Chapter 13: 13.17 (page 329)
Why can photomultiplier tubes not be used with IR radiation?
A photon with energy in the ultraviolet-visible region is required to liberate electrons from the photo emissive surface of a photomultiplier tube.
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At 580 nm, which is the wavelength of its maximum absorption, the complex has a molar absorptivity of 7.00 * 103 L cm -1 mol -1. Calculate
(a) the absorbance of a 4.47 * 10 -5M solution of the complex at 580 nm in a 1.00-cm cell.
(b) the absorbance of a solution in a 2.50-cm cell in which the concentration of the complex is one half that
in (a).
(c) the percent transmittance of the solutions described in (a) and (b).
(d) the absorbance of a solution that has half the transmittance of that described in (a).
A portable photometer with a linear response to radiation registered µA with the solvent in the light path. The photometer was set to zero with no light striking the detector. Replacement of the solvent with an absorbing solution yielded a response of µA. Calculate
(a) the percent transmittance of the sample solution.
(b) the absorbance of the sample solution.
(c) the transmittance to be expected for a solution in which the concentration of the absorber is one third that of the original sample solution.
(d) the transmittance to be expected for a solution that has twice the concentration of the sample solution.
A solution containing of has a transmittance of in a cell at . Calculate the molar absorptivity of at this wavelength.
The following questions concern the relative concentration uncertainty in spectrophotometry.
(a) If the relative concentration uncertainty is given by Equation , use calculus to show that the minimum uncertainty occurs at . What is the absorbance that minimizes the concentration uncertainty? Assume that is independent of concentration.
(b) Under shot-noise-limited conditions, the relative concentration uncertainty is given by Equation . Another form of the equation for the shot-noise-limited case is
where is a constant. Use calculus and derive the transmittance and absorbance that minimize the concentration uncertainty.
(c) Describe how you could experimentally determine whether a spectrophotometer was operating under Case I, Case II, or Case III conditions.
Calculate the absorbance of solutions having twice the percent transmittance of those in Problem 13-2.
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