1.4 Homework Assignment

Question 1

Metal complexation in seawater. The seawater composition is as follows:

1. Please set up CrunchFlow to do the aqueous complexation calculation for the seawater system and identify the dominant species (the top 2 species with the highest concentrations) for the major metals (Na, Ca, Mg, Sr). Is charge balanced? If not, what species should you put to balance charge?
2. Try the keyword “database_sweep” (look it up in the manual). What are the dominant secondary species formed in seawater? Pick the dominant secondary species for each cation. What is the difference by including and not including the dominant secondary species?
3. A Pb-containing solution is accidentally added to a tank of seawater, resulting in a total concentration of all Pb-containing species being 10^-2 mol/kg. All other concentrations remain roughly the same. In calculation, please make sure charge is balanced.
   • Redo the calculation and identify the dominant species for all major metals, including Pb.
   • Note that the total concentrations of Ca and Pb are similar. Do they differ in their dominant species? If there are differences, which parameter leads to such differences? 

Question 2 open carbonate system

Here we assume we have an open carbonate system instead of a closed system. This means H₂CO₃, or CO₂(aq), is in equilibrium with atmospheric CO₂ concentration at P_CO2 of 10^-3.5 atm. Henry’s law constant for CO₂ dissolution is K_H = 10^-1.47 mol/L/atm. Suppose we have a solution with a given pH and H₂CO₃ is solely from gas dissolution, please answer the following questions: 

(Hint: you will need to look into the CrunchFlow manual to know how to set up a solution in equilibrium with a gas phase at given pressure. Look up the table for "Types of Constraints: Aqueous Species" on page 65-66).

1. At pH=7, what are the concentrations of individual species (H₂CO₃, HCO₃^-, CO₃^2-)? 
2. Calculate the concentrations of TIC, H₂CO₃, HCO₃^-, CO₃^2-, H⁺, and OH^-, under pH of 2, 4, 6, 8, and 10.
3. Plot the concentrations of all individual species as a function of pH.
4. Observing from the plot, what are the dominant species under different pH conditions.
5. How does this open system behave differently from the closed system in example 1.1?

Question 3 buffering effect of carbonate system

Imagine you have two closed bottles. One bottle has pure water at a pH of 7.0 (with only H+, OH-, and water). In the other bottle, you have carbonate water as those in example 1.1 at a pH of 7.0 and a total inorganic carbon concentration of 0.001 mol/L. Both systems are charge balanced. If I add 0.001 mol/L of Ca(OH)2, what is the new pH of the two systems when each system reaches their new equilibrium? How does the presence of carbonate species influence pH changes? Think ahead how the two systems might be different before you do the calculation, and check if your calculation confirms your hypothesis. 

HW1 files and solution package [13]

Reference:

Haynes, W.M. (2012) CRC handbook of chemistry and physics. CRC press.
Langmuir, D., Hall, P. and Drever, J. (1997) Environmental Geochemistry. Prentice Hall, New Jersey.
Wolery, T.J., Jackson, K.J., Bourcier, W.L., Bruton, C.J., Viani, B.E., Knauss, K.G. and Delany, J.M. (1990) CURRENT STATUS OF THE EQ3/6 SOFTWARE PACKAGE FOR GEOCHEMICAL MODELING. Acs Symposium Series 416, 104-116.