Chemistry, General/Organic – Quantitative Analysis Lab Methods Source: Conklin et al., J. Chem. Educ. 2014, 91, 889–891 | Lab Experiment
Difficulty: Introductory to Intermediate Prerequisites: Companion study notes on FT-IR Spectroscopy for Ethanol Analysis in Gasoline. Basic algebra (solving a linear equation for x). Familiarity with preparing solutions by volume.
This set of notes focuses on the quantitative side of the ethanol-in-gasoline experiment: how to build a calibration curve from known standards, use it to determine an unknown concentration, and evaluate the quality and limitations of your results. These skills transfer to virtually every instrumental analysis technique you will encounter, from UV-Vis to HPLC to mass spectrometry.
If you understand the IR theory from the companion notes, you are ready for this. The emphasis here is on the measurement workflow, the maths, and the sources of error.
You prepare a series of ethanol-in-hexanes standards at known concentrations, measure the O–H/C–H area ratio for each, and plot these to get a straight line. That line is your calibration curve. Plug in a gasoline sample’s area ratio, solve for x, and you have its ethanol percentage. The method works well but has a detection limit near 3% and is sensitive to how carefully you prepare your standards and define your integration regions.
Absorbance mode
The spectrometer setting in which the IR data are recorded as absorbance (a logarithmic scale proportional to concentration) rather than transmittance (a percentage scale). Absorbance mode is required for quantitative analysis because absorbance is linearly proportional to concentration (Beer’s Law), whereas transmittance is not.
In simple terms, always collect in absorbance mode when you want to measure how much of something is present.
Baseline correction
A software adjustment that subtracts the background signal from the spectrum so that peak areas reflect only the analyte’s absorption. If the baseline drifts or is set inconsistently between samples, your area measurements will be off.
Think of it as zeroing your ruler before you measure.
Peak integration (peak area)
The process of calculating the total area under an absorption band in the spectrum. In this experiment, you integrate the O–H region and the C–H region separately, then take their ratio. The area is more reliable than peak height for quantitative work because it accounts for band broadening.
Limit of detection (LOD)
The lowest concentration of analyte that can be reliably distinguished from a blank (zero-concentration sample). For this method, the LOD is approximately 3% ethanol by volume. Below this, the O–H signal disappears into baseline noise.
Systematic error
An error that consistently shifts results in one direction (always too high or always too low), as opposed to random scatter. In this experiment, the use of denatured ethanol (containing methanol and 2-propanol) and the contribution of ethanol’s own C–H bonds to the reference region are both sources of small systematic error.
R² (coefficient of determination)
A statistical measure of how well a straight line fits the data points. R² = 1.00 means a perfect fit. The calibration curves in this experiment typically have R² values around 0.99, which indicates a very good linear relationship.
Water extraction (verification method)
A separate technique used to cross-check the IR result. You weigh a gasoline sample, shake it with water (ethanol dissolves into the water, alkanes do not), separate the layers, and reweigh the gasoline. The mass difference tells you the ethanol content by mass, which can then be converted to a volume percentage for comparison.
Prepare eight ethanol-in-hexanes solutions at 0.0%, 1.0%, 3.0%, 5.0%, 10%, 30%, 50%, and 85% ethanol by volume.
Volumes are straightforward. For example, 5.0% = 0.50 mL ethanol + 9.50 mL hexanes in a 10 mL total.
Use reagent-grade ethanol ("reagent alcohol": 90.25% ethanol, 4.75% methanol, 5.00% 2-propanol). This is standard laboratory ethanol; the small amount of other alcohols introduces a minor systematic error that is acceptable for educational purposes.
Precision here is critical. Sloppy volume measurements are the single biggest source of student error in this experiment.
Run each standard on the FT-IR spectrometer using an ATR (attenuated total reflectance) sampler.
Collect spectra in absorbance mode (not transmittance).
All sample handling should be done in a fume hood. Hexanes, ethanol, and gasoline are volatile and flammable.
Cover the ATR sample plate during collection. Place waste in an appropriate container.
Using the spectrometer’s software, integrate two regions for each spectrum:
O–H absorption: approximately 3660 to 3045 cm⁻¹
C–H absorption: approximately 3045 to 2760 cm⁻¹
The boundaries and baseline correction settings must be kept identical across all standards and samples. Inconsistency here is a major source of error, especially at low ethanol concentrations.
For each standard, calculate the ratio: (O–H area) / (C–H area).
Plot percent ethanol (x-axis) against the area ratio (y-axis).
Fit a straight line. The representative equation from the paper is y = 0.0268x + 0.0039, with R² = 0.99249.
Check your curve: if one or more points fall clearly off the line, recheck your standard preparation for that concentration.
Collect the FT-IR spectrum of your gasoline sample with the same settings.
Measure the O–H and C–H areas, calculate the ratio.
Solve for percent ethanol using the calibration equation.
The paper reports that a gasoline sample advertised as containing up to 10% ethanol yielded an average result of 10% by this method.
Weigh a sample of gasoline.
Add water and shake. Ethanol dissolves preferentially into the water layer; alkanes do not.
Separate the layers and reweigh the gasoline.
The mass difference gives ethanol content by mass (the paper found approximately 10.5% by mass, which corresponds to about 8.9% by volume).
This result is comparable to the 10% by volume from the IR method.
Note: a trace of ethanol may remain dissolved in the gasoline after extraction, so this method may slightly underestimate.
Calibration line (from representative data):
y = 0.0268x + 0.0039
where y = (Area O–H) / (Area C–H) and x = % ethanol by volume.
Solving for the unknown:
% Ethanol = (y – y-intercept) / Slope = (y – 0.0039) / 0.0268
Volume-to-mass conversion (for comparison with water extraction):
A solution of 10.5% ethanol in hexanes by mass is approximately 8.9% ethanol by volume. The difference arises because ethanol (density ~0.789 g/mL) is less dense than hexanes (density ~0.659 g/mL). When comparing results between the IR method (% by volume) and the extraction method (% by mass), you need to convert.
Calibration curves are the foundation of quantitative instrumental analysis across all of chemistry. The workflow you learn here (prepare standards, measure response, fit a line, determine unknowns) is exactly what clinical labs use to measure blood glucose, what environmental labs use to measure pollutants in water, and what forensic labs use to quantify drugs in biological samples.
In the fuel industry specifically, ASTM standard test methods (such as ASTM D 5845-01) prescribe IR spectroscopy for determining ethanol and other oxygenates in commercial gasoline.
Students often think that if one or two data points fall off the calibration line, the method has failed. It has not. A point off the line usually means that particular standard was prepared incorrectly. The remedy is to reprepare that standard, not to abandon the method.
There is a tendency to assume that the calibration curve should pass exactly through the origin (0, 0). A small positive y-intercept is expected and is handled by the equation. Forcing the line through the origin would introduce error.
Some students confuse precision with accuracy. A tight cluster of student results around 14% for gasoline advertised at 10% is precise but inaccurate, and the inaccuracy traces back to systematic issues with standard preparation.
Students sometimes assume that measuring a higher peak height means more analyte is present. For quantitative work, you use peak area, not peak height, because the width of the absorption band can vary with concentration and hydrogen bonding.
Be able to walk through the full calibration workflow: prepare standards, collect data, plot, fit line, solve for unknown.
Know how to use the calibration equation to calculate percent ethanol from a measured area ratio. Expect a numerical problem on this.
Understand the role of the C–H absorption as a quasi-internal standard and why using a ratio is better than an absolute measurement.
Be able to name at least three sources of error in this experiment (standard preparation, baseline correction, denatured ethanol, C–H contribution from ethanol itself, integration region consistency).
Know the detection limit (approximately 3%) and be able to explain what determines it.
Understand the difference between percent by volume and percent by mass, and be able to explain why the two give different numbers for the same sample.
True or false: Transmittance mode is suitable for quantitative IR analysis. (False – absorbance mode is required because absorbance is linearly proportional to concentration.)
If your calibration equation is y = 0.0268x + 0.0039, what is the slope? (0.0268)
True or false: Peak height is more reliable than peak area for quantitative analysis. (False – peak area is preferred.)
Name one reason the water extraction method might underestimate ethanol content. (A trace of ethanol may remain dissolved in the gasoline after extraction.)
Fill in the blank: The coefficient of determination (R²) for the calibration curves in this experiment is typically around _______. (0.99)
Q: Describe the complete workflow for determining ethanol content in gasoline by FT-IR, from standard preparation to final result.
A: Prepare a series of ethanol-in-hexanes standards at known volume percentages (0% to 85%). Collect an FT-IR spectrum of each in absorbance mode. Integrate the O–H absorption (3660–3045 cm⁻¹) and C–H absorption (3045–2760 cm⁻¹) for each standard. Calculate the area ratio (O–H / C–H). Plot percent ethanol vs. the ratio and fit a linear regression. Then collect the gasoline sample’s spectrum under identical conditions, measure its area ratio, and solve the calibration equation for percent ethanol.
Q: Why are hexanes used as the model solvent rather than ethanol-free gasoline?
A: Hexanes are readily available, inexpensive, safe to handle, and representative of the C5–C12 alkanes in gasoline. Ethanol-free gasoline is hard to find in most areas, and removing ethanol from gasoline by water extraction is time-consuming and exposes users to aromatic hydrocarbons.
Q: A student’s calibration curve has an R² of 0.85. What should they suspect, and what should they do?
A: An R² of 0.85 suggests that one or more standards were prepared inaccurately or that the peak integration regions were set inconsistently between measurements. The student should examine the plot for outlier points, reprepare those standards, and ensure that the same integration boundaries and baseline correction are applied to every spectrum.
Q: The water extraction method gives 10.5% ethanol by mass for a gasoline sample, while the IR method gives 10% by volume. Are these consistent? Explain.
A: Yes. Because ethanol is denser than the alkanes in gasoline, a given mass percentage of ethanol corresponds to a smaller volume percentage. The paper notes that 10.5% by mass in hexanes corresponds to approximately 8.9% by volume. The slight remaining discrepancy is within expected experimental error and may also reflect the trace ethanol that remains in gasoline after water extraction.
Q: List three specific sources of systematic error in this experiment.
A: (1) The reagent ethanol contains methanol and 2-propanol, so the O–H signal is not from pure ethanol alone. (2) Ethanol itself has C–H bonds that contribute to the C–H reference region, meaning the internal standard is not purely from alkanes. (3) The use of hexanes rather than actual gasoline alkanes introduces a small compositional difference.
The calibration-curve method taught here is the same workflow used in UV-Vis spectroscopy (Beer’s Law plots), atomic absorption spectroscopy, HPLC, and gas chromatography. Once you understand it for FT-IR, you understand it for all of them; only the instrument and the specific measurement change.
The concept of an internal standard connects to analytical chemistry more broadly. Whenever you can normalise your analyte signal against a reference signal that is always present and roughly constant, you reduce the effect of run-to-run variability. This principle appears in techniques from NMR to mass spectrometry.
The volume vs. mass percentage distinction comes up throughout physical chemistry and solution preparation. Getting it wrong is a common source of error in any lab that works with solutions.
Calibration curve, standard curve, Beer’s Law, linear regression, R-squared, coefficient of determination, peak area, peak integration, baseline correction, absorbance mode, limit of detection, LOD, internal standard, quasi-internal standard, systematic error, random error, percent by volume, percent by mass, water extraction, ethanol determination, ASTM D 5845, quantitative IR analysis, FT-IR quantitative method, standard solution preparation, unknown concentration, gasoline testing