Experiment 5: Acid-Base Equilibrium: Capacity of Buffers

PURPOSE

To make buffer solutions at a desired pH, test how these solutions react to the addition of strong acid and strong base, and analyze how the capacity of a buffer solution depends on both pH and the absolute concentrations of the conjugate acid-base pair

OBJECTIVES

Learning Objectives

• Learn proper use of electronic pH meters: know how to calibrate, make accurate measurements, and the proper storage

Experimental Objectives

• Make a weak acid buffer solution using two different methods

• Titrate the buffer solutions with strong acid and strong base

• Determine the relative capacity of each buffer solution

• Determine how the capacity of each solution changes with pH

UNDERSTANDING THE BACKGROUND

Strength of Acids and Bases

The strength of an acid/base depends on how much it dissociates when placed into water

• Strong acids dissociate completely in water (HCl, HBr, HI, HNO₃, HClO₄, H₂SO₄) and reaction goes to completion

• Strong bases dissociate completely in water (those containing alkali metals) and reaction goes to completion

• Weak acids and bases do not dissociate completely instead…

  • Weak acids donate a proton to H₂O (bronsted-lowry definition)

  • Weak bases accept a proton from H₂O (bronsted-lowry definition)

  • These reactions remain in equilibrium

The Acid Dissociation Constant Ka and H-H Equation

• The greater the value of Ka, the more an acid dissociates in water

• The acid dissociation equilibrium can be represented by the Henderson-Hasselbalch Equation:

  pH=pKa+\log\frac{\left\lbrack A^{-}\right\rbrack}{\left\lbrack HA\right\rbrack}

  • This equation is used to estimate the pH of buffer solutions

  • Is only valid with in ±1 pH unit of the pKa of the acid

Buffer Solutions

What are buffer solutions?

• Solutions that contain significant amounts of both a weak acid and its conjugate base OR a weak base and its conjugate acid

• These solutions can resist changes in pH when some amount of acid/base is added

  • But these solutions have a limit of how much strong acid/base that can be added before they become overwhelmed and can no longer maintain a stable pH. This is known as the buffer capacity

• Buffer capacity depends on the ratio of the concentrations of the conjugate acid-base pair and on the total absolute concentrations of the conjugate acid-base pair. It is represented by the equation

  Buffer Capacity ==\frac{\Delta V_{added}}{\Delta pH_{solution}}

  • The more volume of strong acid/base required to change the pH of the solution, the higher the capacity of the buffer

• To make a buffer solution the pH and the capacity of the desired buffer must be taken into account

• There are TWO methods of making a weak acid buffer solution that will be explored in this experiment

  1. Using a known volume and molar concentration of weak acid and then adding the correct number of moles of conjugate base

  2. Using a known volume and molar concentration of weak acid and adding the correct amount of strong base

SUMMARY OF THE PROCEDURE

Part 1: Making the two weak acid buffer solutions

• Calculating the correct volumes of each solution to use in order to make an acetic acid/acetate buffer at the pH assigned by the TA

  • Both buffers should be 40 mL total

  • Buffer 1 consists of 0.50 M acetic acid and 0.50 M sodium acetate

  • Buffer 2 consists of 0.50 M acetic acid and 0.50 M sodium hydroxide

• Using the pH electrode

  • Do not wipe tip of electrode with pressure; lightly blot with tissue instead

  • The tip of the electrode is immersed in at least an inch of solution

  • Rinse/blot the electrode between solutions to avoid contamination

  • Always pace back into proper storage solution

Part 2: Titrating buffers to determine buffer capacity

• Record initial pH of solutions

• Titrate 10mL of buffer 1 in small increments with 0.50 M NaOH until pH of solution changes by 2 pH units. Then repeat with 0.50 M HCl

• Titrate 10mL of buffer 2 in small increments with 0.50 M NaOH until pH of solution changes by 2 pH units. Then repeat with 0.50 M HCl

• All titrations should be measured in increments of 0.2 pH units

ANALYZING THE DATA

• Results of titrations were used to graph buffering capacity vs pH (plot of dV/dpH vs pH) using information such as:

  • The change in volume between each data point (dV)

  • The absolute value of the change in pH (dpH)

• The 2 buffers have a maximum capacity at the pH that equals the pKa of the weak acid

  • This is where the concentrations of the weak acid and its conjugate base are equal

• An acetic acid/acetate buffer has a higher buffer capacity when titrated with NaOH (a strong base) compared to HCl (a strong acid)

• Sources of Error include:

  • Rounding during calculation of the ratio of component volumes needed to make the 2 buffer solutions