SPANA
	SED & PREDOM

Concentration units in diagrams

The equilibrium constants in the DATABASE program are for 25°C and 1 bar pressure. Under such conditions the density of water is close to 1 g/cm³ (see Table), and for diluted aqueous solutions molar and molal concentration units are practically the same.

In other conditions, that is, at higher temperature (or pressure), or at higher ionic strength the molal concentrations may differ quite a lot from Molar units.

The programs SED & PREDOM make calculations based on the equilibrium constants, which are in molal scale, and using the activity coefficients, also for molal concentrations. Therefore, the results displayed in the diagrams are always in molal units. But the texts in the axes and header describring the concentration units vary as follows: Because at 25°C the molar and molal concentration units are practically the same, the diagrams report “molar” units if the temperature is 25°C. See this example. But at temperatures above 45°C the texts in the diagrams report molal concentration units. See this example.

If the user wishes to change how the diagram texts report concentrations, it is possible to run the programs SED & PREDOM using special command-line arguments. When using SPANA two keys in the “ini”-file control how the concentrations are displayed:

• Calc_concentration_notation
• Calc_concentration_units

See also:

• temperature effects on equilibrium constants
• activity, molality
• activity coefficients
• activity coefficient models.
• ionic strength
• enter a value for the ionic strength when making a diagram