How to calculate pKa of an acid from a UV/Vis titration with DBU in THF?

Dear Sir. Concerning your issue about the calculation of pKa of an acid from a UV/VIS with DBU in THF. Spectrometry with visible light made it possible to measure pKa values of acid/base indicators and this in turn was extended to the use of UV light to measure pKa’s of other components. A requirement for this UV/pH measurement is the presence of a chromophore close to the ionization site in the molecule. If this is fulfilled, then the spectra of the dissociated and the non-dissociated form can be expected to differ. In principle any wavelength can be used for the determination of pK, except at the isosbestic point at which wavelength of both forms have the same molar absorptivity. The best choice however is a wavelength at which the molar absorbtivities are as different as possible. The method was further improved by measuring the absorption of two different wavelengths at a variable pH. The ratio in absorption at those two wavelengths is plotted against the pH. In this way, a sigmoid curve is obtained and the pKa can be determined from the inflection point as normal. One of the wavelengths has to be assigned to the chromophore and the other wavelength should be invariant under change of pH (if this is possible). By using a 2nd wavelength as reference, change in total concentration will not affect the final result and activity/concentration issues and assumptions are bypassed. For more details see the following below links may help you in your analysis:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3747999/

https://bib.irb.hr/datoteka/305797.sandra.pdf

Thanks

Yang Yu

Dear Researcher,

Calculating the pKa of an acid from a UV/Vis titration involves a systematic approach where the acid in question is titrated with a base, such as 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), in a solvent like Tetrahydrofuran (THF). The process entails monitoring the absorbance change at specific wavelengths as the acid-base reaction occurs, which can be correlated with the dissociation of the acid. Here's a step-by-step guide to determine the pKa:

Preparation of Solutions: Prepare a solution of the acid in THF. Similarly, prepare a solution of DBU in THF. Ensure that the concentrations are known and that the solvent is of high purity to minimize interference with UV/Vis absorption.

Baseline Measurement: Before starting the titration, record the UV/Vis spectrum of the acid solution in THF to establish a baseline for comparison.

Titration Process: Gradually add the DBU solution to the acid solution while continuously stirring. After each addition of DBU, allow the solution to equilibrate, and then record the UV/Vis spectrum. Continue this process until no significant changes in the spectrum are observed, indicating that the acid has been fully deprotonated.

Data Analysis: Analyze the UV/Vis spectra to identify the wavelength(s) at which the most significant changes in absorbance occur. These changes are indicative of the acid undergoing deprotonation.

Plotting the Titration Curve: Plot the absorbance changes at the selected wavelength(s) against the volume of DBU added. The resulting titration curve should exhibit an inflection point corresponding to the equivalence point where the acid is fully deprotonated.

Calculating pKa:Identify the half-equivalence point on the titration curve, which occurs at half the volume required to reach the equivalence point. At the half-equivalence point, the concentration of the acid ([HA]) equals the concentration of its conjugate base ([A-]). Use the Henderson-Hasselbalch equation to calculate the pKa:pKa=pH−log⁡([A−][HA])pKa=pH−log([HA][A−])In non-aqueous titrations, such as those in THF, the concept of "pH" is not directly applicable. Instead, the pKa calculation involves the known concentration of the acid and base, the volume of DBU added at the half-equivalence point, and the assumption that [A-] = [HA] at this point.

Considerations:Ensure that the solvent does not absorb at the monitored wavelengths to avoid interference. The accuracy of the pKa value can be affected by the solvent (THF in this case), as solvent properties can influence acid dissociation. The obtained pKa is specific to the solvent system used.

Final Calculation: Integrate the obtained data with the known concentrations and the DBU volume at the half-equivalence point into the adapted Henderson-Hasselbalch equation to calculate the pKa of the acid in THF.

This methodology requires careful execution and consideration of the specific characteristics of the acid, base, and solvent system used. It provides a valuable technique for understanding the acid-base properties of compounds in non-aqueous environments.

Best regards.

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