Acids, Bases, and the pH Scale

Introduction

The environments within organisms and cells are composed of aqueous (water-based) solutions. The properties of acids and bases, and their influence on pH, are fundamental to understanding biological processes, as they affect molecular shapes and functions.

Preliminaries: Ionization of Water

Ionization of Water

• Ionization is the process by which water molecules dissociate into ions. In water, a small fraction of molecules split to form hydrogen ions (H+) and hydroxide ions (OH-).

• The reaction can be represented as:

• This process is reversible, and the concentrations of H+ and OH- in pure water are both M at equilibrium.

• Most water molecules remain un-ionized; only about 1 out of every 500,000,000 molecules is ionized at any given time.

Acids and Bases: Definitions and Properties

Acids

• Acids are substances that increase the concentration of H+ ions in water, either by donating H+ directly or by dissociating to release H+.

• Strong acids ionize completely or irreversibly in water, releasing all possible H+ ions. Example: Hydrochloric acid (HCl):

• Weak acids ionize incompletely or reversibly, so not all acid molecules donate H+ at once. Example: Acetic acid (CH3COOH):

Bases

• Bases are substances that reduce the concentration of H+ ions in water.

• Strong bases dissociate completely and irreversibly to release OH- ions, which combine with H+ to form water. Example: Sodium hydroxide (NaOH):

• Weak bases (e.g., ammonia, NH3) react reversibly with H+:

The pH Scale

Definition and Calculation

• The pH scale is a logarithmic scale that measures the concentration of H+ ions in a solution.

• Formula:

• Each unit change in pH represents a tenfold change in H+ concentration.

• Pure water has a pH of 7 ( M). Acidic solutions have pH < 7; basic solutions have pH > 7. The standard pH scale ranges from 0 to 14.

Logarithms and pH

• Logarithms are used to simplify the representation of very small H+ concentrations.

• For example, if M, then pH = 8.

• Each step on the pH scale represents a tenfold difference in H+ concentration.

Summary Table: Acids, Bases, and pH

Summary of Major Points

• Water molecules ionize and dissociate reversibly into H+ and OH- ions.

• Acids increase H+ concentration; strong acids ionize completely, weak acids only partially.

• Bases reduce H+ concentration; strong bases dissociate completely, weak bases react reversibly with H+.

• pH is the negative logarithm of H+ concentration; each pH unit represents a tenfold change in H+ concentration.

Examples and Applications

• Example 1: If M, then pH = 7 (neutral solution).

• Example 2: If M, then pH = 4 (acidic solution).

• Example 3: If pH = 9, then M (basic solution).

Practice Questions (Selected)

• If the H+ concentration of a solution is M, what is the pH? Answer: 8

• If the H+ concentration is 0.01 M, what is the pH? Answer: 2

• As H+ concentration increases, OH- concentration decreases, and vice versa.

• Each pH step represents a tenfold difference in H+ concentration.

Additional info: The notes also include practice problems and a summary of key points for exam preparation. Understanding the relationship between acids, bases, and pH is essential for interpreting many biological processes, such as enzyme activity and cellular homeostasis.