Experiment 6: Acid-Base Equilibrium Potentiometric Titration of Weak Acids

PURPOSE

To observe how pH changes throughout both monoprotic and polyprotic weak acid titrations to determine the equivalence point(s) and experimentally determine the Ka for weak acids

OBJECTIVES

Learning Objectives

• Understand the titration of a weak monoprotic acid with a strong base as a function of pH vs the amount of strong base added (titration curve)

• Understand the titration curve for a weak monoprotic acid: know the four regions and how to do acid-base equilibrium calculations for each

• Understand the titration curve for a weak polyprotic acid: know the number of equivalence points and the composition os the solution at different pHs

Experimental Objectives

• Determine the titration equivalence point(s) using a gran plot

UNDERSTANDING THE BACKGROUND

In this experiment we analyzed the weak acid-strong base titration using a pH meter to construct a titration curve.

• The equivalence point of a titration is determined by interpreting this graph

• We used this titration curve to also determine the pKa value for the acid

• The inflection points in both the buffer region and at the equivalence point can be used to calculate the pKa of the weak acid and the equivalence point

A pH meter was used to measure the pH and does so by measuring the difference in electric potential as the titration proceeds, which is why this experiment is called a potentiometric titration

• Essentially the pH meter is voltmeter that acts as a galvanic cell

Phosphoric acid is the polyprotic acid used in this experiment and it is a triprotic acid that dissociates stepwise:

H3PO4 => H2PO4- + H+

H2PO4- => HPO42- + H+

HPO42- => PO43- + H+

• The 3 buffer region inflection points on the titration curve correspond to each of the pKas, however the pKa2 and pKa3 for some weak acids is not discernable if the pH of the solution approaches pH of the titrant base

SUMMARY OF THE PROCEDURE

Part 1: Titration of a weak monoprotic acid

In this experiment, a potentiometric titration was carried out using 0.1 M acetic acid and 0.35 M NaOH, wherein the initial pH of the acid solution was measured before adding the titrant. As the NaOH is incrementally added, continuous monitoring of the pH occured until the equivalence point was reached, following which the titration curve was plotted to identify the buffer regions and the corresponding pKa values. The endpoint of the titration was confirmed through a significant change in pH indicative of the complete neutralization of the weak acid.

Part 1: Titration of a weak polyprotic acid

In this part of the experiment, a similar potentiometric titration was performed using phosphoric acid and 0.35 M NaOH. The initial pH was recorded before titration commenced, with incremental additions of NaOH allowing for continuous pH monitoring. As multiple equivalence points are approached, the titration curve was analyzed to identify distinct buffer regions and corresponding pKa values for each deprotonation step. The endpoint for each stage of the reaction was determined by notable changes in pH, reflecting the neutralization of the protons from the polyprotic acid.

ANALYZING THE DATA

Part 1: Titration of Acetic Acid

• Created three graphs: (1) Titration curve (pH vs. mL NaOH), (2) Modified Gran plot (ΔV/ΔpH vs. volume of base), and (3) Zoomed-in Gran plot near the equivalence point.

• Used the Gran plot to determine the volume of 0.35 M NaOH needed to reach the equivalence point.

• Calculated the molarity of acetic acid and determined its Ka using three methods:

  1. Initial pH and molarity

  2. pH at half-equivalence point

  3. pH at equivalence and concentration of sodium acetate

• Compared Ka values to those in Table 17.5 of Tro.

Part 2: Titration of Phosphoric Acid

• Created two graphs: titration curve and modified Gran plot.

• Determined the molarity of H₃PO₄ from the first endpoint.

• Calculated Ka₁ using:

  1. Initial pH and molarity

  2. pH at half-equivalence point

• Determined Ka₂ from the pH halfway between the first and second equivalence points.

• Compared results to Table 17.10 in Tro