Acids, Bases, and pH

Introduction

Acids and bases are fundamental chemical concepts with significant roles in biological systems. The pH scale is used to measure the acidity or basicity of a solution, which is crucial for maintaining proper cellular and physiological functions.

Examples of Acids, Bases, and Neutral Compounds

Acids

• Definition: Substances that increase the concentration of hydrogen ions (H+) when dissolved in water.

• Examples:

  • Hydrochloric acid (HCl) – found in stomach acid

  • Sulfuric acid (H2SO4) – used in batteries

  • Acetic acid (CH3COOH) – found in vinegar

  • Citric acid – found in citrus fruits

• Properties: Sour taste, can corrode metals, turn blue litmus paper red.

Bases

• Definition: Substances that decrease the concentration of hydrogen ions (H+) or increase hydroxide ions (OH-) in solution.

• Examples:

  • Sodium hydroxide (NaOH) – used in cleaning products

  • Potassium hydroxide (KOH) – used in soap making

  • Ammonia (NH3) – found in household cleaners

• Properties: Bitter taste, slippery feel, turn red litmus paper blue.

Neutral Compounds

• Definition: Substances with a pH of 7, neither acidic nor basic.

• Example: Pure water (H2O)

pH and the pH Scale

Definition and Measurement

• pH: A measure of the hydrogen ion concentration in a solution.

• Formula:

• Scale: Ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Logarithmic Nature: Each unit change in pH represents a tenfold change in [H+]. For example, a solution with pH 3 is 10 times more acidic than one with pH 4.

Hydronium and Hydroxide Ions

• Hydronium ion (H3O+): Formed when a hydrogen ion (proton) associates with a water molecule.

• Hydroxide ion (OH-): Formed when a water molecule loses a proton.

• Relationship: In pure water at 25°C, M, and .

Table: pH, [H+], and [OH-]

Chemical Reactions of Acids and Bases in Water

Acids in Water

• General Reaction:

• Example (Hydrochloric acid):

Bases in Water

• General Reaction:

• Example (Sodium hydroxide):

• Ammonia as a base:

Acid-Base Neutralization

What Happens When an Acid Meets a Base?

• Neutralization Reaction: Mixing an acid and a base produces a salt and water.

• General Equation:

• Example:

Buffer Systems and pH Regulation

Role of Buffers

• Definition: Buffers are substances that minimize changes in pH by absorbing or releasing hydrogen ions.

• Function: Buffers help maintain stable pH in biological systems, which is essential for proper cellular function.

• Mechanism:

  • When pH drops (becomes too acidic), the buffer absorbs H+ ions.

  • When pH rises (becomes too basic), the buffer releases H+ ions.

Bicarbonate Buffer System in Blood

• Importance: Maintains blood pH within the narrow range of 7.35–7.45.

• Components: Carbonic acid (H2CO3) and bicarbonate ion (HCO3-).

• Equilibrium Reaction:

• Example: When acid is added to blood, the buffer system prevents drastic pH changes, unlike pure water.

Acids, Bases, and the Environment

Atmospheric CO2 and Ocean Acidification

• CO2 from human activities dissolves in oceans, forming carbonic acid, which lowers ocean pH (ocean acidification).

• Impact: Affects marine life, especially organisms that build shells or skeletons from calcium carbonate.

Summary Table: Properties of Acids and Bases

Key Equations and Concepts

• pH calculation:

• Relationship between [H+] and [OH-]:

• Neutralization:

Additional info:

• Maintaining pH is critical for enzyme activity and overall cellular health.

• Buffer systems are present in all biological fluids, not just blood.