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

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 mathematicalwere 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.

Interrelationship between the Ancillary Ligand Structure, Acid–Base Properties, and TiO2 Surface Coverage of RuII Dyes

“…The UV/Vis spectra were measured after allowing the pH to stabilize for approximately 1–2 min. The p K a values were determined by an iteration process on OriginPro® using Equation , where A is the measured absorbance value, A i and A f are the initial and final absorbance values, respectively, and all were measured at the wavelength of the highest spectral change. An iteration process was necessary in order to correct the A i and A f values, which can be slightly divergent due to the effects of changing the ionic strength or concentration at the end of the titration.…”

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“…The pK a 0 was determined using Equation (3) [64] and 4 [65], according to the method of Salgado and Vargas-Hern andez [66] (see Fig. 3).…”

A Squaraine Derivative for Cost‐Effective, Quick, and Highly Sensitive Determination of Mercury and Thiols and pH Sensing

“…477 nm (see the Supporting Information). An isosbestic point is observed at 520 nm (pK a calculated 7.734 as log[( A HA − A i )/( A A− − A i )]=0) . Moreover, a decrease in intensity is observed as the pH is increased from neutral to 9.0.…”

“…An isosbestic point is observed at 520 nm (pK a calculated 7.734 as log[(A HA ÀA i )/(A AÀ ÀA i )] = 0). [29] Moreover,adecrease in intensity is observed as the pH is increased from neutral to 9.0. As ar esult,t he reported IndSq could be also used as ap Hc olorimetric probe, similart oo ther reported squaraines.…”