Vernier Software and Technology
Vernier Software & Technology

Testing the Effectiveness of a Buffer

Introduction

A buffer is an aqueous mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. A buffer has a very stable pH. When small amounts of an acid or a base are added to a buffer solution, the pH of the solution changes very little. In many chemical and biochemical systems, buffers are critical. Blood plasma, a natural example in humans, is a bicarbonate buffer that keeps the pH of blood between 7.2 and 7.6.

By design, a buffer is an equilibrium system. An example is a buffer prepared with nitrous acid, HNO2. The weak acid establishes an aqueous equilibrium as shown below:

{\text{HNO}}{_{\text{2}}}{\text{ (aq)}} \to {\text{H}}{^{+}}{\text{ (aq) + NO}}{_{\text{2}}}{^{\text{-}}}{\text{ (aq)}}

The equilibrium constant expression is

K{_{a}} = \frac{[{\text{H}}{^{+}}][{\text{NO}}{_{2}}{^{-}}]}{[{\text{HNO}}{_{2}}]}

To prepare a buffer system with nitrous acid, a conjugate base, such as sodium nitrite (NaNO2), is added. The resulting system is a mixture of HNO2 and NO2 ions. The nitrous acid molecule will buffer the addition of an acid and the nitrite ion from the conjugate will buffer the addition of a base.

A variation of the equilibrium expression above, called the Henderson-Hasselbalch equation, is a very good reference in preparing a buffer solution. For a nitrous acid/sodium nitrate buffer, the Henderson-Hasselbalch equation is shown below:

pH = pK{_{a}} + log \frac{[{\text{NO}}{_{2}}{^{-}}]}{[{\text{HNO}}{_{2}}]}

The pH range in which a buffer solution is effective is generally considered to be ±1 of the pKa.

Objectives

In the Initial Investigation, you will prepare two acetic acid/sodium acetate buffer systems. After examining the factors involved in preparing these buffers and testing their effectiveness, you will design a buffer for a specified pH range as well as test its effectiveness; effectiveness will be assessed based on the amount of acid or base a buffer can absorb before the pH of the buffer solution falls outside a specified pH range.

Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

Option 1

Option 2

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

Vernier Chemistry Investigations for Use with AP* Chemistry

See other experiments from the lab book.

1Investigating Food Dyes in Sports Beverages
2Determining the Copper Content in Brass
3Investigating Water Hardness
4The Acidity of Juice and Soft Drinks
5Separating Molecules
6Identifying an Unknown Substance
7Investigating the Purity of a Mixture
8Determining the Percent Peroxide in a Commercial Product
9Investigating the Components of a Commercial Tablet
10The Effect of Acid Rain on a Marble Structure
11Investigating the Kinetics of a Crystal Violet Reaction
12Investigating Commercial Hand Warmers
13Investigating LeChatelier’s Principle
14Investigating Acid-Base Titrations
15The Buffering Ability of Commercial Products
16Testing the Effectiveness of a Buffer

Experiment 16 from Vernier Chemistry Investigations for Use with AP* Chemistry Lab Book

<i>Vernier Chemistry Investigations for Use with AP* Chemistry</i> book cover

Included in the Lab Book

Vernier lab books include word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.

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