How do pH indicators work 1

Indicators in chemistry

In general, indicators are tools designed to show certain information. A pH indicator is a dye that changes color to indicate whether a solution is acidic or basic. So if we want to know whether a solution is acidic or basic, we add one or two drops of the indicator to the solution and we can tell from the color that the indicator takes on which milieu is involved.

Each indicator changes its color at a certain point (when the pH value is equal to the pKa value). However, this point is not exactly visible to the naked eye, which is why indicators are always indicated.

But how can an indicator show whether it is an acidic or a basic solution? To do this, the dye itself must be a weak base or acid and react with the acids and bases in the solution:

with HInd as an indicator with H + and Ind− as an indicator without H +.

Depending on how high the concentration of oxonium ions is, the indicator is present with or without a proton (the equilibrium is shifted depending on the concentration - see Le Chatelier's principle).

The question now arises as to why indicators change their color as a result of reactions with acids or bases. Most indicators consist of aromatic systems with a large delocalized π-electron system. This means that the indicators are colored. Deprotonation changes the size of the delocalized system, which absorbs light of a different wavelength and thus creates a different color. We can read about the topic of color in the organic chemistry study book in the chapter on dyes.

In principle, there are many different indicators. In the kitchen we find the indicator red cabbage juice. In the laboratory, however, indicators such as phenolphthalein, thymol blue or the universal indicator (combination of different indicators - therefore it has a large number of envelope areas) are used. Which ones are best to use always depends on what purpose we need them for.