2.5.5
pH Curves & Titrations (A2 Only)
Key Titration Techniques
Key Titration Techniques
Here's a quick recap of key titration information.
Glassware
Glassware
- Above are labelled images of a burette and a pipette.
Technique
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
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
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
Strong Acid/Base Titrations
Titrations of a strong acid with a strong base are the simplest to understand.
pH curve
pH curve
- The pH curve pictured shows the change in pH when a strong acid is titrated with a strong base.
The beginning
The beginning
- In the beginning, you can see that the pH is very low, and doesn’t change very quickly.
The middle
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 end
- The curve ends at a very high pH because the solution has excess base present.
Indicator
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
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
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
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
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
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
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
- 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
1Physical Chemistry
1.1Atomic Structure
1.1.1Fundamental Particles
1.1.2Isotopes & Mass Number
1.1.3Mass Spectrometry
1.1.4Electron Shells, Sub-Shells & Orbitals
1.1.5Electron Configuration
1.1.6Ionisation Energy
1.1.7Factors Affecting Ionisation Energies
1.1.8Trends of Ionisation
1.1.9Specific Impacts on Ionisation Energies
1.1.10End of Topic Test - Atomic Structure
1.1.11A-A* (AO3/4) - Atomic Structure
1.2Amount of Substance
1.3Bonding
1.3.1Ionic Bonding
1.3.2Covalent & Dative Bonding
1.3.3Carbon Structures
1.3.4Metallic Bonding
1.3.5Physical Properties
1.3.6Shapes of Molecules
1.3.7Polarity
1.3.8Intermolecular Forces
1.3.9Intermolecular Forces 2
1.3.10End of Topic Test - Bonding
1.3.11Exam-Style Question - Shape of Molecules
1.3.12A-A* (AO3/4) - Bonding
1.4Energetics
1.5Kinetics
1.6Equilibria
2Physical Chemistry 2 (A2 Only)
2.1Thermodynamics (A2 Only)
2.2Rate Equations (A2 Only)
2.3The Equilibrium Constant Kp (A2 Only)
2.4Electrochemical Cells (A2 Only)
2.5Acids & Bases (A2 Only)
2.5.1Brønsted-Lowry Acids & Bases (A2 Only)
2.5.2pH (A2 Only)
2.5.3The Ionic Product of Water (A2 Only)
2.5.4Weak Acids & Bases (A2 Only)
2.5.5pH Curves & Titrations (A2 Only)
2.5.6pH Curves & Titrations 2 (A2 Only)
2.5.7Buffer Solutions (A2 Only)
2.5.8End of Topic Test - Acids & Bases
2.5.9Exam-Style Question - Weak Acids
2.5.10A-A* (AO3/4) - Acids & Bases
3Inorganic Chemistry
3.1Periodicity & Trends
4Inorganic Chemistry 2 (A2 Only)
4.1Period 3 (A2 Only)
4.2Transition Metals (A2 Only)
4.2.1General Properties (A2 Only)
4.2.2Substitution Reactions (A2 Only)
4.2.3Shapes of Complex Ions (A2 Only)
4.2.4Colours of Ions (A2 Only)
4.2.5Variable Oxidation States (A2 Only)
4.2.6Titrations (A2 Only)
4.2.7Homogeneous Catalysts (A2 Only)
4.2.8Heterogeneous Catalysts (A2 Only)
4.2.9End of Topic Test - Transition Metals
4.2.10A-A* (AO3/4) - Transition Metals
4.3Reactions of Ions in Aqueous Solutions (A2 Only)
5Organic Chemistry 1
5.1Introduction
5.2Alkanes
5.3Halogenoalkanes
5.4Alkenes
5.5Alcohols
5.6Organic Analysis
5.7A-A* (AO3/4) - Organic 1
6Organic Chemistry 2 (A2 Only)
6.1Optical Isomerism (A2 Only)
6.2Aldehydes & Ketones (A2 Only)
6.3Carboxylic Acids & Esters (A2 Only)
6.4Aromatic Chemistry (A2 Only)
6.5Amines (A2 Only)
6.6Polymers (A2 Only)
6.7Biological Organic (A2 Only)
6.8Organic Synthesis (A2 Only)
6.9NMR Spectroscopy (A2 Only)
6.10Chromatography (A2 Only)
6.11A-A* (AO3/4) - Organic 2
Jump to other topics
1Physical Chemistry
1.1Atomic Structure
1.1.1Fundamental Particles
1.1.2Isotopes & Mass Number
1.1.3Mass Spectrometry
1.1.4Electron Shells, Sub-Shells & Orbitals
1.1.5Electron Configuration
1.1.6Ionisation Energy
1.1.7Factors Affecting Ionisation Energies
1.1.8Trends of Ionisation
1.1.9Specific Impacts on Ionisation Energies
1.1.10End of Topic Test - Atomic Structure
1.1.11A-A* (AO3/4) - Atomic Structure
1.2Amount of Substance
1.3Bonding
1.3.1Ionic Bonding
1.3.2Covalent & Dative Bonding
1.3.3Carbon Structures
1.3.4Metallic Bonding
1.3.5Physical Properties
1.3.6Shapes of Molecules
1.3.7Polarity
1.3.8Intermolecular Forces
1.3.9Intermolecular Forces 2
1.3.10End of Topic Test - Bonding
1.3.11Exam-Style Question - Shape of Molecules
1.3.12A-A* (AO3/4) - Bonding
1.4Energetics
1.5Kinetics
1.6Equilibria
2Physical Chemistry 2 (A2 Only)
2.1Thermodynamics (A2 Only)
2.2Rate Equations (A2 Only)
2.3The Equilibrium Constant Kp (A2 Only)
2.4Electrochemical Cells (A2 Only)
2.5Acids & Bases (A2 Only)
2.5.1Brønsted-Lowry Acids & Bases (A2 Only)
2.5.2pH (A2 Only)
2.5.3The Ionic Product of Water (A2 Only)
2.5.4Weak Acids & Bases (A2 Only)
2.5.5pH Curves & Titrations (A2 Only)
2.5.6pH Curves & Titrations 2 (A2 Only)
2.5.7Buffer Solutions (A2 Only)
2.5.8End of Topic Test - Acids & Bases
2.5.9Exam-Style Question - Weak Acids
2.5.10A-A* (AO3/4) - Acids & Bases
3Inorganic Chemistry
3.1Periodicity & Trends
4Inorganic Chemistry 2 (A2 Only)
4.1Period 3 (A2 Only)
4.2Transition Metals (A2 Only)
4.2.1General Properties (A2 Only)
4.2.2Substitution Reactions (A2 Only)
4.2.3Shapes of Complex Ions (A2 Only)
4.2.4Colours of Ions (A2 Only)
4.2.5Variable Oxidation States (A2 Only)
4.2.6Titrations (A2 Only)
4.2.7Homogeneous Catalysts (A2 Only)
4.2.8Heterogeneous Catalysts (A2 Only)
4.2.9End of Topic Test - Transition Metals
4.2.10A-A* (AO3/4) - Transition Metals
4.3Reactions of Ions in Aqueous Solutions (A2 Only)
5Organic Chemistry 1
5.1Introduction
5.2Alkanes
5.3Halogenoalkanes
5.4Alkenes
5.5Alcohols
5.6Organic Analysis
5.7A-A* (AO3/4) - Organic 1
6Organic Chemistry 2 (A2 Only)
6.1Optical Isomerism (A2 Only)
6.2Aldehydes & Ketones (A2 Only)
6.3Carboxylic Acids & Esters (A2 Only)
6.4Aromatic Chemistry (A2 Only)
6.5Amines (A2 Only)
6.6Polymers (A2 Only)
6.7Biological Organic (A2 Only)
6.8Organic Synthesis (A2 Only)
6.9NMR Spectroscopy (A2 Only)
6.10Chromatography (A2 Only)
6.11A-A* (AO3/4) - Organic 2
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