Polyprotic substances

Not all acid-base reactions occur in a 1 1 combining ratio (as hydrochloric acid and sodium hydroxide in the previous example). Acid-base reactions with other than 1 1 combining ratios occur between what are termed polyprotic substances. Polyprotic substances donate (as acids) or accept (as bases) more than one proton per formula unit. 

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. 

Refers to substances that crystallize in more than one crystalline arrangement. Polyprotic Acid... 

Polyprotic acids Substances containing more than one acidic proton. 

Many acids can donate more than one proton. Examples are H2CO3, H3PO4, and [Al(H20)6l. These acids are referred to as polyprotic acids. Similarly, bases that can accept more than one proton, for example, OH , C03, a.nd NH, are polyprotic bases. Many important substances, for example, proteins or poly aery lie acids, so-called polyelectrolytic acids or bases, contain a large number of acidic or basic groups. 

The treatment of diprotic and polyprotic acids is more involved than that of mono-protic acids because these substances may yield more than one hydrogen ion per molecule. These acids ionize in a stepwise manner that is, they lose one proton at a time. An ionization constant expression can be written for each ionization stage. Consequently, two or more equilibrium constant expressions must often be used to calculate the concentrations of species in the acid solution. For example, for carbonic acid, H2CO3, we write... 

When the neutralization steps of a polyprotic add or polybasic base are suf-fidently separated, the substance exhibits a titration curve with multiple equivalence points. Figure 17.13 T shows the two distinct equivalence points in the... 

We see from Reactions (5.15b) and (5.16) that H2CO3 (carbonic acid) contributes two protons to water. Substances that contribute more than one proton to water are called polyprotic acids. Other polyprotic acids are H2C2O4 (oxalic acid), H3PO4 (phosphoric acid), and H2SO3 (sulfurous acid). 

Also notice that some acids contain more than one hydrogen ion in their formula. A general term for this kind of acid is polyprotic. Specifically, a substance Hke sulfuric acid (H SO ) is said to be diprotic and a substance Hke phosphoric acid (HjPO ) is said to be triprotic. 

Acids and bases can be identified as monoprotic or polyprotic depending upon the numbers of dissociable protons the acid contains or the number of protons with which the base can combine. Irrespective of whether a substance is monoprotic or polyprotic, the strength of an acid or base refers to the efficiency with which the substance demonstrates the properties of an acid or a base. For an acid, this means the ease with which it donates protons for a base, this means the ease with which it accepts protons. These properties of a substance will be affected by its environment it is easier for an acid to donate protons in a proton-accepting medium. In biological systems, non-membranous environments are essentially aqueous. 

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