Lab Report 7- Determination of Alcohol Content in Wine PDF

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Lab Report 7- Determination of Alcohol Content in Wine...

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Alcohol Content Determination in Wine!

Hannah Strickland

March 14, 2018

Chemistry 355 Section JW

Experiment 7

Introduction: In this experiment, the alcohol content of wine was determined for an unknown sample of wine. Several techniques and methods can be used to determine the alcohol content of wine. In this experiment, this was done by measuring the alcohol percentage using a spectroscopic technique called ultraviolet-visible spectroscopy (UV-Vis). Although this technique is simple to do in a laboratory, it is often less accurate.1 Ethanol, or alcohol, itself is a clear liquid. In order for a UV-Vis spectrophotometer to detect a material, the species must be able to absorb light in the operating region. A species that absorbs light is typically a colored one. Although a wine may appear to be colored, such as red or yellow, the ethanol itself is not colored.1 Thus, in order for ethanol to be detected a reaction must be done. In this experiment, a reaction was done in order to detect ethanol in UV-Vis. Yellow ammonium hexanitrato-cerium(IV) was reacted with colorless ethanol to form red ethoxy-cerium(IV). This red ethoxy-cerium(IV) is able to be detected on the UV-Vis at around 468 nm.1 The ethanol percentage will be able to be detected by finding the absorbance using the Beer-Lamber, shown as equation 1. Equation 1:1

A=εlc

(1)

Experimental: Preparing Ethanol Standards: In this experiment, ethanol standards were made. First, 10 mL of 20% ethanol solution were obtained. Then, by serial dilutions, four more 10 mL solutions with the concentrations of 5%, 7.5%, 10%, and 15% were made. This process was done using a 10 mL graduate pipet, a 10 mL volumetric flask, and deionized water for diluting the samples. Each sample was placed into a labeled test tube. UV-Vis Data Collection: Next, UV-Vis spectroscopy was done. Each solution was scanned from 400 nm to 700 nm on medium. First, a glass cuvette was obtained and a blank composed of diluted hexanitrato-cerium(IV) was done. Then, for each of the diluted ethanol solutions, UV-Vis was done. About 1 mL of the diluted ethanol solution was placed in a cuvette along with 1 mL of the diluted hexanitrato-cerium(IV). The cuvette was immediately placed in the UV-Vis spectrometer. This was done for all five samples. Then, the unknown wine sample was placed into a cuvette and ran.

Constructing Calibration Curve: After UV-Vis was done, Excel was used to plot the absorbance versus the alcohol percentage in an x y scatter plot. A trend line was added to determine the calibration line equation. Then, the equation y = mx + b was solved for x, which was the concentration of ethanol in the wine sample. Results: In this experiment, the concentration of ethanol in an unknown wine sample was determined using UV-Vis. Table 1 shows the data for the preparation of the UV-Vis samples. A graph was obtained from the UV-Vis spectrometer. This graph is shown as graph 1. The graph compares the wavelength versus absorbance for each sample.

Table 1: UV-Vis Data Trial

Concentration Ethanol Added (mL) (%)

Hexanitrato-cerium(IV) Added (mL)

Absorbance

1

20 ~1

~1

0.9904

2

15 ~1

~1

0.6524

3

10 ~1

~1

0.3575

4

7.5 ~1

~1

0.2473

5

5 ~1

~1

0.1250

Graph 1: Graph 1 shows the wavelength versus absorbance for the various samples. The seven lines represent the five ethanol solutions, the hexanitrato-cerium(IV) blank, and the unknown wine solution.

Discussion: Throughout the course of this experiment, a UV-Vis spectrometer was used on various percentage levels of ethanol solutions and then compared to an unknown wine sample in order to determine its ethanol concentration. This data for the ethanol solutions are found in table 1 and graph 1. The absorbance for the unknown wine sample was 0.6356. In order to determine the concentration of ethanol in the unknown wine sample, an x y scatter plot was made, comparing the concentration of ethanol versus the absorbance. This is shown as graph 2. The R2 value for this graph was 0.9942. Because this value is close to 1.0, it indicates that there is little variance in the data obtained.2 A linear line was obtained in the graph, and the linear equation was obtained. This equation was y = 0.05777x — 0.1888. The value x represents the concentration of ethanol in the sample. In order to solve for x, the value y is plugged in, which is the absorbance of the sample. This equation is shown as equation 2, and is solved for the concentration of ethanol in the unknown wine sample. Equation 2: y = 0.05777x — 0.1888 y = 0.6356 0.6356 = 0.05777x — 0.1888 x = 14.2738%

(2)

In solving the equation, it was determined that the ethanol concentration in the unknown sample was 14.2738%. The absorbance value for the unknown was 0.6356. This data roughly matches the data for the 15% ethanol solution, which has an absorbance value of 0.6524. Absorbance vs. Concentration of Ethanol 1 0.9

y = 0.0577x - 0.1888 R² = 0.9942

Absorbance

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 1 2

3 4 5 6

7 8

9 10 11 12 13 14 15 16 17 18 19 20

Concentration of Ethanol (%)

Graph 2: Graph 2 represents the concentration of ethanol versus the absorbance.

Conclusion: In this experiment, the ethanol concentration in an unknown sample of white wine was determined by making five ethanol solutions of different concentrations (20%, 15%, 10%, 7.5%, and 5%). Then, reacting these samples with a hexanitrato-cerium(IV) solution which forms a ethoxy-cerium(IV) solution, which can be detected using UV-Vis. Then, an unknown sample of wine was also ran through the UV-Vis and these results were compared to the ethanol solutions. The concentration of ethanol in the wine solution was determined by solving for x in the linear equation of graph 2, which compares the concentration of ethanol to the absorbance. The absorbance of the unknown was 0.6356 and the concentration was 14.2738%. An error that could have occurred in this experiment would be due to the fact that hexanitrato-cerium(IV) reacts quickly with ethanol. If these samples were not put quickly enough into the UV-Vis spectrometer, the absorbance results could have been inaccurate.1

Citations: 1. Vyazovkin, Sergey. Department of Chemistry. CHEM 355: Quantitative Analysis Student Laboratory Manual. 2. Frost, J. Regression Analysis: How Do I Interpret R-squared and Assess the Goodness-of-Fit? http://blog.minitab.com/blog/adventures-in-statistics-2/regressionanalysis-how-do-i-interpret-r-squared-and-assess-the-goodness-of-fit (accessed Mar 18, 2018)....