Titration Techniques

Setting Up the Equipment (PAG 6)

Titrations require highly precise glassware to ensure accurate volume measurements.

• First, a volumetric pipette is used to transfer an aliquot (a fixed, highly precise volume, typically $25.0$ cm³) of the unknown solution into a conical flask. The pipette is filled using a safety filler until the bottom of the meniscus rests exactly on the calibration line.
• The pipette must first be rinsed with the solution it will measure, while the flask is rinsed only with distilled water.
• Next, a burette is filled with the standard solution. The burette must be rinsed with the solution it will hold to prevent residual water from diluting the concentration.
• Safety Precaution: When filling the burette, it should be placed below eye level (e.g., by placing the clamp stand on the floor or a low stool). This prevents any acid or alkali from splashing into the face or eyes if a spill occurs.
• The burette is filled using a filter funnel. This funnel must be removed before starting the titration; otherwise, extra drops may fall into the burette during the experiment and falsely increase the reading.
• The jet space (the narrow tip below the tap) must be completely full of liquid. If air bubbles are trapped inside, they will fill during the titration, leading to an inaccurately high reading.

Choosing the Right Indicator

To see exactly when neutralisation is complete, an indicator must be added to the conical flask.

• Only a single-colour indicator is suitable for titrations.
• Universal indicator is unsuitable because it does not have a single, sharp colour change; instead, it produces a gradual colour spectrum over a wide pH range.
• Phenolphthalein is colourless in acid and pink in alkali. The endpoint is reached at the first permanent faint pink colour (if adding alkali to acid) or the first permanent loss of pink (if adding acid to alkali).
• Methyl orange is red in acid and yellow in alkali. Its endpoint is a distinct orange.
• A white tile is always placed underneath the conical flask so that the colour change is clearly visible.

The Step-by-Step Procedure

1. Use a pipette to transfer $25.0$ cm³ of the alkali into a conical flask and add a few drops of phenolphthalein.
2. Fill the burette with acid, ensure the jet space is full, remove the funnel, and record the initial volume to two decimal places.
3. Perform a rough titration by opening the burette tap and adding the acid in small portions. Continuously swirl the conical flask to ensure thorough mixing.
4. Close the tap as soon as the indicator changes from pink to colourless and record the final volume.
5. Repeat the process accurately. As you approach the rough estimate volume, add the acid dropwise.
6. The endpoint is reached when there is a permanent colour change that persists after swirling.
7. Repeat this accurate titration until you achieve at least two concordant results.

Calculating the Mean Titre

• The titre is the exact volume of solution added from the burette to reach the endpoint.
• It is calculated by subtracting the initial reading from the final reading: $\text{Final Volume} - \text{Initial Volume} = \text{Titre}$.
• Burette readings should always be recorded to two decimal places, typically ending in $.00$ or $.05$.
• When calculating the overall mean titre, only use your concordant results and entirely exclude the rough attempt.

Worked Example

A student performs a titration and records the following titres: $24.80$ cm³ (rough), $24.35$ cm³, $24.40$ cm³, and $24.70$ cm³. Calculate the mean titre.

Step 1: Identify the concordant results.

• The concordant results are $24.35$ cm³ and $24.40$ cm³ because they are within $0.10$ cm³ of each other.

Step 2: Calculate the mean of these concordant titres.

• $\frac{24.35 + 24.40}{2} = 24.375$

Step 3: Round to an appropriate number of decimal places (usually 2).

• Mean titre = $24.38$ cm³