Acidity and Basicity Scales

Acidity and basicity scales are used to measure the relative acidity or basicity (alkalinity) of substances. These scales provide a quantitative way to compare the strength of acids and bases.

Acidity Scale

• pH Scale: The pH scale measures the acidity or alkalinity of a solution. It ranges from 0 to 14, where:
  • pH < 7: acidic solution (lower pH indicates higher acidity)
  • pH = 7: neutral solution (equal concentrations of H⁺ and OH^– ions)
  • pH > 7: basic (alkaline) solution (higher pH indicates higher basicity)
• pK_a Values: The pK_a value is a measure of the strength of an acid in solution. It is the negative logarithm (base 10) of the acid dissociation constant (K_a​). Lower pK_a values indicate stronger acids.

Basicity Scale

• pOH Scale: The pOH scale is similar to the pH scale but measures the concentration of hydroxide ions (OH^–) in solution. It also ranges from 0 to 14, where:
  • pOH < 7: basic solution (lower pOH indicates higher basicity)
  • pOH = 7: neutral solution
  • pOH > 7: acidic solution (higher pOH indicates higher acidity)
• pK_b Values: The pK_b value is a measure of the strength of a base in solution. It is the negative logarithm (base 10) of the base dissociation constant (K_b​). Higher pK_b values indicate weaker bases.

pH Scale and pOH Scale

pH and pOH are two important measurements used to describe the acidity or alkalinity of a solution. They are related to each other through the autoionization of water.

1. pH: pH is a measure of the concentration of hydrogen ions (H⁺) in a solution. It is defined as the negative logarithm of the hydrogen ion concentration: pH = −log[H⁺]
2. pOH: pOH is a measure of the concentration of hydroxide ions (OH^–) in a solution. It is defined as the negative logarithm of the hydroxide ion concentration: pOH = −log[OH^–]

Relationship Between pH and pOH

The pH and pOH of a solution are related through the ion product of water (K_w​), which is the equilibrium constant for the self-ionization of water:

K_w​ = [H⁺][OH^–]

At 25°C (298 K), the value of K_w​ is approximately 1.0×10^-14 (mol/L)². This means that in pure water at this temperature, the concentration of H⁺ ions is equal to the concentration of OH^– ions, and both are 1.0×10^-7 mol/L.

Taking the negative logarithm of both sides of the K_w​ expression:

−logK_w ​= −log([H⁺][OH^–])

14 = −log[H⁺]−log[OH^–]

Since −log[H⁺] = pH and −log[OH^–] = pOH,

we can rewrite the equation as: 14 = pH + pOH

This equation shows the relationship between pH and pOH. Their sum is always equal to 14 in aqueous solutions at 25°C. Therefore, if you know the pH of a solution, you can easily calculate its pOH, and vice versa.

The ionization of water is process of dissociation water molecules dissociate into ions when dissolved in water. In pure water, a small fraction of water molecules undergo ionization, forming equal concentrations of positively charged hydrogen ions (H⁺) and negatively charged hydroxide ions (OH^–).

In this article, we will learn in detail about the chemistry of water ionization, exploring its exchange principles, equations, factors affecting it, and its implications.