2.5.5

# pH Curves & Titrations (A2 Only)

Test yourself

## Key Titration Techniques

Here's a quick recap of key titration information.

### Glassware

• Above are labelled images of a burette and a pipette.

### Technique

• In a titration, you want to calculate the concentration of an acid or base by reacting it with a known amount of base or acid. To do this:
• Add a measured amount of one to the other.
• Use a balanced equation to calculate the concentrations.

### Accuracy

• In a titration, you want to make sure you have accurate results.
• Accurate results are achieved by repeating your titration until you get concordant results.
• Concordant results are values that are within 0.1 cm3 of each other.

### Indicators

• To find the endpoint of a titration, you use an indicator.
• An indicator changes colour when all of the unknown reactant is used up.

## Strong Acid/Base Titrations

Titrations of a strong acid with a strong base are the simplest to understand.

### pH curve

• The pH curve pictured shows the change in pH when a strong acid is titrated with a strong base.

### The beginning

• In the beginning, you can see that the pH is very low, and doesn’t change very quickly.

### The middle

• In the middle of the graph, there is a sharp change and the pH rises very rapidly. This is known as the equivalence point.
• At the equivalence point, the acid is just about neutralised. Adding any more base causes a rapid shift to a basic solution.

### The end

• The curve ends at a very high pH because the solution has excess base present.

### Indicator

• When doing an acid/base titration, you need to use an indicator to find the equivalence point.
• A good indicator will change colour dramatically over a small pH range.
• If the small pH range includes the range of the equivalence point, it’s a great indicator for the reaction.

### Examples of good indicators

• The two main indicators are phenolphthalein and methyl orange.
• Phenolphthalein changes from colourless to pink from pH 8.3 to pH 10.
• Methyl orange changes colour from red to yellow between pH 3.1 and pH 4.4.
• Either of these are well suited for a strong acid/base titration.

## Calculations of Titrations

We have seen before how to carry out titration calculations using titre values and balanced equations. We shall recap these calculations.

### Information provided

• Before starting your calculation you will be given:
• The balanced equation for the reaction.
• The volume and concentration of the titre.
• The volume of the other reactant.
• You will, most often, be asked to find the concentration of this other reactant.

### First step

• Calculate the moles of the titre using the two pieces of information we know about the titre: the volume and the concentration. Use the equation:
• Moles = volume x concentration

### Second step

• Find the number of moles of the other component:
• A titration is stopped at the exact point of neutralisation so we can say that the mole ratio will be the same as in the balanced equation.
• Examples:
• If the mole ratio (from equation) is 1:1, the component will have the same number of moles as the titre.
• If the mole ratio (from equation) is 2:1, the component will have the twice as many moles as the titre.

### Third step

• Find the concentration of the other component:
• We can rearrange the equation:
• Moles = concentration x volume
• To get:
• Concentration = moles ÷ volume

### Units

• Units are very important in titration calculations!
• Moles → mol
• Volume → dm3
• Concentration → moldm−3
• Forgetting to convert the volume from cm3 to dm3 is where most people make mistakes!
• 1 cm3 = 0.001 dm3