AS & A-Level Chemistry 11 — AS practical measurement and chemical analysis

Accurate measurements require selecting appropriate apparatus: pipettes, burettes, volumetric flasks, balances, thermometers, gas syringes for gas collection, and stopwatches for timing. Burette readings are recorded to the nearest $0.05 \mathrm{\; cm^3}$, and thermometers with $1 \mathrm{\; ^\circ C}$ divisions are read to the nearest $0.5 \mathrm{\; ^\circ C}$. Zero checks must be performed on balances to avoid systematic errors. When reading volumes, align the bottom of the meniscus with the calibration mark at eye level. Rinsing a burette or pipette with the solution it will contain prevents dilution, whereas a volumetric flask is rinsed only with distilled water.

Preparing a standard solution requires the quantitative transfer of a solute into a volumetric flask, adding washings, and making the volume up to the mark. Solutions can also be prepared by exact dilution of a stock solution. Precision refers to the closeness of repeat readings; accuracy is how close a measurement is to the true value. Finer instrument resolution reduces uncertainty, and taking repeat readings minimises random variation but cannot fix a systematic error.

In titrations, a rough titre establishes the approximate volume. Accurate titrations are performed dropwise near the endpoint until two concordant results (usually within $0.10 \mathrm{\; cm^3}$) are obtained. The mean titre is calculated using only these concordant values. The equivalence point occurs when stoichiometric amounts of reactants combine; the endpoint is when the indicator changes colour. Errors like a wet burette will lower the titrant concentration and artificially increase the titre.