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By Bruce Rosner
Micro Essential Laboratory, Inc.
The year 2009 marks the 100th anniversary of Danish chemist Søren Peder Lauritz Sørensen’s introduction of the pH concept. pH is a measure of the hydrogen ion concentration in an aqueous solution. Essentially, pH measures acidity and alkalinity on a scale from 0.0 to 14.0. Pure water at room temperature (77° F) has a neutral pH, measuring 7.0.
The measurement of pH is critical to many fields, including medicine, biology, chemistry, and environmental science, and thus has many applications in daily life. Some examples of these applications are ensuring that the foods we eat are safe, protecting the environment from harmful industrial wastewater, and creating safe and effective medicines.
You can obtain a quick, accurate, and economical measure of pH by using universal indicator paper, commonly called pH paper. It contains a combination of indicators that change color when they come in contact with hydrogen and hydroxide ions. Acid contains hydrogen ions; bases contain hydroxide ions.
When you dip pH paper in a sample liquid, different amounts and types of ions produce different colors. Wide-range pH papers, such as the Hydrion® Spectral pH Paper, provide accurate measurement at full pH unit (1.0) divisions within the entire pH range of pH 0.0 to 14.0. Short-range pH papers, such as the Lo-Ion Test Kit, provide accurate pH measurements at smaller divisions, typically 0.5 pH units, within a shorter pH range, e.g., pH 0.0 to 6.0.
Should you use pH papers or liquid pH indicators? Here are some points to keep in mind. pH papers allow you to:
Teaching students about pH measurement can be fun and simple. Here’s an exercise that challenges students to think about the relative acidity and alkalinity of various liquids.
Note: This exercise uses acids and bases, so please make certain to use safety goggles and gloves, particularly when handling strong acids (HCl) and strong bases (NaOH or KOH). And be sure to dispose of all solutions properly.
The use of pH measurement can also be associated with environmental studies. Students can compare the pH of water from various sources such as an ocean, a lake, a stagnant puddle, rain, a tap, a purification process, a bottler, a seltzer manufacturer, and so on. Suggestions for investigations include:
Short-range pH paper, e.g., the Lo-Ion Test Kit (pH 3.0 to 6.0), is recommended for environmental studies because it is sensitive enough to pick up small differences in pH.Teacher tip: If cost is a concern, buy pH paper rolls and tear off the smallest length of paper you can use. Or, purchase pH paper strips and tear to make shorter strips.
pH paper provides an accurate, inexpensive method of determining pH. You can use it to teach lessons on topics relevant to daily life such as medicine, biology, chemistry, and environmental science. Experiment with different ranges and types of pH paper to find the one that best suits your needs—and have fun!