pH in the X-axis

The chemical system H⁺, CO₃²⁻, Cl⁻ is used to create diagrams using the simplified HKF model to illustrate the effect of different options for the ionic strength when making a diagram.

Ionic strength fixed = 0

Ionic strength fixed = 1.5

Ionic strength calculated

For this diagram, where the ionic strength is calculated, electroneutrality is assumed, and the large concentration of chloride is compensated by a fictive equivalent amount of positive ions (for example sodium), so that the calculated ionic strength is in this case ≈ 3 all over.

Ionic strength calculated, pH = 1 to 13

In this diagram [Cl⁻] = 0.

• The calculated ionic strength at the left of the diagrapm (pH = 1) is ≈ 0.13 (because at pH = 1, [H⁺] ≈ 0.1, electroneutrality is assumed).
• At pH = 7 the calculated ionic strength is 0.008 (at pH = 7, [H⁺] ≈ 10⁻⁷).
• At pH = 13, I ≈ 0.16 ([OH⁻] = 0.13, electroneutrality is assumed).

Purbaix diagrams

A Pourbaix diagram is a Eh-pH predominance area diagram, see Example 10. If the ionic strength is calculated, large variations occur, as in the diagram between pH 1 and 13 shown above for CO₃²⁻.

Furthermore, large concentrations of O₂(aq) and of H₂(aq) are calculated:

	2 H2O    + O2(aq) + 4 e− + 4 H+		log K° ≈ −86 ≈ log[O2(aq)] − 4 pH − 4 pe
	2 H+ + 2 e−    H2(aq)		log K° ≈ −3.1 = log[H2(aq)] + 2 pH + 2 pe

At Eh = +1 V, pe = +16.9, and [O₂(aq)] becomes ≈ 10^+33.6 at pH = 13.

Similarly, at Eh = −1 V, pe = −16.9, and [H₂(aq)] becomes ≈ 10^+28.7 at pH = 1.

Such large concentrations of O₂(aq) and of H₂(aq) are physically impossible.

Ionic strength calculated

In a Pourbaix diagram the dashed lines indicate the equilibrium at 1 bar pressure between H₂O and either O₂(g) (upper dashed line) or H₂(g) (lower dashed line). Above the upper dashed line impossibly large concentrations of O₂(aq) are obtained, and likewise, below the lower dashed line impossible concentrations of H₂(aq) are calculated.

Predominance area diagrams

The chemical system H⁺, Na⁺, Cl⁻, Fe³⁺ was modified in SPANA to exchange components for reactions, in this case Na⁺ was exchanged for NaCl and Fe³⁺ for FeCl₃. In this way electrically neutral components may be used in the axes of the diagram. The crystalline oxides of Fe(III) are removed to simulate a system where aqueous solutions are mixed in a laboratory experiment. The concentration of H⁺ is large enough to prevent the precipitation of amourphous Fe(III) hydroxides (remember that amorphous solids are more soluble than crystalline.) The following diagrams use the simplified HKF model, and they illustrate the effect of different options for the ionic strength when making a diagram:

Ionic strength fixed = 0

Ionic strength fixed = 0.5

Ionic strength calculated

Because the components in the axis are electrically neutral, and a background concentration of HCl = 0.1 is given, electroneutrality prevails all over the diagram.

The calculated ionic strength at the bottom left of the diagram is ≈ [HCl], that is, ≈ 0.1, while at the top-right corner the ionic strength is controlled by the high concentration of NaCl and FeCl₃, and by the complex formation, I ≈ 3.5.