1.1 This standard covers 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?9do not interfere. Salts react if their hydrolysis constants are larger than 10?9.
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 proo?ng 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 SH/T 0688, but their data is in a certain relation.
2 Normative References
The following standards contain provisions which, through reference in this text, constitute provisions of this standard. Unless otherwise specified in the standard, the standards below shall be valid and current.
GB/T 6682 Water for Analytical Laboratory Use - Specification and Test Methods
GB/T 7304 Petroleum Products and Lubricants; Determination of Neutralization Number; Potentiometric Titration Method
SH/T 0688 Petroleum Products and Lubricants-Determination of Base Number-Potentiometric Titration Method
3 Definitions
This standard adopts the definitions as follows.
3.1 Acid number
It refers to the quantity of base of sample that is required to titrate a sample of 1g to a speci?ed 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 speci?ed 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 Summary of Test Method
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 hydroxide solution, using methyl orange as an indicator.
