pH scale. An experimental approach to the math behind the pH chemistry

Abstract We present an experimental activity designed for upper secondary students, where a pH scale is constructed taking advantage of the ability of a pH indicator to display different colors according to the pH of the medium. To build the scale, two solutions are prepared: 1 M HCl (pH = 0) and 1 M NaOH (pH = 14). Each original solution undergoes six consecutive 1/10 dilutions, producing acidic solutions with pH values of 1, 2, 3, 4, 5, and 6; and basic solutions with pH values of 13, 12, 11, 10, 9, and 8. Pure water (nominal pH = 7) serves as the reference. Upon adding the indicator, a beautiful rainbow of colors appears in the solution containers. To connect the experimental results with its mathematical representation, a written exercise is provided for students. This activity allows them to visually understand that, even though the change in pH is only one unit, the change in H + concentration is 10 times greater or smaller. Thus, pH is an exponential function, best expressed in logarithmic terms.