Exploring Buffer Capacities: A Laboratory Experiment on Buffer Solutions, Summaries of Chemistry

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Peter William Atkins, Loretta Jones, Leroy

Laverman,

Chemical Principles

Table 1. Buffer solution’s response to addition of 2.0 mL 0.050 M HCl

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Lab: Buffers

Abstract

The purpose of this lab was to explore the use of buffers. Buffers are used to maintain the pH of

a system at a constant value. Their make-up involves a weak acid and a base. For a specific

buffer, the pH depends on the equilibrium constant and the other elements of the Henderson-

Hasselbalch equation,

A

−¿

mol

mol HA

pH = pKa + log ¿

. A buffer’s capacity is to what extent the buffer can

resist a change in pH if a strong base or a strong acid is added. This is interpreted through the

equation,

A

¿

F =[ HA ] +

[ ¿

)

β ∝¿

, which says that the capacity is directly proportional to the

concentration. This lab involved three buffers, labeled, E, F, and G, each created through a

mixture of weak acids and conjugate bases. After creating the buffer, the capacities were

measured by adding HCl and NaOH. The experiment found that Buffer E had the best capacity

and Buffer G had the worst. With this data obtained, ICE tables were created to find the mean

pKa value (also using the Henderson-Hasselbalch equation). The resulting mean pKa value was

found to be 4.88 with a standard deviation of 0.225, and the mean Ka value was 1.3 × 10

• 5

This experiment involved certain possible errors that may have occurred, such as the use of a

pipette that may have resulted in inaccurate pouring of acids, and graduated cylinders that were

used that may have had solutions from previous experiments that offered the possibility of

contamination. Despite these possible errors, the experiment succeeded in its application and

clearly taught the concept of buffers and its capacities.

Data

Table 1. This table contains the values resulting from the buffers after adding HCl.

Sample Calculations

*ICE tables were used to calculate values in Table 3

Ice Table for E+HCL

HA H

2

O A

H

3

O

Initial 0.002 -- 0.002 0.

Change 0.001 -- - 0.0001 - 0.

Equilibrium 0.0021 -- 0.0019 0

Discussion

1. Looking at Table 1, the pH of a buffer prior to adding an acid or a base depends on the

formal concentration. In Table 1, E, F and G all have formal concentrations that are 0.1,

0.01, and 0.001, respectively, and have difference initial pH’s (4.71, 4.89, and 5.36,

respectively). They may have the same liquids, but because they have different

concentrations, and therefore, different formal concentrations, their initial pH values are

different.

2. Buffer E is the best buffer because when observing the change in pH, Buffer E had only

0.04 (regarding the addition of HCl). The buffer with the smallest change in pH is the

best buffer because the buffer capacity is the extent to which a buffer can resist a change

in pH. Buffer G is the worst buffer because Buffer G suffered a pH change of 2.

(regarding the addition of HCl), which is high compared to other buffers. This means that

the higher change in pH means the buffer capacity is not that good. When looking at the

addition of NaOH, Buffer E again had a small pH change of 0.1, compared to the pH

change of Buffer G which was 5.38. Therefore, Buffer E is the best buffer and Buffer G is

the worst buffer, and both have been ranked as so in the tables.

3. The buffer capacity was indeed destroyed by the addition of the acid and base. This is

because in trials G + HCL and G + NaOH, the pKa could not be calculated. In these two

cases, the limiting reagent is either HA or A

, which eventually reached zero (discovered

in the ICE tables), meaning the pKa cannot be calculated in the equation,

A

−¿

mol

mol HA

pH = pKa + log ¿

. If the pKa cannot be calculated, it means that weak acids and weak

bases are not present, meaning the system is destroyed. This means that the buffer

capacity was destroyed as well, since the weak base or weak acid was not present.

4. The purpose of this lab was to explore the concept of buffers and determine which buffer

was the best and worst in the experiment. This purpose was achieved, since it was found

that Buffer E was the best buffer and Buffer G was the worst buffer, based on their

changes in pH. The experiment also taught the Henderson-Hasselbalch equation, which

resulted in successful application through the data tables. The concept of buffer capacity

was also explored, and a pKa value of 4.88 with an SD of 0.225 was obtained. A Ka value

of 1.

×

• 5

was found. Because the experiment involved the use of a pipette,

possible errors include inaccurate pouring of acids, which could affect the data. This can

be solved through the use of machine pipettes, which factor out human error when

pipetting liquids. Another error could be leftover solutions from previous experiments

affecting the data of this experiment. This can be prevented by making sure to rinse and

clean all materials needed in the lab with distilled water.