What is the key feature of a chemical that makes it useful as a pH indicator?
To answer this question, we first have to ask ourselves, what exactly is a pH indicator? Before we do that, let’s imagine that we have a solution of hydrochloric acid. Hydrochloric acid is a strong acid and produces solutions with very low pHs. This means that they are acidic. They have a high concentration of H⁺. The higher the concentration of H⁺, the lower the pH. Such acidic solutions are potentially dangerous. One of the main reasons they’re dangerous is because they look exactly like water. Water is neutral and nontoxic. Meanwhile, a solution of hydrochloric acid could have a pH as low as one or even zero. Such solutions could cause dangerous chemical burns if they get in touch with your skin or cause you to go blind if they get in your eye.
Obviously, sticking your hand in a solution to test whether it’s acidic or not is not a good idea. So what can we do to keep ourselves safe? One answer is pH indicators. pH indicators are complex chemicals. Some made naturally and some made synthetically. When added to a solution, they change colour. Generally, pH indicators will turn one colour when the pH is low and turn a different colour when the pH is high. The pH range typically spans between zero and 14. And different indicators are useful across different ranges.
Phenolphthalein is a very popular indicator which has no colour at all but pH is between zero and eight. Above eight, it starts to turn pink. The top end of the pH scale — 12, 13, and 14 — indicate solutions which are strongly basic. These can also be quite dangerous. So adding phenolphthalein to a solution will tell you whether the pH is above or below, around nine. If it turns pink, the solution is more basic. If it turns colourless, the solution is more acidic.
There are other indicators useful for different ranges. For instance, methyl orange is red below about 3.5 and yellow above. Round about 3.5, it’s a mixture of red and yellow and appears orange. And bromothymol blue is blue in basic solutions and yellow in acidic ones, turning a dark red in very acidic solutions. So bromothymol blue is an example of an indicator that exhibits more than two colours. Of course, pH indicators aren’t just about safety. They’re are also particularly useful for finding reaction endpoints, where a quick change in the pH might cause the indicator to change colour completely. For instance, in an acid–base titration.
So how do we describe this key feature of a pH indicator? The key feature of a chemical that makes itself useful as a pH indicator is that its colour changes with varying pH. There are other factors that make a good indicator. Low reactivity is one, meaning that it’s not going to interfere and cause other chemical reactions. It should simply sit around and observe and indicate the pH.
Another factor is that pH indicators should be soluble in water, since that’s what they’re testing. But, in actual fact, a lot of pH indicators aren’t particularly soluble in water. So, they’re first dissolved in ethanol and then mixed with water. Another potential factor is cost. However, this is only a minor factor because indicators are generally used in very small amounts, a few drops of a one percent or less solution. In spite of these other factors, a chemical would be useless as a pH indicator if it did not change colour with varying pH. Therefore, this is the key feature.