Introduction to titrations

A titration is a procedure used by chemists to determine the concentration of a particular solute in a solution.

To understand how titrations work, let’s do an example.

Let's say we are given an aqueous solution of hydrochloric acid.

We do not know the concentration of the solution, but we do know its volume, which is 20 milliliters.

To perform a titration, we need a piece of equipment called a burette. Into this burette, we add a 0.100 molar solution of aqueous sodium hydroxide.

Since we know the concentration of our sodium hydroxide solution, we call it our “known” solution.

The known solution is also called the “titrant.”

Since we don't know the concentration of our HCl solution, we call it our “unknown” solution.

To do a titration, we are going to add the known solution to the unknown.

Before we start doing this, we need to carefully record the initial volume of the solution in our burette.

The numerical value of the initial volume does not matter, it just matters that we know the exact starting point so that we can compare it to our ending volume later.

We also need to add into the hydrochloric acid solution a few drops of phenolphthalein, which is an acid base indicator.

Phenolphthalein is colorless when it is in an acidic environment.

But in a basic environment, the indicator will turn the solution pink.

Next, we open up the stopcock on the burette and allow the sodium hydroxide to gradually drip into the hydrochloric acid solution.

As the sodium hydroxide is added, the aqueous solution of sodium hydroxide reacts with the aqueous solution of hydrochloric acid in an acid-base neutralization reaction to form an aqueous solution of sodium chloride and water.

The net ionic equation leaves out all of the ions that don't participate in the reaction.

Remember that those ions are called are spectator ions.

Since sodium ions and chloride ions are present in solution both before and after the acid-base reaction, they are the spectator ions.

Therefore, we can take out the spectator ions to give the net ionic equation of OH minus plus H plus goes to H2O.

So, as we add hydroxide ions, we are neutralizing the H plus ions in the unknown.

As soon as we have neutralized all of the H plus ions, the unknown solution will no longer be acidic.

Any additional hydroxide ions will form a basic solution and the phenolphthalein indicator will turn the solution pink.

This color change indicates the endpoint of the titration and means that all of the acid present in the unknown has been completely neutralized.

In the titration, when we see this color change, we should immediately close the stopcock on the burette.

At this point, our next step is to record the final volume of the known solution in our burette.

We subtract this final volume from the initial volume to find the total volume of base that was needed to neutralize the acid.