AAT Bioquest

What are acid-base indicators?

Posted September 28, 2022


Acid-base indicators are chemicals that are used to determine whether a solution is acidic, basic, or neutral or in other words. They help to indicate approximately where a solution falls on the pH scale. Acidic solutions have a pH of less than 7, while basic or alkaline solutions have a pH greater than 7, and aqueous solutions have a pH of 7, which is neutral. An acid-base indicator may be either a weak acid or a weak base. They are widely used in titration to identify the end point of titration and determine approximately where a solution falls on the pH scale. On disassociating with water, the acid-base indicator exhibits a color change because of the change in concentration of hydrogen (H+) or hydroxide (OH-) ions in water. An acid-base indicator is useful over a specific pH range so it may be a good choice to test one solution but ineffective for another solution. Moreover, some indicators are only able to determine the approximate pH of an acid or base but are unable to identify the acid or base. Litmus, Methyl Orange, Thymol Blue and Phenol Red are some of the more commonly used acid-base indicators. 

Additional resources

Investigation of Acid-Base Indicator Property of Plumbagin from Plumbago zeylanica Linn

Weak Acid pH Calculator