Core Concepts about Indicators
Acid and Base indicators are molecules which change color in different pH environments. They are incredibly useful for quickly identifying the pH of a solution. In this article, the basics of Indicators in the context of pH will be covered.
Topics Covered in Other Articles
Be sure to read the following articles:
pH Scale and Acid/Base Overview
The pH scale is a measure of how acidic or basic a solution of water is. Acidity refers to having a high concentration of hydrogen ions (sometimes also called 
or protons), while basicity (or alkalinity) refers to having a low concentration of hydrogen ions. These concentrations of protons are usually very small, so in order to make the values a little more friendly, scientists usually take the negative logarithm of this concentration to give the pH. Lets try an example:
Water, at 100 degrees Celsius (boiling) has 
moles of hydrogen ions per liter of solution. Taking the negative logarithm of this value gives us 
. A much friendlier value. pH usually occurs on the range of 0-14. Low values of pH represent high concentrations of hydrogen ions, while high pH values represent low concentrations of hydrogen ions. It can seem backwards to some people, but remember that solutions with low pH values (less than 7 ) are acidic.
What Are Indicators
Indicators simply are molecules that change slightly depending on the pH of the solution they are in. Most organic molecules actually do this, but to be considered an indicator, these different forms (isomers) of the molecule must change color. This color-changing property is a bit more rare, but there are still plenty of indicators out there. For example, one of the most common indicators, phenolphthalein, turns from colorless to pink at a pH of 9.
This color change can seem like magic to some people, so it is important to investigate why different pH solutions cause different colors in these molecules. Color is really just light at different frequencies. Different bonds in molecules will vibrate when exposed to their favorite frequency of light, so by changing the bonds in the molecules at different pH values, the light that makes it through the molecule’s absorption spectrum can be controlled.
To add one additionally layer of complexity, these changes in the structure of the molecule need to be brought about by either water, or the two species water splits into: hydrogen ions and hydroxide ions. Thus, these rearrangements usually happen on reasonably acidic and basic functional groups, and indicators usually have aromatic parts of the molecule to permit electrons to move more freely though them to change the bonds. An animation of the rearrangements in phenolphthalein is shown below, from pH values ranging from 0 to 14:
Indicators need to be soluble in aqueous solution across the majority of the pH scale.
How does Equilibrium Factor Into Acid/Base Indicators, and Choosing the Right Indicator
Acid and base indicators are usually used to detect the endpoint of acid and base titrations. To be effective, the right indicator must be used that changes color as close to the endpoint as possible. When titrating a strong acid or base, the endpoint should occur at a pH of 7. Indicators that change color at pH 7 must have a pKa or pKb of their functional group that reacts with the water, hydrogen ion, or hydroxide at 7 as well. This is because a small change in pH near the endpoint will create the most sensitive change in the indicator molecule since the pKa is the same. Because indicators are actually acids and bases themselves, remember to only use a little bit, to not impact the pH. Phenolphthalein, which has a pKa of 9, changes color at pH 9, not 7, is a horrible choice for strong acid/base titrations.
This also holds for weak acids and bases. However, the equivalence point of these titrations does not always occur at pH 7. By the same reasoning as above, an indicator with a pKa or pKb near the pH of the equivalence point would work best. A chart summarizing the most popular indicators, their color changes, and the pH of the color change is below:
Notice something curious about the last indicator? Universal indicator is a very common indicator that can be used for almost all pH values. However, it is a mixture of indicators, which mix and change color at different solution pH values, creating the continuous spectrum of colors.