Titration of Strong Acids and Alkalis

The Titration Procedure

Once the apparatus is prepared, the titration is carried out in a specific sequence to find the end-point of the neutralisation reaction.

1. Transfer the acid: Use the pipette and a safety filler to transfer exactly $25.0\text{ cm}^3$ of the unknown acid into the conical flask. Touch the tip of the pipette against the surface of the liquid to ensure the exact volume is delivered.
2. Add the indicator: Place the flask on a white tile (to see colour changes clearly) and add 2–3 drops of a single indicator, such as phenolphthalein or methyl orange.
3. Prepare the burette: Fill the burette with the known alkali, ensuring the jet space below the tap is full of liquid and free of air bubbles. Remove the funnel so that rogue drops do not fall in during the experiment. Record the initial volume by reading the bottom of the meniscus at eye level.
4. The rough titration: Add the alkali from the burette while constantly swirling the flask until the indicator changes colour. This gives an approximate titre.
5. Accurate titrations: Repeat the entire process. When you get within $2\text{ cm}^3$ to $3\text{ cm}^3$ of your rough titre, add the alkali dropwise while swirling. Record the final burette reading to two decimal places (ending in $.00$ or $.05$).

Choosing the Right Indicator

A single indicator must be used for strong acid/strong alkali titrations because it provides a sharp colour change exactly at the moment of neutralisation. Universal indicator is not suitable because it is a mixture of indicators that changes colour gradually over a wide pH range.

• Phenolphthalein: Colourless in acid, pink in alkali. When adding alkali to acid, the end-point is reached when the solution turns from colourless to a faint, permanent pink.
• Methyl orange: Red in acid, yellow in alkali. When adding alkali to acid, the end-point is the intermediate orange colour.

Concordant Results and Mean Titre

To minimise the effect of random errors, titrations must be repeated until you achieve concordant titres.

• Results are only considered concordant if they are within $0.10\text{ cm}^3$ of each other.
• When calculating the mean titre, you must only average the concordant results.
• The initial "rough" titration and any anomalous results must be excluded from the mean calculation.

Calculating Concentration

Once you have your mean titre, you can calculate the unknown acid concentration.

$\text{Moles (mol)} = \text{Concentration (mol/dm}^3) \times \text{Volume (dm}^3)$

$\text{Concentration (mol/dm}^3) = \frac{\text{Moles (mol)}}{\text{Volume (dm}^3)}$

Important: Always convert volumes from $\text{cm}^3$ to $\text{dm}^3$ by dividing by 1000 before starting.

Worked Example

A student titrates $25.0\text{ cm}^3$ of hydrochloric acid ($\text{HCl}$) of unknown concentration. The acid is neutralised by a mean titre of $18.40\text{ cm}^3$ of $0.150\text{ mol/dm}^3$ sodium hydroxide ($\text{NaOH}$). Calculate the concentration of the acid. (Balanced equation: $\text{HCl} + \text{NaOH} \rightarrow \text{NaCl} + \text{H}_2\text{O}$)

Step 1: Convert all volumes to $\text{dm}^3$.

• $\text{Volume of NaOH} = 18.40 / 1000 = 0.0184\text{ dm}^3$
• $\text{Volume of HCl} = 25.0 / 1000 = 0.0250\text{ dm}^3$

Step 2: Calculate the moles of the known substance ($\text{NaOH}$).

• $\text{Moles of NaOH} = 0.150\text{ mol/dm}^3 \times 0.0184\text{ dm}^3 = 0.00276\text{ mol}$

Step 3: Use the molar ratio to find the moles of the unknown acid ($\text{HCl}$).

• The balanced equation shows a $1:1$ ratio.
• $\text{Moles of HCl} = 0.00276\text{ mol}$

Step 4: Calculate the unknown concentration.

• $\text{Concentration of HCl} = 0.00276\text{ mol} / 0.0250\text{ dm}^3 = 0.1104\text{ mol/dm}^3$
• Final answer: $0.110\text{ mol/dm}^3$ (to 3 significant figures).