Acid-base chemistry polyprotic acids

In addition, the equilibria on the left hand side of eqn (8.20) can sometimes be defined by the dissociation constants Kdissj (j = 1, 2,. .., m), that are very familiar from acid-base reactions (the acidity constants Kgj for polyprotic acids are in fact dissociation equilibrium constants). The relationships between all these constants are given by the sets of eqn (8.21). In certain cases, it is more convenient to use one type of equilibrium constants or another, and the choice is typically made based on the simplicity of the mathematical equations describing a system of interest. Definitions and uses of all equilibrium constants in determination of concentrations of all species present in a complex system are given in many monographs on coordination chemistry. ... 

From an acid-base titration curve, we can deduce the quantities and pK.d values of acidic and basic substances in a mixture. In medicinal chemistry, the pATa and lipophilicity of a candidate drug predict how easily it will cross cell membranes. We saw in Chapter 10 that from pKa and pH, we can compute the charge of a polyprotic acid. Usually, the more highly charged a drug, the harder it is to cross a cell membrane. In this chapter, we learn how to predict the shapes of titration curves and how to find end points with electrodes or indicators. 

Use your Weh browser to connect to http //chemistry.brookscole.com/skoogfac/. From the Chapter Resources menu, choose Web Works. Locate the Chapter 13 section, and click on the link to the Virtual Titrator. Click on the indicated frame to invoke the Virtual Titrator Java applet and display two windows the Menu Panel and the Virtual Titrator main window. To begin, click on Acids on the main window menu bar, and select the diprotic acid o-phthalic acid. Examine the titration curve that results. Then click on Graphs/Alpha Plot vs. pH and observe the result. Click on Graphs/Alpha Plot vs. mL base. Repeat the process for several monoprotic and polyprotic acids, and note the results. 

Borkovec, M., and G. J. M. Koper. 1994b. Ising models of polyprotic acids and bases. The Journal of Physical Chemistry 98, no. 23 6038-6045. doi 10.1021/jl00074a034. 

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