Eriochrome Black T is one of those chemicals I mostly remember for its name. It’s the kind of CSI-ey name that just rolls off the tongue. It has the unusual property of “complexometric” indication.
It’s normally blue, but on binding to calcium or magnesium (the divalent cations largely responsible for water hardness), it turns red. I have fond memories of playing with this in my analytical chemistry lab in undergrad. The usual pat explanation is that it’s a water hardness indicator (see the movies on that page!). I’m sure it was, at one time, but I have a hard time imagining Joe EPA setting up his buret full of eriochrome black to measure every municipality’s water. Anyone know if it’s still used?
Have a good weekend.