Final answer:
When EDTA has chelated all zinc ions, the indicator Eriochrome Black T changes its color from wine-red to blue, as the complexation displaces the indicator and restores its original color.
Step-by-step explanation:
When sufficient EDTA has been added to complex with all of the zinc ions, the result is that the indicator Eriochrome Black T (EBT) changes color. Initially, EBT displays a wine-red color in the presence of free metal ions. However, once all the zinc ions have been complexed by the EDTA, the color of the solution changes from red to blue. This happens because EDTA has a stronger affinity for the metal ions than EBT, so it displaces the indicator from the metal ion complex, rendering the EBT free in solution. Upon losing its association with the metal ion, EBT reassumes its original blue color.
It is important to note that the co-ordination number of the EDTA-metal ion complex is 6. This means that six coordinate bonds are formed between the negatively charged EDTA and the positively charged metal ion. Zinc ions have a +2 charge in solution, as depicted in the reactions where they form colorless Zn2+ ions. The overall charge of the complex comes from balancing the metal ion charge with the charge on the EDTA ion.