Polyprotic acids species distribution

In this chapter you ll learn how to calculate species distributions of polyprotic weak acid species, how to apply Gran s method for the estimation of end-points in titrations and a general method for the calculation of titration curves. 

To illustrate the variation in composition of an aqueous solution of a polyprotic weak acid species, it is useful to plot a species distribution curve such as the one for citric acid shown in Figure 20-1. The parameter plotted versus pH for each species is a, the fraction of the total citric acid concentration in the form of a particular species. 

For the single, monoprotic acids and bases in the above example, the distribution and logarithmic concentration diagrams are rather simple, yet they clearly show the relative and absolute concentrations respectively of the various species present. Such diagrams become all the more useful when we consider more complicated systems, such as polyprotic acids and bases, where it otherwise becomes increasingly difficult to envision what happens as a function of pH. We will do so in exercises 4.5 and 4.6. 

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