Na. SO4. Ca. Mg. Fe. Cl. HCO3. Reaction calculations. Saturation Indices. Solution Definition and Speciation Calculations. Speciation calculation. Seawater: units are ppm. IS.1. Questions. What is the approximate molality of Ca? What is the approximate alkalinity in meq/kgw?
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Default GFW is defined in 4th field of SOLUTION_MASTER_SPECIES in database file.
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1. Write the mass-balance equation for calcium in seawater.
2. Write the mass-action equation for the reaction CO2 + H2O = HCO3- + H+.
3. Write the mass-action equation for question 2 in log form.
4. Calculate the equilibrium constant by using the log activities from the speciation results.
5. Assuming activity of water = 1, at what pH will [CO2] = [HCO3-]? “” indicates activity.
6. What is the activity coefficient of HCO3- in seawater? CO3-2?
pH, Carbon, Alkalinity
1. How does the pH change when CO2 degasses during an alkalinity titration?
2. How does pH change when plankton respire CO2?
3. How does pH change when calcite dissolves?
pH = 6.3 + log((HCO3-)/(CO2))
1.Reset all to false
2. Set pH to true
Concentration in mmol/kgw
1. Make speciation calculations for these 9 solution compositions with SOLUTION _SPREAD.
2. Make a table of pH, (CO2), (HCO3-), (CO3-2) with SELECTED_OUTPUT. Plot pH vs. concentrations in Excel; it is easiest to open the selected-output file in Wordpad and cut and paste into Excel.
Concentration in meq/kgw
1. Make speciation calculations for these 6 solution compositions with SOLUTION _SPREAD.
2. Use SELECTED_OUTPUT to make a table of pH, (CO2), (HCO3-), (CO3-2), total C (use TOTALS tab). Plot pH vs. concentrations in Excel.
1. Write a definition of total carbon(4) (sometimes called total CO2 or TDIC) in terms of (CO2), (HCO3-), (CO3-2).
2. Write a definition of alkalinity in terms of (CO2), (HCO3-), (CO3-2).
3. Write a definition of alkalinity in terms of (CO2), (HCO3-), (CO3-2), (OH-).
Fe+2 = Fe+3 + e-
pe = log( [Fe+3]/[Fe+2] ) + 13
HS- + 4H2O = SO4-2 + 9H+ + 8e-
pe = log( [SO4-2]/[HS-] ) – 9/8pH + 4.21
N2 + 6H2O = 2:NO3- + 12H+ + 10e-
pe = 0.1log( [NO3-]2/[N2] ) –1.2pH + 20.7
pe = 16.9Eh, Eh platinum electrode measurement
1. Write an equation for pe from the equation for oxidation of NH4+ to NO3-, log K for reaction is –119.1.
Hint: Chemical reaction has NH4+ and H2O on the left-hand-side and NO3-, H+, and e- on the right-hand-side.
Double click to get list of redox couples
Must have analyses for chosen redox couple
Use SOLUTION to run these 6 solutions.
1. For each solution
2. What pe is calculated for solution 6?
3. In solution 6, given the following equation, why is the pe not 13?
pe = log( [Fe+3]/[Fe+2]) + 13
4. For pH > 5, it is a good assumption that the measured iron concentration is nearly all Fe(2) (ferrous). How can you ensure that the speciation calculation is consistent with this assumption?
Charge Balance and Adjustments to Phase Equilibrium
Select pH or major ion
Add saturation index for mineral, log partial pressure for gas
Define SI or log partial pressure
Don’t forget to set the units!
Concentration in mg/L
1. Calculate the carbon concentration that would be in equilibrium with the atmosphere (log P(CO2) = -3.5.
Concentration in mg/L
SI < 0, Mineral should dissolve
SI > 0, Mineral should precipitate
SI ~ 0, Mineral reacts fast enough to maintain equilibrium
SI(FeS) = log[Fe]+log[SO4-2]-8pH-8pe-log K(FeS)
SI(FeS) = log[Fe]+log[HS-]+pH-log K(FeS)
Apatite from Stumm and Morgan:
Ca5(PO4)3(OH) = 5Ca+2 + 3PO4-3 + OH-
Apatite from Wateq: log K = -55.4
Log Ks especially uncertain for aluminosilicates
Examine solution compositions in spreadsheet “speciation.xls”.
Calculate saturation indices.
What can you infer about the hydrologic setting, mineralogy, and possible reactions for these waters?