GES 166/266
Soil Chemistry
Winter 2004
NAME: _____________________
LECTURE HOMEWORK 2
Visual MINTEQ -- Carbonate System
DUE FRIDAY FEBRUARY 27th
The carbonate system is extremely important, being central to respiration, photosynthesis,
pH buffering, carbon budgets, and carbonate rock formation. CO2 is the primary cause of
global warning, and carbonate rocks and dissolved species are the most important buffers
of pH shifts in the natural environment. The carbonate system, however, can be
challenging to predict without a full consideration of all species and the specific deviation
from ideal conditions. Fortunately, with current computational power (and ease) this is a
manageable problem. To make things even easier for us, there are a number of chemical
speciation programs available. Visual MINTEQ is one such program and offers an easy
means to fully speciate the carbonate or other systems. Our first assignment using
MINTEQ will therefore be to further explore the carbonate system.
Open Visual MINTEQ and do the following exercises.
1.
First the closed system without calcite present (the total carbonate is held
constant).
Specify a total CO32- and Ca2+ concentration of 200 mg/L
(note: you have multiple options to choose from in the input file. When using mg/L, MINTEQ will
convert your units to molarity and report this in the output) .
Run MINTEQ
a) What is the resulting pH?
b) Are there species in the output that we had not considered in the carbonate
equilibria earlier (e.g. ion pairs)?
c) Is the system oversaturated with respect to calcite?
2. Now let’s look at a closed system with calcite present.
Specify CO32- = 1 mg/L and Ca2+ = 1 mg/L (you need to have them defined)
Now go to the menu bar (at the top) and select “Solid phases and excluded species”
From the drop-down menu choose “Specify infinite solid phases”
Then scroll down until you find “Calcite” and select it.
Run MINTEQ
a) What is the pH this time?
b) What is the saturation index for calcite?
c) What are the concentrations of Ca2+, H2CO3, HCO3-, and CO32-?
3. Consider a situation where the pH is buffered my constituents other than just
calcite. Use the exact input as in #2, but this time fix the pH at 7.0.
Run MINTEQ
a) What is the saturation index for calcite?
b) What are the concentrations of Ca2+, H2CO3, HCO3-, and CO32-?
4. All right, predictably the next situation is to move into an open system.
Specify CO32- = 1 mg/L
Now go to the menu bar (at the top) and select “Gases”
From the drop-down menu choose “Specify F1”
The “Specify fixed CO2 …” will already be chosen (default), so just click the “add”
button.
Run MINTEQ
a) What is the resulting pH?
b) What are the concentrations of H2CO3, HCO3-, and CO32-?
c) Try it again with PCO2 = 3.5 x 10-3 atm.
5. Now we need to move into a more advanced (but still easy) stage of MINTEQ and
speciate the total carbonate across a pH range. To do this we will use a nifty feature in
MINTEQ called the ‘sweep’ function.
First, set the input for an open system at normal atmospheric levels of carbon dioxide (as
in #4)
Now go up and click the ‘Multi-problem/Sweep’ button on the top menu bar.
Click the ‘sweep (only one component is varied)’ button,
Specify ‘20’ in the ‘number of problems’ box,
Specify a start value of ‘3’ and an ‘increment’ of 0.5,
Select the 3 carbonate species in the ‘add component’ section, being sure to specify
“concentration”. Just for kick, also select the CaHCO3+ ion pair (and any other species
you wish). This will output this selected data for each sweep to a special file for ease of
plotting.
Click ‘Save and Back’
RUN MINTEQ
Go to output,
Click ‘Selected sweep results’ button, then save the file to excel,
Go to excel and plot your data as an X-Y scatter plot with pH on the X axis and the
carbonate species on the Y axis.
Change the Y axis scale to ‘logarithmic’ scale.
Pretty cool, huh?
6. OK, one last scenario. Let’s go with a sweep of an open system with calcite
present.
Set atmospheric levels of CO2
Allow calcite to be an infinite solid
Now sweep across a pH range from 3 to 12. For the sweep output, select the 3 carbonate
species in the ‘add component’ section, being sure to specify “concentration”. Also,
again select the CaHCO3+ ion pair (and any other species you wish). This will output this
selected data for each sweep to a special file for ease of plotting.
Click ‘Save and Back’
RUN MINTEQ
Oops, you probably just got an error message. MINTEQ doesn’t like to have the number
of fixed species exceeding the number of unknown components. So, set Cl- at a level of
0.1 mM and repeat the try.
Go to output,
Click ‘Selected sweep results’ button, then save the file to excel,
Go to excel and plot your data as an X-Y scatter plot with pH on the X axis and the
carbonate species on the Y axis.
Change the Y axis scale to ‘logarithmic’ scale.
Note the level of total carbonate in the system as pH increases!