Measuring the "Power of Hydrogen Ions" (pH)

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.

Paper or liquid pH indicators

Should you use pH papers or liquid pH indicators? Here are some points to keep in mind. pH papers allow you to:

1. Save the results. pH paper strips can be pasted in your lab notebook.
2. Minimize the loss of sample. pH paper strips require just a drop or 2 of sample to measure pH. Liquid indicators require more volume of sample to see the color change clearly.
3. Measure pH with more resolution. pH paper strips can measure to 0.2 pH units. Most liquid pH indicators offer 1.0 pH unit resolution at best.

pH paper exercise

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.

Materials

• pH paper
• Several sets of eleven 3 × 5″ index cards; label each set as follows: card 1, “Lye”; card 2, “Bleach”; card 3, “Ammonia”; card 4, “Milk of Magnesia”; card 5, “Baking Soda”; card 6, “Pure Water”; card 7, “Milk”; card 8, “Coffee”; card 9, “Tomato Juice”; card 10, “Vinegar”; and card 11, “Lemon Juice”
• Small beakers or plastic cups containing actual samples of the liquids named on the cards

Procedure

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.

1. Divide the class into small groups and have each group put on safety equipment.
2. Give each group a card set.
3. Ask the groups to organize their cards based on the predicted pH of the liquids listed on them (acidic, basic, or neutral), and record their answers. You can make it more challenging by asking the groups to assign a number from 0.0 to 14.0 (the pH scale) to each card.
4. Have the groups test an actual sample of each liquid using pH paper and record the results. Which group was most accurate in its predictions?

pH paper and environmental studies

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:

• Does the lake water show any evidence of acidity from pollution as compared to tap water?
• Is bottled water any “purer” than tap water? How does it compare to purified water?
• How does saltwater from the ocean compare to the other samples?
• How do gasses in the earth’s atmosphere affect the pH of rainwater?

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.

Conclusion

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!