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Mastering Titration Curves: Your Guide to Acid-Base Chemistry
Unlock the secrets of titration curves and boost your chemistry knowledge. Learn to analyze pH changes, identify equivalence points, and understand buffer zones for success in your studies and beyond.

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Now Playing:Titration curves – Example 0a
Intros
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  1. Using titration data in graphs
  2. Using titration data in graphs
    How do graphs explain titration?
  3. Using titration data in graphs
    Different titration curves: strong acid/strong base.
Examples
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  1. Explain the use of suitable indicators for combinations of strong and weak acids and bases.
    Below is a table with information on three common pH indicators used in titrations.

    Indicator

    Acid (protonated) color

    Base (deprotonated) color

    pH range

    Phenolphthalein

    Colorless

    Pink

    8.2-12.0

    Bromothymol blue

    Yellow

    Blue

    6.0-7.6

    Alizarin yellow

    Yellow

    Red

    10.0-12.0

    1. If phenolphthalein is used, what type of acid and what type of base is likely being used in the titration experiment?

    2. Sketch a titration curve for a titration of this type, showing the range where the equivalence point is expected.

Introduction to acid-base theory
Jump to:NotesConceptFAQsPrerequisites
Notes

In this lesson, we will learn:

  • To recall the types of titration curves and their general shapes.
  • How to choose an appropriate indicator for the type of titration being performed.

Notes:

  • Titration is an important technique in finding the concentration of a chemical sample.
    Depending on what combination of strong/weak acid/base you are using, when plotting volume of added titre (the chemical with known concentration) against pH from a titration run, you will get distinct graph shapes.

  • You need to be careful with the descriptions of a titration: the chemical with unknown concentration is being titrated by the titer, which is the chemical with known concentration.
    For example in the sentence “The titration of chemical A by chemical B”, A is the sample (with unknown concentration) in the beaker that you want to investigate, and B is the known chemical being added from the burette.

  • The simplest titration curve is a titration of a strong acid/base by a strong base/acid. The following phases occur in order as titer volume increases:
        • An initial ‘horizontal’ phase of extremely little change in pH as titer volume increases.
        • A ‘vertical’ phase when close to the equivalence point, where a very small addition of titer causes a large change in pH. Around the equivalence point, pH changes nearly ‘vertically’ on the graph (rising in acid titrated by base and dropping in base titrated by acid) as opposed to nearly ‘horizontally’ outside this region. The equivalence point in a strong acid and strong base titration, whichever way around, should occur at pH 7.
        • A second horizontal phase with a levelling-off, like the beginning, after the equivalence point.
    • See the graph below, where:
        • Va / Vb = volume of acid or base required to reach the equivalence point:
    • The titration of a weak acid by a strong base has a graph with these phases in sequence as titer volume increases:
        • An initial rise in pH seen with the first small amounts of titer,
        • A horizontal levelling off phase (like in strong acid by strong base), where very little pH change occurs. This is known as the buffer region, where pH is staying nearly constant due to the weak base equilibria being disturbed and re-established as per Le Chatelier’s principle.
        • A nearly vertical rise near the equivalence point, like the curve with strong acid/strong base titrations. The equivalence point in these titrations will occur at a pH greater than 7 (basic conditions).
        • Another near horizontal level-off after the equivalence point where very little pH change occurs.

      See the graph below, where:
        • VB = volume of strong base required to reach equivalence point.
        • V1/2 = half the value of VB
        • pH1/2 = pH at V1/2:
    • The titration of a weak base with strong acid has all the same features as the curve of a weak acid with strong base except it is has been turned upside down
        • An initial drop in pH seen with the first small amounts of titer,
        • A horizontal levelling off phase, where very little pH change occurs with titer being added. This is known as the buffer region, where pH is staying nearly constant due to the weak base equilibria being disturbed and re-established as per Le Chatelier’s principle.
        • A nearly vertical pH drop around the equivalence point, similar to the curve with strong acid/strong base titrations. The equivalence point in these titrations will occur at a pH lower than 7 (acidic conditions).
        • Another near horizontal level-off after the equivalence point where very little pH change (a very slow drop) occurs.

      See the graph below, where:
        • VA = volume of strong acid required to reach equivalence point.
        • V1/2 = half the value of VA
        • pH1/2 = pH at V1/2:
  • The Va / Vb measurements, along with can be used to find the concentration of the unknown acid

  • Knowing which type of titration you’re doing is important for your choice of pH indicator. When performing a titration you will need to use an indicator that has a ‘pH range’ that changes color through the equivalence point. Depending on whether you have a strong acid and base or strong/weak acid/base, this gives you specific options.
    • Titrating a strong acid with a strong base needs an indicator with a range covering pH 7, for example bromothymol blue which has a pH range of 6.0 – 7.6. This ensures the equivalence point (where a massive change in pH occurs with minimal titer being added) will be flagged by the color.
    • Titrating a weak acid with a strong base means the equivalence point will be at a pH above 7, so pick an indicator with a pH range above 7. Phenolphthalein is a very good choice for titrating acids with strong bases as it has a pH range from 8.2-12.
    • Titrating a weak base with a strong acid means your equivalence point will be at a pH below 7, so you need an indicator with a pH range below 7 too. Methyl red is a good choice with a pH range of 6.2-4.4.
Concept

Introduction to Titration Curves

Welcome to our exploration of titration curves, a fundamental concept in chemistry! Titration curves are graphical representations of acid-base titrations, providing crucial insights into the behavior of solutions during the titration process. These curves are essential for understanding the relationship between pH and the volume of titrant added. By visualizing the titration curve, chemists can easily identify the equivalence point, where the amount of acid and base are exactly equal. This concept is vital for various applications in analytical chemistry, environmental science, and biochemistry. Our introductory video will guide you through the basics of titration curves, making this complex topic more accessible. You'll learn how to interpret different regions of the curve, recognize buffer zones, and determine the strength of acids or bases. Whether you're a high school student or pursuing a college degree, mastering titration curves will enhance your understanding of acid-base chemistry and equip you with valuable analytical skills.

FAQs

Here are some frequently asked questions about titration curves:

1. What does a titration curve tell you?

A titration curve provides information about the pH changes during a titration process. It shows the relationship between the volume of titrant added and the pH of the solution. From this curve, you can determine the equivalence point, buffer regions, and the strength of acids or bases involved in the reaction.

2. What are the four parts of a titration curve?

The four main parts of a titration curve are: 1) Initial pH region, 2) Buffer region, 3) Equivalence point region, and 4) Excess titrant region. Each part provides specific information about the titration process and the properties of the substances involved.

3. How do you draw a titration curve?

To draw a titration curve, plot the pH of the solution on the y-axis against the volume of titrant added on the x-axis. Record pH values at regular intervals as you add the titrant. Connect the points to create a smooth curve, paying special attention to the region near the equivalence point where the pH changes rapidly.

4. Why is a titration curve S-shaped?

A titration curve is S-shaped due to the changing rate of pH as titrant is added. The curve is relatively flat at the beginning and end, where the solution resists pH changes. The steep middle section represents the rapid pH change near the equivalence point. This shape reflects the buffering action and the sudden neutralization at the equivalence point.

5. How do you interpret the shape of a titration curve?

The shape of a titration curve provides information about the strength of the acid and base involved. A steep curve near the equivalence point indicates a strong acid-strong base titration. A more gradual curve suggests weak acid or weak base involvement. The location of the equivalence point (above, at, or below pH 7) also indicates the type of titration (e.g., weak acid-strong base, strong acid-strong base).

Prerequisites

Understanding titration curves is a crucial skill in chemistry, but to truly grasp this concept, it's essential to have a solid foundation in certain prerequisite topics. One of the most important prerequisites for mastering titration curves is the acid dissociation constant (Ka). This fundamental concept plays a pivotal role in understanding the behavior of acids and bases during titration processes.

The acid dissociation constant, often referred to as Ka, is a quantitative measure of the strength of an acid in solution. It represents the extent to which an acid dissociates into its constituent ions in water. When studying titration curves, knowing the Ka value of the acid being titrated is crucial for predicting and interpreting the shape of the curve.

Titration curves graphically represent the change in pH during a titration process. The acid dissociation constant directly influences the shape and key features of these curves. For weak acids, which have smaller Ka values, the titration curve will have a more gradual slope in the buffer region. In contrast, strong acids with larger Ka values will produce curves with steeper slopes and more pronounced equivalence points.

Understanding the relationship between Ka and titration curves allows students to: 1. Predict the pH at different points during the titration 2. Identify the equivalence point and half-equivalence point 3. Calculate the concentration of the acid or base being titrated 4. Choose appropriate indicators for specific titrations

Moreover, the acid dissociation constant concept is essential for comprehending buffer solutions, which are often encountered in titration experiments. Buffers resist changes in pH, and their behavior can be explained using Ka values and the Henderson-Hasselbalch equation, which is derived from the acid dissociation constant.

By mastering the concept of Ka, students can more easily interpret titration data, perform calculations related to acid-base equilibria, and understand the underlying principles of pH changes during titrations. This knowledge is not only crucial for academic success but also has practical applications in various fields, including environmental science, biochemistry, and pharmaceutical research.

In conclusion, a solid understanding of the acid dissociation constant (Ka) is indispensable for students aiming to excel in their study of titration curves. It provides the necessary foundation for interpreting experimental results, predicting chemical behavior, and solving complex problems in acid-base chemistry. By investing time in mastering this prerequisite topic, students will find themselves better equipped to tackle the intricacies of titration curves and related concepts in their chemistry studies.