GB/T 4945-2002Standard test method for acid and base number of petroleum products and lubricants by colour-indicator titration (English PDF)

1.1 This Standard is applicable to the determination of acid or basic constituents in

petroleum products and lubricants soluble or nearly-soluble in mixtures of toluene and

isopropyl alcohol. It is applicable for the determination of acids or bases whose

dissociation constants in water are larger than 10-9; extremely weak acids or bases

whose dissociation constants are smaller than 10-9. If their hydrolysis constants are

larger than 10-9, then there is influence.

Notes.

1 In new and used oils, the constituents considered to have acidic characteristics include organic and inorganic

acids, esters, phenolic compounds, lactones, resins, salts of heavy metals, and addition agents such as

inhibitors and detergents. Similarly, constituents considered to have basic properties include organic and

inorganic bases, amino compounds, salts of weak acids (soaps), basic salts of poly acidic bases, salts of

heavy metals, and addition agents such as inhibitors and detergents.

2 This test method is not suitable for measuring the basic constituents of many basic additive-type lubricating oils.

Test method in SH/T 0688 may be used for this purpose.

1.2 This test method can be used to indicate relative changes that occur in an oil

product during use under oxidizing conditions. Although the titration is made under

specified conditions, the method is not able to measure an absolute acidic or basic

property that may be used to predict performance of an oil product under service

conditions. No general relationship between bearing corrosion and acid or base

numbers is known.

Note. Oils, such as many cutting oils, rust proofing oils, and similar compounded oils, or dark-colored oils, that are

not able to be analyzed for acid number by this test method due to obscurity of the color-indicator end point,

may be analyzed by test method in GB/T 7304. The acid numbers obtained by this color-indicator test

method need not be numerically the same as those obtained by test method in GB/T 7304, the base

numbers obtained by this color indicator test method need not be numerically the same as those obtained by

test method in SH/T 0688, but their data is in a certain relation.

3.1 Acid number

It refers to the quantity of base of sample that is required to titrate a sample of 1g to a

specified end point, expressed in mgKOH/g.

3.2 Base number

It refers to the quantity of acid of sample that is required to titrate a sample of 1g to a

specified end point, expressed in mgKOH/g.

3.3 Strong acid number

It refers to the quantity of base of sample that is required to titrate a hot water extract

of 1g sample to a yellow end point using methyl orange solution, expressed in

mgKOH/g.

3.4 Used oil

It refers to any oil that has been in a part of equipment (for example, an engine,

gearbox, transformer, or turbine) whether operated or not.

4.1 To determine the acid or base number, the sample shall be dissolved in a

mixture of toluene and isopropyl alcohol containing a small amount of water, and the

resulting single-phase solution shall be titrated at room temperature with standard

alcoholic base or alcoholic acid solution, respectively, to the end point indicated by the

color change of the added p-naphtholbenzein solution (orange in acid and dark green

in base). To determine the strong acid number, a separate portion of the sample is

extracted with hot water and the aqueous extract shall be titrated with potassium

7.4 Distilled water. it shall be adopted in accordance with the specification of Type III

in GB/T 6682.

7.5 Potassium hydrogen phthalate solution. standard reagent.

7.6 Barium hydroxide solution. analytical reagent.

7.7 Methylbenzene solution. analytical reagent.

8.1 Preparation of solution and solvent

8.1.1 Hydrochloric acid and isopropanol standard solution (0.1mol/L)

Mix 9mL of hydrochloric acid solution (note 1) with 1000mL of isopropyl alcohol,

accurately take 8mL of 0.1mol/L potassium hydroxide - isopropanol standard solution,

and then dilute it with 125mL of carbon dioxide-free water to obtain the titrant which

shall be standardized by adopting potentiometric titration method. Standardize

frequently enough to guarantee the standardization error less than 0.0005 mol/L (Note

2).

Notes.

8.1.2 Methyl orange indicator solution

Dissolve 0.1g of methyl orange in 100mL of water.

8.1.3 P-naphtholbenzein indicator solution

The p-naphtholbenzein must meet the specifications given in Annex A. Prepar...

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