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Chapter 17: Problem 22

What are the criteria for choosing an indicator for a particular acid-base titration?

Short Answer

Expert verified
The criteria for choosing an indicator for an acid-base titration are: (1) The pH range over which the indicator changes colour, which should correspond to the equivalence point of the titration; and (2) A distinct colour change over a narrow pH range for better precision. Some common indicators include litmus, phenolphthalein, and methyl orange, and their use would depend on the pH at the equivalence point of the specific titration.

Step by step solution

01

Understanding Acid-Base Titrations

Acid-base titration is a process used to determine the concentration of an unknown acid or base. It is performed by neutralizing the acid or base with a solution of a base or acid of known concentration. An indicator is used to show when the reaction has been completed, by its colour change.
02

Identifying the Role of an Indicator

An indicator is a substance that changes color near the endpoint of a chemical reaction. The choice of indicator is very crucial for accurate results in a titration.
03

Recognizing the Criteria for Choosing an Indicator

The choice of an indicator depends mainly on the pH range over which it changes colour, which should correspond to the equivalence point of the titration, i.e., the point at which the reactants have reacted in stochiometrically equivalent quantities. Moreover, the indicator should have a distinct colour change over a narrow range of pH, for better precision.
04

Identifying Different Indicators and Their pH Ranges

Common indicators and their pH ranges are: Litmus: changes from red in acidic solution to blue in alkaline solution with a neutral point at pH 7. Phenolphthalein: colorless in acidic solution and becomes pink in alkaline solution with change at pH 8.2-10. Methyl Orange: changes from red in acidic solution to yellow in alkaline solution with change at pH 3.1 - 4.4. The selection of these indicators would depend on the pH at the equivalence point of the titration.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Indicator Selection
Indicator selection is crucial in acid-base titrations, as it determines the accuracy and reliability of your results. An indicator is a chemical that changes color at a specific pH range. Picking the right one ensures you know exactly when the titration has reached its equivalence point.

To select an indicator, consider the equivalence point of your titration. For example:
  • If the equivalence point is in a basic pH range, phenolphthalein may be suitable as it changes color around a pH of 8.2-10.
  • In a more acidic range, methyl orange, which changes color between pH 3.1 - 4.4, could be a better choice.
  • Litmus can be used for titrations near a neutral pH, turning red in acidic and blue in basic solutions.
Think about the specifics of your reaction and which indicator will best highlight the transition point effectively.
pH Range
The pH range over which an indicator changes color is critical in its selection for titrations. Different indicators have different pH transition ranges. This range must align with the expected pH at the equivalence point of the titration.

For example:
  • Phenolphthalein changes color in the pH range of 8.2 to 10. It's ideal for strong acid with strong base reactions where the expected equivalence point is basic.
  • Methyl orange has a color change from pH 3.1 to 4.4, suitable for strong acid with weak base titrations.
It's the correspondence between the indicator's pH transition range and the titration equivalence point that ensures accuracy in determining when the reaction is complete.
Equivalence Point
The equivalence point in a titration is the point at which the amount of titrant added is stoichiometrically equivalent to the amount of substance in the sample. This is often the goal of the titration process, and knowing this helps choose the right indicator.

During an acid-base titration, it's crucial that the indicator's color change occurs precisely at the equivalence point, ensuring that the reaction completion is easy to identify. The equivalence point can differ between titrations:
  • With strong acids and bases, the equivalence point pH tends to be around 7.
  • With strong acid and weak base reactions, it falls below 7.
  • With strong base and weak acid titrations, it's above 7.
Understanding where the equivalence point lies ensures precise and correct titration results.
Color Change
Color change of an indicator signals the transition point during a titration. It's vital that the color change is clear and distinct for accurate identification of the equivalence point. This clear shift aids in the visual evaluation of the titration process.

For instance:
  • Phenolphthalein turns from colorless to pink, making it easy to see the shift in basic solutions.
  • Methyl orange changes from red to yellow, providing a sharp transition in acidic ranges.
Color change must be swift and visible within a narrow pH range to avoid errors in determining the endpoint, ensuring that the titration result is accurate and reliable.
Titration Accuracy
Titration accuracy depends greatly on the correct selection of the indicator and a precise recognition of the equivalence point. This precision involves:
  • Selecting an indicator with a color change that corresponds closely to the pH of the titration's equivalence point.
  • Ensuring that any color change is dramatic and distinct to minimize interpretation errors.
Accurate titrations allow the determination of unknown concentrations with confidence. However, errors can arise if improper indicators are chosen or if the precise point of color change is overlooked. By considering these aspects diligently, you enhance the accuracy and reliability of your titration results.

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Most popular questions from this chapter

A 5.00 -g quantity of a diprotic acid is dissolved in water and made up to exactly \(250 \mathrm{~mL}\). Calculate the molar mass of the acid if \(25.0 \mathrm{~mL}\) of this solution required \(11.1 \mathrm{~mL}\) of \(1.00 \mathrm{M} \mathrm{KOH}\) for neutralization. Assume that both protons of the acid are titrated.

Write the formation constant expressions for these complex ions: (a) \(\mathrm{Zn}(\mathrm{OH})_{4}^{2-},\) (b) \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}^{3+}\) (c) \(\mathrm{HgI}_{4}^{2-}\).

A student wishes to prepare a buffer solution at \(\mathrm{pH}=\) \(8.60 .\) Which of these weak acids should she choose and why: HA \(\left(K_{\mathrm{a}}=2.7 \times 10^{-3}\right),\) HB \(\left(K_{\mathrm{a}}=4.4 \times\right.\) \(10^{-6}\) ), or HC \(\left(K_{\mathrm{a}}=2.6 \times 10^{-9}\right) ?\)

Which of these solutions has the highest \(\left[\mathrm{H}^{+}\right]\) : (a) \(0.10 \mathrm{M} \mathrm{HF},\) (b) \(0.10 \mathrm{M} \mathrm{HF}\) in \(0.10 \mathrm{M} \mathrm{NaF},\) or (c) \(0.10 \mathrm{M}\) HF in \(0.10 \mathrm{M} \mathrm{SbF}_{5}\) ?

The ionization constant \(K_{\mathrm{a}}\) of an indicator HIn is \(1.0 \times\) \(10^{-6}\). The color of the nonionized form is red and that of the ionized form is yellow. What is the color of this indicator in a solution whose \(\mathrm{pH}\) is \(4.00 ?\)
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