CIE iGCSE Co-ordinated Sciences-C12.2 Acid-base titrations- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C12.2 Acid-base titrations – Study Notes
CIE iGCSE Co-ordinated Sciences-C12.2 Acid-base titrations – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe an acid–base titration to include the use of a:
(a) burette
(b) volumetric pipette
(c) suitable indicator - Describe how to identify the end-point of a titration using an indicator
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Acid–Base Titration
An acid–base titration is an experimental technique used to determine the concentration of an acid or alkali by reacting it with a base or acid of known concentration, using an indicator to show the endpoint.
(a) Burette
- A burette is a long, graduated glass tube with a tap at the bottom.
- It is used to deliver variable, precise volumes of the solution of known concentration (the titrant).
- In a typical titration, the burette is filled with the acid or the alkali of known concentration.
(b) Volumetric Pipette
- A volumetric pipette is used to measure and transfer a fixed, accurate volume of the solution of unknown concentration into a conical flask (e.g. 25.0 cm³).
- A pipette filler is used to safely draw up the liquid.
(c) Suitable Indicator
- An indicator shows when neutralisation has occurred (the endpoint).
- Common indicators:
- Phenolphthalein: Colourless in acid, pink in alkali.
- Methyl orange: Red in acid, yellow in alkali, orange at endpoint.
- Universal indicator is not used because it has too many colour changes, making the endpoint unclear.
Procedure:
- Use a volumetric pipette to measure 25.0 cm³ of alkali and transfer it to a conical flask.
- Add a few drops of a suitable indicator (e.g. phenolphthalein or methyl orange).
- Fill the burette with acid of known concentration, making sure the meniscus is read at eye level.
- Slowly add the acid from the burette to the alkali, swirling the flask continuously.
- Stop when the indicator changes colour sharply (endpoint).
- Repeat the titration several times until concordant results (within 0.1 cm³) are obtained.
Key Idea: The volume of acid (from the burette) required to neutralise the known volume of alkali (from the pipette) is used to calculate the concentration of the unknown solution.
Example :
25.0 cm³ of sodium hydroxide solution was titrated with hydrochloric acid of concentration 0.100 mol/dm³. The mean titre was 24.8 cm³. Calculate the concentration of the sodium hydroxide solution.
▶️ Answer/Explanation
Step 1: Balanced equation: \(\mathrm{HCl + NaOH \;\;\rightarrow\;\; NaCl + H_2O}\).
Step 2: Moles of \(\mathrm{HCl} = \dfrac{0.100 \times 24.8}{1000} = 0.00248 \, \text{mol}\).
Step 3: Ratio HCl : NaOH = 1 : 1 → moles of NaOH = 0.00248 mol.
Step 4: Concentration of NaOH = \(\dfrac{0.00248}{25.0/1000} = 0.0992 \, \text{mol/dm}^3\).
Final Answer: The concentration of NaOH = \(0.099 \, \text{mol/dm}^3\) (3 s.f.).
Identifying the End-Point of a Titration
The end-point of a titration is the stage at which the reaction between the acid and the base is just complete. It is detected using a suitable indicator that changes colour sharply at neutralisation.
How to Identify the End-Point:
- Add a few drops of a suitable indicator to the solution in the conical flask (e.g. alkali with indicator, acid in burette).
- Swirl the flask while slowly adding the solution from the burette.
- As neutralisation is reached, the indicator will change colour sharply and permanently.
- The exact moment this colour change occurs is the end-point of the titration.
Common Indicators:
- Phenolphthalein: Colourless in acid, pink in alkali → endpoint when the pink colour just disappears.
- Methyl orange: Red in acid, orange at endpoint, yellow in alkali.
Key Idea: The endpoint is shown by a distinct colour change in the indicator, not a gradual one.
Example :
In a titration between hydrochloric acid and sodium hydroxide, phenolphthalein was used as the indicator. Describe how the endpoint is identified.
▶️ Answer/Explanation
Step 1: At the start, the sodium hydroxide solution is alkaline, so phenolphthalein is pink.
Step 2: As hydrochloric acid is added from the burette, the alkali is gradually neutralised.
Step 3: At the endpoint, the pink colour just disappears, leaving the solution colourless.
Final Answer: The endpoint is identified when phenolphthalein changes from pink to colourless.