A study of the limitations of ultraviolet spectral data for the determination of dissociation constants of organic nitrogeneous bases Public Deposited

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  • Ultraviolet spectroscopic methods for the determination of dissociation constants can only be applied to those compounds having protonatable sites conjugated with or inherent in a chromophoric group of the compound. The determination of the dissociation constants of organic nitrogeneous bases by ultraviolet spectroscopic methods was found to be only reliable for those compounds having a single protonatable site in or conjugated with the chromophoric group of the heterocycle. With such compounds both methods, (I) those based on taking the extinction coefficient profile over the entire pH range at selected wavelengths, and (II) those based on a measurement of the pH at which the λmax, value was approximately the average for the protonated and unprotonated forms at a selected wavelength, were in fair agreement with one another and with dissociation constants determined by potentiometrics, conductometric and other methods. With those compounds having two protonatable sites, both weakly basic, there was found to be approximately fair agreement between methods I and II and with those determined by other procedures, but not in all cases. Method II did not lend itself to the measurement of more than a single constant when several constants were present in a compound Those compounds having two protonatable sites, one weakly basic and the other weakly acidic, also gave approximately fair agreement between methods I, II and published data. The amino acids likewise gave only fair approximations with methods I and II and those recorded in the literature. With compounds having three or more protonatable sites in the chromophore there was poor agreement between methods I and II as well as with those determined by other procedures. Method I was found to be capable of measuring several dissociation constants when more than one occurred with a given compound. Moreover, the most reliable information was obtained by taking the extinction coefficient profile over the entire pH range at 10 mμ intervals over the entire ultraviolet spectra. This was especially helpful for measuring dissociation constants which were of approximately the same order associated with a given chromophoric group. Method I gave a number of dissociation constants for compounds having several protonatable sites within the chromophoric group for certain compounds which had not been measurable by other procedures. Included in these observations was evidence of the deprotonation of amino substituent in alkaline media in a fairly large number of instances. A number of unreported dissociation constants of known compounds are herein documented.
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