Complexometric Titration of Zn with EDTA

In this lab, you will determine the Zn concentration of a Zn standard and an unknown ZnCl2 solution.

Reagents Needed:

pH 10 buffer solution
Eriochrome Black T indicator solution
0.01 M EDTA Solution

Preparation of the 0.01 M EDTA Solution

Dry Na2EDTA•2H2O at 80 &Mac251;C to remove excess water. After cooling in a dessicator, weigh close to 0.01 moles into a 1 L volumetric flask. Fill the flask about half full and swirl to dissolve the EDTA salt (this will take some time). Fill to the mark after dissolution.

Preparation of the Eriochrome Black T Indicator

Titration Procedure

To 50 mL of the known Zn solution and unknowns, add 5-10 mL of pH 10 buffer and 2 drops of the Eriochrome Black T indicator. Titrate with EDTA until the indicator changes from red to blue. You may overrun the first titration and need to discard it (color transisiton is not as sharp as in pH titrations). Three good runs should be sufficient to standardize the EDTA solution.

Repeat the above procedure with the unknown Zn solution and the river water.

Questions: What causes the indicator to change color? What can you say about the river water compared to the unknown based on this titration? Can you determine the amount of Zn in the river water from this titration?

This experiment was developed by Michael R. Jordan at Oklahoma Baptist University. Reproduction by printing and photocopying for instructional use by educational institutions is permitted.