Chapter 4

### LEARNING OBJECTIVES

After completing this chapter, the reader should be able to:

• ► Define a strong electrolyte, a weak electrolyte, and a nonelectrolyte.

• ► Differentiate between a cation and an anion.

• ► Define an acid, a base, and the different types of salts.

• ► Recognize and give examples of pharmaceutically important acids, bases, and salts.

• ► Describe the ionization equilibria of strong and weak electrolytes.

• ► Calculate and interpret the ionization constants of weak acids and bases (Ka, Kb, pKa, and pKb).

• ► Calculate the pH and pOH and the concentrations of the components of aqueous solutions of weak electrolytes.

• ► Calculate the pH and pOH of mixtures of weak acids and bases, diprotic acids, and ampholyte solutions.

• ► Calculate the percent ionization of weak electrolytes as a function of pH.

• ► Describe the physiologic implications of percentage of ionization of weak electrolytes in the absorption, transport, and excretion of drugs (pH partition hypothesis).

• ► Define a buffer solution and list its components.

• ► Calculate the pH of a buffer solution.

• ► Define and calculate the buffer capacity of a buffer solution.

• ► Calculate the quantity of ingredients needed to prepare a buffer solution.

• ► Recognize the various types of buffer solutions and their methods of preparation.

• ► Define ionic strength and understand its effect on ionization.

### ELECTROLYTES VERSUS NONELECTROLYTES

An electrolyte is a substance (an acid, a base, or a salt) that in an aqueous solution ionizes to positive ions (cations) and negative ions (anions). A salt such as sodium chloride (NaCl) is an example of an electrolyte. In an aqueous solution, the ions formed are Na+ (a cation) and Cl (an anion). Therefore, for each molecule, two independent entities are formed in solution. In an aqueous solution, electrolytes exhibit the following important properties that differentiate them from nonelectrolytes:

• They exhibit anomalous colligative properties compared with nonelectrolytes (see Chapter 3).

• They can conduct an electrical current.

• They tend to show rapid chemical reactions compared with nonelectrolyte solutions.

Electrolytes can be classified into two groups—strong and weak—depending on their ability to ionize in an aqueous solution. Strong electrolytes are substances that are completely ionized in water. Hydrochloric acid, for example, is a strong electrolyte, and its degree of ionization is pH-independent. The weak electrolytes, in contrast, ionize only partly in water. Atropine, phenobarbital, and sulfadiazine, and most other drugs, are examples of weak electrolytes. Nonelectrolytes are substances that do not ionize in water at all and therefore do not conduct an electric current in solution. Examples of nonelectrolytes include sucrose, fructose, urea, and glycerol.

### IMPORTANCE OF IONIZATION IN PHARMACY

Many drugs and pharmaceutical ingredients are organic weak electrolytes. Their degree of ionization is an important physicochemical property and has extensive physiologic implications. Depending on the pH ...

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