Chem.Eq.Diagr  Chemical Equilibrium Diagrams / Tutorials

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:
  
d [H+]bound

d (log aH+)
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:

  
d [H+]bound

d (log aH+)
 
 = 
 
Δ [H+]bound

Δ (−pH)
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:

Titration_citr = 0
Titration_citr

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:

Titration_citr_H-affinity


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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:

Proton_affinity_diagram_Mixture
Note that the peaks are located at the pH corresponding to the acid constant.


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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:

Proton_affinity_diagram_Al
However, the corresponding logarithmic diagram:

Log_diagram_Al
shows that several protonation reactions are included in each peak.

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