ANALYTICAL cHEMISTRY - II

MSCM201

To determine pKa of the weak acid using pH meter.

The strength of the acid is experimentally measured by determining its equilibrium constant or dissociation constant (Ka).
For example, acetic acid ionizes as,

CH₃COOH ↔ CH₃COO⁻ + H⁺

Ka = [H⁺] [CH₃COO⁻] / CH₃COOH

pKa is the mode of expressing the acid strength, i.e., pKa = − log Ka, and pKa is determined by measuring the changes in pH of the acid solution at different amounts of base added.
During the titration of the weak acid with a strong base, initially, the pH of the solution rises gradually, then more rapidly and very sharply at the equivalence point for a very small quantity of the base added.
After the equivalence point, pH increases slightly on further addition of the base. The equivalence point is determined by plotting the change in pH at different amounts of base added.
According to Henderson – Hesselbach equation, 𝐩𝐇= 𝐩𝐊𝐚 + 𝐥𝐨𝐠 [𝐒𝐚𝐥𝐭] / [𝐀𝐜𝐢𝐝]
At the half equivalence point, [Salt] = [Acid] and therefore, pH at the half equivalence point is pKa of the weak acid.
For, Finding the change in pH during acid–base titration potentiometer is employed. For an acid–base titration, the indicator electrode has to be pH sensitive. Therefore glass electrode is used as the indicator electrode. A saturated calomel electrode is used as the reference electrode.

Burette, Pipette, Beakers, Glass rod, Digital potentiometer with provision for measuring pH, Acetic acid solution, Standard NaOH solution, etc.

In a 100 mL clean borosil beaker, take 25 mL of the standard weak acid solution using a 25 mL pipette.
Immerse the pH cell electrode into the solution.
Fill the standard strong base solution into the burette up to the mark.
Before commencing the titration, the pH reading has to be noted for the standard weak acid solution. It takes a little time for stabilization.
Use a clean glass rod for stirring purposes. The initial reading is noted.
For each addition of 0.5 ml of standard strong base solution, the stabilized pH reading is noted after stirring the solution using the glass rod.
The pH of the solution will rise with every addition. After the sudden jump in the pH readings, take four-to-five more readings.
The readings are recorded in the observation table.
Plot a graph between Δ𝐩𝐇 / Δ𝐕 versus the volume of the standard NaOH solution added.
Also, plot a graph between the observed pH v/s the volume of the standard NaOH solution added.

Determination of pka of weak acid.mp4

The pKa of the weak acid is ________.

G H Jeffery, J Bassett, J Mendham, and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition
S. Suzanne Nielsen, Food analysis, 4th Edition

Dr. Viraj Bhanvadia,

Assistant Professor, Chemistry,

viraj.bhanvadia@gsfcuniversity.ac.in

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