16.6 Titrations and Neutralization Calculations
Learning Objectives
By the end of this section, you will be able to:
- Explain the use of acid-base indicators in determining pH of a solution and outline how it is used in a titration experiment.
- Know the steps of an acid-base titration experiment.
- Perform neutralization calculations involving strong acids and strong bases.
In the previous section, neutralization reactions were introduced. This section will examine the basics of acid-base titrations and perform neutralization calculations between strong acids and strong bases.
Titrations
When hydrochloric acid is reacted with sodium hydroxide, an acid/base mole ratio of 1:1 is required for full neutralization.
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
If instead the hydrochloric acid were reacted with barium hydroxide, the mole ratio would be 2:1.
2HCl(aq) + Ba(OH)2(aq) → BaCl2(aq) + 2H2O(l)
Two moles of HCl are required to completely neutralize one mole of Ba(OH)2. In a balanced neutralization equation, the moles of H+ ions supplied by the acid will be equal to the moles of OH− ions supplied by the base. The equivalence point is the point in a neutralization reaction when the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.
In the laboratory, you may need to determine the concentration of an acid or a base whose concentration is not known. This can be accomplished by performing a controlled neutralization reaction. A titration is an experiment in which a solution, whose concentration is known, is gradually added to a measured volume of another solution in order to determine its concentration. Typically, the titrant (the known solution) is added from a buret to a known volume of the analyte (the unknown concentration). Many titrations are acid-base neutralization reactions, although other types of titrations can also be performed.
In order to perform an acid-base titration, the chemist must have a way to visually detect that the neutralization reaction has reached the equivalence point. An indicator is a substance that has a distinctly different colour when in an acidic or basic solution. A commonly used indicator for strong acid-strong base titrations is phenolphthalein. Solutions in which a few drops of phenolphthalein have been added, turn from colourless to brilliant pink as the solution turns from acidic to basic (Figure 16.6a).
The steps in a titration reaction are outlined below.
- A measured volume of an acidic solution (the analyte) whose concentration is unknown is added to an Erlenmeyer flask.
- Several drops of an indicator are added to the acid and mixed by swirling the flask.
- A buret is filled with a basic solution (the titrant) of known molarity.
- The stopcock of the buret is opened and base is slowly added to the acid while the flask is constantly swirled to ensure mixing. The stopcock is closed at the exact point at which the indicator just changes colour. If the colour change does not remain