To begin a discussion about acid-base titrations, we must first recall that there are several definitions of acids and bases. For the purpose of this exercise, we will consider the Arrhenius definition of acids and bases, in which an acid is a proton (H+) donor and a base produces hydroxide (OH-) in solution. When an acid reacts with a base, the products of this reaction are water and a salt. Note that salt here does not only mean table salt (NaCl), but can refer to any ionic compound. For an example, consider the reaction of hydrochloric acid (HCl) with the base, potassium hydroxide (KOH): HCl(aq) + NaOH(aq) -> KCl(aq) + H2O(l)
As this equation shows, the reaction between HCl and NaOH forms water and the salt, KCl. The concentration of an acid, specifically H3O+, or a base, specifically OH-, in a solution will cause that solution to have a specific pH. Recall that pH is a logarithmic scale that is based on the concentration of H3O+ in solution. The pH scale ranges from 0-14, with an acidic solution having a pH lower than 7 on the pH scale and a basic solution having a pH greater than 7 on the pH scale.
The word titration comes from the Latin word "titulus", which means inscription or title. The French word title means rank. Therefore, Titration means the determination of concentration or rank of a solution with respect to water with a pH of 7. Titration is used for determining how much of an analyte in moles (or millilmoles) is in a solution. Analyte is the term for the dissolved unknown in the titration experiment. This is done by slowly adding a standard solution, or a reagent of known concentration, until the titration is determine to be complete. This typically occours after the titration has passed an equivalence point, or when the amount of reagent equals, chemically, the amount of analyte. The equilvalence point is not something that is typically observed however because around the equlvalence point, one drop before and there is no change in pH and the next drop changes the pH sometimes by 3-4 units. The equilvalence point is between these two drops and the closer these two drop are to each other the better the quantification of the analyte.
The standard solution is usually added from a graduated vessel called a burette. The process of adding standard solution until the reaction is just complete is termed as titration and the substance to be determined is said to be titrated.
All chemical reactions cannot be considered as titrations. A reaction can serve as a basis of a titration procedure only if the following conditions are satisfied:
1. The reaction must be a fast one.
2. It must proceed stoichiometrically.
3. The change in free energy (ΔG) during the reaction must be sufficiently large for spontaneity of the reaction. 4. There should be a way to detect the completion of the reaction.