A2-Level Acids and Bases
Titration curves show how the pH of an acidic (or alkaline) solution changes as a certain volume of alkali (or acid) is added.
The end point of a titration is when an indicator changes colour at a certain pH.
The equivalence point in a titration is the point when the concentration of H⁺(aq) ions is the same as (or ‘equivalent’ to) the concentration of OH⁻(aq) ions.
Equivalence points are not always pH 7, which means different indicators that change colour at a pH very close to the equivalence point must be used.
When carrying out a titration, pH can be plotted against the volume of acid (or alkali) added to an alkali (or acid) in a graph. This gives a standard shaped graph that shows how the pH changes depending on the volume of acid or alkali added. The shape of the graph produced is called a titration curve. Their shape looks straightforward and easy to explain, but there are some key details that must be understood in order to fully ‘read’ a titration curve.
The most confusing terms for A-level chemistry students are normally the ‘end point’ and the ‘equivalence point’ – they are not the same!
During a titration, an indicator is used that changes colour at a certain pH. When the solution reaches this pH, the indicator changes colour – this indicates when the titration is ‘complete’. The end point of a titration is when the indicator changes colour. Different indicators can change colour at different pH values, this is why the same indicators aren’t always used for different titrations.
Indicators should be chosen that change colour at a pH that falls within the sharp peak area of a titration curve.
The purpose of a titration is to find the concentration of an unknown acid or alkali. To do this, the amounts of acid and alkali need to cancel each other out in solution. The equivalence point of a titration is when the amount of hydrogen ions, H⁺ ions, is equivalent to the amount of hydroxide ions, OH⁻ ions in solution.
Why the difference between the two (the end point and the equivalence point)?
The end point is not a chemical point – it is simply a colour change that indicates a certain pH of solution. The equivalence point is a ‘chemical’ point that refers to equal amounts of acid and alkali in the solution, not the pH of the solution.
In a titration, we want the end point to be as close to the equivalence point as possible. If a strong acid is titrated with a strong base, the resulting solution has a pH of 7. If we use an indicator that changes colour at pH 7, the end point will be the same as the equivalence point of the solution.
Water and sodium chloride are the only products produced, neither of which affects pH. The solution is pH 7.
However, if a strong acid is titrated with a weak base or a weak acid is titrated with a strong base, the solution produced does not have a pH of 7. The solution is not neutral, even though the moles of H⁺ ions and base are the same. If an indicator that changes colour at pH 7 was used, the end point of the titration would not be the same as the equivalence point.
Ammonium and chloride ions are produced, at the equivalence point there is no more HCl or NH , but NH⁺ and Cl⁻. Ammonium ions release H⁺ ions in solution (they actually react with water to produce H O⁺ ions).
This means the solution becomes slightly acidic, even though there is no original acid left in the solution. To get the solution to reach a pH of 7 more NH would have to be added, and now the amount of NH added is not the same as the amount of acid at the start.
When the indicator changes colour (end point) at pH 7, the amounts of acid and base are no longer equivalent. The end point and the equivalence point are not the same. A different indicator needs to be used, which changes colour at the pH value when the equivalence point is reached.
Titration Curve Shapes
There are four shapes of titration curves that you need to be able to explain.
In the example shown, alkali is added to an acid. As the starting solution is acidic, the pH of the curve starts low. As alkali is added, the pH of the solution begins to increase. This is because the acid reacts with the alkali, so the H⁺ ion concentration in the solution decreases.
The pH does not change rapidly, because there is a simple buffer solution formed. In the above equation, A⁻ is formed, which is the conjugate base of the weak acid, HA. This means we have excess HA and A⁻ in the solution, as well as H⁺ ions from dissociation of the acid. A buffer system is present. As buffer systems resist changes to pH, the pH change is slow. The titration curve shows this as the ‘buffering’ region.