Instant Notes: Analytical Chemistry

Section C – Analytical reactions in solution

C8 GRAVIMETRY

Gravimetry Gravimetry is one of the ‘classical’ techniques of analysis, and although less
frequently used now, it is of value when an accurate reference method is
required for comparison with an instrumental technique.
If an element is present in a mixture, for example, silver in a sample of nickel,
one way of separating it is to dissolve the metal completely in a suitable solvent.
In this example, the metal mixture could be dissolved in concentrated nitric acid
and a reagent added that would react with the silver to produce a precipitate,
which for silver might be a sodium chloride solution:

Ag(s) +Ni(s) +HNO 3 (sol) =AgNO 3 (sol) +Ni(NO 3 ) 2 (sol)

AgNO 3 (sol) +NaCl(sol) =AgCl(s) +NaNO 3 (sol)

The silver chloride is precipitated completely, and may be filtered off since both

nickel nitrate and nickel chloride are very soluble in water. The precipitate will

be wet and may contain traces of nickel in solution, so must be thoroughly

washedand dried,as discussed below.

Since weighing may be carried out readily and accurately in almost all labora-

tories, gravimetry is often used as a reference method. Analysis of major compo-

nents of metal samples such as steel, and of minerals and soils may be carried

out by gravimetric methods, but they often involve lengthy separations and are

Key notes

Gravimetry is the analytical technique of obtaining a stable solid

compound, of known stoichiometric composition so that the amount of

an analyte in the sample may be found by weighing.

This involves treatment of an analytical sample, usually in solution, to

obtain a quantitative amount of an insoluble compound of known

composition.

The precipitate must be as pure as possible. Substances that are similar

and might precipitate under the same conditions must be removed, and

the analysis must be carried out so that no impurities are co-precipitated.

If precipitation is carried out from solution, the solid precipitate will have

solvent associated with it. This must be removed. Heating near the

boiling point of the solvent will do this, and further heating may be

needed to obtain a more stable compound whose formula is known.

The procedures of weighing the container initially and with the final

sample are most important.

Related topics Complexation, solubility and Thermogravimetry (G1)

redox equilibria (C6)

Gravimetry

Precipitation

Purification

Drying and heating

Weighing