5.16 : Gravimetry: Inorganic And Organic Precipitating Agents

In gravimetry, the precipitant is chosen carefully to obtain a pure solid that can be easily filtered. Common inorganic precipitants can be used to determine several cations and anions. In some cases, the formation of the same precipitate can be used to determine the cation and the anion. For example, the reaction of barium and chromate ions to give barium chromate is used to determine both barium and chromate. However, precipitates such as hydroxides, oxalates, and metal ammonium phosphates are first converted to a weighable form. Precipitation methods can also be applied to determine organic functional groups such as organic halides, carbonyl, alkoxy groups, aromatic nitro, azo, and phosphate.

Organic precipitants are usually more selective than their inorganic counterparts and yield sparingly soluble precipitates with high molecular masses. A small number of analyte ions will yield a large amount of precipitate. For example, sodium tetraphenylborate is a near-specific precipitant for potassium and ammonium ions, yielding ionic precipitates. Several organic precipitants contain multiple functional groups that can bond with the cation to generate five- or six-membered rings called chelates. Typical chelating agents include 8-hydroxyquinoline and cupferron.