Selecting an Indicator for a Redox Titration

The most important class of indicators for redox titrations are substances that do not participate in the redox titration, but whose oxidized and reduced forms differ in color. When we add a redox indicator to the titrand, the indicator imparts a color that depends on the solution’s potential. As the solution’s potential changes with the addition of titrant, the indicator changes oxidation state and changes color, signaling the end point. As illustrated here, the change in color occurs over a range of potentials centered on the indicator’s standard state reduction potential. The size of this range is ±0.05916/n volts where n is the number of electrons in the indicator’s oxidation or reduction reaction.

Figure9.39

As with other titrations, the indicator is chosen so that the change in color signaling the titration’s end point is close to the equivalence point. The titration curve shown here is for the titration of 50.0 mL of 0.100 M Fe2+ with 0.100 M Ce4+ (which has a symmetric equivalence point). The end point transitions for the indicators diphenylamine sulfonic acid and ferroin are superimposed on the titration curve. Because the transition for ferroin is too small to see on the scale of the x-axis—it requires only 1–2 drops of titrant to complete the change in color—its color change is expanded to the right. Ferroin is the better choice of indicator for this titration.

Figure9.40

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