Analysis of Ethanol in Gasoline by Gas Chromatography and Fourier-Transform Infrared Spectroscopy

The addition of ethanol into gasoline has long been a method to help the hydrocarbons of gasoline burn smoother in an internal combustion engine by adding oxygen into the fuel. Gas stations list ethanol content in gasoline can be up to 10% ethanol but no more than 10% ethanol. By using the methods of standard addition, using known concentrations and volumes of spikes of ethanol into the gasoline, we may extrapolate back the original concentration of ethanol in the mixture of gasoline. Quantifying these results with gas chromatography, we received 10 (±1) volume percent ethanol at the 95% confidence interval and looked for the 1.87-minute retention time. Similarly, we constructed the same samples under the same environments and ran them using Fourier-Transform Infrared Spectroscopy and looked for the peak near 1100 cm. Using infrared spectroscopy, it was determined that the ethanol content in gasoline was 13 (±2) volume percent ethanol. Both methods were examined at the 95% confidence interval and subsequent Ftest and Ttest confirm that these two methods of analysis for ethanol are statically different. Introduction Internal combustion engines, such as the ones in automobiles, intake air from the environment to burn a fuel to produce work. The fuel used would be gasoline but when gasoline is bought from the gas station, 100% pure gasoline is not being purchased. The actual composition of the gasoline being purchased is near 90% where the rest of the 10% is from the added ethanol. Ethanol has been added to because it oxygenates the fuel for internal combustion engines to create more work. Since the fuel is already oxygenated with ethanol, the fuel will burn easier and cleaner compared to 100% gasoline. The structure of ethanol (fig. 1) confirms this theory by having a very small structure with an alcohol group attached. This alcohol will be the oxygen used to help the fuel burn when added in small amounts of up to 10% by weight. The oxygenated fuel will help the engine always contain some amount of oxygen to burn for the engine. Gasoline, in itself, is not a pure substance but a mixture of many different lengths of hydrocarbons ranging from 4 to 12 carbons. The general reaction for the combustion of a hydrocarbon from gasoline is as follows in equation 1. 2 C8H18 + 25 O2 → 16 CO2 + 18 H2O (1) Knowing that gasoline is a mixture of hydrocarbons with added ethanol, it will be challenging to select the amount of ethanol due to the amount of compounds in the solution. Current techniques such as a calibration curve, UV-Vis spectrophotometry, and atomic adsorption spectroscopy are not the most optimal instruments to use to analyze the composition of ethanol in gasoline. Techniques used in this experiment are designed to work around the obstacle of having multiple similar compounds in the mixture. Fourier Transform Infrared spectroscopy (FT-IR) and gas chromatography (GC) are the instruments better suited for this analysis. Using these techniques with standard addition will allow the determination of ethanol in a mixture of hydrocarbons. Standard addition is similar to standard calibration in which a graphical analysis approach is taken. Standard addition employs adding known concentrations and quantities of standard into the analyte which may contain many unknown compounds in unknown concentrations. Measuring 5 different test tubes that all contain the same amount of analyte and increasing amounts of percent standard added, the initial concentration of a compound can be found. In this experiment, known quantities of percent ethanol are Fig. 1 The Lewis structure and condensed structure of