Samples of motor oil and creosote are subjected to three-dimensional fluorescence spectroscopy to document the shifting of excitation/emission maxima as the solutions undergo serial dilutions. Effects such as self-quenching of individual polycyclic aromatic hydrocarbons (PAHs) and energy transfer between PAHs combine to produce large red-shifts in the resulting fluorescence emission spectra. The concentration-imposed complications are also demonstrated for several synthetic ternary PAH mixtures and a diesel residue mimic.
3-D topographic surfaces (“topos”)
can be generated
to visualize how pH behaves during titration and dilution procedures.
The surfaces are constructed by plotting computed pH values above
a composition grid with volume of base added in one direction and
overall system dilution on the other. What emerge are surface features
that correspond to behavior in aqueous solutions. Equivalence point
breaks become cliffs that pinch out with dilution. Buffer effects
become plateaus. Dilution alone generates 45° ramps. Limitations
of the Henderson–Hasselbalch equation can be seen by noting
the conditions over which a plateau remains relatively flat. Because
dissociation is driven by dilution, the surfaces demonstrate when
the solution of a weak acid becomes indistinguishable from that of
a strong acid. Surfaces are presented for hydrochloric acid, HCl (a
strong acid); acetic acid, CH3COOH (a weak monoprotic acid);
oxalic acid, HOOCCOOH (a weak diprotic acid) and l-histidine
dihydrochloride, C6H9N3O2·2HCl (a weak triprotic acid). Supplementary materials include
suggested use of topos in lecture, as worksheets and in support of
laboratory activities for first-year college courses and third-year
analytical chemistry courses. Also provided is pH TOPOS, the macro-enabled
spreadsheets that quickly generate surfaces for any mono-, di-, or
triprotic acid desired. Only a change of acid dissociation constants, K
a values, is required.
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