Neutralisation (titration) required practical

Aim: To find the volume of a strong acid needed to exactly neutralise a known volume of a strong alkali, and use it to work out an unknown concentration.

Variables

• Dependent: Volume of acid added from the burette to reach the end point
• Control: Volume of alkali used (25.0 cm³ measured with a pipette); Concentration of the alkali and of the acid; Type of indicator and number of drops used; Temperature of the solutions

Equipment

• Burette, clamp and stand
• 25.0 cm³ volumetric pipette and pipette filler
• Conical flask
• Small funnel for filling the burette
• White tile
• Indicator (phenolphthalein or methyl orange)
• Dilute hydrochloric acid (HCl) and sodium hydroxide solution (NaOH)

Method

1. Use a pipette and pipette filler to transfer exactly 25.0 cm³ of sodium hydroxide solution into a clean conical flask.
2. Add a few drops of indicator and place the flask on a white tile under the burette.
3. Rinse the burette with a little of the acid, then fill it using a funnel; remove the funnel and run a little acid through so the tip has no air bubble.
4. Read the starting volume at eye level from the bottom of the meniscus and record it to 2 decimal places.
5. Add acid quickly while swirling the flask to get a rough (range-finder) titre, then note the volume where the colour changed.
6. Repeat with fresh alkali, adding acid quickly until about 2 cm³ from the rough end point, then drop by drop until one drop gives a permanent colour change.
7. Record the final burette reading and calculate the titre (final reading minus initial reading).
8. Repeat until you have at least two concordant titres (within 0.10 cm³ of each other) and calculate their mean.

Safety

• Wear eye protection: sodium hydroxide is corrosive to eyes and skin, and acids are irritants.
• Always use a pipette filler; never pipette by mouth.
• Fill the burette below eye level (for example by lowering it or standing it on the bench) to avoid splashes into the face.
• Mop up any spills straight away and wash skin that is splashed with plenty of water.

Results and calculations

Record initial and final burette readings and the titre for each run in a table, then calculate the mean of the concordant titres (ignore the rough titre). Use moles = concentration (mol/dm³) × volume (dm³), the mole ratio from the balanced equation (for HCl + NaOH → NaCl + H₂O it is 1 : 1), then concentration = moles ÷ volume.

Common mistakes and improvements

• Overshooting the end point by adding acid too fast; slow down to single drops near the end point and swirl continuously.
• Reading the burette from above or below; read at eye level from the bottom of the meniscus.
• Leaving an air bubble in the burette tip or water in the burette, which gives wrong volumes; rinse the burette with the acid first and run some through before starting.
• Including the rough titre or non-concordant titres in the mean; only average titres within 0.10 cm³ of each other.

Exam tips

• Know the indicator colours: phenolphthalein goes from pink in alkali to colourless at the end point; methyl orange is yellow in alkali and red in acid. Do not use universal indicator because its colour changes gradually.
• Explain the white tile (to see the colour change clearly) and swirling (to mix the reactants so the end point is not overshot).
• Show every step in concentration calculations and remember to convert cm³ to dm³ by dividing by 1000.
• Be able to pick concordant results from a table and calculate the mean to 2 decimal places.
• Higher tier may ask you to convert mol/dm³ into g/dm³: multiply by the relative formula mass (M_r).