Why indicators change color

The indicator must change color at the pH which corresponds to that endpoint. The indicator changes color because of its own neutralization in the solution. Different indicators have different transition intervals, so the choice of indicator depends on matching the transition interval to the expected pH of the solution just as the reaction reaches the point of complete neutralization.

Phenolphthalein changes from colorless to pink across a range of pH 8. As the pH increases - the intensity of the colour of HIn decreases and the equilibrium is pushed to the right.

Therefore the intensity of the colour of In - increases. 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. Additionally, pH indicators can be found in nature, so therefore their presence in plants and flowers can indicate the pH of the soil from which they grow.

Nature contains several natural pH indicators as well: for example, some flower petals especially Roses and Hydrangeas , certain fruits cherries, strawberries and leaves can change color if the pH of the soil changes. See figure 7. Refer to the table of Universal Indicator Color change figure 1 in the introduction for clarification.

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.

Here is a closer look of the pH papers before and after dipping them in the lemon juice and cleaning detergent Figure 10 :. Here is a simple demonstration that you could try in the lab or at home to get a better sense of how indicator paper works. Make sure to always wear safety glasses and gloves when performing an experiment! She finds 5 brands that she really likes, but since she never took any introductory chemistry classes, she is unsure about which one to purchase.

The first has a pH of 3. This is a hydrolysis problem, but the following method employs the general principle of equilibrium. Phenolphthalein in the table above has a p K ai value of 9. A parade of the color intensities is shown below:. The equivalence point is when the color changes most rapidly, not when the solution has changed color. Improper use of indicators will introduce inaccuracy to titration results.

Indicators change color gradually at various pH. Let us assume that the acid form has a blue color and the basic form has red color. The variation of colors at different pH is shown below. The background color affects their appearance and our perception of them. If a solution has a color matching this, the pH would be the same as the p K ai of the indicator, provided that the conjugate forms of the indicator have the BLUE and RED colors.

Learning Objectives Explain color changes of indicators. Determine the acidic dissociation constants K a or K ai of indicators.