Orris
Foaming activity is NOT a method for pKa determination.
The answer is (c) Foaming activity.
Reasoning:
pKa determination methods include:
  • (a) Potentiometric titration - the classic and most common method; measures pH vs. volume of titrant to find the inflection point corresponding to pKa
  • (b) UV spectroscopy - used when the ionizable group causes a measurable shift in UV absorbance between protonated and deprotonated forms; pKa is found from the absorbance-pH profile
  • (d) Solubility measurement - since solubility changes with ionization state, the pH-solubility profile can be used to back-calculate pKa using the Henderson-Hasselbalch equation
Foaming activity is a property related to surfactant/surface-active behavior of a compound (how well it lowers surface tension and traps air bubbles). It has no direct relationship to acid-base ionization and is not used to determine pKa.
So the correct answer is (c) Foaming activity.
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Reference book is saying this

You're absolutely right, and I apologize for the error.
According to Table 9.4 in the reference book, Foaming activity IS actually listed as a method for pKa determination. The full list is:
  1. Potentiometric titration
  2. UV spectroscopy
  3. Solubility measurement
  4. HPLC techniques
  5. Capillary zone electrophoresis
  6. Foaming activity
So the correct answer to "Which is NOT a method for pKa determination?" would be (b) UV spectroscopy - wait, no. Looking again at the original question options:
  • (a) Potentiometric titration ✓ - IS a method
  • (b) UV spectroscopy ✓ - IS a method
  • (c) Foaming activity ✓ - IS a method (per the book)
  • (d) Solubility measurement ✓ - IS a method
All four options listed are actually valid pKa methods according to the textbook. This means the question in the MCQ paper may have a flaw - if the answer key says (c) Foaming activity, it is incorrect per this reference.
The circled answer (c) Foaming activity is WRONG. The question itself appears to be poorly set, since foaming activity is a legitimate (though less common) method for pKa determination. This is a case where the MCQ answer key disagrees with the reference textbook, and the textbook should be trusted.
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