Last week, I did titration of Zn with EDTA experiment. I used calmigte as the indictor. The endpoint of this titration is supposed to be blue. The color change go from wine-red to violet to Blue. I did a mistake by stopping the titration at the violet color. Is there anyway to estimate how many mL I am away from the Blue end-point. Thanks.

The only thing I can suggest is to use the numbers you have, assume that is the end point, and calculate the amount of Zn in your sample. Then weigh an amount of Zn or Zn salt equal to the amount you have calculated as being in your sample, add the same materials as you did to the unknown, titrate to the same color that you titrated to the first time, then count the drops of titrant from that point in the titration to the suggested end point color. That won't get it exactly but it should be a close approximation. The problem with this estimation and that you have only your "memory" as to the color you titrated to the first time. And I can tell you that the eyes are not that good at remembering a color that is not a sharp color change. My advice to students who did this was to start over. If this is a lab report you are doing, I suspect starting over is not an option; hence you want to estimate. And you may not have the option, even, of doing the estimation procedure I've outlined above. The simple answer to your question initially is "no."

To estimate how many mL you are away from the blue endpoint, you can use a technique called back titration. Here's how you can do it:

1. Obtain another sample of the zinc solution and add an excess amount of EDTA solution to it. Make sure the excess is greater than what is required to form the complex with zinc.

2. Next, add a few drops of calmagite indicator to the mixture. The calmagite color will be changed to violet just as it did in your original titration.

3. Now, slowly add a standard solution of zinc sulfate (of known concentration) to the mixture while stirring, until the violet color starts to turn blue. Note the volume of the standard zinc sulfate solution added.

4. From the recorded volume, you can estimate how many mL you were away from the blue endpoint in your original titration experiment.

For example, if you had to add an additional 2 mL of the standard zinc sulfate solution in the back titration to observe the blue endpoint, then you can estimate that you were 2 mL away from the blue endpoint in your original titration.

It's important to note that this estimation is not as accurate as performing the titration until the blue endpoint is observed directly. However, it can give you a rough idea of how close you were to the endpoint and can help improve your titration technique in the future.