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<p>The acidity constant (pKa) represents the pH value at which the univalent acid is 50% dissociated, and the univalent base is 50% protonated. The pKa can be determined experimentally by various techniques, and/or predicted using software programs. The present study aims to develop a spectrophotometric approach for determining pKa values, using drugs with reported pKa values in a wide range [Fluconazole (low pKa), Azathioprine (moderate pKa), Salbutamol sulphate (high pKa), and Metronidazole (low pKa)]. This will help ensure the method is broadly applicable across diverse pH conditions, particularly in the physiological pH range. A stock solution of each drug was prepared and diluted with pH buffers (pH 1.2 to 11.3) to obtain working standard solutions in a defined concentration range, separately for each drug. These solutions were analysed spectrophotometrically at respective wavelength of absorption maxima at room temperature [Fluconazole (50&#xa0;µg/mL) at 261&#xa0;nm, Azathioprine (10&#xa0;µg/mL) at 279&#xa0;nm, Salbutamol sulphate (50&#xa0;µg/mL) at 276&#xa0;nm, Metronidazole (20&#xa0;µg/mL) at 319&#xa0;nm, and Abacavir (10&#xa0;µg/mL) at 285&#xa0;nm]. The pKa values for Fluconazole (2.11), Azathioprine (7.8), Salbutamol sulphate (9.19), and Metronidazole (2.53), calculated by the Albert-Serjeant method, were found to be consistent with the reported pKa values. Further, the proposed method was applied to estimate the pKa of Abacavir (5.87), which was consistent with the published predicted value of 5.8. Therefore, the present study provides practical insights for accurate pKa estimation using a spectrophotometric approach, especially when experimental data is lacking.</p>

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Application of a UV-spectrophotometric method for determination of acidity constant (pKa) of drugs using the Albert-Serjeant equation

  • 期刊论文

  • 发表日期：

    2026年4月28日

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  • Vijeth N. Bhat,
  • Vaibhav Namdeo,
  • Abhishek N. Waykos,
  • Vinod L. Gaikwad

摘要

The acidity constant (pKa) represents the pH value at which the univalent acid is 50% dissociated, and the univalent base is 50% protonated. The pKa can be determined experimentally by various techniques, and/or predicted using software programs. The present study aims to develop a spectrophotometric approach for determining pKa values, using drugs with reported pKa values in a wide range [Fluconazole (low pKa), Azathioprine (moderate pKa), Salbutamol sulphate (high pKa), and Metronidazole (low pKa)]. This will help ensure the method is broadly applicable across diverse pH conditions, particularly in the physiological pH range. A stock solution of each drug was prepared and diluted with pH buffers (pH 1.2 to 11.3) to obtain working standard solutions in a defined concentration range, separately for each drug. These solutions were analysed spectrophotometrically at respective wavelength of absorption maxima at room temperature [Fluconazole (50 µg/mL) at 261 nm, Azathioprine (10 µg/mL) at 279 nm, Salbutamol sulphate (50 µg/mL) at 276 nm, Metronidazole (20 µg/mL) at 319 nm, and Abacavir (10 µg/mL) at 285 nm]. The pKa values for Fluconazole (2.11), Azathioprine (7.8), Salbutamol sulphate (9.19), and Metronidazole (2.53), calculated by the Albert-Serjeant method, were found to be consistent with the reported pKa values. Further, the proposed method was applied to estimate the pKa of Abacavir (5.87), which was consistent with the published predicted value of 5.8. Therefore, the present study provides practical insights for accurate pKa estimation using a spectrophotometric approach, especially when experimental data is lacking.