Abstract: The content of petroleum pollutants in environmental water is one of the indicators that reflect water quality. This article uses three wavelengths to quantitatively test the oil content in water, which is convenient for sample testing and accurate data.
The petroleum in the environment comes from the pollution of industrial wastewater and domestic sewage. Oils form an oil film on the surface of the water, which affects the gas exchange between air and water; oil dispersed in water and adsorbed on particles or existing in water in an emulsified state, when decomposed by microorganisms, will consume dissolved oxygen in the water and easily deteriorate the water quality .
Mineral oil is a mixture of alkanes, naphthenes and aromatic hydrocarbons. This article refers to "GB / T16488-1996" Determination of Water Quality Petroleum, Animal and Vegetable Oils Infrared Photometric Method "" and chooses three-wavelength infrared spectroscopy to measure surface water. The measurement results are accurate and the use of "standard oil" is avoided.
principle:
Oil in water is a mixture of alkanes, naphthenes and aromatic hydrocarbons, which can be extracted with carbon tetrachloride to determine the total extract. Then, after extracting polar substances such as animal and vegetable oils with magnesium silicate, the petroleum content is determined. The infrared spectra of petroleum and animal and vegetable oils are absorbed at 2930cm-1, 2960cm-1 or 3030cm-1, and their contents can be calculated based on the absorbance values ​​at the above three wavenumber positions.
Experimental conditions:
Instruments and accessories:
FTIR-650 Fourier Transform Infrared Spectrometer
1cm quartz cuvette
Reagents:
Carbon tetrachloride (CCl4): for environmental protection, Tianjin Benchmark Reagent Co., Ltd .;
N-hexadecane [CH3 (CH2) 14CH3] analytical grade: Chengdu Kelong Chemical Reagent Factory;
Granulane (2,6,10,14-tetramethylpentadecane) analytical purity: Beijing Bailingwei Technology Co., Ltd .;
Toluene (C6H5CH3) analytical grade: Tianjin Jiangtian Chemical Technology Co., Ltd .;
Analytical purity of anhydrous sodium sulfate (Na2SO4): Beijing Chemical Plant;
Sodium chloride (NaCl) analytical grade: Tianjin Chemical Reagent Co., Ltd .;
Hydrochloric acid (HCl) analytical grade: Tianjin Chemical Reagent Factory No.1.
Sample preparation:
Transfer all the water samples to a separatory funnel, wash the sampling bottle with 20ml of carbon tetrachloride, the washing liquid is incorporated into the separatory funnel, adjust the pH ≤2, add 20g of sodium chloride, shake thoroughly for 2min and let it stand, extract The liquid flows through a glass sand core funnel covered with 10mm anhydrous sodium sulfate, and the filtrate is collected with a volumetric flask. Take 20ml of carbon tetrachloride for extraction again, wash the glass sand core funnel with an appropriate amount of carbon tetrachloride, and put the extraction solution and the washing solution together in a volumetric flask. Mark with carbon tetrachloride to the score and shake well.
The measurement results:
1. Determination of correction coefficient:
Use carbon tetrachloride as a solvent, and configure a concentration of 100 mg / L n-hexadecane, 100 mg / L pristane, and 400 mg / L toluene solution. Use carbon tetrachloride as the reference solution, using a 10 mm × 10 mm cuvette. , Measure the absorbance A2930, A2960, A3030 of the three solutions at 2930cm-1, 2960cm-1 and 3030cm-1 respectively. The absorbances of these three solutions at the above wavenumbers satisfy the formula:
C = X · A2930 Y · A2960 Z (A3030- A2930 / F),
In the formula:
C- the content of the compound in the extraction solvent, mg / L;
A2930, A2960, A3030-The absorbance value measured under each corresponding wave number;
X, Y, Z-correction coefficient corresponding to the absorbance of each CH bond;
The correction factor for F-aliphatic hydrocarbons to aromatic hydrocarbons, namely the ratio of the absorbance of n-hexadecane at 2930 cm-1 and 3030 cm-1;
For n-hexadecane (H) and pristane (P), since their aromatic hydrocarbon content is zero, that is, A3030-A2930 / F = 0, then F = A2930 (H) / A3030 (H);
C (H) = X × A2930 (H) Y × A2960 (H);
C (P) = X × A2930 (P) Y × A2960 (P);
Thus, the values ​​of X, Y, and F can be obtained.
For toluene (T), there are: C (T) = X · A2930 (T) Y · A2960 (T) Z [A3030 (T)-A2930 / F], from which the Z value can be obtained.
After calculation, the correction coefficients X, Y, Z, and F values ​​are: 126.6, 242.5, 1575, and 63, respectively.
2. Inspection of correction coefficient:
According to the volume ratio of 5: 3: 1, accurately measure n-hexadecane, pristane and toluene to prepare mixed hydrocarbons. Accurately weigh an appropriate amount of mixed hydrocarbons to configure a series of solutions with different concentrations, measure their absorbance at A2930, A2960, A3030 and calculate the concentration and recovery of mixed hydrocarbons.
to sum up
When using "three-wavelength" infrared spectroscopy to determine the oil content in water, it can effectively avoid the measurement error caused by the sudden change of the relative content of the compound of the characteristic group in the sample. Comparing the concentration of the mixed hydrocarbon calculated according to the national standard formula with the true value, the recovery rate meets the requirements of environmental testing and is an ideal method for determining oily substances.
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