pH curves

Overview of pH Curves

A pH curve is a graph depicting the change in pH of an acidic or a basic solution upon the addition of a basic or acidic solution, respectively.
It is a visual representation of an acid-base titration, showing the pH changes that occur during the mixing process.
The graph typically showcases a smooth curve, reflecting the slow but constant change in pH.

Components of a pH Curve

The x-axis typically illustrates the volume of the titrant (base or acid) added.
The y-axis demonstrates the pH of the solution.
A visual inspection can identify the equivalence point or the end point of the titration, the point at which equal amounts of acid and base have reacted.
The buffer region is the area of the curve where the pH doesn’t change significantly upon the addition of small amounts of acid or base.

Characteristics of pH Curves

The curve tends to increase gradually at first, then sharply in the middle (near the equivalence point), and finally levels off.
There’s a sharp vertical change in pH at the equivalence point, known as the vertical section, where a small change in the volume of added titrant leads to a large change in pH.
The point halfway up the vertical section corresponds to the point where the pH equals the pKa of the weak acid or the pKb of the weak base.

Determining Various Quantities using a pH Curve

The equivalence point can be used to calculate the concentration of the unknown solution using the formula c1v1=c2v2, where c1 and v1 represent the concentration and volume of the known solution, and c2 and v2 represent the concentration and volume of the unknown solution.
The half equivalence point is the point where half the amount of titrant needed to reach equivalence has been added. This point can be used to determine the pKa or pKb of the weak acid or base.

Types of pH Curves

Monoprotic acids or bases give a single equivalence point and thus form a single hump in their pH curves.
Polyprotic acids or bases give two or three equivalence points and thus form multiple humps in their pH curves. These acids or bases can donate or accept more than one proton per molecule.

Key Points

pH curves enable us to understand the behaviour of acid-base titrations effectively.
They help in distinguishing between strong and weak acids/bases and are crucial in determining the pKa or pKb and the concentration of the unknown solution.