Spectrophotometric Determination of the pKa, Isosbestic Point and Equation of Absorbance vs. pH for a Universal pH Indicator

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 aci...

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Bibliographic Details
Published inAmerican journal of analytical chemistry Vol. 5; no. 17; pp. 1290 - 1301
Main Authors Salgado, L. E. Vidal, Vargas-Hernández, C.
Format Journal Article
LanguageEnglish
Published 01.12.2014
Subjects
Absorbance
Calibration
Constants
Deviation
Indicators
Mathematical analysis
Spectrophotometry
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Summary: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 lambda = 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 plus or minus 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.
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ISSN:2156-8251
2156-8278
DOI:10.4236/ajac.2014.517135