Abstract
The pKa and the isosbestic point of the universal pH indicator Carlo Ebra 1-11 (catalog number 45712) were determined using UV-Vis spectrophotometry. Aqueous buffer solutions with pHs ranging from 3.83 to 10.85 were mixed. Four methods—two graphical and two mathematical— were used to estimate the acid dissociation constant (pKa) and isosbestic point using absorbance measurements. The equation for the dependence of the absorbance on pH at λ = 600 nm was obtained using calibration curves. The resulting average pKa of the four methods was 8.277 with a standard deviation of 0.1728. The results obtained using the mathematical methods were very similar, with a deviation of 0.0014; the average pKa determined using these methods was 8.263 ± 0.001. The literature contains no previous reports of the pKa of this indicator. The isosbestic point occurs at a wavelength of 494 nm, with an absorbance of 0.46.
Highlights
The value of the acid dissociation constant is an important parameter that indicates the degree of ionization of molecules in solution at different pH values
Neither the chemical composition of this indicator nor its dissociation constant was provided by the manufacturer
An equation was obtained for the relationship between absorbance and pH with a very low dispersion, and the isosbestic point was determined
Summary
The value of the acid dissociation constant (pKa) is an important parameter that indicates the degree of ionization of molecules in solution at different pH values. Physical and biological properties of natural and synthetic compounds are governed by the interactions of acidic and basic groups. In such compounds, How to cite this paper: Salgado, L.E.V. and Vargas-Hernández, C. (2014) Spectrophotometric Determination of the pKa, Isosbestic Point and Equation of Absorbance vs pH for a Universal pH Indicator. Several groups [3]-[6] have investigated the behaviors of different drugs, including those used against cancer, and the dependence of their behaviors on their pKa values. Novel pKa determination methods that use pH colorimetric indicators have been reported [7]-[9]
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