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LAB in Acid Base Reactions, Balancing Equations, Titrations, Indicators, Strong vs Weak, Graphing, pH, Equivalence Point. Last updated October 24, 2019.

### Summary

In this lab, students graphically observe a plot of "micro" acid/base titrations and determine the equivalence point of each plotted curve.

High school

### AP Chemistry Curriculum Framework

This laboratory experiment supports the following units, topics and learning objectives.

• Unit 4: Chemical Reactions
• Topic 4.2: Net Ionic Equations
• TRA -1.B: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
• Topic 4.5: Stoichiometry
• SPQ-4.A: Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.
• Topic 4.6: Introduction to Titration
• SPQ- 4.B: Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.
• Unit 8: Acids and Bases
• Topic 8.2: pH and pOH of Strong Acids and Bases
• SAP-9.B: Calculate pH and pOH based on concentrations of all species in a solution of a strong acid or a strong base.
• Topic 8.3: Weak Acid and Base Equilibria
• SAP-9.C: Explain the relationship among pH, pOH, and concentrations of all species in a solution of a monoprotic weak acid or weak base.
• Topic 8.4: Acid-Base Reactions and Buffers
• SAP-9.D: Explain the relationship among the concentrations of major species in a mixture of weak and strong acids and bases.
• Topic 8.5: Acid-Base Titrations
• SAP-9.E: Explain results from the titration of a mono- or polyprotic acid or base solution, in relation to the properties of the solution and its components.
• Topic 8.7: pH and pKa
• SAP-10.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

### Objectives

By the end of this lesson, students should be able to

• identify strong and weak acids and bases.
• identify the equivalent point of a titration reaction.
• write a balance acid/base reaction

### Chemistry Topics

This lesson supports students’ understanding of

• Equivalence point
• Titrations
• Acid base reactions
• Strong vs weak acids/bases

### Time

Teacher Preparation: one hour initial chemical and label prep, 10 minutes thereafter

Lesson: one class period

### Materials

For each group:

Small-scale pipets of the following solutions:

• Buffer solutions (pH 1-12)
• Acetic acid (CH3COOH)
• Hydrochloric acid (HCl)
• Sodium hydroxide (NaOH)
• Ammonium hydroxide (NH4OH)
• 2 small test tubes
• 2 large test tubes
• Transparency paper
• Safety goggles

### Safety

• Always wear safety goggles when working with chemicals in a laboratory setting.
• When working with acids and bases, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals

### Teacher Notes

• If you are working with limited supplies, each lab group can perform one portion of the experiment and share their data with the class.

### Background

A titration curve is a graph that shows how the pH of a solution changes as a function of the amount of added titrant. You obtain data for a titration curve by titrating a solution and measuring the pH after every drop of added titrant. For example, you can contrast a titration curve for the titration of hydrochloric acid with sodium hydroxide by measuring the pH after every added drop of NaOH. A plot of the pH on the y-axis versus drops of NaOH on the x-axis tells you how the pH changes as the titration proceeds.

A titration curve gives you a lot of information about a titration. For example, you can use the curve to identify the pH of the equivalence point. The equivalence point is the point on the titration curve where the moles of acid equal the moles of base. The midpoint of the steepest part of the curve (the most abrupt changes in pH) is a good approximation.

### Materials

Small-scale pipets of the following solutions:

• Buffer solutions (pH 1-12)
• Acetic acid (CH3COOH)
• Hydrochloric acid (HCl)
• Sodium hydroxide (NaOH)
• Ammonium hydroxide (NH4OH)

### Procedure

Construct a pH meter for use in this experiment: Place one drop of each of the numbered pH solutions on a transparency and use as a color guide for this experiment. Record the color observed in the data table below.

Colorimetric pH Meter

 pH 1 pH 2 pH 3 pH 4 pH 8 pH 7 pH 6 pH 5 pH 9 pH 10 pH 11 pH 12

Part A: Strong Acid/Strong Base & Weak Acid/Strong Base

Set 1:

1. Add 10 drops of 1-M HCl to a test tube.
2. Add one or two drops of universal indicator and 10 drops of distilled water.
3. Add 1-M NaOH drop by drop. Stir after each addition and using the colors of the reference pH chart, record the approximate pH in the test tube after each drop.
4. Continue adding NaOH until the color turns a dark violet and stays violet after one additional drop. You may not need 15 drops.
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Set 2:

1. Add 10 drops of 1-M HC2H3O2 to a test tube.
2. Add one or two drops of universal indicator and 10 drops of distilled water.
3. Add 1-M NaOH drop by drop. Stir after each addition and using the colors of the reference pH chart, record the approximate pH in the test tube after each drop.
4. Continue adding NaOH until the color turns a dark violet and stays violet after one additional drop. You may not need 15 drops.
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Part B: Strong Base/Strong Acid & Weak Base/Strong Acid

Set 3:

1. Add 10 drops of 1-M NaOH to a test tube.
2. Add one or two drops of universal indicator and 10 drops of distilled water.
3. Add 1-M HCl drop by drop. Stir after each addition and using the colors of the reference pH chart, record the approximate pH in the test tube after each drop.
4. Continue adding HCl until the color turns a dark red and stays red after one additional drop. You may not need 15 drops.
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Set 4:

1. Add 10 drops of 1-M NH4OH to a test tube.
2. Add one or two drops of universal indicator and 10 drops of distilled water.
3. Add 1-M HCl drop by drop. Stir after each addition and using the colors of the reference pH chart, record the approximate pH in the test tube after each drop.
4. Continue adding HCl until the color turns a dark red and stays red after one additional drop. You may not need 15 drops.
 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

### Analysis

Use graph paper to construct a titration curve for both microtitrations from part A.

1. Label and plot the pH (y-axis) versus the number of drops of NaOH added (x-axis).
2. Connect the points (you should see a smooth “S” curve).
3. Make a key to indicate which line is the plot of set 1 and which is set 2.
4. Draw a small horizontal line about half way up the steepest portion of the curve. Label this line as the equivalence point for each plotted curve.

Use graph paper to construct a titration curve for both microtitrations from part B.

1. Label and plot the pH (y-axis) versus the number of drops of HCl added (x-axis).
2. Connect the points (you should see a smooth “S” curve).
3. Make a key to indicate which line is the plot of set 3 and which is set 4.
4. Draw a small horizontal line about half way up the steepest portion of the curve. Label this line as the equivalence point for each plotted curve.
5. Indicate whether each chemical used in this lab was a strong or weak acid or base.
Hydrochloric acid (HCl) _________________________
Sodium hydroxide (NaOH) _______________________
Ammonium hydroxide (NH4OH) ________________
Acetic acid (CH3COOH) _______________________
6. Write a balanced equation for each of the reactions you carried out.
7. Set 1& 3 (yep, same reaction in both parts):
Set 2:
Set 4:

8. Examine each of the curves plotted on the graphs from Part A and Part B. What is the approximate pH at the equivalence point when using each of the following:
Strong Acid/Strong Base_______Weak Acid/Strong Base_____________

Strong Base/Strong Acid _________Weak Base/Strong Acid_____________
Explain why you observed the equivalence at these pHs.

Use the following words or symbols to make each paragraph read correctly. Some words may be used more than once.
H3O+ OH- acidic basic increase decrease 7
9. Both titration curves in part A begin in the ___________ range because the ion present in greater concentration is ___________. As you added base, the __________ ion concentration increased, causing the pH value to ________. The equivalence point of a strong acid with a strong base is expected to be pH ____________. The pH of the equivalence point of a weak acid and strong base is in the _______________ range.
10. Both titration curves in part B begin in the ___________ range because the ion present in greater concentration is ___________. As you added acid, the __________ ion concentration increased, causing the pH value to ________. The equivalence point of a strong base with a strong acid is expected to be pH ____________. The pH of the equivalence point of a weak base and strong acid is in the _______________ range.
11. Sketch a titration curve if a student has a weak acid and adds drops of weak base.