An indicator can show show the pH of a solution when added in small amounts. If the pH is below 7, then it is an acid. if it is above 7, it is an alkali (to test pH, the solution must be dissolved in water, therefore it should be described as an alkali rather than a base), and it's neutral if it is exactly 7. The strength of the acid or alkali increases as the pH reading gets further away from 7. (This is not if an acid or alkali is strong or weak .)
List of commonly used indicators
Universal indicator is the most commonly used indicator. It gives a full scale of 0 to 14 pH, and allows the pH to be found with a good level of accuracy. It turns dark red when strongly acidic, yellow when weakly acidic, green when neutral, light blue when weakly alkali, and dark blue/purple when strongly alkali.
Litmus paper is an indicator used to measure pH. There are two type of litmus paper - red and blue. When the red is
in the presence of an acid. it remains red, and when in the presence of an alkali it turns blue. When blue litmus paper is in an acid it changes colour to red, and when in an alkali remains blue. When either paper is in pure water. it remains the original paper colour.
Phenolphthalein is colourless in acidic solutions and turns pink/purple in alkali solutions. The stronger the alkali the darker the pink, which may look purple.
Phenolphthalein is insoluble in water, but can be dissolved in ethanol.
Methyl orange is a pH indicator that gives fire colors: Red with acids, yellow with neutrals and orange with bases.
Methyl orange is often used in titrations because of its clear colour change. Because it changes colour at the pH of a mid-strength acid, it is usually used in titrations for acids. Unlike a universal indicator, methyl orange does not have a full spectrum of colour change, but has a sharper end point.