The most important indicators are litmus, phenol-phthalein, and methyl-orange. Litmus is a weak acid, red in colour, the salts of which are blue. When dissolved in water, the molecule is hardly at all ionised, hence the red colour of the acid is alone visible. If a base such as sodium hydroxide is added, which, in aqueous solution, is largely ionised into Na.Aq and -OH.Aq, the hydroxyl ions combine with that portion of the hydrogen ions of the litmus acid which exist in solution ; when these are withdrawn, more hydrogen ions take their place, and the solution acquires the colour of the ion of the litmus acid, viz., blue.* Conversely, if an acid be added to a base in which the blue litmus ions are present, the hydrogen ions of the acid combine with the hydroxyl ions of the base, forming water, so long as any are present; after they are all in combination they convert the ion of the litmus acid into the red acid, non-ionised, and there is a marked colour-change. As the colours of the litmus acid and of its ion are both very bright, the presence of a mere trace of the indicator suffices. Phenol-phthalein, like litmus, is also a weak acid, that is, it is hardly ionised at all in dilute solution ; the acid is colourless, but the ions are pink, hence the addition of a trace of free alkali causes the colourless solution to become pink. But this indicator gives results only with strong bases, like the hydroxides of the alkalies ; with ammonium hydroxide, present in a solution of ammonia in water, it is not a good indicator, for NH₄OH is too weak a base, i.e. the hydroxyl and ammonium ions are present in too small amount to liberate the ions of the phenol-phthalein, unless much ammonium hydroxide is present in solution. Hence the presence of a trace of free ammonium hydroxide is not revealed by that indicator. Phenol-phthalein is therefore serviceable only with strong bases, but it may be used for weak acids. Methyl-orange, on the other hand, is a comparatively strong acid ; with a weak base it forms the ions of a salt, and it may therefore be used for weak bases like ammonium hydroxide, or for strong bases like the hydroxides of the alkali metals ; but it is too strong an acid to serve well as an indicator of excess of a weak acid, such as carbonic or acetic acid. Its colour-change is from orange to orange-pink.