The term pH measures how acidic or basic a substance is when dissolved in water. It is a fundamental concept in chemistry, biology, and environmental science, influencing everything from soil health to human physiology. The letters “pH” stand for “Potential of Hydrogen,” referring to the concentration of hydrogen ions (\(H^+\)) in a solution. Measuring the pH value assesses this concentration, and pH paper offers a low-cost, immediate way to determine this value.
The Appearance of Unused Universal pH Paper
Universal pH paper is typically a light, uniform color before use. The most common appearance is a pale yellow, light orange, or creamy tan. This initial color is the base state before the chemical components are activated by a liquid. The paper is made of cellulose, which acts as an absorbent matrix for indicator chemicals. These strips are saturated with a complex mixture of dyes that collectively cover the entire pH range, and the faint color results from the combined dry colors of this mixture.
The Chemical Mechanism Behind Color Change
The color change observed on pH paper results from a chemical reaction between indicator dyes and the hydrogen ions in the tested solution. The universal indicator is a blend of multiple chemical dyes, such as methyl red, bromothymol blue, and thymol blue, each acting as a weak acid or base. These indicator molecules exist in chemical equilibrium, allowing them to switch between two different forms. In an acidic solution, the high concentration of hydrogen ions causes the dye molecules to gain a proton (\(H^+\)), a process called protonation. Conversely, in a basic or alkaline solution, the low hydrogen ion concentration causes the dye molecules to lose a proton (deprotonation). Since each form—protonated and deprotonated—has a distinctly different molecular structure, this structural change results in a visible shift in color. The collective color change provides a specific hue corresponding to a particular pH value.
Interpreting the Universal pH Scale
After the pH paper is dipped into a substance, the resulting color is immediately compared against a standardized color chart. The chart organizes the pH scale, which runs from 0 to 14, into three primary zones. The acidic zone (pH below 7) generally produces colors in the red, orange, and yellow spectrum. For example, a strong acid (pH 1-2) will turn the paper bright red, while a weaker acid like orange juice (pH 3-4) results in a more subdued orange or yellow.
The neutral point (pH 7), characterized by pure water, typically produces a green or greenish-yellow color. This color signifies that the concentration of hydrogen ions is balanced with the concentration of hydroxide ions. Moving beyond neutral, the alkaline or basic zone (pH above 7) causes the paper to change to shades of blue, indigo, and violet.
A weak base like baking soda solution (pH 8-9) results in a blue color. Conversely, a strong base such as oven cleaner (pH 12-14) will cause the paper to turn a deep violet or purple. By matching the exact shade on the paper to the corresponding shade on the reference chart, one can quickly determine the approximate strength of the acid or base being tested.