What Is a Titration Curve? — Overview & Parts

Titration Curves

In a titration, a known solution of acid or base is added to another solution of base or acid with an unknown concentration. The reaction is monitored using a colorimetric indicator or pH meter. The data from the titration can be recorded and organized to create a pH titration curve.

This is a plot of the solution pH (y-axis) vs. the added volume of titrant (x-axis). When the data is plotted an important feature can be found, the equivalence point. The equivalence point is when the acid and base are in stoichiometrically equal amounts.

What is a titration curve?

A titration curve is a graph that describes what happens during a titration. It graphs the changes in pH of a solution (the analyte) based on the volume of an acid or base (the titrant) added to the solution.

Ultimately a it can tell us when an analyte is being deprotonated or protonated, when the reaction reaches completion, and even help us determine the concentration of an unknown analyte.

Variables given through a curve:

$\color{#50BF14}{\text{y-axis}}$: pH

$\color{#50BF14}{\text{x-axis}}$: volume of titrant added

$\color{#50BF14}{\text{equivalence point}}$: where the $\text{(moles of acid)=(moles of base)}$ and the solution is neutralized

$\color{#50BF14}{\text{half equivalence point}}$: the point at which the $pH=pK_{a}$

An indicator is used to determine the equivalence point of the solution. There is a very specific way of choosing your indicator. An indicator is chosen specifically based on the pH of the equivalence point because we want to be able to tell once the reaction has reached completion.

Titrations, ultimately, can tell us the concentration of an unknown solution based off of the titrant completely neutralizing the analyte, i.e. reaching the equivalence point. If we know the volume and the concentration of the titrant added, we can determine the unknown concentration of the analyte based on the balanced chemical reaction that happens and using the formula:

$$\frac{(M_{1}V_{1})}{n_{1}} = \frac{(M_{2}V_{2})}{n_{2}}$$

Where M is the molar concentration, V is the volume, and n is the stoichiometric ratio found in the balanced chemical equation. All this equation is, is a fancy way of saying $\text{(moles of A)=(moles of B)}$ in terms of all of the variables found in a titration. How? Let’s look at the units. When $n_{1}$ and $n_{2}$ are equal, $(M_{1}V_{1})=(M_{2}V_{2})$ and molarity (mol/L) times volume (L) equals moles. Therefore, $\text{(moles of 1)=(moles of 2)}$. The variables in this equation are molarity, volume, and the stoichiometric ratio because a specific concentration of titrant is used to neutralize an unknown analyte, a volume of titrant is added to a volume of analyte, and the reaction is progressing in the way described by the chemical equation, respectively.

Using a titration curve, we are additionally able to find the $pK_{a}$ of the analyte solution, which is the point at which the analyte is deprotonated. When the pH is lower than the $pK_{a}$ the analyte is protonated and when the pH is higher than the $pK_{a}$ the analyte is deprotonated. This is very important in the titrations of polyprotic acids where there are multiple $pK_{a}$ values for the dissociation of each proton.

There are four main possible curves for the different titration scenarios listed below. These are the same scenarios that can be found in the titration of a base to an acidic analyte.

1. Titration of a strong acid to a weak base
2. Titration of a strong acid to a strong base
3. Titration of a weak acid to a weak base
4. Titration of a weak acid to a strong base

(Video) Titrations and Titration Curves

Video By: Chad's Prep

In this video, Chad's Prep explains titration curves, the equivalence point, and how these are both affected by different conditions, like the strength of the acid and base. It also shows how the curve can show us the buffer range and capacity for the solution.