Amphiprotic substances

Complex systems include solutions containing two acids or bases, which contain or consume two or more protons, and amphiprotic substances that act as both acids and bases. A characteristic of all such systems is that two or more equilibria must be considered in describing their behavior. As a consequence, the techniques for pH data derivation are often more complex than for simple systems. 

Amphiprotic substance, when dissolved in a suitable solvent, behaves both as a weak acid and as a weak base. If either its acidic or its basic character predominates sufficiently, titration of the species with a strong base or a strong acid may be feasible. 

Amphiprotic substances Species that can either donate protons or accept protons, depending on the environment. 

Some substances can act as an acid in one reaction and as a base in another. For example, H2O is a Bronsted-Lowry base in Equation 16.3 and a Bronsted-Lowry acid in Equation 16.5. A substance capable of acting as either an acid or a base is called amphiprotic. An amphiprotic substance acts as a base when combined with something more strongly acidic than itself and as an acid when combined with something more strongly basic than itself. 

A Bronsted-Lowry acid is a substance that donates a proton to another substance a Bronsted-Lowry base is a substance that accepts a proton from another substance. Water is an amphiprotic substance, one that can function as either a Bronsted-Lowry acid or base, depending on the substance with which it reacts. 

What is the difference between an amphoteric substance and an amphiprotic substance ... 

Similarly, cations (H2NCH2CH2NH3 ), anions (HC204"), and neutral molecules can all act as bases. Certain substances such as H2O and SH behave as acids as well as bases, and are called ampholytes or amphiprotic substances. 

The pairs of compounds NH4" - NH3 and H30" - H2O are called conjugate acid-base pairs. Water can lose a proton as well as gain one and is therefore called an amphiprotic substance. 

In Section 12-1 the autoprotolysis of the amphiprotic substance water was discussed. Every solvent whose molecules contain one or more protons and one or more unshared electron pairs in outer shells can act as an amphiprotic solvent. One of its molecules can donate a proton to a sufficiently strong base or accept a proton from a sufficiently strong acid. For example, perchloric acid dissolved in pure sulfuric acid undergoes the following reaction, in which H2SO4 acts as a base ... 

In these equations we see that water can gain a proton while acting as a base, and lose a proton while acting as an acid. Therefore all amphiprotic substances are amphoteric, because when they donate a proton they are acting as an acid, and when they accept a proton they are... 

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