Experiment 6: Properties of Buffer Solutions

Buffers are aqueous solutions with the special property of resisting changes to their pH when
either acid or base are added. A simple buffer is a mixture of a weak acid HA and its conjugate
base A-
.
() ⇌ +() + −() (1)
↑

↑

When made with equal moles of each, the acid and the conjugate base, a buffer is at its most
effective with a pH equal to the pKa of the weak acid.
[] = [−], = (Conditions when a buffer is at its most effective) (2)
Buffer solution pH can be calculated by using the Henderson-Hasselbalch equation:
= − log �
[]
[−]
� (Using concentration of a weak acid and its conjugate base) (3)
= − log �

−� (Using moles of a weak acid and its conjugate base) (4)
When nHA = nA- initially the moles of both can be called Y and when the moles of strong acid
or base added to the buffer they can be called x. When strong acid is added to a buffer solution
it can react with the conjugate base, A-
.
−() + () → , = − log �
+
−
� (Strong acid added) (5)
The reaction reduces the amount of A- in the solution while increasing the amount of weak acid
HA:
The converse is true when strong base is added.
() + () → −(), = − log �
−
+
� (Strong base is added) (6)
The mole amount of strong acid or base that must be added to change the pH of the buffer in
either direction by one unit (pH = pKa ± 1) is called its buffer capacity. The buffer capacity can
also be calculated using the Henderson-Hasselbalch equation and by solving for x.
= − 1, ℎ − 1 = − log �
+
−
� (when acid is added) (7)
= + 1, ℎ 1 = − log �
−
+
� (when base is added) (8)
2
In this experiment we will study how well buffer solutions can resist changes to their pH and
provide context to this property by comparing them to pH changes in an unbuffered solution.
This work will be done in three parts:
1. Preparation of buffer solutions
2. Study of buffer solution properties
3. Comparison of buffered and unbuffered solutions
General Procedure
Note: It is advisable to use a stirring bar to stir your solutions and ensure homogeneity during pH
measurements
Materials
50 mL Beaker x 2 10 mL Graduated Cylinder x 1 Magnetic Stirring Bar x 1
100 mL Beaker x 3 25 mL Graduated Cylinder x 1 Hot Plate with Stirrer x 1
Dropper x 1 50 mL Graduated Cylinder x 1
Part 1: Preparation of Buffer Solutions
1. Obtain 50.0 mL of 0.10 M acetic acid in the following way:
a. Use a 150 mL beaker and fill approximately halfway with 0.10 M acetic acid
from the bottle located in the fume hood
b. From that sample fill a 50 mL graduated cylinder to the top mark
2. Determine how many grams of sodium acetate are needed to make the buffer in
the following way:
a. Calculate how many moles of acetic acid are in 50.0 mL of 0.10 M acetic acid
b. Convert the number of moles obtained in 2a. to grams of sodium acetate (the
molar mass of sodium acetate is 82.0 g)
c. Label a 150 mL beaker “Buffer Stock” and transfer to it the amount of sodium
acetate calculated (or as very close to it as possible) in 2b.
3. Make and test your buffer solution in the following way:
a. Transfer the 50.0 mL of acetic acid from the graduated cylinder into the beaker
labeled “Buffer Stock” and ensure that all the sodium acetate is dissolved by
stirring
b. Thoroughly rinse your pH meter with distilled water prior to any measurement
and then measure the pH of your stock solution
c. Rinse and dry the 50 mL graduated cylinder
Part 2: Study of Buffer Solution Properties
4. Label two empty 50 mL beakers “Acid Study 1” and “Base Study 1” and pour
exactly 20.0 mL from the “Buffer Stock” into them using a 25 mL graduated
cylinder (a total of 40.0 mL). Immediately pour the remaining 10.0 mL of buffer
into the 50 mL graduated cylinder and dilute to the 50.0 mL mark to use in step 6
a. Fill a 10 mL graduated cylinder with 0.10 M HCl
b. Add a total of 5.0 mL of 0.10 M HCl in 1.0 mL increments to the beaker labeled
“Acid Study 1” using a dropper and measure the pH of the solution after each
addition (your pH meter probe can remain in the solution during the pH study)
c. Rinse the 10 mL graduated cylinder and the dropper a few times and dry
d. Repeat steps 4a and 4b using 0.10 M NaOH instead and add to the beaker
labeled “Base Study 1”