EDTA forms 1:1 complexes with-several metal ions. Titrations that are used to determine the amount of a metal ion in a solution can be performed in multiple ways. In lab, you performed a direct titration of Ca2+ and Mg2+ using EBT as an indicator. This particular indicator is red when a metal ion is bound to it and blue in its free form. Some metal ions (like Cu+ and Al3+) bind irreversibly to EBT making them unsuitable for a direct titration.
You have 500.0 mL of a solution that contains an unknown amount of Al3+. You decide to adapt the procedure for an EDTA titration so you can figure out how much Al3+ is in your solution. You take 25.00 mL of the unknown solution and mix it with 30.00 mL of 0.01544 M EDTA. A few drops of EBT indicator are added and the resulting solution requires 48.84 mL of 5.00 x 108 M Mg2+. The formation constant for the Mg•EDTA complex is 6.17 x 108. The formation constant for the Al•EDTA complex is 2.0 x 1016.
1. What is the color of the solution when the indicator is added?
2. What is the color of the solution when the endpoint is reached?
3. How many moles of Al3+ were in the 25.00 mL sample of unknown?