| MadSci Network: Chemistry |
Dear Clare, I am sorry it has taken so long to answer your questions. As for how pyrocatechol violet (PV) indicator works you will have a good idea after reading "Metal Ion Indicators" at the following web page: http://www.chem.uky.edu/courses/che115/metalind.html.
The details there are for an EDTA titration of calcium ion using eriochrome black indicator. However, in your Cu ion titration, the PV indicator works in a very similar way. The PV has one color when bonded to the Cu ion. This occurs when you add the indicator. When enough EDTA has been added during titration to bind with all of the "unbound" Cu ion, the next small amount of EDTA displaces the PV from its copper complex. That is because the affinity of EDTA for Cu ion is stronger. The free, or uncomplexed, PV has a different color. (In your case the solution is green.)
Your second question is answered by the same web page. Read the "Auxiliary Complexing Agent" paragraph. Pyridine prevents the precipitation of copper hydroxide before the EDTA is added. It also acts as a buffer.
More information on complexometric titrations can be found at http://falcon.sbuniv.edu/~ggray.wh.bol/CHE3345/chp15.html.
Clare, if you would like to see the structural formula for pyrocatechol violet go to Sigma-Aldrige Catalog's search page. Type in pyrocatechol violet in box 3 at the left and hit go. Another page will appear with info on PV. There you can select an option to see the formula. A lot more info can be found at Sigma-Aldrige. You will need Adobe Acrobat Reader to see this page.
I hope these sources help with your chemical investigation.
Try the links in the MadSci Library for more information on Chemistry.