Titrations of acids with bases.

The idea

A titration measures the exact volume of one solution that reacts with a fixed volume of another. A standard solution is made by weighing the solid, dissolving it, transferring the solution and all the washings to a volumetric flask, making up to the graduation mark with distilled water, and inverting the stoppered flask repeatedly to mix. A pipette delivers 25.0 cm³ into a conical flask and the burette delivers the other solution. Rinse the pipette and the burette with the solutions they will contain, so that residual water cannot dilute them, but rinse the conical flask with distilled water only, because rinsing it with acid adds extra moles and raises the titre. Remove the funnel from the burette before starting, since drops falling from it decrease the titre. Repeat until titres agree within 0.10 cm³.

What earns the marks

• Rinsing the pipette with the solution it will deliver, so water cannot dilute it.
• Removing the funnel from the burette before titrating, because drops falling in would decrease the titre.
• Transferring the solution and its washings to a volumetric flask and making up to the graduation mark.
• Inverting the stoppered flask several times to mix.
• Using concordant titres, agreeing within 0.10 cm³, to calculate the mean.

Common mistakes and how to put them right

• Rinsing the conical flask with the acid, which adds extra moles and raises the titre.

• Writing that leaving the funnel in the burette increases the titre, when examiners require 'decreases'.

• Making the standard solution up to the mark in a beaker or a measuring cylinder.

• Including the rough titre in the mean.

• Reading the burette from the top of the meniscus instead of the bottom.

Exam tips

• Distilled water in the conical flask changes nothing, because it does not change the number of moles present; say so explicitly if asked.
• State titres to two decimal places, ending in 0 or 5, because a burette is read to the nearest 0.05 cm³.