Chemistry – Acids, bases and salts - The characteristic properties of acids and bases | e-Consult

Alkalis are bases, and their effect on indicators is a direct consequence of their ability to accept protons (H⁺) or donate hydroxide ions (OH^-).

Litmus: Litmus paper is a mixture of lichens and produces a red colour in acidic solutions and a blue colour in alkaline solutions. Alkalis turn red litmus paper blue. This occurs because the hydroxide ions (OH^-) in the alkali react with the acidic components of the litmus dye, changing its colour to blue. The OH^- neutralizes the H⁺ ions present in the litmus dye.

Thymolphthalein: Thymolphthalein is a pH indicator that is colourless in acidic and neutral solutions, but turns pink/purple in alkaline solutions. Alkalis cause thymolphthalein to change colour because the hydroxide ions (OH^-) in the alkali react with the acidic proton on the thymolphthalein molecule, causing a structural change that results in the pink/purple colour. The reaction involves the removal of a proton from the thymolphthalein molecule by the hydroxide ion.

Methyl Orange: Methyl orange is yellow in acidic solutions and orange in alkaline solutions. Alkalis cause methyl orange to turn orange because the hydroxide ions (OH^-) in the alkali react with the acidic proton on the methyl orange molecule, changing its molecular structure and resulting in the orange colour. The OH^- removes the proton, altering the electronic properties of the molecule and shifting the absorption of light to a longer wavelength (towards orange).

In summary, alkalis increase the concentration of hydroxide ions in solution, which react with the acidic components of the indicators, causing a colour change.