Acids and Bases: Fundamental Properties, Theories, and pH Measurement

Properties of Acids

• Solutions often have a sour taste.
• They attack marble (CaCO₃) and other calcareous rocks.
• Metals react with acids, evolving hydrogen gas.
• Concentrated solutions destroy organic matter.
• They conduct electric current.
• Neutralize bases.
• Produce salts with bases.
• Change the color of acid-base indicators to red.

Properties of Bases

• Solutions often have a bitter taste.
• React with fats to form soaps, producing a soapy feel on skin.
• Generate insoluble solids with certain metals.
• Concentrated solutions destroy organic matter.
• They conduct electric current.
• Neutralize acids.
• Produce salts with acids.
• Change the color of acid-base indicators to blue.

Acid-Base Theories

Arrhenius Theory of Ionic Dissociation

In 1884, Svante Arrhenius (1859-1927) postulated, in his theory of ionic dissociation, the existence of negatively and positively charged ions in solutions of acids, bases, and salts to explain their electrical conductivity.

Limitations of the Arrhenius Theory

• Restricted to aqueous solutions.
• Does not account for substances (e.g., NH₃) that lack OH⁻ ions but exhibit basic properties.
• Does not explain why some salt solutions have acidic or basic properties.
• Does not explain ionic species that have acidic properties.
• Cannot explain neutralization reactions between gases.

Brønsted-Lowry Theory: Conjugate Acid-Base Pairs

In 1923, Johannes Brønsted and Thomas Lowry proposed a more comprehensive concept of acids and bases. In this theory, acids and bases are considered interrelated. According to this theory, reactions are called proton-transfer reactions. A limitation is that it does not explain the nature of acids without protons in their formula.

Lewis Acid-Base Theory

In 1923, G.N. Lewis introduced a new theory based on the ability of molecules to form covalent bonds by donating or accepting electron pairs.

Key Strong Acids

• Perchloric acid (HClO₄)
• Hydroiodic acid (HI)
• Hydrobromic acid (HBr)
• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Nitric acid (HNO₃)

Acid-Base Indicators

These are substances that change the color of their solutions in response to variations in pH. Indicators are typically weak organic acids or bases that exist in equilibrium with their conjugate forms.

• Methyl Orange: pH = 2 (red), pH = 7 (orange)
• Phenolphthalein: pH = 6 (colorless), pH = 10 (rose)

Measuring pH with pH Meters

To accurately determine the pH value of a solution, an instrument known as a pH meter is used. It consists of a glass electrode sensitive to changes in proton concentration.