Acid–base titrations
The Purpose of Titration
Titration is a precise experimental technique used to determine the unknown concentration of an acid or alkali (base) by reacting it with a solution of known concentration. This known solution is called the standard solution. The goal is to find the exact volume of one solution required to completely neutralise a specific volume of the other.
This process relies on the stoichiometry of the neutralisation reaction. For example, in the reaction between hydrochloric acid and sodium hydroxide: HCl + NaOH \rightarrow NaCl + H_2O. One mole of acid reacts exactly with one mole of alkali. By knowing the volume and concentration of the standard solution used, we can calculate the unknown concentration.
Apparatus Roles in Titration
To perform a titration accurately, specific glassware is used for distinct roles. Understanding why each piece of apparatus is chosen is critical for exam questions.
| Why it is used (Reasoning) |
|---|
| It has fine graduations allowing for precise measurement of variable volumes. It allows control over the flow rate, especially near the end-point. |
| Unlike a measuring cylinder, a volumetric pipette is calibrated to deliver one specific volume (e.g., 25.0 \text{ cm}^3) with very high accuracy and low uncertainty. |
| The conical shape allows the contents to be swirled easily without splashing liquid out, ensuring thorough mixing of the acid and alkali. |
| It provides a white background that makes the colour change of the indicator much easier to see clearly. |
Suitable Indicator and End-Point
An indicator is a weak acid or base that changes colour depending on the pH of the solution. In a titration, we must choose an indicator that changes colour sharply at the pH corresponding to the equivalence point (where moles of acid = moles of base).
How to use the indicator: Add 2–3 drops of a suitable indicator (e.g., methyl orange or thymolphthalein) to the solution in the conical flask before starting the titration. Do not add it to the burette.
Identifying the end-point: The end-point is the point at which the indicator changes colour permanently. This visual signal tells you to stop adding the titrant from the burette. Ideally, the end-point coincides with the equivalence point.
Why not Universal Indicator? Universal indicator is not suitable for titrations because it changes colour gradually over a wide pH range (many different colours). This makes it impossible to identify a sharp, distinct end-point. A suitable indicator must have a sharp colour change.
Typical Titration Procedure
Consider titrating dilute hydrochloric acid (unknown concentration) against aqueous sodium hydroxide (known concentration, 0.1 \text{ mol dm}^{-3}).
| Action |
|---|
| Use a volumetric pipette to transfer exactly 25.0 \text{ cm}^3 of the aqueous sodium hydroxide into a clean conical flask. |
| Place the conical flask on a white tile on the bench. |
| Add 2–3 drops of methyl orange indicator to the alkali in the flask. (The solution will turn yellow). |
| Fill a clean burette with the dilute hydrochloric acid. Record the initial burette reading. |
| Slowly add the acid from the burette to the flask while swirling the flask continuously. |
| As the colour begins to change, slow down to adding drop-by-drop. Stop when the colour changes permanently (e.g., from yellow to orange for methyl orange). This is the end-point. |
| Record the final burette reading. The titre volume is \text{Final Reading} - \text{Initial Reading}. |
⚠︎ Apparatus Confusion and Indicator Choice
Mistake 1: Using a measuring cylinder for the fixed volume. Students often think any glassware can measure volume. However, for titrations, the initial volume of the analyte must be extremely precise. A volumetric pipette is required because it has a much lower percentage uncertainty than a measuring cylinder.
Mistake 2: Rinsing the conical flask with the solution being titrated. You should never rinse the conical flask with the alkali (or acid) you are about to put in it. If you do, extra drops of the solution will remain on the walls, increasing the number of moles present. This leads to a larger titre volume and an inaccurate result.
Mistake 3: Using Universal Indicator. Examiners frequently ask why universal indicator is not used. The correct answer is that it shows too many colour changes (gradual change), making the end-point indistinct. You must specify an indicator with a sharp change, like methyl orange or phenolphthalein.
Describing the End-Point and Colour Changes
When to use this tip: When asked to describe how to identify the end-point or name a suitable indicator.
Examiner Acceptance: Examiners look for specific keywords regarding the nature of the colour change. Do not just say 'the colour changes.' You must state that the change is sharp or permanent. For example, if using methyl orange in an acid titrated into alkali, the correct description is 'yellow to orange' (or red). If using thymolphthalein in an alkali titrated into acid, the change is 'colourless to blue'.
Why this matters: The phrase 'sharp colour change' directly addresses the requirement for precision. A gradual change implies uncertainty about exactly when the reaction is complete. Stating the specific colours (e.g., 'yellow to orange') demonstrates you know the specific indicator's properties.
Example Answer: 'Add methyl orange indicator. The end-point is reached when the solution changes colour sharply from yellow to orange.'
Explaining the Use of a White Tile
When to use this tip: When asked why a white tile is placed under the conical flask.
Examiner Acceptance: The key concept is contrast. You must explain that the white background makes the colour change easier to see or more distinct.
Why this matters: Titration relies on visual detection. If the colour change is subtle, a dark bench might hide it, leading to overshooting the end-point. The white tile ensures maximum visibility of the transition.
Example Answer: 'The white tile provides a contrasting background so that the colour change of the indicator is easier to see clearly.'
Past Paper Style Questions
Q:
Describe how you would use a burette and a volumetric pipette in an acid-base titration.
A:
The volumetric pipette is used to measure a precise, fixed volume of the solution (analyte) into the conical flask. The burette is used to deliver the other solution (titrant) drop-by-drop until the end-point is reached.
Q:
Explain why universal indicator is not suitable for use in a titration.
A:
Universal indicator undergoes a gradual colour change over a wide pH range (showing many colours). A titration requires an indicator with a sharp, distinct colour change to accurately identify the end-point.
Q:
State one reason why the conical flask should not be rinsed with the sodium hydroxide solution before the titration.
A:
Rinsing with the solution would leave extra drops of alkali in the flask, increasing the volume/moles of alkali. This would result in a larger titre (volume of acid) being used, leading to an inaccurate calculation.
Q:
Name a suitable indicator for the titration of hydrochloric acid with sodium hydroxide and state the colour change at the end-point.
A:
Methyl orange. Colour change: Yellow to Orange (when acid is added to alkali) or Red to Yellow (when alkali is added to acid). Note: Phenolphthalein (Colourless to Pink) is also acceptable.
Q:
Why is a white tile placed under the conical flask during a titration?
A:
To provide a white background that makes the colour change of the indicator easier to see clearly.