beaker.

6.6.2 Determination

6.6.2.1 Dissolve the sample (6.6.1) in 50 mL of 4-methyl-2-pentanone (6.2.1).

Insert the electrode of the pH meter (6.3.1); start the magnetic stirrer (6.3.2); titrate to the equivalence point with potassium hydroxide solution (select 6.2.2 or 6.2.3 according to the estimated acidity).

Note 12: The equivalence point usually corresponds approximately to a certain pH value and can be determined by observing the turning point of the neutralization curve by graphical method; the equivalence point can also be calculated by the pH change value (the first differential of the functional relationship of the added potassium hydroxide isopropanol solution is the maximum value, or the second differential is equal to zero).

6.6.2.2 The turning point of cottonseed oil with high gossypol content cannot be

measured. In this case, the pH value corresponding to the equivalence point when oleic

......