pH Scale and Indicators - SS1 Chemistry Lesson Note

The pH scale and indicators play a vital role in understanding and quantifying the acidity or alkalinity of solutions. pH is a fundamental concept in chemistry and is widely used in various scientific disciplines, including chemistry, biology, environmental science, and medicine. We will delve into the pH scale, the concept of acidity and alkalinity, and the use of indicators in pH measurement:

 

pH Scale:

The pH scale is a logarithmic scale that measures the acidity or alkalinity of a solution. It ranges from 0 to 14, with 7 being considered neutral. Values below 7 indicate acidity, while values above 7 indicate alkalinity. Each unit on the pH scale represents a tenfold difference in the concentration of hydrogen ions (H+) in the solution.

 

Acidity and Alkalinity:

Acidity and alkalinity are measures of the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in a solution. Acidity refers to the presence of an excess of hydrogen ions, while alkalinity refers to the presence of an excess of hydroxide ions. Acids increase the concentration of hydrogen ions, while bases increase the concentration of hydroxide ions.

 

pH Indicators:

pH indicators are substances that undergo a distinct colour change depending on the acidity or alkalinity of the solution. They provide a visual indication of the pH of a solution. Common pH indicators include litmus paper, phenolphthalein, bromothymol blue, and universal indicator. Different indicators have different pH ranges over which they exhibit colour changes, allowing for a broad spectrum of pH measurements.

 

Acid-Base Indicator Colour Changes:

The colour change exhibited by pH indicators is a result of a change in the molecular structure of the indicator molecule caused by the presence of hydrogen ions or hydroxide ions. Each indicator has a specific pH range over which it changes colour. For example:

 

-       Litmus paper: It turns red in acidic solutions and blue in alkaline solutions.

 

-       Phenolphthalein: It is colourless in acidic solutions and pink in alkaline solutions.

 

-       Bromothymol blue: It is yellow in acidic solutions, greenish-blue in neutral solutions, and blue in alkaline solutions.

 

-       Universal indicator: It is a mixture of different indicators that provides a continuous colour spectrum, allowing for more precise pH measurements.

 

pH Measurement:

pH can be measured using pH metres or pH indicator papers. pH metres provide a numerical value for pH, while pH indicator papers provide a colour change that can be compared to a colour chart to estimate the pH. In both cases, the indicator or the electrode detects the concentration of hydrogen ions in the solution, which is then converted to a pH value.

 

Understanding the pH scale and indicators is crucial in various scientific disciplines:

-       Chemistry: pH is fundamental in acid-base chemistry, titrations, and understanding reaction kinetics.

 

-       Biology and Medicine: pH plays a crucial role in maintaining the pH balance in biological systems, including blood, cells, and tissues. It is also important in various physiological processes and medical diagnostics.

 

-       Environmental Science: pH is used to monitor and assess the quality of water bodies, soil, and ecosystems, as well as the effects of pollution, acid rain, and climate change.