HOMEWORK #7

 

  1. This question will require you to go to the Chemistry library to find some solution stability constants. There are several good resources in the references section there, including Critical Solubility Constants (Smith and Martell). Be sure to reference the one you use.

 

Given the attached geothermal water, each person is to calculate the concentration of their assigned metal complex based on the K or β value they obtain from the reference material. Be sure to state the ionic strength and temperature you that you are assuming.  b) Compare your value to that determined using PhreeqC using the Minteq database. c)  A more typical chloride geothermal water has a much lower pH.  How does the concentration of your complex change if you decrease the pH to 4.5?

 

 

Geothermal water and metal complexes excel file: HW7 water.xls

 

  1. Plot the changes in log activity of Al:OH complexes and AlTotal for a water in equilibrium with gibbsite as a function of pH.  How do changes in Al:OH complex concentrations affect the total solubility of gibbsite?  Use the diagram in chapter 8 of your textbook (A&P) as a guide.

 

  1. Based on your graphs from #2, would you expect total aluminum concentrations to be very high in most natural waters?  If the water pH was lowered dramatically, as can happen due to acid mine drainage, what would happen to the total aluminum concentration (assuming gibbsite is present)?  If this water was then mixed with less acidic water, what would happen?

 

 

  1. EDTA or ethylenediaminetetraacetic acid is a hexadentate ligand molecule used for complexing metal ions.  It is a polyprotic acid containing four carboxylic acid groups (acidic hydrogens are red) and two amine groups with lone pair electrons (green dots). EDTA is able to form ligands with dissolved metals on a 1:1 molar ratio. The classic structural formula is given below.  

 

(from http://www.chm.bris.ac.uk/motm/edta/edtah.htm)

 

Listen to the EDTA song (http://www.geocities.com/le_chatelier_uk/edta.html) and read the lyrics. What kind of bonds do hard and soft ligands like to form?

What are some examples of each? What does the author mean by “it’s a positive delta S”?