Molecule of the Day

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.


  1. #1 Adam
    August 25, 2006

    Eriochrome Black T? Isn’t that a hip-hop group out of East LA?


    Thank you; I’ll be here all week. Tip your server! Try the Quorn!

  2. #2 jes
    August 26, 2006

    The method is still used, at least in my state.

  3. #3 Snarfevs
    August 26, 2006

    I have a sneaking suspicion that it may do funky things to DNA…

  4. #4 Fadima
    May 10, 2007

    Do u have the equation of complexation of EBT with Mg??

    thank u

  5. #5 Tophe
    September 12, 2007

    Eriochrome Black T is the indicator, not the titrant. Typically you would titrate the solution with a chelator such as EDTA. Once all the Mg and Ca has been complexed by the EDTA, the EBT will turn blue again.
    (As an aside, EBT is also known as Solochrome Black.)

  6. #6 ningegowda
    February 7, 2009

    why it is black in color

  7. #7 Anonymous
    December 7, 2010

    whta is “t” in EBT???

  8. #8 iqbal safi
    August 14, 2011

    Why it is called Eriochrome Black, as it is blue in colour? and what T for is?

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