Table 5.18 Some homogeneous precipitation processesminimum solubility. The presence of ammonium salts and hydrogen ions increases the solubility so that
the precipitant (NH 4 ) 3 PO 4 must be added in only small excess, and the pH must be maintained high (pH
= 11–12). Supersaturation is an additional problem, and quantitative precipitation is only achieved after
the mixture has been allowed to stand for an extended time, e.g. overnight.
(c)—
Precipitate Purity
H 3 PO 4 , NH 4 H 2 PO 4 , (NH 4 ) 2 HPO 4 , (NH 3 ) 3 PO 4 , Mg(H 2 PO 4 ) 2 , MgHPO 4 , Mg 3 (PO 4 ) 2 , basic Mg phosphates,
Mg(OH) 2 , MgCl 2 and NH 4 Cl are all possible contaminants. Of these only NH 4 Cl which is volatile and
MgHPO 4 which ignites to Mg 2 P 2 O 7 are of no consequence. Amongst the rest are a number of
compounds (including the precipitant itself) which have crystallinity similar to the precipitate, and are
coprecipitated by isomorphic inclusion. The magnitude of the coprecipitation problem is such that, to
overcome it, dissolution and reprecipitation of the initial precipitate is required. This is done by using
the minimum amount of dilute hydrochloric acid and a very small amount of ammonium phosphate.
Concentrated ammonia solution is then added to complete the reprecipitation.
The use of an organic precipitant may be exemplified by reference to the employment of 8-
hydroxyquinoline (Oxine), to determine aluminium. Aluminium oxinate, Al(C 9 H 6 NO) 3 , can be
quantitatively precipitated from