The chart below shows different indicators along with the pH range that shows a color change for that indicator. Because color changes and intensities can be subjective from one person to another, an indicator is not an excellent choice if pH precision is required for a particular experiment. In those cases, a pH meter would be a better choice.
Choosing an appropriate indicator for a reaction will mean the difference in having a reaction that works vs. a reaction that is a failure. Let us look at some examples:
Strong acid and strong base neutralization reaction:
A strong acid and a strong base neutralization reaction will result in a salt and water. This salt solution will have a pH of 7. By looking at the chart above, an indicator that changes color in a range that includes 7 could be bromothymol blue or phenol red.
Strong acid and weak base reaction:
A strong acid reacting with a weak base will result in a weak acid as the product. This means that the pH will be less than 7. An indicator that changes color in that range would be methyl orange or methyl red.
Weak acid and strong base titration:
A weak acid reacting with a strong base will result in a weak base as the product. This means that the pH will be greater than 7. An indicator that changes color in that range would be phenolphthalein or thymolphthalein.
Due to the complication demonstrated above in choosing appropriate indicators, scientists fabricated a universal indicator. By combining several indicators with color changes over a range of pHs, universal indicator was born. A universal indicator is one indicator solution that has a variation of color changes over a pH range of 1-14. Using a universal indicator can take the mystery out of choosing appropriate indicators.