Gravimetric and classical methods

Why it matters

Before instruments, chemists measured composition with a balance. Gravimetric analysis is still used where accuracy and traceability matter most: sulfate in cement and water, loss on drying and residue on ignition in pharmaceuticals, ash and moisture in foods, and reference measurements that calibrate other methods. Because the result depends on masses and molar masses, a good gravimetric method needs no calibration curve. Classical methods also teach the habits (constant mass, blanks, stoichiometric factors) that every instrumental method relies on.

Safety and scope. Acids, bases, silver salts and other reagents in this lesson are hazardous. Bench work happens only in a teaching or employer lab, with your instructor or supervisor, after the lab's chemical-hygiene training (OSHA laboratories, checked on 2026-10-23). The activity is a data exercise: never attempt these procedures at home.

The core idea

In precipitation gravimetry, the analyte is converted into a sparingly soluble compound of known composition, which is filtered, washed, dried or ignited, and weighed:

1. Dissolve a weighed sample.
2. Add a precipitating reagent in slight excess (for example silver nitrate to precipitate chloride as AgCl).
3. Digest the precipitate (keep it warm) so particles grow larger and purer.
4. Filter through a weighed crucible, wash away soluble impurities.
5. Dry or ignite to constant mass: weigh, heat again, weigh again until two weighings agree within a set tolerance (often 0.3 mg).

The result uses a gravimetric factor (GF) that converts the mass of precipitate into the mass of analyte:

GF = (molar mass of analyte x a) / (molar mass of precipitate x b)
% analyte = mass of precipitate x GF / mass of sample x 100

where a and b balance the atoms of interest (for chloride as AgCl, a = b = 1).

Worked example: chloride as AgCl

A 0.5012 g sample gives 0.2866 g of AgCl.

• GF = 35.45 / 143.32 = 0.2474
• Mass of Cl = 0.2866 x 0.2474 = 0.07090 g
• % Cl = 0.07090 / 0.5012 x 100 = 14.14 %

Worked example: a factor with a ratio

Iron is often weighed as Fe2O3 after ignition. Each Fe2O3 contains two iron atoms:

• GF = (2 x 55.845) / 159.69 = 0.6994
• If 0.3510 g of Fe2O3 comes from a 1.2000 g ore sample: % Fe = 0.3510 x 0.6994 / 1.2000 x 100 = 20.46 %

Forgetting the 2 would halve the answer.

Other classical methods

• Volatilization gravimetry: measure mass lost or gained on heating. Loss on drying gives moisture; residue on ignition (sulfated ash) gives inorganic content in drug substances.
• Kjeldahl nitrogen: digest a sample in sulfuric acid to convert organic nitrogen to ammonium, distill the ammonia, and titrate it. Protein is estimated as nitrogen x a conversion factor (6.25 for many foods). It is classical chemistry, but it is still a regulatory reference method.
• Electrogravimetry: deposit a metal onto a weighed electrode and weigh it.

Sources of error

• Solubility losses: some precipitate dissolves, especially with too much wash water. Washing with a dilute electrolyte reduces this.
• Co-precipitation: impurities trapped in or on the precipitate. Digestion and reprecipitation help.
• Incomplete drying or hygroscopic precipitates gaining water from the air; cool in a desiccator before weighing.
• Light sensitivity: AgCl decomposes slowly in light, so it is kept in subdued light.
• Balance technique: weigh warm crucibles and convection currents give false readings. Always let them cool to room temperature.

A reagent blank carried through the whole procedure corrects for any precipitate formed from impurities in the reagents.

Common mistakes

• Using the wrong atoms ratio in the gravimetric factor.
• Weighing once and assuming constant mass.
• Reporting more significant figures than the balance or sample mass supports.
• Skipping the blank.

Quick recap

• Precipitate, digest, filter, wash, dry or ignite to constant mass, weigh.
• GF = (M analyte x a) / (M precipitate x b); % analyte = mass precipitate x GF / sample mass x 100.
• Loss on drying, residue on ignition and Kjeldahl nitrogen are classical methods still used in regulated labs.
• Control solubility, co-precipitation, moisture and blanks.

Try it

Open the Do tab. A Python calculator is written except for the percent-analyte formula.