Indicators for Complexation Titrations

Most indicators for complexation titrations are organic dyes—known as metallochromic indicators—that form stable complexes with metal ions. The indicator, Inm, is added to the titrand’s solution where it forms a stable complex with the metal ion, MInn–. As we add the EDTA titrant it reacts first with free metal ions, and then displaces the indicator from MInn. If MInn and Inm have different colors, then the change in color signals the end point.

The accuracy of an indicator’s end point depends on the relative strength of the metal–indicator complex and the metal–EDTA complex. If the metal–indicator complex is too strong, the change in color occurs after the equivalence point. If the metal–indicator complex is too weak, however, the end point occurs before we reach the equivalence point.

Most metallochromic indicators also are weak acids. One consequence of this is that the conditional formation constant for the metal–indicator complex depends on the titrand’s pH. This provides some control over an indicator’s titration error because we can adjust the strength of a metal–indicator complex by adjusted the pH at which we carry out the titration. Unfortunately, because the indicator is a weak acid, the color of the uncomplexed indicator also changes with pH. The illustration below, for example, shows the color of the indicator calmagite as a function of pH and pMg, where H2In, HIn2–, and In3– are different forms of the uncomplexed indicator, and MgIn is the Mg2+–calmagite complex. Because the color of calmagite’s metal–indicator complex is red, its use as a metallochromic indicator has a practical pH range of approximately 8.5–11 where the uncomplexed indicator, HIn2–, has a blue color.

Figure9.30

Conditions to the right of the dashed line, where Mg2+ precipitates as Mg(OH)2, are not analytically useful for a complexation titration.

The photo shown here shows the actual colors of calmagite during the titration of Mg2+ with EDTA; the indicator is: (a) red prior to the end point due to the presence of the Mg2+–indicator complex; (b) purple at the titration’s end point; and (c) blue after the end point due to the presence of uncomplexed indicator.

Figure9.32The illustration below—which shows titration curves for the titration of 50 mL of 10–3 M Mg2+ with 10–3 M EDTA at pHs levels of 9, 10, and 11—demonstrates the importance of pH when using calmagite as an indicator. An early end point is likely at a pH of 9 and a late end point is likely at a pH of 11; setting the pH to 10 ensures that the end point and equivalence point are nearly identical.

Figure9.33

About Author

This entry was posted in Illustration, Photo and tagged , , , , . Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *