7.12: Titration Experiment

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A large amount of current research involves the development of biodiesel fuels. Often this material can be made from used vegetable oils. The vegetable oil is treated with lye to create the biofuel. In the oils is a variable amount of acid that needs to be determined, so that the workers will know how much lye to add to make the final fuel. Before the lye is added, the native vegetable oil is titrated to find out how much free acid is present. Then, the amount of lye added can be adjusted to take into account the amount needed to neutralize these free acids.

Titration Experiment

In the neutralization of hydrochloric acid by sodium hydroxide, the mole ratio of acid to base is 1:1.

\[\ce{HCl} \left( aq \right) + \ce{NaOH} \left( aq \right) \rightarrow \ce{NaCl} \left( aq \right) + \ce{H_2O} \left( l \right)\nonumber \]

One mole of \(\ce{HCl}\) would be fully neutralized by one mole of \(\ce{NaOH}\). If instead the hydrochloric acid was reacted with barium hydroxide, the mole ratio would be 2:1.

\[2 \ce{HCl} \left( aq \right) + \ce{Ba(OH)_2} \left( aq \right) \rightarrow \ce{BaCl_2} \left( aq \right) + 2 \ce{H_2O} \left( l \right)\nonumber \]

Now two moles of \(\ce{HCl}\) would be required to neutralize one mole of \(\ce{Ba(OH)_2}\). The mole ratio ensures that the number of moles of \(\ce{H^+}\) ions supplied by the acid is equal to the number of \(\ce{OH^-}\) ions supplied by the base. This must be the case for neutralization to occur. The equivalence point is the point in a neutralization reaction where the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.

In the laboratory, it is useful to have an experiment where the unknown concentration of an acid or a base can be determined. This can be accomplished by performing a controlled neutralization reaction. A titration is an experiment where a volume of a solution of known concentration is added to a volume of another solution in order to determine its concentration. Many titrations are acid-base neutralization reactions, though 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 occurred. An indicator is a substance that has a distinctly different color 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 colorless to brilliant pink as the solution turns from acidic to basic. The steps in a titration reaction are outlined below.

A measured volume of an acid of unknown concentration 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 base solution 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 color.

The standard solution is the solution in a titration whose concentration is known. In the titration described above, the base solution is the standard solution. It is very important in a titration to add the solution from the buret slowly, so that the point at which the indicator changes color can be found accurately. The end point of a titration is the point at which the indicator changes color. When phenolphthalein is the indicator, the end point will be signified by a faint pink color.

Figure \(\PageIndex{1}\): Phenolphthalein in basic solution. (CC BY-NC; CK-12)

Summary

The equivalence point is the point in a neutralization reaction where the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.
A titration is an experiment where a volume of a solution of known concentration is added to a volume of another solution in order to determine its concentration. (Many titrations are acid-base neutralization reactions.)
An indicator is a substance that has a distinctly different color when in an acidic or basic solution.
The process for carrying out a titration is described.