When testing solutions to determine whether they are acidic or basic, why do chemists use a mixture of indicators rather than a single indicator solution?

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jerichorayel | College Teacher | (Level 2) Senior Educator

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Indicators that were used in the laboratories vary on at what pH the color is going to change. For example:

Phenolphthalein is changes from colorless to pink at pH range 8.30-10

Methyl red indicator changes from red to yellow at pH range of 4.4-6.2

Alizarine Yellow R indicator changes from yellow to red at pH range of 10.2 – 12.0


Chemists often use different kinds of indicators because to identify the possible acidity of the unknown solution. To better explain this topic, let me show you an example. Suppose you have solved an unknown solution of pH 4.

You cannot determine the exactly if it is an acid or base if you use Alizarine Yellow R because Alizarine Yellow R has the color yellow until near 10. All we know that if we use Alizarine Yellow R, the pH of the solution is 10.2 below. Next we use Phenolphthalein, we cannot see any color difference since at pH 4 the phenolphthalein is colorless. Now we already know that the unknown is below pH 8.30. Next we can try Methyl red or any other indicator until we get finally found the pH of the unknown solution.