CEE 680

18 December 1995


Closed book, three pages of notes allowed.

Answer all questions. Please state any additional assumptions you made, and show all work. If you don't have time to complete a section, please describe how you would solve the problem (without using MINEQL).

Use of Ethylenediaminetetraacetic Acid (EDTA) as a complexing agent.

1. (25%) In addition to being a hexadentate ligand, EDTA can also bind with up to 6 protons. Using the equilibrium constants from below, sketch a pC vs pH (or Log C vs pH) diagram for a concentration of 1.0 mM of total EDTA.

2. (20%) EDTA is often used in laboratory experiments and analytical methods for the purpose of binding transition metals that would otherwise interfere with some desired reaction. For this purpose, it is common to use high concentrations of EDTA. What would the pH and Y-4 be if you added 1.0 mM of Na4Y (sodium salt of EDTA) to a liter of pure water?

3. (25%) Determine the free copper and free manganese concentration after adding 0.001 mM CuCl2 and MnCl2 to the above EDTA solution.

4. (20%) EDTA can also be used to simulate the behavior or natural organic matter. In this case we would be looking at much lower concentrations. If you had a water that was well buffered at pH 12.0, and you added 0.01 mM Na4Y, and 0.01 mM total Mn(+II), what would the complete manganese speciation be? Assume that there are no hydroxide species formed.

5. (10%) Repeat question #4, but this time let's presume a more realistic pH of 7.0 instead of 12.0.

Important Equilibria


Log K

Y-4 + H+ = HY-3


Y-4 + 2H+ = H2Y-2


Y-4 + 3H+ = H3Y-


Y-4 + 4H+ = H4Y


Y-4 + 5H+ = H5Y+


Y-4 + 6H+ = H6Y+2


Y-4 + Cu+2 = CuY-2


Y-4 + Mn+2 = MnY-2