UV Spectrophotometry for Oil-in-Water Determination

Oil contamination in water is a critical environmental concern. Petroleum hydrocarbons entering water bodies not only impair aesthetic quality but also deplete dissolved oxygen and threaten aquatic ecosystems. Accurate determination of oil content is therefore essential for water quality assessment and pollution source control. Among the various analytical techniques available, ultraviolet (UV) spectrophotometry has emerged as one of the most widely adopted methods for routine monitoring of oil in surface water, groundwater, and seawater.

Regulatory Background and Method Development

The adoption of UV spectrophotometry as a standard method in China is closely tied to international environmental commitments. For many years, the determination of petroleum oils in water relied on infrared spectrophotometry using carbon tetrachloride as the extraction solvent. 

However, carbon tetrachloride was listed as a controlled substance under the Montreal Protocol on Substances that Deplete the Ozone Layer, and China committed to ceasing its use in laboratory applications from 1 January 2019. To address this regulatory imperative, the Ministry of Ecology and Environment issued the national standard Water Quality – Determination of Petroleum Oils – UV Spectrophotometry (Trial) (HJ 970-2018), which took effect on 1 January 2019. This standard replaced the previously used extraction solvent with n-hexane, a more environmentally benign alternative.

Principle of the Method

The UV spectrophotometric method is founded on the Lambert-Beer law. Under acidic conditions (pH ≤ 2), oil substances in the water sample are extracted with n-hexane. The extract is dehydrated with anhydrous sodium sulfate and then passed through magnesium silicate to adsorb polar substances such as animal and vegetable oils. 

The absorbance of the purified extract is measured at a wavelength of 225 nm using a quartz cuvette. The concentration of petroleum oils is directly proportional to the absorbance value, allowing quantitative determination through a calibration curve.

The method specifically targets aromatic hydrocarbons containing conjugated double bonds and benzene rings, which exhibit characteristic absorption in the ultraviolet region. This selectivity makes UV spectrophotometry particularly suitable for monitoring petroleum-derived oils, though it is less responsive to saturated hydrocarbon-dominated oils.

Analytical Procedure

The analytical workflow comprises several sequential steps. Sample collection is followed by acidification to pH ≤ 2 with hydrochloric acid. If analysis cannot be completed within 24 hours, samples must be stored at 5 °C or below and analysed within seven days. For extraction, the entire sample is transferred to a separatory funnel, and 25.0 mL of n-hexane is added. 

mixture is shaken vigorously for two minutes and allowed to stand for phase separation. The organic extract is then passed through anhydrous sodium sulfate for dehydration and treated with magnesium silicate for adsorption cleanup. Finally, absorbance is measured at 225 nm using a 2 cm quartz cuvette, with n-hexane serving as the reference blank.