UV-Vis

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Northern Illinois University *

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231

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Chemistry

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Dec 6, 2023

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pdf

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UV-Vis Experiment: Building a Calibration Curve for Tartrazine in Cola 1. Full absorbance spectrum of Tartrazine measured by UV-Visible spectrophotometer Concentration = 2.02*10 -5 M λ max = 425 nm 2. Instruments and relevant settings The Avantes UV-Vis spectrophotometer software was used. A 170-800 nm diode-array absorbance spectrophotometer was used to measure the absorbance of samples.

3. Calibration curves Calibration curve for all solutions Calibration curve for linear dynamic range

The linear range was chosen by firstly removing the most highly concentrated solution from the data. Then, the R 2 value for the linear regression line was examined as more solutions high in concentration were removed. The R 2 value indicates how well the linearized regression line approximates the data. The closer to 1 this value is, the better the regression line represents the data. Since this line is linear, as the R 2 value approaches 1, the linearity of the data increases. A R 2 value of exactly 1 would indicate that the data’s relationship is perfectly linear (this is only the case when the regression line is linear itself). Removing the two most highly concentrated solutions from the data resulted in the R 2 value increasing from around 0.95 to 0.99, indicating that those solutions were not a part of the linear dynamic range. Any other removals of data points resulted in the R 2 value decreasing, indicating that the data’s relationship was moving further away from linearity. Another method used to double check this was calculating the percent difference of the absorbance measured and the absorbance predicted by the regression line. Any difference greater than 10% indicates that the data point does not belong in the linear dynamic range. Calculations necessary in building the calibration curves are included in Appendix. 4. Residual plot The plot of Tartrazine concentration residuals shows the difference between the measured absorbance values for each solution concentration and the absorbance values fitted via the linear regression line. This shows that the measured and fitted absorbances had some discrepancies.

However, these discrepancies were slight, showing that the linear regression line accurately approximated the data. 5. Molar absorptivity of Tartrazine From literature: At λ max = 425 nm, ε = 2.63*10 4 L*mol -1 *cm -1 Calculated using calibration curve: At λ max = 425 nm, ε = 2.22*10 4 L*mol -1 *cm -1 These values are quite similar, showing that data was accurately collected and fitted. Calculation included in Appendix. 6. LOD and LOQ for Tartrazine concentration using blank measurement LOD = 2.06*10 -6 mol/L LOQ = 6.88*10 -6 mol/L Calculations included in Appendix. 7. Concentrations of unknowns and Gatorade [Unknown 1] = 3.45*10 -5 ± 3.47* 10 -6 mol/L [Unknown 2] = 9.71*10 -5 ± 8.67* 10 -6 mol/L [Gatorade] = 3.20*10 -5 ± 3.477* 10 -6 mol/L Calculations included in Appendix. The concentrations of unknown 1 and gatorade fall into the linear dynamic range, which meant they were able to be calculated using the calibration curve. Since the concentration of unknown 2 fell out of the linear dynamic range, a diluted solution was used in order to be able to utilize the calibration curve. All of these concentrations are much higher than the LOD, which means their absorbances were accurately measured, thus, the calculated concentrations can be determined to be plausible. The LOD makes sense in this context because it is low enough to properly detect the errors associated with each absorbance measurement and accurately detect the absorbances of each solution well within its means.

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