|
CEE 680 |
UMass Amherst |
Water
Chemistry
Notes on the Use of the PC
version of
MINEQL+
A. Determine the complete
species composition from the addition of 5x10-3 moles of NaCN to 1
liter of water.
1. When you launch the program,
it goes directly to the “Select Components for Calculation” screen. If it doesn’t, click on “select components”
from the “Model” drop-down menu. From
here you select the Type-I components by checking the boxes next to desired
species. The components you should
select are: H2O (almost always a
selection), H(+), CN(-) and Na(+). Note
that OH(-) is not a selection, because it can be obtained by taking an H(+)
away from H2O so that it is not a independent substance. The same is true for HCN (it can be obtained
by combining CN(-) and H(+).)
2. Click the “Scan THERMO”
button at the top. This selects from the
thermodynamic database the equilibrium constants you will need and presents
them in a table entitled “Type-II Aqueous Species”. If this table does not automatically appear,
find it by going to menu bar and selecting: Model>Tableau>Aqueous
Complexes. Now move the cursor to the
total concentration line at the bottom.
Keep H2O and H(+) at zero, but type in 5E-3 for the concentration of
CN(-) and the same for Na(+).
3. Escape from the “Aqueous
Species” table by hitting the “Close” button.
Select “Fixed Entities” (i.e., Type-III species) directly from the
tableau switch screen and hit “yes”. If
the tableau switch screen doesn’t automatically appear, you’ll need to go to
the menu bar and select: Model>Tableau>Fixed Entities. You will now see the “Type III Fixed
Entities” screen. Highlight the pH line[1]
and click on the “move” key. Choose to
move pH to “species not considered (Type-VI)”[2],
and click on “Move”. This is necessary,
because we don’t want to force the pH to stay at any particular level. In fact, one of the purposes of this problem
is to determine what the final pH will be.
4. Escape from “Fixed Solids” by
hitting the “close” button. Now select
“No” in the Tableau switch screen if it pops up. The “run time manager” screen may then appear. If it doesn’t, you’ll need to select it by
going to the menu bar adn selecting: Model>Calculate. On the “run time manager” screen, you should type
in an “output data name” and select “Run”.
Keep “m corrections” (ionic
strength corrections) off for now, and keep the temperature at 25°C.
Shift over to the “file output” line and select a name for the file
where the results of your run will be stored.
Then click on “run”.
5. After the run is complete
(should only take a second or two), the “output manager” screen may
automatically appear. If it doesn’t,
retrieve it by going to: Model>Output Manager. Select your output file name in the drop-down
box, if it’s not already there. Under
“output types” select “component groups”.
Under “data object” select H(+) and click on “view”. A table of species containing an exchangeable
proton (i.e., H+) will appear. Copy down
the relevant information. Ignore the
line labeled “pH”. Repeat this process,
but this time choose “CN(-)” under the “data object” category. This time you will see data on all species
that contain the cyanide group (i.e., CN).
You
should get the following concentrations:
|
Species |
Concentration |
|
H+ |
3.63e-11 |
|
OH- |
0.000278 |
|
HCN |
0.000278 |
|
CN- |
0.00472 |