Why is the indicator EBT shows wine red at beginning and blue end?

EBT (Eriochrome Black T) is an indicator that is commonly used in complexometric titrations, particularly for determining the concentration of metal ions such as calcium and magnesium. The color change of EBT indicator involves a shift from wine red to blue, not the other way around. Here's the explanation:

Initial Color (Wine Red):

At the beginning of the titration, when the concentration of metal ions is high, EBT forms a complex with the metal ions, which shifts the equilibrium towards the metal-EBT complex. This complex exhibits a wine red color.

Color Change to Blue:

As the titration progresses, and more of the metal ions are complexed with EDTA (the titrating agent), the concentration of free metal ions decreases. This causes the equilibrium to shift back towards the free EBT form. Free EBT exists as a dianion, which has a blue color. Therefore, as the metal ions are increasingly complexed by EDTA, the solution gradually turns from wine red to blue.

The exact pH range at which this color change occurs depends on the metal ion being titrated and the complexing properties of EDTA.

In summary, the initial wine red color of EBT is due to the formation of a metal-EBT complex, while the change to blue signifies the depletion of free metal ions and the presence of free EBT dianions. The color change serves as an endpoint indicator in complexometric titrations, signaling the completion of the reaction between the metal ions and EDTA.