Acknowledgement: This page is a modified version of a page found at http://homepages.ius.edu/DSPURLOC/c121/week13.htm

Eriochrome Black-T Indicator Color Changes

Since the reactants and products in the titration between hard water and EDTA (equation 1) are all colorless, an indicator is needed to allow us to visually see the end point (or equivalence point) of the titration, i.e. the point at which moles of Ca²⁺ = moles of EDTA. Eriochrome Black-T is the indicator we will use.

Equation 1.         Ca⁺²_(aq)     +   EDTA^-2_(aq)       à    CaEDTA_(aq)  

How Eriochrome Black-T Works

When Eriochrome Black-T complexes with Mg²⁺ ions, it produces a PINK-RED solution. The indicator is BLUE when the it is not complexed with Mg²⁺ (equation 2) and the solution is basic.

As EDTA is added to hard water during the titration it reacts with the divalent ions (e.g. Ca²⁺ and Mg²⁺) present in the hard water. Once all the divalent ions in solution have reacted with EDTA, the EDTA reacts with the Mg²⁺ ions that are complexed with the Eriochrome Black-T indicator, thus causing the solution to turn BLUE.

                Equation 2.        In-Mg_(aq)              à          In^2-_(aq)  +  Mg²⁺_(aq)   

                                            (Pink)                                     (Blue Endpoint)

The three pictures show the color changes as the end point of the titration is reached. There is only a 1 drop difference of 0.010 M EDTA between the first and second pictures and between the second and third pictures. Two or three seconds were allowed for colors in the second and third pictures to develop after adding the additional drop. In each case the solution was thoroughly mixed. This color change from wine red to violet to blue is due to the color changes of the indicator Eriochrome Black-T.

Sample Problem

Suppose you are using EDTA with a molarity of .0080 for the titration. You titrate 50.00 mL of a water sample using 10.68 mL of EDTA. Calculate the concentration of Ca²⁺ ion in the water sample in moles per liter and in parts per million, ppm CaCO₃?

Answers: 0.0017 M Ca²⁺  or  170 ppm CaCO₃

Useful information:

Ca⁺²_(aq)     +   EDTA^-2_(aq)       à    CaEDTA_(aq)  

ppm = mass fraction x 10^{6         or}