The Universal Indicator (UI) is a chemical tool designed to measure the acidity or basicity of an aqueous solution using the pH scale, which runs from 0 to 14. UI is a multi-component solution that functions by changing color in response to the concentration of hydrogen ions (H\(^+\)) present in the liquid. This visual color shift provides a direct way to determine the approximate pH level of the solution being tested.
The Specific Color Change in Acid
When Universal Indicator is introduced into an acidic solution (pH below 7), the resulting color will be a shade of red, orange, or yellow. The indicator is a mixture of several different chemical dyes, each changing color at a specific point on the pH scale. In acidic conditions, the indicator molecules absorb hydrogen ions, which alters their chemical structure. This change causes them to reflect light differently, yielding the characteristic warm colors.
How Acid Strength Affects the Hue
The exact color produced by the Universal Indicator provides a clear visual distinction between strong and weak acids. A strong acid, such as hydrochloric acid, releases a high concentration of hydrogen ions, resulting in a very low pH value (typically 1 to 3). When UI is added to a strong acid, the indicator turns a deep, true red color.
In contrast, a weaker acid, like citric acid or vinegar, releases fewer hydrogen ions, leading to a higher pH value (usually 4 to 6). The UI in these weaker acidic solutions will display colors ranging from orange to yellow. A moderately strong acid (pH 3 to 4) often produces an orange hue, while a slightly acidic solution (pH 5 to 6) will appear yellow.
Universal Indicator Across the Entire pH Scale
Observing the Universal Indicator’s response across the entire pH scale reveals its full utility. If a solution is neutral (pH 7), the indicator will turn a distinct green color. This green represents a balance between the hydrogen ions and hydroxide ions in the solution, such as in pure water.
As the solution becomes basic or alkaline, with a pH above 7, the colors transition into the cooler end of the spectrum. For weak bases, which have a pH between 8 and 10, the indicator will display blue colorations. This is due to the indicator losing hydrogen ions to the solution’s excess hydroxide ions.
Strong bases (pH 11 to 14), like sodium hydroxide, cause the Universal Indicator to turn indigo, violet, or deep purple. The wide range of distinct colors makes UI preferred over single-dye indicators, such as litmus paper. Litmus paper only gives a binary result (red for acid or blue for base), while the Universal Indicator provides an approximate numerical pH value through its full spectrum of color changes.