Relative Concentrations of Complex Species

This material addresses concentration calculations for substances with complex equilibria involving a common species.

For example, calculating distributions of all the ionized forms produced from poly-protic acids is a problem commonly found in chemistry and biochemistry. We show herein that this distribution is not impossible to calculate. With a little effort toward understanding the mathematics, the student may become fluent in equilibrium calculations.

The central idea is to formulate a set of equations that describe the fraction of a complex species in a particular form. The fractional amount of species in this particular form is called the a for that species. To see the form of the a, as well as how they are derived, for acid species click here. To see an alternate chemical form, e.g., a metal-ligand complex, click here.

Calculations of the fractional concentration is facilitated by using the formal concentration unit. One reason why formal concentration makes the relative concentrations of ionized species easier to specify is that formal concentration is the sum of molar concentrations of all forms of the species. This fact, makes the formal concentration very useful in mass balance expressions as well.

What now?

• Click here to see how we can use the principle of mass conservation to calculate the degree of ionization of a simple acid.
• or here to see how the alpha are formulated for complex acids
• How about their use in pH calculations?
• Click here to see how the alpha are used to determine pH-dependent solubilities of salts like carbonates
• Clicking here gets you to a page describing alpha formulation for metal ion-ligand solutions