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Quantifying Acidity in Heterogeneous Systems: Biphasic pKa Values
Andre Leesment1, Sigrid Selberg1, Merili Tammiste1
1Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia.
Analytical Chemistry
|February 23, 2022
Summary
Aqueous pKa values inaccurately estimate acidities in biphasic systems. This study introduces biphasic pKa (pKaow) for accurate acidity measurements in lipophilic environments, crucial for catalysis and sensors.
Area of Science:
- Physical Chemistry
- Analytical Chemistry
Background:
- Aqueous pKa values are often used to approximate acidities of lipophilic compounds in biphasic systems.
- This approximation is flawed because H+ ions concentrate in the aqueous phase, while acids and their anions partition into the organic phase.
Purpose of the Study:
- To introduce and validate the biphasic pKa concept for accurate acidity determination in biphasic systems.
- To provide a more reliable method for quantifying acidities of lipophilic compounds relevant to various chemical applications.
Main Methods:
- Determined biphasic pKa (pKaow) values for 35 diverse acids in a 1-octanol:water system.
- Utilized UV-vis and NMR spectroscopy for direct pKaow measurements.
- Applied Debye-Hückel theory to extrapolate apparent, concentration-dependent pKaow values to concentration-independent results.
Main Results:
- Demonstrated that aqueous pKa values lead to significantly biased acidity estimations in biphasic systems.
- Successfully determined concentration-independent biphasic pKa values for a range of acids.
- Validated the biphasic pKa concept as a superior method for acidity quantification in such systems.
Conclusions:
- Aqueous pKa is not justified for assessing acidities of lipophilic species in biphasic environments.
- The biphasic pKa concept offers a more accurate and reliable approach.
- This work provides essential data and methodology for fields utilizing lipophilic acids in multiphase systems.
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