PART IV: Measuring Buffer Capacity

This is part of our Virtual Buffers Lab series: Understanding, Making, and Using Buffers

This lab includes the complete titration procedure used to determine the buffering capacity of a solution.

 

Learning Objectives

Figure 1. Learning Objectives for Part IV: Measuring Buffer Capacity

Students will be able to…

Prepare a buffer solution by accurately calculating and measuring the required amounts of both the acid and the base components. Use a burette set-up to administer precise quantities of acid and base to the buffer solution and accurately record the resulting pH. Analyze data by graphing and by identifying key data points. Calculate buffer capacity using key data points recorded in the lab. Use evidence from lab data to verify or falsify a theory.

 

Key Vocabulary

• Buffer
• Buffer capacity
• Titration
• pH
• Conjugate acid-base pair

 

Documents

• Click here for PDF of student handout. 

TEACHERS: Request an Answer KEY for this lab activity (Part IV), as well as the other lab activities in this "Understanding, Making, and Using Buffers" virtual lab.

 

Part IV: Measuring Buffer Capacity

Measuring Buffer Capacity 

Buffer capacity (β) is a measure of the amount of acid or base that can be added to a solution without changing the pH of the solution by more than 1 pH value divided by the pH.

 

 

Buffer capacity is determined experimentally using a titration procedure which measures the pH of a solution as acid or base is added in small increments. (To review or practice the titration procedure, go to PART II: Practicing Titrations.)

In this lab, the buffer capacity of a particular buffer solution will be determined.

 

MISSION 

Choose one of the following scenarios from the Buffer Challenge (See Part III: Determining Buffer Components for a Desired pH). Create a buffer solution to meet the requirements of the selected scenario. Test the buffer capacity using the following procedure.

Scenario 1: Prepare a buffer for an antibiological agent which is designed for use in the human body. This buffer should have a pH of 7.2 ± 0.5 with the ability to stay within one pH unit of this target when strong acid or base is added.

Scenario 2: Prepare a buffer for an antifungal agent which is designed for use against a fungus that attacks food sources that grow in acidic soil. This buffer should have a pH of 4.7 ± 0.5 with the ability to stay within one pH unit of this target when strong acid or base is added.

Scenario 3: Prepare a buffer for an antifungal agent which is designed for use against a fungus that attacks food sources in basic soil. This buffer should have a pH of 9.2 ± 0.5 with the ability to stay within one pH unit of this target when strong acid or base is added.

Scenario 4: Prepare a buffer for an antiviral agent which is designed for use against a strain of virus that attacks drug-producing bacteria that survive and grow in acidic environments. This buffer should have a pH of 3.1 ± 0.5 with the ability to stay within one pH unit of this target when strong acid or base is added.

 

✔CHECKPOINT: Check your buffer recipe with your teacher before completing the procedure below.

 

VIRTUAL PROCEDURE

Table 1. Virtual Procedure for Testing Buffer Solutions with Acid and Base

Scenario	Adding 0.20 M HCl	Adding 0.20 M NaOH
Scenario 1: Antibiological Agent
Scenario 2: Antifungal Agent for acidic soil
Scenario 3: Antifungal Agent for basic soil		BEING RECORDED...Check back soon!
Scenario 4: Antiviral Agent

 

WET LAB PROCEDURE

1. Prepare 100-mL of buffer solution using the amounts of solution and salt determined appropriate for your target pH.
2. Label a 250-mL beaker “ACID” and measure 50-mL of your buffer solution into this beaker. 
3. Label a second 250-mL beaker “BASE” and measure 50-mL of your buffer solution into that beaker. Set the BASE beaker aside to use later.
4. Use the pH meter’s electrode (or probe) to measure the pH in the ACID beaker. Record this initial pH in the data table.
5. To complete the titration, 0.20 M hydrochloric acid (HCl) will be added to the buffer solution in varying increments using a buret. (The purpose of varying the amount of acid added is to create a smooth curve by adding smaller amounts when the pH changes noticeably.) 
   1. Add the first indicated amount of 0.20 M HCl to the buffer solution.
   2. Record the precise volume added by reading the buret’s measurement. (The volume measurements may be exactly on the line, in which case –.00 should be recorded; however it is ok if the meniscus ends up slightly above or below the line, in which case you should record a proper estimated value.)
   3. Stir the solution thoroughly and allow the pH probe to stabilize, then record the pH data.
   4. Repeat the procedure for all the amounts of HCl listed below until you reach a total of 50.00 mL added. 
6. Flush the resulting mixture (buffer and HCl) down the sink with copious amounts of water. Rinse the beaker well and set out to dry. Rinse the pH probe well with distilled water.
7. Use the pH meter’s rinsed probe to measure the pH in the BASE beaker. Record this initial pH in the data table. (This value should be the same as your initial pH for the acid titration.)
8. Repeat the titration procedure described in Step 5 using 0.20 M sodium hydroxide (NaOH).
9. When finished, flush the resulting mixture (buffer and NaOH) down the sink with copious amounts of water. Rinse the beaker well and set out to dry. Rinse the pH probe and replace in the storage bottle with pH buffer solution.

Figure 2. Incremental volume additions for testing buffer capacity.

INCREMENTAL VOLUME (mL)	TOTAL AMOUNT ADDED (mL)
5.00	5.00
5.00	10.00
2.00	12.00
2.00	14.00
2.00	16.00
1.00	17.00
1.00	18.00
...continue adding 1.00 mL increments until a total of 30.00 mL is added	... 30.00
2.00	32.00
...continue adding 2.00 mL increments until a total of 40.00 mL is added	... 40.00
5.00	45.00
5.00	50.00

 

ANALYSIS

1. Graph the pH vs. Total amount added (mol) for both HCl and NaOH.
2. Determine how many moles of acid and base were neutralized by finding the data point that is approximately 1 pH value removed from the initial pH.
3. If the theory that the acids added to a buffer solution are neutralized by the conjugate base component of a buffer and the bases added are neutralized by the conjugate acid, what ratio would you expect to exist between the moles of acid/base neutralized and the moles of the conjugate present in the solution?
   1. Verify the theory that acids added to a buffer solution are neutralized by the conjugate base component of the buffer by comparing the amount in moles of HCl neutralized to the amount in moles of conjugate base (CB) in the buffer solution. 
   2. Then verify the theory by comparing the moles of NaOH neutralized to the amount of conjugate acid (CA) in the solution.
4. Calculate the buffering capacity for both HCl and NaOH using the equation:

 

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Navigate to the different pages of the Virtual Buffer Lab series: Understanding, Making, and Using Buffers

 

This lab was created with support from the Ren Research group at Purdue University with funding from the National Science Foundation grant NSF CHE 2102049.