Method for determining the acid number of petroleum products

The acid number is the amount of base used when titrating the specimen to the specified endpoint in a specified solvent, measured in KOH, as mg/g
.

The acid value mainly reflects the corrosivity of the metal and the refining depth or degree of deterioration of the oil in the process of extraction, transportation, processing and use of petroleum products, which can be used to control the quality of lubricating oil in formulation research, and can sometimes be used to determine the degradation of lubricating oil in the process of use, but the acid number as the lubricating oil scrapping index should be determined
by experience.

At present, there are many types of acid number determination methods, one is acid-base titration, such as GB/T264-1983 "Determination of Acid Value of Petroleum Products"; The other category is potentiometric titration, such as the national standard GB/T7304-2014 "Determination of Acid Value of Petroleum Products - Potentiometric Titration"
.
The following describes the test methods
of the two national standards.

1.
Acid number-potentiometric titration method for petroleum products

Two methods
are introduced in the national standard GB/T7304-2014.
Method A: It is mainly suitable for the determination of the acid value of petroleum products and lubricants that can be dissolved and basically dissolved in the mixed solvent of toluene and sewage isopropanol, and its precision is established in the range of acid number (in terms of KOH) of 0.
1mg/g ~ 150mg/g; Method B is suitable for the determination of the
acid number of biodiesel and biodiesel blended fuels with low acidity and large differences in solubility.
The main method is to dissolve the specimen in a titration solvent, use potassium hydroxide isopropanol standard solution as the titrant for potentiometric titration, and the electrode is a glass indicator electrode and a reference electrode or composite electrode
.

Its advantage is that according to the nature and characteristics of different samples, in the sample processing, the selection of test conditions, the adjustment of instruments and equipment, the preparation and calibration of reagents, the judgment of test results and other aspects of targeted provisions and requirements, in order to adapt to the particularity of the sample, and to obtain more accurate results
.

2.
Acid number of petroleum products - acid-base titration

GB/T264-1983 is acid-base titration, which is mainly used for the determination
of acid number of petroleum products.
The acidic components in the specimen are extracted using boiling ethanol and then titrated
with potassium hydroxide ethanol solution.
Acid-base titration is easy to operate, but the disadvantage is that it is not easy to observe the color change for additives or darker oils, and there is a certain error
.

3.
Comparison of test methods

This method requires high reproducibility, and the final result takes the arithmetic mean of the two results as the acid number
of the specimen.