Fractions and Distribution Diagrams

The ionization fraction can be determined from the mass balance and equilibrium equations. For example, consider HOCl, which has a pK of 7,5 at 25°C. Neglect ionic strength effects. 

Solving for [HOC1]/Ct,oci/ that is, the fraction of Cj.oci which is [HOCl], we obtain 

The subscript on a refers to the number of protons that the species has released. If ionic strength effects are important, iCa = [H ][OCl ]/[HOCl] (= KJiyn+yoc]-)) should be used in place oi in Eqs, 4-69 and 4-70. 

Using Eqs. 4-69 and 4-70, or alternatively the table of a versus pH in Appendix 1, we can determine ofo ciiid at various pH values and plot them in a distribution diagram as a versus pH (Fig. 4-9). Distribution diagrams for the multiprotic acids, H2S and H3PO4, are shown in Fig. 4-10 such curves can be obtained using either the table or the general equations for a in Appendix 1. 

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