pH Curves & Titrations (A2 Only)
Chemistry: AQA A Level, topic 2.5.5
Key Titration Techniques
Here's a quick recap of key titration information.
Glassware
- Above are labelled images of a burette and a pipette.
%204.2.5.1%20-%20Titration%20apparatus%20FERTILISERS,w_800.png)
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.
,w_800.jpg)
pH curve
- The pH curve pictured shows the change in pH when a strong acid is titrated with a strong base.
,w_800.jpg)
The beginning
- In the beginning, you can see that the pH is very low, and doesn’t change very quickly.
,w_800.jpg)
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.
,w_800.jpg)
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.
%203.2.5.2%20-%20moles,%20concentration%20and%20volume%20equation%20triangle,w_800.png)
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
Test yourself on pH Curves & Titrations (A2 Only)
Which of the following is the definition of concordant results in titrations?
What do you need to use to find the equivalence point?
A basic solution would turn phenolphthalein what colour?