DETERMINATION OF ALIPHATIC AND AROMATIC HYDROCARBONS

Abstract :

This method is based on absorption spectra in UV (aromatic hydrocarbons) or IR (aliphatic hydrocarbons) of pure analyzed components and calibrating mixtures of well defined fraction components. The basis of the presented method is the correlation equation between spectra of tested mixtures and spectra of their pure components. The method was tested in UV and IR regions. Benzene solutions in carbon tetrachloride in five different concentrations were tested. Similarly IR spectra of five concentrations of a mixture of Isooctane and n-Hexadecane in CCl4 were examined. The presented method is rapid because it omits the time – consuming stage of preparation and cleanup of samples required for conventional GC method. The analysis time is shorter than 5 min. if spectra of pure components were measured earlier and applied.

Introduction :

Petroleum hydrocarbons (PHCs) are common site contaminants, but they are not generally regulated as hazardous wastes. Petroleum products are complex mixtures of hundreds of hydrocarbon compounds, ranging from light, volatile, short-chained organic compounds to heavy, long-chained, branched compounds. The exact composition of petroleum products varies depending upon

(1) The source of the crude oil (crude oil is derived from underground reservoirs which vary greatly in their chemical composition) and

(2) The refining practices used to produce the product.

Due to the perceived toxicity and carcinogenic character of the aromatic hydrocarbons, the presence of these materials is carefully monitored in all areas where they might enter the human food chain. The analysis of water for aromatic hydrocarbons, particularly surface water in those areas where contamination might take place, is a common assay made by the public analyst. It is essential to be able to measure concentrations in the ppb levels, and thus GC method employing a high sensitive detector is essential. Nevertheless, even if a high sensitivity detector is employed, some sample concentration will be necessary to measure contaminants at such low levels. One method is the purge and trap procedure, using a solid adsorbent to remove the hydrocarbon vapors, which is costly. Gas chromatography (GC) methods do provide some information about the product type. Most methods involve a sample preparation procedure followed by analysis using GC techniques.

Materials & Method :

Samples :

Furnace oil and Oil emulsion.

Solvents:

HPLC grade Carbon tetrachloride, n-Hexane.

Standards:

For Aromatic component – HPLC grade Benzene.

1% stock standard solution was prepared in HPLC grade CCl4. This stock solution was used to prepare the working standard solution of 0.01%, 0.02%, 0.03%, 0.04% and 0.05%.

For Aliphatic component – HPLC grade Isooctane, n-Hexadecane.

First a mixture of 1% stock standard solution was prepared in HPLC grade CCl4. This stock solution was used to prepare the working standard solution of 0.01%, 0.02%, 0.03%, 0.04% and 0.05%.

Analysis :

(a)  2.5 g of oil sample was taken, extracted with n-Hexane for Oil & Grease following standard APHA 21st Edition 5520B method followed by APHA 21st Edition 5520F method for Hydrocarbon part. Simultaneously 2.5g of the same sample was extracted with CCL4 and analyzed using FTIR. The Hydrocarbon content obtained by both the methods is in excellent correlation.

(b)  Aliphatic component by FTIR – A standard curve has been drawn with the working standard solutions for aliphatic component using FTIR. The solutions were taken in 10 mm path length cell and scanned from 3000 – 2821 cm-1 range. Positive correlations among different concentrations have been observed. Measurement of the sample was done against this standard curve.

(c) Aromatic component by FTIR – A standard curve has been drawn with the working standard solutions for aromatic component using FTIR. The solutions were taken in 10 mm path length cell and scanned from 3116 – 2996 cm-1 range. Positive correlations among different concentrations have been observed. Measurement of the sample was done against this standard curve.

(d) Aromatic component by UV – A standard curve has been drawn with the working standard solutions for aromatic component using UV Spectrophotometer. The solutions were taken in 10 mm path length cell and measured at 267 nm wavelength. Positive correlations of different concentrations have been observed. Measurement of the sample was done against this standard curve.

Results & Discussion :

  • The same sample was analyzed APHA method and by FTIR. The value obtained for Hydrocarbon from both the methods show excellent correlations. Value obtained from gravimetric method was 54.24% whereas by FTIR it was 54.62%.
Type of Sample Concentration of Aliphatic component by FTIR Concentration of Aromatic component by FTIR Total Hydrocarbon obtained by Gravimetric Method Total Hydrocarbon obtained by IR
Furnace Oil 34.64% 19.60% 54.24 % 54.62 %
Oil Emulsion 0.33 % 0.2 % 0.50 % 0.52 %

 

  • Presence of aliphatic component was characterized by C – H stretching absorption band at 3000 – 2821 cm-1 range. Calibration curve was obtained with a correlation coefficient of 0.9997 (Fig. 1). The spiked sample shows a concentration of 0.0228% with a recovery of 91.2%.

  • Presence of aromatic component was characterized by C – H stretching absorption band at 3116 – 2996 cm-1 range. Calibration curve was obtained with a correlation coefficient of 0.9964 (Fig. 2). The spiked sample shows a concentration of 0.0224% with a recovery of 89.6%.

 

  • Presence of aromatic component in the spiked sample was also measured using UV spectrophotometer. This shows a concentration of 0.0222% with recovery of 88.8%. This was quantified against a calibration curve with a correlation coefficient of 0.9988 (Fig. 3).

Conclusion :

Spectroscopy is one of the major analytical tools for analyzing hydrocarbons qualitatively as well as quantitatively. An FTIR spectrum is a very sensitive, reliable and less time consuming technique for hydrocarbon analysis. This method is simple, cost effective and faster than conventional GC method which requires a time taken sample preparation technique.

We are thankful to Prof. Barun Gupta for his continuous encouragement, guidance and valuable suggestions. 

Contributed by: Ms. Nandita Das

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