How the Color Change of the Indicator Happens
The color change of a pH indicator is caused by the dissociation of the H + ion from the indicator itself. Recall that pH indicators are not only natural dyes but also weak acids. The dissociation of the weak acid indicator causes the solution to change color. The equation for the dissociation of the H + ion of the pH indicator is show below (Figure 4).
\[HIn + H_2O \rightleftharpoons H_3O^+ + In^- \tag<4>\]
- \(HIn\) is the acidic pH indicator and
- \(In^-\) is the conjugate base of the pH indicator
It is important here to note that the equation expressed in figure 4 is in equilibrium, meaning Le Chatelier's principle applies to it. Thus, as the concentration of \(H_3O^+\) (H + ) increases or decreases, the equilibrium shifts to the left or right accordingly. An increase in the \(HIn\) acid concentration causes the equilibrium to shift to the right (towards products), whereas an increase of the \(In^-\) base concentration causes the equilibrium to shift to the left (towards reactants).
pH Ranges of pH Indicators
pH indicators are specific to the range of pH values one wishes to observe. For example, common indicators such as phenolphthalein, methyl red, and bromothymol blue are used
to indicate pH ranges of about 8 to 10, 4.5 to 6, and 6 to 7.5 accordingly. On these ranges, phenolphthalein goes from colorless to pink, methyl red goes from red to yellow, and bromothymol blue goes from yellow to blue. For universal indicators, however, the pH range is much broader and the number of color changes is much greater. See figures 1 and 2 in the introduction for visual representations. Usually, universal pH indicators are in the paper strip form.
Graphing pH vs. the H + (\(H_3O^+\)) Concentration
Indicators in Nature
In the lemon juice experiment, the pH paper turns from blue to vivid red, indicating the presence of \(H_3O^+\) ions: lemon juice is acidic. Refer to the table of Universal Indicator Color change (figure 1 in the introduction) for clarification. A video is included below (figure 8).
The household detergent contained a concentrated solution of sodium bicarbonate, commonly known as baking soda. As shown, the pH paper turns a dark blue: baking soda (in solution) is basic .Refer to the table of Universal Indicator Color change (figure 1 in the introduction) for clarification. A video is included below (figure 9).
Here is a closer look of the pH papers before and after dipping them in the lemon juice and cleaning detergent (Figure 10):