Proton affinity diagrams
This diagram type can only be selected in SPANA with the
advanced program level (in the menu Preferences / General options).
A proton affinity curve made with Spana will show the derivative of bound
H+ ions with respect to the logarithm of the
H+ activity:
this can be plotted against pH, for example. Remember that
log aH+ = −pH.
The amount of bound H+ is calculated as:
|
[H+]bound |
= |
[H+]TOT
− [H+] − [OH−] |
This is the amount of H+ bound in different species,
for example in HCO3− or in
Al(OH)4−.
The derivative is approximated numerically:
If a value for the ionic strenght
is not given in Spana when
making the diagram, then pH = −log [H+].
The resulting diagram shows a peak for each protonation reaction,
and it can be more useful than the corresponding titration curve. Unfortunately
peaks may overlap, and a diagram’s interpretation is never unique.
Examples:
Citric acid
Select citrate (cit3−) and H+
as components in DATABASE. In SPANA make sure you have selected the
advanced program level (in the menu Preferences / General options).
Make and compare the titration curves without and with citrate:
It is difficult to tell what kind of ligand is present only by looking at these
curves. By taking the derivative, making a proton affinity diagram,
it becomes clear that there are three protonation sites:
Mixture
Select oxalate (ox2−), carbonate
(CO32−) and H+
as components in DATABASE. In SPANA make sure you have selected the
advanced program level (in the menu Preferences / General options).
When a mixture of ligands is presents (in this case ox2− and
CO32−) in the proton affinity diagram
the height of the peaks is proportional to the concentration of the corresponding
ligand. For example:
Note that the peaks are located at the pH corresponding to the acid constant.
Al(III) hydrolysis
An example where several peaks are overlapping in a proton affinity diagram.
Select Al3+ and H+
as components in DATABASE. In SPANA make sure you have selected the
advanced program level (in the menu Preferences / General options).
Select the menu Run /
Modify chemical system.
Double click on each of the crystalline phases (both Al(OH)3(cr) and
AlOOH(cr)) and deactive them.
Save the file. With this change only the more soluble amorphous hydroxide can
precipitate.
Make a proton affinity diagram for [Al(III)]TOT =
10−6M. With this low concentration the Al-hydroxide
does not precipitate. The diagram shows two peaks:
However, the corresponding logarithmic diagram:
shows that several protonation reactions are included in each peak.