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How does Eriochrome Black-T React?

Since the reactants and products in the titration between hard water and EDTA are all colorless, an indicator is needed to allow us to visually see the end point (or equivalence point) of the titration, i.e. the point at which moles of Ca2+ = moles of EDTA. Eriochrome Black-T is the indicator we will use.

When Eriochrome Black-T complexes with Mg2+ ions, it produces a PINK-RED solution. The indicator is BLUE when the it is not complexed with Mg2+ and the solution is basic.

As EDTA is added to hard water during the titration it reacts with the divalent ions (e.g. Ca2+ and Mg2+) present in the hard water. Once all the divalent ions in solution have reacted with EDTA, the EDTA reacts with the Mg2+ ions that are complexed with the Eriochrome Black-T indicator, thus causing the solution to turn BLUE.

Eriochrome Black-T

The three pictures show the color changes as the end point of the titration is reached. There is only a 1 drop difference of 0.010 M EDTA between the first and second pictures and between the second and third pictures. Two or three seconds were allowed for colors in the second and third pictures to develop after adding the additional drop. In each case the solution was thoroughly mixed. This color change from wine red to violet to blue is due to the color changes of the indicator Eriochrome Black-T.

 

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