These are called acid-base indicators . Many indicators do not have a sharp color change as a function of pH. Most acid-base indicators can exist as either an acid or a base. These two forms have different colors. The relative concentration of the two different forms is sensitive to the pH of the solution. Thus, if we know the pH at which the indicator turns color, we can use this color change to determine whether a solution has a higher or lower pH than this value.
Strong acids are strong electrolytes.
The pH of a solution of a monoprotic acid may usually be calculated directly from the initial molarity of the acid. Strong bases are strong electrolytes and dissociate completely in solution. Not all bases contain the OH ion. Ionic metal oxides, hydrides, and nitrides are basic. The oxide, hydride, and nitride ions are stronger bases than hydroxide.