Analysis of Food Dyes in Beverages
Beer's Law (simulation from PhET)
Brief Summary
- The color of a solution shown is the complement to the light it absorbs, refer to the color wheel
- Absorbance quantifies how much light is absorbed at a certain wavelength, is the common logarithm of the ratio of incident (I_0) to transmitted (It) light intensity through a material (I_0/I_t)
- In a certain concentration range, absorbance is proportional to the length of light path (b = 1 cm for standard cuvette) and molarity (c) of the colored species, Abs = abc, where a is a constant called molar absorptivity (Beer's Law)
- Another great interactive platform about the Beer's Law can be found here (shiny-app written by R)
- Practice problems from USNCO: N2020-Q8, N2015-Q6, N2009-Q8, N2000-Q6
Determination of a Dye Concentration in a Sports Drink
Briefly Summary
- the idea of using Beer's Law to quantify or semi-quantify color species in solutions is called colorimetry
- theoretically, absorbance at any wavelength should follow the Beer's Law, but the wavelength with a maximum absorbance (λ_max) is commonly used for measurement
- A calibration curve (λ_max vs molarity) is needed for any spectrophotometric determination to get the molarity of target sample by plotting its absorption
- when holding cuvette, fingerprints should NOT be put on the more transparent side (light path), otherwise, the absorption measured is higher than actual value
- Another good video to refer to is here
- Practice problems from USNCO: L2019-Q10, N2017-Q9, L2015-Q29, N2010-Q5, N2009-Q4, L2009-Q5, N1999-Q4
Acid-Base Titrations and Titration Curve
Setting up and Performing a Titration
Brief Summary
- titration is used to measure the accurate molarity of an acid or a base based on stoichiometry: c1V1 = c2V2
- typically, the ~20.00 mL analyte is transferred using a volumetric pipette to a Erlenmeyer flask, the titrant (with known molarity) is added through a buret dropwise
- titrant may need to be standardized prior to use, such as HCl(aq), NaOH(aq), etc., which might change molarity after standing up
- dionized water is used to rinse the buret tip and the internal wall of the flask, which wont' affect the titration
- phenolphthalein, methyl red, methyl orange are common used indicator to show the end point with a sharp color change in 1-2 drops
- both volumetric pipette and buret need to be rinsed using the corresponding solution prior to use
Different Titration Curves
Brief Summary
- a few concepts to know: equivalence point vs end point, half-equivalence point, pH window, transition pH range of indicator, etc.
- pH can change 3-6 units (pH window) with 1-2 drops of titrant is added when approaching to the equivalence point
- end point is determined by the indicator, so choosing an appropriate indicator is critical for titrations, transition range of indicator needs to fit in the pH window in the titration curve
- when strong base titrating weak acid, pH at half equivalence point equal to the pKa of the weak acid, and the titration curve after the equivalence point overlaps with that of strong base titrating strong acid
- when strong base titrating weak acid, the pH changes slowly at half-equivalence point, which is called buffer region
- not all equivalence points have significant pH windows for polyprotic acids, such as the 3rd eq point of H3PO4
- Problem problems from USNCO: a bundle, N2016-PIII-L2
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