Acids, Bases & Electrochemistry
Volumetric Analysis: Acid-Base Titrations
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to define key terms associated with volumetric analysis, including titration, standard solution, primary standard, equivalence point, and end point.
- ✓By the end of this lesson students will be able to describe the experimental procedure for carrying out an acid-base titration, including the correct use and rinsing of apparatus.
- ✓By the end of this lesson students will be able to explain the importance of using a primary standard and the process of standardising a secondary standard solution.
- ✓By the end of this lesson students will be able to select an appropriate indicator for a given acid-base titration.
- ✓By the end of this lesson students will be able to perform calculations to determine the concentration of an unknown solution from titration data, including those involving stoichiometry.
Key concepts
Volumetric analysis is a quantitative analytical method that involves determining the concentration of a substance by accurately measuring the volume of a solution of known concentration required to react completely with it.
Titration is a laboratory technique in volumetric analysis where a solution of accurately known concentration (the titrant) is gradually added from a burette to a solution of unknown concentration (the analyte) in a conical flask until the chemical reaction between them is complete.
A standard solution is a solution whose concentration is accurately known. It is essential for volumetric analysis.
A primary standard is a substance that is used to prepare a standard solution directly by weighing a precise amount and dissolving it in a known volume of solvent. Key characteristics include high purity, stability (does not react with air or absorb moisture), high molar mass (to minimise weighing errors), solubility in water, and non-toxicity. Examples include anhydrous sodium carbonate (Na₂CO₃) for standardising acids and hydrated oxalic acid (H₂C₂O₄·2H₂O) for standardising bases.
A secondary standard is a solution whose concentration is not accurately known when prepared, and therefore must be determined by titration against a primary standard solution. Examples include hydrochloric acid (HCl) and sodium hydroxide (NaOH) solutions, as they are hygroscopic or volatile and cannot be weighed accurately to prepare a primary standard.
Standardisation is the process of accurately determining the concentration of a secondary standard solution by titrating it against a primary standard solution.
The equivalence point (or stoichiometric point) is the theoretical point in a titration where the amount of titrant added is chemically equivalent to the amount of analyte in the sample, meaning the reaction is complete according to the balanced chemical equation. The end point is the point at which the indicator changes colour, signalling the completion of the reaction. An appropriate indicator is chosen so that its end point is as close as possible to the equivalence point.
An indicator is a substance, usually a weak organic acid or base, that changes colour over a specific pH range. The choice of indicator depends on the pH at the equivalence point of the titration. For strong acid/strong base titrations, methyl orange or phenolphthalein can be used (equivalence point at pH 7). For strong acid/weak base titrations, methyl orange is suitable (equivalence point at pH < 7). For weak acid/strong base titrations, phenolphthalein is suitable (equivalence point at pH > 7).
Concordant results are a set of titre values that are in close agreement with each other, typically within 0.1 cm³. The average of these concordant titres is used in calculations to ensure accuracy.
Molarity (concentration in mol L⁻¹) is a measure of the concentration of a solution. Calculations in volumetric analysis often involve using the formula n = cV (moles = concentration × volume) and applying stoichiometry from a balanced chemical equation.
Key facts to remember
- 1Volumetric analysis is a quantitative method based on accurate volume measurements.
- 2A primary standard must be pure, stable, have a high molar mass, and be soluble. Anhydrous sodium carbonate is a common primary standard for acids.
- 3Secondary standards (like HCl and NaOH) must be standardised against a primary standard.
- 4The equivalence point is the theoretical point of complete reaction; the end point is where the indicator changes colour.
- 5For accurate results, all glassware (burette, pipette, volumetric flask) must be rinsed correctly with the appropriate solutions.
- 6Concordant titre values (within 0.1 cm³) are essential for calculating a reliable average titre.
- 7The formula n = cV (moles = concentration × volume) is fundamental for all titration calculations.
- 8Always write a balanced chemical equation to determine the mole ratio between reactants.
Worked examples
Example 1
A 0.050 M solution of anhydrous sodium carbonate (Na₂CO₃) was prepared. 25.0 cm³ portions of this solution were titrated with a hydrochloric acid (HCl) solution. The average titre required was 20.0 cm³. Calculate the concentration of the HCl solution.
Answer
The concentration of the HCl solution is 0.125 mol L⁻¹.
Remember to convert volumes from cm³ to L by dividing by 1000 before using them in the n=cV formula.
Example 2
25.0 cm³ portions of a sodium hydroxide (NaOH) solution were titrated with the standardised hydrochloric acid (HCl) solution from the previous example (assume its concentration is 0.125 mol L⁻¹). The average titre required was 22.5 cm³. Calculate the concentration of the NaOH solution.
Answer
The concentration of the NaOH solution is 0.113 mol L⁻¹.
Always ensure your final answer is given to an appropriate number of significant figures, usually three for Leaving Cert Chemistry calculations.
Common mistakes
- ✗Failing to rinse the burette, pipette, and conical flask correctly, leading to inaccurate concentrations.
- ✗Not reading the burette at eye level, causing parallax error in volume measurements.
- ✗Using the wrong indicator for a specific acid-base titration, resulting in an inaccurate end point.
- ✗Including non-concordant titres when calculating the average titre, which compromises accuracy.
- ✗Incorrectly applying the stoichiometric ratio from the balanced equation in calculations.
Exam tips
- ★Always start by writing the balanced chemical equation for the reaction; this is crucial for determining the mole ratio.
- ★Show all steps in your calculations clearly, including units, as partial marks are awarded for correct working.
- ★Pay close attention to significant figures in your final answer, typically three significant figures for Leaving Cert calculations.
- ★Be familiar with the practical steps and safety precautions for performing a titration, as these are often examined in theory questions.
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