（1） Gravimetric method

Gravimetric method is a commonly used method, which is not limited by the type of oil, but the operation is cumbersome and the sensitivity is low. It is only suitable for the determination of more than 10mg / L oil-bearing water samples.

The principle of the method is to acidify the water sample with sulfuric acid, extract the mineral oil with petroleum ether, remove the petroleum ether by evaporation, weigh the residue and calculate the content of mineral oil. This method refers to the total amount of substances that can be extracted by petroleum ether in water, which may contain heavy petroleum components and cannot be extracted. When the solvent is removed by evaporation, the light oil will also be lost.

（2） Infrared spectrophotometry

Carbon tetrachloride was used to extract oil from water, and then the total extract was determined. The extract was adsorbed by magnesium silicate. After removing polar substances such as animal and vegetable oil, the oil in the filtrate could be determined.

The contents of total extract and petroleum were calculated by absorbance a2930, a2960 and a3030 at 2930cm-1 (stretching vibration of C-H bond in CH2 group), 2960cm-1 (stretching vibration of C-H bond in CH3 group) and 3030cm-1 (stretching vibration of C-H bond in aromatic ring), respectively. The content of animal and vegetable oil is calculated according to the difference between total extract and oil content.

The main points of the method are as follows: first, the total extract of water sample is extracted by carbon tetrachloride or flocculation enrichment (water sample with low content of petroleum substances), and the extract is divided into two parts: one part is used for the determination of total extract; the other part is used for the determination of total extract; The other part is adsorbed by magnesium silicate and used for the determination of petroleum substances. Then, carbon tetrachloride was used as solvent to prepare a certain concentration of n-hexadecane, 2,6,10,14-tetramethylpentadecane and toluene solutions, and their a2930, a2960 and a3030 were determined respectively

Where: ρ—— The content of some substance in the prepared solution, mg / L;

A2 930, A2 960 and A3 030 -- absorbance of three substance solutions at corresponding wavelengths;

10. Y, Z -- correction coefficient of absorbance;

F -- correction coefficient of the influence of aliphatic hydrocarbon on aromatic hydrocarbon, i.e. the ratio of absorbance of n-hexadecane at 2930cm-1 and 3030cm-1.

Finally, determine the absorbance a12930, a12960 and A13030 of the total extract of the water sample, and the absorbance a22930, a22960 and a23030 of the extract after removing animal and vegetable oil, and calculate the total extract content in the water sample according to the following three formulas ρ 1 (mg / L), petroleum substances ρ 2 (mg / L) and animal and plant content ρ 3（mg/L）：

Where: V0 -- constant volume of extraction water sample solvent, ml;

VW -- volume of water sample, ml;

D -- dilution ratio of extraction solution;

L -- optical path of the cuvette used to determine the correction coefficient, cm;

L -- optical path of the cuvette used in the determination of water sample, cm.

This method is suitable for the determination of petroleum, animal and vegetable oils in various kinds of water. The detection limit of the method is 0.1mg/l when the sample volume is 500ml and the optical path is 4cm; When the sample volume is 5L, 0.01mg/l can be detected after enrichment.

（3） Non dispersive infrared absorption method

In this law, methyl (- CH3) and methylene (- CH2 -) of petroleum substances are used in the near infrared region (3.4 μ m) It can be used as the basis for the determination of oil content in water samples. The standard oil can be extracted with petroleum ether from polluted water. According to the composition characteristics of crude oil in China, mixed petroleum hydrocarbon can also be used as standard oil, and its composition is: hexadecane ∶ isooctane ∶ benzene = 65 ∶ 25 ∶ 10 (V / V).

During the determination, the water sample was acidified with sulfuric acid, demulsified with sodium chloride, and then extracted with trichlorotrifluoroethane. The extracted liquid was filtered through anhydrous sodium sulfate layer, constant volume, and injected into infrared analyzer to measure its content.

All organic substances containing methyl and methylene will interfere. If there are animal, vegetable oils and fatty acids in water samples, they should be separated in advance. In addition, some heavy components in petroleum are insoluble in trichlorotrifluoroethane, which leads to low determination results.