Acids, Alkalis and Titrations

What an indicator is

• An indicator is a substance that changes colour to show whether a solution is acidic, neutral or alkaline
• A two-colour indicator shows one colour in acid and a different colour in alkali — its colour flips sharply at the end-point of an acid–alkali reaction
• A universal indicator is a mixture of several indicators that goes through a continuous range of colours, giving a rough numerical pH rather than a single switch

Common two-colour indicators

• Phenolphthalein — stays colourless in acid; flips to a clear pink in alkali
• Methyl orange — looks red while the solution is acidic; turns yellow once the solution becomes alkaline
• Litmus — red in acid, blue in alkali; supplied either as a solution or as red and blue paper strips for testing drops of liquid or moist gases
• Phenolphthalein and methyl orange give sharp colour changes and are the indicators used in titrations (see section 3)
• Litmus solution is not used in titrations because its colour change passes through a fuzzy purple band rather than a single clean transition

The pH number line

• The pH scale is a number scale that runs from 0 (most acidic) to 14 (most alkaline)
• pH = 7 is neutral (pure water at room temperature)
• The scale measures the concentration of H⁺ ions in solution; the more H⁺ ions, the lower the pH
• Rough guide:

Using universal indicator

• A few drops of universal indicator are added to the solution under test
• The resulting colour is matched against a printed chart that maps each colour band to a pH value
• Universal indicator gives only an estimate; the exact colours vary between brands, so the supplied chart must be used
• Because the change is gradual, universal indicator is not suitable for titrations — the end-point would be impossible to pin down