Acids, Alkalis and Titrations

What a titration is and what it tells you

• A titration is a quantitative procedure used to find how much of one solution exactly reacts with a measured volume of another
• For acid–alkali titrations, the question usually asked is: what volume of acid (of known concentration) neutralises a fixed volume of alkali (of unknown or known concentration)?
• From the titration result you can work out the moles that reacted, the concentration of the unknown solution, or the volume of acid needed in a salt preparation

Method (acid in burette, alkali in flask)

• Pipette a fixed, known volume (e.g. 25.0 cm³) of the alkali into a clean conical flask, using a pipette and pipette filler
• Add 2–3 drops of a suitable indicator (phenolphthalein or methyl orange) to the alkali
• Fill the burette with the acid; record the initial reading by lining up the bottom of the meniscus with the scale, reading to the nearest 0.05 cm³
• Position the conical flask directly under the burette tip, sitting it on a white tile so any colour change shows up clearly against the pale background
• Do one rough run first — open the tap and let the acid run in steadily, swirling the flask, and close the tap as soon as the indicator changes colour. Record the rough titre
• For each accurate run, deliver the acid quickly to about 1 cm³ short of the rough titre, then add it drop by drop with swirling between each addition, stopping the moment the indicator flips colour
• Record the final reading to the nearest 0.05 cm³; the titre is the final reading minus the initial reading
• Repeat until two titres agree to within 0.1 cm³ — these are concordant results — and take their mean as the answer

Sample results

• Runs 1, 2 and 3 are within 0.1 cm³ of each other and are taken as concordant
• Mean titre = (22.10 + 22.15 + 22.15) ÷ 3 = 22.13 cm³
• The rough run is excluded from the mean — it is used only to find the end-point quickly so that the careful runs can be timed

Common reasons accurate runs miss

• Filling the burette while a funnel is still in place — drops from the funnel after the start give a false initial reading
• Air bubble trapped in the burette's jet below the tap — the bubble dislodges during the run and the volume reading is off
• Reading the meniscus from above or below eye level — parallax error of up to 0.1 cm³
• Adding the acid too fast near the end-point — the indicator flips colour past the equivalence point and the titre is too large