Standard solution

Malay: Larutan piawai · Chinese: 标准溶液

A standard solution is a solution whose concentration is accurately known, prepared using a volumetric flask. The phrase “accurately known” is the heart of the definition: a standard solution is not just any solution, but one whose molarity you can trust to enough precision to use it in a titration. It is the reference against which an unknown solution is measured.

A worked example gives the method. To make 250 cm³ of a 0.10 mol dm⁻³ sodium carbonate solution, you calculate and weigh the exact mass needed, dissolve it fully in a beaker with a little distilled water, transfer every drop into a 250 cm³ volumetric flask (rinsing the beaker so none is lost), and top up carefully to the calibration mark with distilled water until the bottom of the meniscus sits on the line. Stopper and invert to mix. Because the mass and the final volume are both exact, the molarity is exact.

The confusion to avoid is thinking any dissolved solid gives a standard solution. It does not: making it up in a beaker or a measuring cylinder gives only an approximate volume, so the concentration is not accurately known and the solution is not “standard”. The volumetric flask, with its single calibration mark for one exact volume, is what makes the difference. A second slip is overshooting the mark with water, which dilutes the solution below the intended molarity and cannot be undone.

Within this chapter the standard solution is what a titration needs on one side to find the concentration on the other, and it draws directly on molarity and the mole. Our teachers take you through one full preparation so each step, especially the rinsing and the meniscus, becomes second nature.