Teaching the Concept of pH to Students

It is very important that students learn and understand the concept of pH because they will be using this concept in various subject areas in high school and college such as chemistry, biology, environmental science, and related areas. They will also use the concept of pH in their daily lives. Understanding the concept of pH will help them describe the nature of aqueous solutions – whether a particular solution is acidic, basic, or neutral. It is highly recommended that science teachers explain in a clear manner the concept of pH and the various ways on how to measure it. They should emphasize the practical applications of pH concept so that students will appreciate it as an important chemistry concept.

pH is a measure of the acidity (H⁺) or basicity (OH^-) (alkalinity) of a particular solution. pH is technically defined as the negative logarithm of hydrogen ion concentration, symbolized as pH=-log[H+]. It is the ratio of hydrogen ions to hydroxide ions. It was introduced to avoid writing the molarity (moles of hydrogen ion per liter of solution) of a solution in an awkward manner. For example, a solution with 0.00000007 M of hydrogen ions can be written as a solution with pH 7. The concept of pH was first described by the Danish biochemist named Søren Peder Lauritz Sørensen of Carlsberg Laboratory in 1909. Some references state that the “p” in pH stands for “power”, others say that it stands for the German of “power” which is “potenz”, while others say that it is a constant.

The pH scale has a range of 0 to 14. Seven is considered the neutral pH where the concentration of hydrogen ions is equal to the concentration of hydroxide ions. Water has a pH of 7 at room temperature. When the pH is below 7, it means that the solution is acidic and the concentration of hydrogen ions exceeds the concentration of hydroxide ions. If the concentration of hydroxide ions is greater than the concentration of hydrogen ions, the solution is said to be basic or alkaline. An aqueous solution with pH of 6 is considered slightly acidic while a pH of 0 is considered extremely acidic. A solution with pH greater than 7 is considered basic where pH 14 is considered extremely basic.

The pH paper is probably the most widely used kit for measuring pH. It is a strip of paper that contains organic dyes. These organic dyes undergo color change when it is dipped in a solution. The color change is an indicator of the approximate pH of the solution. The pH paper is referred to a chart that illustrates the resulting color change for every pH value. If pH papers are available, students can use them to get the pH of various solutions. For example, they can test the pH of soft drinks, dish washing liquids, lime juice, etc. pH papers are inexpensive and portable but they cannot indicate accurately the pH readings of solutions being tested. More reliable measurement of pH is through the use of pH meter that uses electrodes to measure the pH of a solution. If pH meter is available, the teacher can demonstrate how to use it to the students. The pH meter has a manual that the teacher can refer to.

The concept of pH can be used by the student to describe the liquid solutions that he or she uses daily. The student will be able to differentiate an acidic solution from a basic solution.

A good understanding of pH will help the students comprehend science concepts that uses the pH term. The student will learn that a slight change in the blood pH can be fatal; that extremely acidic or basic solutions can burn the skin; that the acid rain can be agriculturally disastrous; that dramatic change of pH in aquatic environments can kill fishes and so on.

Brown and Lemay. 2005. Chemistry: The Central Science. Prentice Hall.