Chapter 15: Q. 15.1 (page 384)
Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum. Which more closely resembles an absorption spectrum?
The detailed answer is given in difference step.
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The following lifetimes were measured for the chloride quenching of quinine sulfate given in Example 15-1. The fluorescence intensities are given in the example.
(a) Plot fluorescence intensity versus [Cl-].
(b) Plot the ratio of intensity to lifetime, F-t versus [Cl-].
(c) Develop a normalization factor to correct the measured fluorescence intensity to that of the solution without quencher.
(d) Plot on the same graph F versus [Cl-] and Fcorr versus [Cl-] .
Iron(II) ions catalyze the oxidation of luminol by H2O2. The intensity of the resulting chemiluminescence has been shown to increase linearly with iron(II) concentration from 10-10 to 10-8 M.
Exactly 1.00 mL of water was added to a 2.00-mL aliquot of an unknown Fe(II) solution, followed by 2.00 mL of a dilute H2O2 solution and 1.00 mL of an alkaline solution of luminol. The chemiluminescence from the mixture was integrated over a 10.0-s period and found to be 12.7. To a second 2.00-mL aliquot of the sample was added 1.00 mL of a 3.27 x 10-5 M Fe(II) solution followed by the same volume of H2O2 and luminol. The integrated intensity was 27.9. Find the concentration of Fe(II)
in the sample.
The following volumes of a standard 10.0 ppb F- solution were added to four 10.00-mL aliquots of a water sample: 0.00, 1.00, 2.00, and 3.00 mL. Precisely 5.00 mL of a solution containing an excess of the strongly absorbing Al-acid alizarin garnet R complex was added to each of the four solutions, and they were each diluted to 50.0 mL. The fluorescence intensities of the four solutions were as follows:
(a) Explain the chemistry of the analytical method.
(b) Construct a plot of the data.
(c) Use the fact that the fluorescence decreases with increasing amounts of the F- standard to derive a relationship like Equation 1-3 for multiple standard additions. Use that relationship further to obtain an equation for the unknown concentration cx in terms of the slope and intercept of the standard-additions
plot, similar to Equation 1-4.
(d) Use linear least squares to find the equation for the line representing the decrease in fluorescence relative to the volume of standard fluoride Vs.
(e) Calculate the standard deviation of the slope and intercept.
(f) Calculate the concentration of F- in the sample in parts per billion.
(g) Calculate the standard deviation of the result in (e).
The determination in Problem 15-9 was modified to use the standard-addition method. In this case, a 3.925-g tablet was dissolved in sufficient 0.10 M HCl to give 1.000 L. Dilution of a 20.00-mL aliquot to 100 mL yielded a solution that gave a reading of 415 at 347.5 nm. A second 20.00-mL aliquot was mixed with 10.0 mL of 50-ppm quinine solution before dilution to 100 mL. The fluorescence intensity of this solution was 503. Calculate the percentage of quinine in the tablet.
Quinine is one of the best-known fluorescent molecules, and the sensitivities of fluorometers are often specified in terms of the detection limit for this molecule. The structure of quinine is given next. Predict the part of the molecule that is most likely to behave as the chromophore and fluorescent center.
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