Describe acid-base titration. What is the equivalence point?

In the context of acid-base titration, the equivalence point is a pivotal moment that marks the completion of a chemical reaction between an acid and a base. It reflects the point at which the quantity of the acid (or base) originally in the solution has fully reacted with the base (or acid) being added from the burette. Understanding the equivalence point is crucial for accurate titrations.

The equivalence point is where the number of moles of the titrant (the solution of known concentration) is chemically equivalent to the number of moles of the analyte (the solution of unknown concentration). If you were to express this in a mathematical equation, it would look like this:
\[\text{moles of titrant} = \text{moles of analyte}\].

For strong acid-base reactions, the equivalence point is characterized by a neutral solution, with a pH of 7.00. However, when titrating a weak acid with a strong base or vice versa, the equivalence point may occur at a pH above or below 7, depending on the strengths of the reacting acid and base. A pH meter or an appropriate pH indicator can be used to identify when this point has been reached.

A stoichiometric reaction is at the heart of the titration process. It refers to the ideal ratio where the quantities of reactants combine in exact proportions as dictated by the balanced chemical equation—no more, no less. It's like a recipe where the ingredients need to be in precise amounts to produce the desired number of product servings.

When performing a titration, it's the stoichiometric point that dictates the volume of titrant required to reach equivalence. The balanced chemical equation provides the mole ratio of the reactants, which is needed to calculate the necessary volumes for the stoichiometry of the reaction.
\[\text{Reactant A} + \text{Reactant B} \rightarrow \text{Product}\]

In practice, the stoichiometric calculations are the backbone of determining the endpoint of the titration. With a perfect stoichiometric match, all the reactant molecules interact completely, leaving none behind. This is essential information when assessing the purity of a substance or its concentration in a solution.

A pH indicator is a chemical that changes color depending on the pH level of the solution it is in, providing a visual representation of the acidity or basicity of the solution. During titrations, pH indicators are essential tools. The choice of indicator depends on the strength of the acid and base involved and the expected pH range at the equivalence point.

For instance, litmus is an elementary pH indicator that turns red in acidic conditions and blue in basic ones. However, there are more precise indicators like phenolphthalein, which is clear in acidic solutions but turns pink as soon as the solution becomes slightly basic. Bromothymol blue is another example that transitions from yellow to blue across a different pH range. These color changes signal the approach or arrival at the equivalence point.
\[\text{Acidic solution} \rightarrow \text{pH Indicator} \rightarrow \text{Basic solution}\]

It's important to note that the indicator should have a visible color change as close to the equivalence point as possible for the most accurate results. Sometimes, a combination of indicators, known as a pH indicator blend, is used to cover a broader pH range or provide more distinct endpoints.