In a titration of hardness using EDTA (ethylenediaminetetraacetic acid) and Eriochrome Black T indicator, the color change from wine-red to blue signifies the formation of the EDTA-metal complex. However, if the solution returns to purple after the blue color change, it indicates the presence of excess metal cations.
The purple color indicates the presence of uncomplexed metal ions that are not bound to the EDTA. These metal ions can interfere with the color change and result in the observed color reversal.
The cation responsible for this problem is usually calcium (Ca2+). Calcium ions have a strong affinity for EDTA and can form complexes, but in the presence of excess calcium, the formation of EDTA-calcium complexes becomes less favorable. As a result, the excess calcium can displace the EDTA from the complex, leading to the purple coloration.
To overcome this issue, it is necessary to ensure that all the calcium ions have reacted with EDTA before reaching the endpoint. This can be achieved by adding a sufficient excess of EDTA to complex all the calcium ions present in the solution. This excess of EDTA ensures that all calcium ions are sequestered, preventing the purple color from reappearing. By titrating until the blue color remains stable, you can be confident that all calcium ions have been complexed.
Recalculating the hardness may not be necessary if the endpoint is reached with a stable blue color, indicating that all the calcium has reacted with EDTA. However, if the solution turns purple again after reaching the blue color, it would be prudent to perform additional titrations with a higher volume of the EDTA solution to ensure complete complexation of the calcium ions. This will provide a more accurate determination of the hardness.