pH Scale: Understanding Acidity and Basicity

Definition

The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14, where 7 is neutral.

Explanation

pH stands for “potential of hydrogen” or “power of hydrogen” and represents the concentration of hydrogen ions ($H^+$) in a solution. A higher concentration of $H^+$ ions indicates a more acidic solution, while a lower concentration indicates a more basic or alkaline solution. The pH scale provides a convenient way to express this concentration, making it easier to compare and understand the relative acidity or alkalinity of different substances.

Core Principles and Formulae

The pH scale is logarithmic, meaning each whole number change in pH represents a tenfold change in hydrogen ion concentration. The pH is calculated using the following formula:

$pH = -log_{10}[H^+]$

Where $[H^+]$ is the concentration of hydrogen ions in moles per liter (M).

Therefore:

pH < 7: Acidic
pH = 7: Neutral
pH > 7: Basic (Alkaline)

The concentration of hydroxide ions ($OH^-$) is related to the hydrogen ion concentration through the ion product of water ($K_w$):

$K_w = [H^+][OH^-] = 1.0 \times 10^{-14}$ (at 25°C)

We can also calculate pOH, which measures the hydroxide ion concentration:

$pOH = -log_{10}[OH^-]$

And the relationship between pH and pOH is:

$pH + pOH = 14$ (at 25°C)

Examples

Acidic: Lemon juice (pH ~ 2), Vinegar (pH ~ 3), Stomach acid (pH ~ 1.5-3.5)
Neutral: Pure water (pH ~ 7)
Basic (Alkaline): Baking soda solution (pH ~ 8), Soap (pH ~ 9-10), Ammonia solution (pH ~ 11)

Common Misconceptions

pH does not equal concentration. pH is a measure of the negative logarithm of the hydrogen ion concentration. A pH of 1 is not the same as having 1 mole per liter of hydrogen ions.
Acids and bases always react immediately and completely. While strong acids and bases react readily, weaker acids and bases might react slowly or incompletely.
A substance is either an acid or a base. Many substances can act as both acids and bases (amphoteric) depending on the situation. Water is a prime example.

Importance in Real Life

The pH scale is critical in numerous aspects of everyday life:

Agriculture: Soil pH affects nutrient availability for plants. Farmers adjust soil pH with lime (to raise pH) or sulfur (to lower pH) to optimize crop yields.
Biology/Medicine: pH levels in the human body (e.g., blood pH) are tightly regulated. Deviations from normal pH can lead to serious health problems (acidosis or alkalosis).
Food Science: pH influences food preservation, flavor, and texture. Pickling and fermentation rely on controlling pH.
Environmental Science: pH monitoring is used to assess water quality in lakes and rivers, and to detect acid rain.
Chemistry/Industry: Controlling pH is essential in many chemical reactions, manufacturing processes, and wastewater treatment.

Fun Fact

The color of a substance in a pH indicator solution changes depending on the pH. For example, litmus paper turns red in acidic solutions and blue in basic solutions.

History or Discovery

The pH scale was introduced by the Danish biochemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909. He initially defined pH as a means of expressing the hydrogen ion concentration in brewing processes.

FAQs

What is the difference between a strong acid and a weak acid?

A strong acid completely dissociates (ionizes) in water, meaning it fully releases its hydrogen ions. A weak acid only partially dissociates.

How do I measure pH?

pH can be measured using pH indicators (like litmus paper or universal indicator), or more accurately using a pH meter.

Why is pH important in the human body?

The body’s pH balance (primarily in blood) affects the function of enzymes, the transport of oxygen, and overall cellular processes. Maintaining a pH within a narrow range (7.35-7.45 for blood) is crucial for survival.