Gravimetric and Volumetric Analysis: Principles and Sources of Error

Gravimetric analysis

The analyte is converted quantitatively into a compound of known composition, which is isolated, dried and weighed. The amount of analyte follows from the mass and the stoichiometry.

What the precipitate must be

• Very insoluble, so essentially none is lost to the solution.
• Of known and constant composition after drying.
• Easily filtered, which means coarse crystals rather than a fine gelatinous solid.
• Pure, or at least free of contaminants that cannot be removed by washing.

Precipitating from hot dilute solution with slow addition gives larger crystals with less trapped impurity, so those conditions are specified for a reason rather than by convention.

Volumetric analysis

A solution of accurately known concentration is added until reaction is complete, and the volume required gives the amount of analyte.

Requirements for a usable titration

• The reaction must be fast, so the end point is reached promptly.
• It must be stoichiometric, with a single known equation and no side reactions.
• It must be quantitative — essentially complete at the equivalence point.
• There must be a way to detect the end point sharply.

Primary standards

A primary standard is a substance from which a solution of accurately known concentration can be made by weighing alone. It must be of high purity, stable in air, non-hygroscopic, and preferably of high molar mass so that weighing errors are proportionally small.

Reagents that fail these criteria — those that absorb moisture or react with air — are standardised against a primary standard rather than made up by weighing. Explaining why a particular reagent cannot be a primary standard is a common question.

Sources of error compared

Drying to constant mass is worth emphasising: the precipitate is dried, weighed, dried again and reweighed until successive masses agree. A single weighing does not establish that drying was complete.

Choosing between them

Gravimetric analysis is generally more accurate but slower, and it requires a suitable precipitating reaction. Volumetric analysis is faster and more widely applicable, and its accuracy depends chiefly on the standardisation and the sharpness of the end point.

Both remain reference methods, used to calibrate instrumental techniques rather than being replaced by them — a point worth making when a question asks why classical methods are still taught.

Frequently asked questions

What is digestion and why is it done?

Standing the precipitate in contact with its mother liquor, usually warm. Small crystals dissolve and larger ones grow, giving a purer precipitate that filters more easily.

Why wash a precipitate with a solution containing a common ion?

To suppress its solubility during washing, so less is lost. Washing with pure water dissolves a measurable amount of a sparingly soluble solid.

Why must a substance be non-hygroscopic to be a primary standard?

Because absorbed moisture makes the weighed mass unreliable, so the concentration calculated from it would be wrong.

Why dry a precipitate to constant mass?

Because a single weighing cannot confirm that all solvent has been removed. Agreement between successive weighings is the evidence that drying is complete.