31090_Julie_Richards_RICHARDS_STNDADDITION_DISCUSSION_29861_62351859.docx

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Unformatted text preview: Spectroscopic Determination of Allura Red Concentration in KoolAid Spectroscopy was utilized to form a standard additions curve for Allura Red, and then linear regression of the curve was performed to find the concentrations of Allura Red in Kool Aid. Kool Aid concentrations found by the multipoint calibration and standard addition methods were then compared. Absorbance Spectra of Allura Red Figure 1 shows the absorbance spectra for Allura Red that was used to determine the wavelength of maximum absorbance. The solution with the highest concentration of Allura Red had a maximum absorbance that occurred at 503.5 nanometers. All absorbance data for the remainder of the experiment was collected at this wavelength. γmax is the wavelength of light that is characteristic of Allura Red. Using this wavelength to measure absorbance of Allura Red in solutions containing compounds in addition to Allura Red ensures the highest sensitivity and minimizes deviations from Beer’s law. Formation of a Standard Additions Curve Figure 2 shows the linear regression of the standard addition curve from trial 1 that was created based on the average corrected absorbance of known Allura Red concentrations added to solution. Visually, the curve appears to be of high quality. All five points fall close to the trendline, and the slope is linear. The R 2 was 0.997. This value also reflects a high quality curve because it shows that the data strongly represents the linear fit. Being so close to 1, the R 2 value means almost all of the variability of the data is explained around the mean. Since a linear regression of the trendline will be used to determine unknown concentrations, the high R 2 indicates that there will be a low amount of error in the calculated unknowns. The error bars of the standard addition curve are considerably small, reflecting precision in all five data points. Precise data also reflects a high quality curve. Data points that are imprecise do not properly identify random error and can lead to a widespread result. In summary, the standard addition curve was of high quality because of the accuracy of the linear fit to the data, and the precision of the data points. Although small, the error that does exist is probably mostly due to instrumental error, such as volume and measurement uncertainties. Allura Red Concentration in Kool Aid & Comparison to Manufacturer’s Value The standard addition spectroscopic method determined the concentration of Kool Aid to be 3.581 x 10-5 ± 0.225 x10-5 M at the 95% confidence level. This interval fell on the upper end of the curve, right between the fourth and fifth data points. In contrast to the multipoint calibration method, this does not mean the method has more error than if it had fallen on the middle of the curve. The manufacturer’s value of 1.32E-4 M Kool Aid did not fall within the confidence interval that was obtained from the standard addition curve. In fact, the manufacturer’s value did not fall anywhere close to the curve. It was much higher. This could be due to FDA guidelines that require the company to report the maximum possible concentration,. It could also be due to the powdered form of the Kool Aid used in the lab. The manufacturer’s value may be for a different form of Kool Aid. The calculated confidence interval could still be an accurate representation of the Allura Red concentration in Kool Aid. The Manufacturer’s value could also be for a flavor of Kool Aid that has a higher concentration of Allura Red than the Grape used in this lab, such as Cherry. Allura Red concentration found by Standard Addition vs Multipoint Calibration An F test compared the variances of the Allura Red concentrations in Kool Aid found by the standard addition and multipoint calibration methods. The F test found an F that was greater than the F critical (195.15>9.12). Therefore it was found that the variances were unequal. Unequal variances made sense considering two different methods were used to calculate the concentrations. In general, the standard addition method has a smaller range of values over which it can obtain a precise measurement. It may also be important to mention that the stock solution of Allura red was used up after trial 1 of the standard additions. A new stock with a different concentration was made. This caused the following four trials to have different standard addition concentrations than the first. This may have increased variance. A case 2 student’s t-test assuming unequal variances was performed. The ttest found a t stat greater than t critical (3.96>2.77). Therefore it was assumed that the Allura Red concentrations found in Kool Aid by the two methods were not statistically the same. The mean for the standard addition method was greater than the mean for the multipoint calibration method (3.58>3.26). The standard addition method found a mean that was significantly greater than the mean found by the multipoint calibration method. This made sense as the standard additions method is an extrapolation and the multipoint calibration method is an interpolation. The standard additions method is useful when the matrix composition is unknown or may interfere with the analyte. Therefore it controls for the matrix effect. However it does this by assuming a small amount of standard will not alter the composition of the sample. The added standard plus the analyte already in the matrix are calibrated to the concentration of the standard only. By contrast, the multipoint calibration method uses the interpolatic method of calibrating known concentrations and the absorbance that correspond and placing the unknown on that curve. Since the matrix composition of Kool Aid is known and does not seem to interfere with absorbance values, it can be assumed that the Allura Red concentrations found by the multipoint method were more accurate. Conclusion In conclusion, the standard additions curve created at 503.5 nanometers for Allura Red and Kool Aid was of high quality and introduced low error. The linear regression of this curve calculated Kool Aid concentrations that were lower than the manufacturer’s value. This could be due to a number of reasons. More Kool Aid samples need to be measured to make a more conclusive statement. Kool Aid was found to have significantly more Allura Red when analyzed by the standard addition method than the multipoint calibration method. Figure 1 1 0.8 0.6 Absorbance 0.4 0.2 0 380 480 580 680 780 880 -0.2 Wavelength (nm) The absorbance spectra for a solution of Allura Red at all wavelengths in the in the visible light spectrum. The maximum absorbance (0.881) occurs at wavelength 503.5 nanometers. This is shown by the peak in the graph. Figure 2 1.2 1 0.8 Absorbance f(x) = 19707.07x + 0.27 R² = 1 0.6 0.4 0.2 0 5.0E-6 1.0E-5 1.5E-5 2.0E-5 2.5E-5 3.0E-5 3.5E-5 4.0E-5 Concentration The visual linear regression of the standard addition curve for the blank corrected average absorbance of 8.38E-6 M, 1.68E-5 M, 2.10E-5M, 2.93E-5M, and 3.77E-5 M Allura Red. The slope of the graph is 19,707. The R^2 value is 0.997. The x intercept is 1.36E-5. The concentrations are in moles per liter. The uncertainty in the slope is 624.26. The uncertainty in the y intercept is 0.0154. The propagated uncertainty is 0.0141. Sources (2013) Laura J. Stevens , John R. Burgess , Mateusz A. Stochelski , and Thomas Kuczek, Amounts of Artificial Food Colors in Commonly Consumed Beverages and Potential Behavioral Implications for Consumption in Children. ...
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