Determination of Oil in Water by Ultraviolet Spectrophotometry

Key words: oil in water; Ultraviolet spectrophotometry; Macylab Instrument www.macylab.com; UV-1100,UV-1200

Ⅰ Purpose of the experiment

Deepen the understanding of oil pollution in the environment, master the analysis method and technology of oil, and learn to use ultraviolet spectrophotometer.

Ⅱ experimental principle

Oil in the water comes from the decomposition of higher organisms or plankton, as well as pollution from industrial wastewater and domestic sewage. The oil floating on the surface of water affects the oxygen exchange at the air-water interface. Oil dispersed in water, partly adsorbed on suspended particles, or present in an emulsified state in water, partly dissolved in water. Oil in water can be oxidized and decomposed by microorganisms, thus consuming dissolved oxygen in water and worsening water quality.

Gravimetric method is a common analytical method, it is not limited by the variety of oil, the measured oil can not distinguish between mineral oil and vegetable oil. Gravimetric method is accurate, but the operation is complicated and the sensitivity is poor, and it is only suitable for the determination of oil products above 5mg/L. Ultraviolet spectrophotometry is simpler than gravimetry. The substances with conjugated systems contained in petroleum have characteristic absorption peaks in the UV region. The main absorption wavelength of aromatic compounds with benzene rings is 250 ~ 260nm, and the main absorption wavelength of compounds with conjugated double bonds is 215 ~ 230nm. The two absorption peaks of general crude oil are 225 and 256nm, and the absorption peaks of other oil products such as fuel oil and lubricating oil are similar to crude oil. The wavelength of this method was selected as 256nm, the minimum detection concentration was 0.05mg/L, and the upper limit was 10mg/L.

Ⅲ  Instruments and reagents

1, ultraviolet spectrophotometer (with 1cm quartz colorimeter). American analysis instrument UV-1100,UV-1200

2, 1L separator funnel.

3. 25mL volumetric bottle.

4, petroleum ether (60 ~ 90℃) or n-hexane: purified after use, light transmittance of more than 80%. If it is not pure, it can be purified by the following method.

Purification: 0.30 ~ 0.15mm (60 ~ 100 mesh) coarse pore microspheres of silica gel and 0.246 ~ 0.125mm (70 ~ 120 mesh) neutral chromatography alumina were activated at 150 ~ 160℃ for 4h, and then put into a glass column with a diameter of 2.5cm and a length of 75cm while warm, so that the silica column was 60cm high and covered with a 5cm thick alumina layer. The petroleum ether is collected in the reagent bottle after passing through the column. With water as the reference, the light transmittance at 256nm should be greater than 80%.

5, Oil standard reserve liquid: Prepared with No. 20 heavy diesel oil, No. 15 oil or other certified standard oil. Accurately weigh 0.1000g of standard oil and dissolve in petroleum ether, transfer to 100mL volumetric bottle, and dilute with petroleum ether to the line, the solution contains 1.00mg oil per milliliter, and store in the refrigerator for use.

6. (1+1) sulfuric acid.

7. Sodium chloride.

8. Anhydrous sodium sulfate (baked at 300℃ in Muffle oven for 1h in advance, bottled after cold).

Ⅳ  Experimental steps

1, The drawing of the standard curve diluted the oil standard reserve liquid with petroleum ether to a standard liquid containing 0.100mg oil per milliliter. Add oil standard liquid 0.20, 0.50, 1.00, 2.00, 3.00, 5.00, 7.00, 10.00mL to 8 10mL volumetric bottles successively and dilute with petroleum ether to the mark. The corresponding concentrations are 2.00, 5.00, 10.00, 20.0, 30.0, 50.0, 70.0, 100.0mg/L. Finally, the absorbance of the standard series was measured with a 1cm quartz cuvette at the wavelength of 256nm and petroleum ether was used as the reference liquid, and the standard curve was drawn.

2, Pour all 500mL of water sample into a 1000mL liquid separation funnel, add 5mL (1+1) sulfuric acid (if the water sample has been acidified during sampling, do not add) and 20g sodium chloride, plug and shake well, wash the sampling bottle with 15mL petroleum ether, and move the lotion into the liquid separation funnel, shake fully for 2min (pay attention to deflating), and stand for stratification. The lower water sample was put into the original sampling bottle, the upper petroleum ether was put into a 25mL volumetric bottle, and then 10mL petroleum ether was added. The water sample was extracted once again, and the extraction solution was combined in the volumetric bottle. Add petroleum ether to line and shake well. If there is water in the volumetric bottle or turbidity, a small amount of anhydrous sodium sulfate can be added to dehydrate.

3, at the wavelength of 256nm, with 1cm quartz cuvette, to dearomatic petroleum ether as a reference, determine its absorbance, and find out the corresponding concentration value on the standard curve.

Ⅴ Results and discussion

1, C oil (mg/L) = C*V2/V1

Where: C -- the corresponding oil concentration (mg/L) found on the standard curve;

V1 -- volume of water sample to be measured (mL); V2 - petroleum ether constant volume (mL).

2, the use of petroleum ether should be mixed in a larger container, the use of the same light transmittance of petroleum ether, draw standard curve and sample determination, otherwise it will be due to different blank values and errors.

3, The samples collected must be representative. When measuring only oil in the emulsified state of water, avoid oil floating on the water surface. Water samples are generally taken 20 to 50cm below the surface of the water. If you want to sample with the oil film, pay attention to the depth of the water, the thickness of the oil film and the covered area.

4, the sampling bottle should be a constant volume (such as 500 or 1000mL) clean glass bottle, clean with solvent, do not wash with soap. Each time sampling, water should be filled to the scale line.

5, in order to preserve the water sample, before collecting the sample, sulfuric acid can be added to the bottle [add 5mL (1+1) sulfuric acid per liter of water sample] to make the pH of the water sample < 2, inhibit microbial activity, and store at low temperature (< 4℃). At room temperature, the sample can be stored for 24h.