**Acid-base titrations**
The neutralisation reaction between an acid and a base can be very useful. If an acidic solution of known concentration (a standard solution) is added to an alkaline solution until the solution is exactly neutralised (i.e. it has neither acidic nor basic properties), it is possible to calculate the exact concentration of the unknown solution. It is possible to do this because, at the exact point where the solution is neutralised, chemically equivalent amounts of acid and base have reacted with each other. This type of calculation is called **volumetric analysis**. The process where an acid solution and a basic solution are added to each other for this purpose, is called a **titration**, and the point of neutralisation is called the **end point** of the reaction. So how exactly can a titration be carried out to determine an unknown concentration? Look at the following steps to help you to understand the process.

**Step 1:**

A measured volume of the solution with unknown concentration is put into a flask.

**Step 2:**

A suitable indicator is added to this solution (bromothymol blue and phenolpthalein are common indicators).

**Step 3:**

A volume of the standard solution is put into a burette (a measuring device) and is slowly added to the solution in the flask, drop by drop.

**Step 4:**

At some point, adding one more drop will change the colour of the unknown solution. For example, if the solution is basic and bromothymol blue is being used as the indicator in the titration, the bromothymol blue would originally have coloured the solution blue. At the end point of the reaction, adding one more drop of acid will change the colour of the basic solution from blue to yellow. Yellow shows that the solution is now acidic.

**Step 5:**

Record the volume of standard solution that has been added up to this point.

**Step 6:**

Use the information you have gathered to calculate the exact concentration of the unknown solution. A worked example is shown below.

Comments:"Accessible versions of this collection are available at Bookshare. DAISY and BRF provided."