We know the color changes that we see in the
nature all around us. We also notice the color changes in
iced tea when lemon juice is added. In the study of acid
base chemistry, we use litmus paper to indicate if a solution
is acidic, basic, or neutral based on the color changes. The
substances in plant products such as tea, red cabbage or
grapes react with acids (or bases), resulting in changes at
the molecular level which causes their colors to be different
at different pH levels. All pH indicators, such as litmus
paper, change colors depending upon whether they donate
or accept protons, (acids are proton donors and bases are
proton acceptors). Therefore, pH indicators are themselves
acids or bases. Indicators work because they are weak acids
which, when in solution, exist in equilibrium with their
conjugate base. The acid and its conjugate base each have
different colors, and as the equilibrium shifts from one
direction to the other, the color of the indicator solution
changes. Some indicators exhibit only two colors and some
exhibit a wide range. Each indicator must be individually
studied to determine its behavior as a function of pH.