Use Figure 19.9 to find an indicator for these titrations: (a) 0.10 M CH3 NH2 (see Appendix C) w

step 1 To determine which indicator will work best for these titrations, we need to determine what the

Use Figure 19.9 to find an indicator for these titrations:...

Use Figure 19.9 to find an indicator for these titrations:
(a) $0.10 \mathrm{M} \mathrm{CH}_{3} \mathrm{NH}_{2}$ (see Appendix C) with $0.10 \mathrm{M} \mathrm{HCl}$
(b) $0.50 M$ HI with $0.10 M$ KOH

Key Concepts

Titration of a Weak Base with a Strong Acid

When a weak base is titrated with a strong acid, the product is a salt that hydrolyzes in water, generating an acidic solution at equivalence. The equivalence point’s pH is determined by the acid dissociation constant (Ka) of the conjugate acid. Thus, when selecting an indicator for such titrations, its transition range should fall within the acidic region where the pH at equivalence is expected.

Indicator Transition Range

This concept refers to the narrow pH interval over which an indicator undergoes its color change. The proper selection of an indicator requires matching its transition range to the expected pH shift at the titration equivalence point. This ensures that the observed color change accurately signals the completion of the reaction.

Titration of a Strong Acid with a Strong Base

In a strong acid–strong base titration, both reactants are completely dissociated in solution, and the reaction proceeds to a neutralization that leads to an equivalence pH near 7. Consequently, indicators chosen for such titrations should have transition ranges that include the neutral pH region to accurately mark the endpoint.

Acid–Base Indicators

Acid–base indicators are chemicals that change color in response to changes in pH. They exist in two forms—one in acidic conditions and one in basic—each having a different color. The indicator’s transition range (usually about ±1 pH unit around its pKa) should be chosen so that the color change occurs very near the titration’s equivalence point, assuring accurate endpoint detection.

Equivalence Point in Titrations

The equivalence point is reached when the amount of titrant added exactly neutralizes the analyte in solution. Its pH depends on the strengths of the acids and bases involved. For example, titrations involving a weak base with a strong acid yield an equivalence point with a pH that is typically below 7 because the resulting conjugate acid hydrolyzes, while strong acid–strong base titrations typically reach an equivalence point near neutral.

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