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An additional problem is encountered when the isolated solid is non-
stoichiometric. For example, precipitating Mn2+ as Mn(OH)2, followed by heating
to produce the oxide, frequently produces a solid with a stoichiometry of MnOx,
where x varies between 1 and 2. In this case the nonstoichiometric product results
from the formation of a mixture of several oxides that differ in the oxidation state
of manganese. Other nonstoichiometric compounds form as a result of lattice de-
fects in the crystal structure.6
Representative Method The best way to appreciate the importance of the theoreti-
cal and practical details discussed in the previous section is to carefully examine the
procedure for a typical precipitation gravimetric method. Although each method
has its own unique considerations, the determination of Mg2+ in water and waste-
water by precipitating MgNH4PO4 ⋅ 6H2O an