DIY Indicators

Acids and bases (alkalis) form solutions that have a pH of <7 or >7 respectively. The pH is a measure of acidity and is defined as -log₁₀[H⁺]. Indicators are substances which exhibit different colours in acid and base(alkaline) solutions. They are often coloured organic substances such as litmus, which is extracted from lichen. Litmus turns red in acids and blue in alkalis. Indicators are normally weak acids:

HInd(aq) ? H+(aq) + Ind^-_(aq)

Where HInd is a weak acid and Ind^- is its conjugate base; both have different colours. The change in colour results from a change of structure as a H⁺ is removed from or attached to the molecule. In acid solution, HInd predominates; in alkali solution, the H⁺ ions are removed from the equilibrium, more HInd ionises and the conjugate base Ind^- predominates.

To read more click here to download Dr Hal's fact sheet on DIY Indicators (900kb pdf file)