PRINCIPLES OF WATER CHEMISTRY

METALS IN AQUEOUS SYSTEMS

HOMEWORK-08

PLEASE NOTE THE UNUSUAL DUE DATE FOR THIS HOMEWORK

DUE ON MONDAY 11/30/2020, at 5:00 PM

• Should be typed since using MINTEQ
• If typing, using “Word”, select “INSERT” and “ EQUATION” and then “ INSERT NEW EQUATION” to type in chemical and mathematical formulas.
• SUBMISSION should be THROUGH CANVAS ONLY. Email submissions are not be accepted

Question (100 points) – Using MINTEQ and following the solution strategy given in Lecture-35, solve the following problem by describing in details the different steps used to address the 4 points below.

A water with a pH of 8.1 contains 2.0 meq/L of alkalinity, 0.2 mmol/L of TOTPO₄, and a negligible concentration of Ca²⁺. In this water, the alkalinity (ALK) is all from PO₄ and CO₃ species. If lime (Ca(OH)₂) is added to this aqueous solution with the goal of removing 95% of TOTPO₄ by precipitation of Ca₅OH(PO₄)_3(s):

1. Draw a log C – pH diagram showing Ca²⁺ activity in equilibrium with each of two solids: CaCO₃(s) and Ca₅OH(PO₄)₃(s), for the initial TOTCO₃ and TOTPO₄. Which solid will precipitate first?
2. Plot pH vs. lime dose, if no solid precipitates. Consider additions of 0 to 5×10^-4M lime. Add a curve to the plot from part (a) showing log(Ca²⁺) vs. pH during the titration.
3. Repeat part (b), allowing for precipitation of solids. How much lime must be added, and what will the pH be, when the TOTPO₄ target is reached?
4. If more lime is added, can the second solid precipitate? If so, under what conditions? If no, why not?