Page 263 - Modern Analytical Chemistry

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246 Modern Analytical Chemistry

An additional problem is encountered when the isolated solid is non-
stoichiometric. For example, precipitating Mn 2+ as Mn(OH) 2 , followed by heating
to produce the oxide, frequently produces a solid with a stoichiometry of MnO x ,
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 Mg 2+ in water and waste-
6H
water by precipitating MgNH 4 PO 4× 2 O and isolating Mg 2 P 2 O 7 provides an in-
structive example of a typical procedure. 2+ 7

Representative Methods Description of Method. Magnesium is precipitated as MgNH 4 PO 4× 2 O using
Method 8.1
Determination of Mg in Water and Wastewater
6H
(NH 4 ) 2 HPO 4 as the precipitant. The precipitate’s solubility in neutral solutions
(0.0065 g/100 mL in pure water at 10 °C) is relatively high, but it is much less soluble
in the presence of dilute ammonia (0.0003 g/100 mL in 0.6 M NH 3 ). The precipitant is
2+
from potential interferents is
not very selective, so a preliminary separation of Mg
necessary. Calcium, which is the most significant interferent, is usually removed by
its prior precipitation as the oxalate. The presence of excess ammonium salts from
the precipitant or the addition of too much ammonia can lead to the formation of
Mg(NH 4 ) 4 (PO 4 ) 2 , which is subsequently isolated as Mg(PO 3 ) 2 after drying. The
precipitate is isolated by filtration using a rinse solution of dilute ammonia. After
filtering, the precipitate is converted to Mg 2 P 2 O 7 and weighed.

2+
Procedure.
Transfer a sample containing no more than 60 mg of Mg
into a
600-mL beaker. Add 2–3 drops of methyl red indicator, and, if necessary, adjust the
volume to 150 mL. Acidify the solution with 6 M HCl, and add 10 mL of 30% w/v
(NH 4 ) 2 HPO 4 . After cooling, add concentrated NH 3 dropwise, and while constantly
stirring, until the methyl red indicator turns yellow (pH > 6.3). After stirring for
5 min, add 5 mL of concentrated NH 3 , and continue stirring for an additional 10 min.
Allow the resulting solution and precipitate to stand overnight. Isolate the
precipitate by filtration, rinsing with 5% v/v NH 3 . Dissolve the precipitate in 50 mL
of 10% v/v HCl, and precipitate a second time following the same procedure. After
filtering, carefully remove the filter paper by charring. Heat the precipitate at 500 °C
until the residue is white, and then bring the precipitate to constant weight at
1100 °C.

Questions
1. Why does the procedure call for a sample containing no more than 60 mg of
2+
Mg ?
A sample containing 60 mg of Mg 2+ will generate approximately 600 mg, or
0.6 g, of MgNH 4 PO 4× 2 O. This is a substantial amount of precipitate to work
6H
with during the filtration step. Large quantities of precipitate may be difficult
to filter and difficult to adequately rinse free of impurities.
—Continued