Titration is the procedure of identifying the quantity (moles or concentration) of a compound (which is called the analyte) using a well known reaction. In order to titrate a substance, there must be a reaction it participates in, and the end point of that reaction must be identifiable. Also, the quantity of the other reactants (the titrants) must be known. Acid-base reactions and redox reactions are most often used for titration.
Acid-base titrations are commonly used because they are simple and their end points can be easily detected using an indicator or a meter. For example, suppose we have a hydrochloric acid solution whose concentration we do not know of. We want to know its molar concentration (let it be ). So, we prepare a sodium hydroxide solution, and slowly mix it into the acid solution: The acid-base reaction equation above implies that the equivalence point is when the number of moles of added equals the number of moles of In this case the end point (or equivalence point) would be the point at which so we slowly add until the meter reads If the is when of has been added, then we have where must equal Thus now we know that the initial molar concentration of the hydrochloric acid solution was
Titration can also be done by using a well known redox reaction. In some cases the equivalence point can be detected using the bare eye for reactions involving color changes. For reactions that do not, a redox indicator or potentiometer may be used.
The oxidation of using potassium permanganate is a well known redox reaction. Below is the reaction equation: Note that moles of is oxidized to become while one mole of permanganate is reduced to form Thus the ferrous ion and permanganate ion react with a ratio. The use of permanaganate is very handy, due to the fact that its solution is bright purple, making it a good indicator. Suppose we have of potassium permanganate solution, and we want to know the concentration of in another solution. We slowly add the ferrous solution (which is colorless) to the permanganate solution, until the purple color disappears. If the color disappeared when of ferrous solution was added, then we have Since the ratio must equal it must be true that