Simple Acid/Base Example for Version 3.01

A. Determine the complete species composition from the addition of 5x10^-3 moles of NaCN to 1 liter of water.

1. Go to the “Data/Run” menu and choose “components”. Select the Type-I components by highlighting the desired species and hitting “return”. 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. While still in the Type-I components mode, hit F2. This selects from the thermodynamic database the equilibrium constants you will need and presents them in a table entitled “Type-II Chemical Complexes”. Now hit F2 three times to bring the cursor to the total concentration line. 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 “Complexes” table and select “Fixed Solids” (i.e., Type-III species). Highlight the pH line and hit “F3” which is the “move” key. Choose to move pH to “Type-VI” species. 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” and select “Run”. Keep “m corrections” (ionic strength corrections) off, 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 select “run”.

5. After the run is complete (should only take a second or two), select “output manager” (found under “Data/Run”). Under “MINEQL object” select your output file name. Under “member” select “S1.H(+)”. Then under “row type” select “species” and under “display criteria” choose “run1”. This will display the concentrations of all species containing an exchangeable proton (i.e., H+). Copy down the relevant information. Ignore the line labeled “pH”. Repeat this process, but this time choose “S1.CN(-)” under the “member” 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⁺	3.52e^-11
OH^-	0.000286
HCN	0.000286
CN^-	0.00471