[luce]

im doing coursework on water quality and am stuck! basically i steam distill a water sample from various rivers, and collect the product in boric acid. i then titrate the distillate with sulphuric acid and the methyl red-bromocresol green mixed indicator. just wondering if you know why the methyl red- bromocresol green indicator changes colour when titrated with diltute sulphuric acid. also im investigating the hardness of water i.e. magnesium and calcium conc. in water- any ideas on why the EDTA and erichrome black T indicator causes the colour change in the titration? i understand that EDTA is a ligand and that the endpoint occurs when there is no free ions...or something like that- but still don't understand the actual colour change i.e. is it something to do with d shells splitting?? your help would be grately appreciated!
thank you
lucy

[Mitch]

The color comes from the indicator, and is due to its extended conjugated pi-system.