2-napthol is a stronger acid in its lowest excited singlet state compared to its ground state.
It exhibits a short wavelength fluorescence emission band originating from its free acid state and a long
wavelength originating from its conjugate base. The excited states ionization constant KA can be
determined by a fluorometric titration method or a dynamic method of the determining rate constants
of the deprotonation protonation reaction from of the excited state.
Uno determined from an analysis of fluorescent decay curves of the free acid conjugate base.
this is related to the ground state KA and spectral frequency shift of the free acid deprotonation
the ground state PK of two napthol is 9.5 while the excited state ranges from 2.5 to 3.5.
In this experiment students will determine the ground state ionization constant KA by analyzing acidic
basic and buffered solutions in the pH range of 9.1 to 9.7.
At 350 nm, we know that at high basic pH, the napthoxide ion will show an absorbance peak while the
free acid will not, and at 316 nm, at high acidic pH, the free acid will show an absorbance peak while the
napthoxide ion will not. Buffered solutions at pH within that range will show an intermediate
absorbance spectrum. Therefore, we are able to obtain the pKa of 2-napthol from analyzing the
absorbance spectra of a range of buffered pH.
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The forester cycle method will be used to determine the excited state ionization constant as free two
napthol undergoes spectral frequency shift upon do you protonation to its conjugate base.
In the first part of this experiment students found the absorption Spectra of prepared buffered solutions
of two naphthol. This is shown in the figure one.
Method
To make the solutions for this experiment students used made solutions of two napthol in 200 proof
ethanol. we used 50 milliliter volumetric flasks and A5 milliliter pipette to measure out accurate
amounts of solutions to carry out series dilution. 14.9 milligrams of naphthol was measured out and
dissolved thoroughly.
I forgot the name of the UV vis instrument but we used it. We took measurements at a wavelength of
350 nm. This wavelength was chosen because the free acid does not absorb at 350 nanometers
therefore the concentration of the two N oxide ion can be determined from the absorption.
The following equation gives the relationship between PKA and PKA star and the transition frequency of
the free acid and the conjugate base