limited number of purposes, such as the determination of halides, these methods are invaluable. Silver
nitrate is the most widely used reagent giving rise to silver salts which are characteristically sparingly
soluble.
The end points of precipitation titrations can be variously detected. An indicator exhibiting a
pronounced colour change with the first excess of the titrant may be used. The Mohr method, involving
the formation of red silver chromate with the appearance of an excess of silver ions, is an important
example of this procedure, whilst the Volhard method, which uses the ferric thiocyanate colour as an
indication of the presence of excess thiocyanate ions, is another. A series of indicators known as
adsorption indicators have also been utilized. These consist of organic dyes such as fluorescein which
are used in silver nitrate titrations. When the equivalence point is passed the excess silver ions are
adsorbed on the precipitate to give a positively charged surface which attracts and adsorbs
fluoresceinate ions. This adsorption is accompanied by the appearance of a red colour on the precipitate
surface. Finally, the electroanalytical methods described in Chapter 6 may be used to scan the solution
for metal ions. Table 5.12 includes some examples of substances determined by silver titrations and
Table 5.13 some miscellaneous precipitation methods. Other examples have already been mentioned
under complexometric titrations.
Table 5.12 Substances determined by precipitation titrations with Ag+Table 5.13 Miscellaneous precipitation titrations
Analyte Reagent Precipitate
Cl–, Br– Hg 2 (NO 3 ) 2 Hg 2 Cl 2 , Hg 2 Br 2Pb(NO 3 ) 2 PbSO 4 , PbMoO 4
Zn2+ K 4 Fe(CN) 6 K 2 Zn 3 [Fe(CN) 6 ] 2Pb(OAc) 2 Pb 3 (PO 4 ) 2 , PbC 2 O 45.2—
Gravimetry
Summary
Principles
Solution reaction between analytes and reagents to give sparingly soluble products; filtration, drying or
ignition of precipitates; electrolytic deposition of metals; weighing.