A Cations Ability to React with Water

Polyatomic cations containing one or more protons can be considered the conjugate acids of weak bases. The NH4 ion, for example, is the conjugate acid of the weak base NH3. Thus, NH4 is a weak acid and will donate a proton to water, producing hydro-nium ions and thereby lowering the pH  

Many metal ions react with water to decrease the pH of an aqueous solution. This effect is most pronounced for small, highly charged cations like Fe and as illustrated by the K values for metal cations in TABLE 16.6. A comparison of Fe and Fe values in the table illustrates how acidity increases as ionic charge increases. 

Notice that values for the 3-1- ions in Table 16.6 are comparable to the values for familiar weak acids, such as acetic acid (fQ, = 1.8 X 10 ). The ions of alkali and alkaline earth metals, being relatively large and not highly charged, do not react with water and therefore do not affect pH. Note that these are the same cations found in the strong bases (Section 16.5). The different tendencies of four cations to lower the pH of a solution are illustrated in T FIGURE 16.15. 

I Interaction between and oxygen of bound H2O molecule weakens O—H bonds 

Why do we need to use two different acid-base indicators in this figure  

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