pH Scale and Indicators


The pH scale is a significant concept in chemistry that allows us to quantify the acidity or alkalinity of a solution. This scale, in tandem with indicators—substances that change color depending on the pH of the environment they are in—gives us the tools to measure and understand the acid-base properties of various substances in our world, from our own bodies to the oceans.

Background Context and Historical Significance:

The term “pH” stands for “potential of hydrogen.” Introduced in 1909 by the Danish biochemist Søren Peder Lauritz Sørensen, the pH scale was developed to provide a precise and straightforward way of expressing acidity levels. Before the introduction of the pH scale, scientists described solutions as ‘acidic’ or ‘basic’, but these descriptions lacked precision.

Detailed Content:

  1. The pH Scale:
    • Definition: A logarithmic scale ranging from 0 to 14, used to specify the acidity or basicity of an aqueous solution.
    • Values:
      • Acidic Solutions: pH less than 7.
      • Neutral Solutions: pH of 7 (e.g., pure water at 25°C).
      • Basic (or Alkaline) Solutions: pH greater than 7.
    • It’s important to note that the pH scale is logarithmic. This means a solution with a pH of 4 is ten times more acidic than one with a pH of 5.
  2. Indicators:
    • Definition: Substances that have different colors in acidic and basic environments.
    • Common Indicators:
      • Litmus: Blue in basic solutions and red in acidic solutions.
      • Phenolphthalein: Colorless in acidic solutions and pink in basic solutions.
      • Bromothymol blue: Yellow in acidic solutions, blue in basic solutions, and green in neutral solutions.
      • Universal indicator: A mixture of indicators that display a range of colors across the pH scale.
  3. Application of the pH Scale:
    • Biology: pH plays a crucial role in many biological processes. For instance, human blood is slightly alkaline with a pH around 7.4. Deviations can be harmful.
    • Agriculture: The pH of soil can affect plant growth. Some plants prefer acidic soils while others thrive in alkaline environments.
    • Environmental Science: Monitoring the pH of natural waters can indicate pollution or other ecological changes. For example, acid rain can lower the pH of lakes, harming aquatic life.

Patterns and Trends Associated with the Topic:

  • Strong Acids and Bases: These tend to have pH values at the extreme ends of the scale. For instance, hydrochloric acid (a strong acid) can have a pH close to 0, while sodium hydroxide (a strong base) can have a pH near 14.
  • Weak Acids and Bases: These have pH values that are less extreme. A weak acid might have a pH of 3 or 4, while a weak base might register at 9 or 10.

Influential Figures or Works Pertinent to the Lesson:

  • Søren Peder Lauritz Sørensen: The Danish biochemist who first introduced the concept of pH at the Carlsberg Laboratory in 1909.


The pH scale, along with indicators, offers an insightful look into the world of acids and bases. It provides a toolset for various fields, from medicine to environmental science, to quantify and respond to changes in acidity and alkalinity.