INTERVIEWER: Quantitative additions of a solution of known concentration to one of unknown concentration resulting in a chemical reaction is called titration. Acid-based titrations are among the most common. When a standard sodium hydroxide solution is added to a hydrochloric acid solution, the neutralization reaction occurs. A pH meter is employed to measure the acidity of the solution. To illustrate, let's add 0.100 molar base to precisely 50 milliliters of a 0.100 molar acid solution. The pH increases slowly at first. As the amount of base added approaches that required for complete neutralization, that is the stoichiometrically equivalent amount, the pH begins to increase more rapidly. The last few drops of added base change the pH from about three to seven. The pH is seven when precisely 50 milliliters of base have been added. We are then at the equivalence point, or in point, at which the stoichiometric amount of base required to neutralize the acid has been added. Addition of one drop of NaOH beyond that needed to neutralize the acid causes the pH to increase from seven to about 10, thus, the endpoint of the titration can be determined with high precision.