Titrations

To carry out a titration accurately, follow these steps:

1. Use a volumetric pipette and a pipette filler to accurately measure a fixed volume (e.g., $25\text{ cm}^3$) of alkali into a conical flask. Rinse the pipette with the alkali first to prevent dilution, and rinse the flask with distilled water.
2. Add a few drops of a suitable indicator to the flask and place it on a white tile to see the colour change more clearly.
3. Fill a burette with the acid. Ensure the burette is rinsed with the acid first, place it below eye level to fill it safely, ensure the jet space is full (no air bubbles), and remove the funnel so extra drops do not fall in.
4. Record the initial volume, reading from the bottom of the meniscus at eye level.
5. Perform a "rough" titration to find the approximate endpoint (the exact point of neutralisation).
6. Repeat the titration accurately: swirl the flask constantly and add the acid drop-wise near the endpoint to avoid overshooting.
7. Calculate the titre by subtracting the initial reading from the final reading.

Choosing the Right Indicator

Universal indicator produces a beautiful rainbow of colours across the pH scale, but this is exactly why it is useless for a titration. Titrations require an indicator with a sharp colour change to pinpoint the exact moment of neutralisation. Universal indicator has a gradual colour change, making it impossible to identify a precise endpoint.

For AQA GCSE, you must use one of two specific indicators, depending on the acid and alkali being used. Phenolphthalein is pink in an alkali and becomes colourless in acid or neutral solutions. Methyl orange is yellow in an alkali, red in an acid, and reaches a distinct orange colour at the endpoint.

Concordant Results and Calculating the Mean

How do scientists know their results are reliable? By repeating the experiment until the data becomes highly consistent. In a titration, you must repeat the procedure until you achieve concordant results. These are titre volumes that are within $0.10\text{ cm}^3$ of each other.

Results must be recorded to two decimal places, ending in a '0' or '5' (e.g., $23.45\text{ cm}^3$). Obtaining concordant results increases the precision of your data, which improves the accuracy of the final calculation. When calculating your final mean, you must exclude your rough titration and any anomalies.

$$\text{Mean Titre} = \frac{\text{Sum of Concordant Titres}}{\text{Number of Concordant Titres}}$$

Titration Calculations (Higher Tier Only)

Finding the volume is only half the battle; the real goal is using that volume to unlock the mystery concentration. Concentration can be expressed as a molar concentration ($mol/dm^3$) or a mass concentration ($g/dm^3$). You must always convert your volume from $cm^3$ to $dm^3$ by dividing by 1000.

The core formulas required are:

$$n = c \times V$$ $$\text{conc in } g/dm^3 = \text{conc in } mol/dm^3 \times M_r$$

(Where $n$ is moles, $c$ is concentration in $mol/dm^3$, $V$ is volume in $dm^3$, and $M_r$ is the relative formula mass)

Worked Example:

$25.0\text{ cm}^3$ of $0.10\text{ mol/dm}^3$ $NaOH$ reacts with $20.0\text{ cm}^3$ of $H_2SO_4$. Calculate the concentration of $H_2SO_4$ in $g/dm^3$. (The balanced equation is $H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O$ and the $M_r$ of $H_2SO_4$ is $98$)

Step 1: Calculate the moles of the known substance ($NaOH$).

• $V = 25.0 / 1000 = 0.025\text{ dm}^3$
• $n = 0.10 \times 0.025 = 0.0025\text{ mol}$

Step 2: Use the mole ratio from the balanced equation to find the moles of the unknown substance.

• The ratio of $H_2SO_4 : NaOH$ is $1:2$.
• Moles of $H_2SO_4 = 0.0025 / 2 = 0.00125\text{ mol}$

Step 3: Calculate the molar concentration ($mol/dm^3$) of the unknown.

• $V = 20.0 / 1000 = 0.020\text{ dm}^3$
• $c = 0.00125 / 0.020 = 0.0625\text{ mol/dm}^3$

Step 4: Convert the concentration to $g/dm^3$.

• Concentration in $g/dm^3 = 0.0625 \times 98 = 6.13\text{ g/dm}^3$ (to 3 sig figs).