Page 264 - Modern Analytical Chemistry

1400-CH08 9/9/99 2:17 PM Page 247

Chapter 8 Gravimetric Methods of Analysis 247

2. Why is the solution acidified with HCl before the precipitant is added?
6H
The HCl is added to ensure that MgNH 4 PO 4× 2 O does not precipitate when
3–
the precipitant is initially added. Because PO 4 is a weak base, the precipitate is
soluble in a strongly acidic solution. If the precipitant is added under neutral or
basic conditions (high RSS), the resulting precipitate will consist of smaller, less
pure particles. Increasing the pH by adding base allows the precipitate of
6H
MgNH 4 PO 4× 2 O to form under more favorable (low RSS) conditions.
3. Why is the acid–base indicator methyl red added to the solution?
The indicator’s color change, which occurs at a pH of approximately 6.3,
indicates when sufficient NH 3 has been added to neutralize the HCl added at
the beginning of the procedure. The amount of NH 3 added is crucial to this
procedure. If insufficient NH 3 is added, the precipitate’s solubility increases,
leading to a negative determinate error. If too much NH 3 is added, the
precipitate may contain traces of Mg(NH 4 ) 4 (PO 4 ) 2 , which, on ignition, forms
Mg(PO 3 ) 2 . This increases the mass of the ignited precipitate, giving a positive
determinate error. Once enough NH 3 has been added to neutralize the HCl,
6H
additional NH 3 is added to quantitatively precipitate MgNH 4 PO 4× 2 O.
4. Explain why the formation of Mg(PO 3 ) 2 in place of Mg 2 P 2 O 7 increases the mass
of precipitate.

The desired final precipitate, Mg 2 P 2 O 7 , contains two moles of Mg, and the
impurity, Mg(PO 3 ) 2 , contains only one mole of Mg. Conservation of mass,
therefore, requires that two moles of Mg(PO 3 ) 2 must form in place of each mole
of Mg 2 P 2 O 7 . One mole of Mg 2 P 2 O 7 weights 222.6 g. Two moles of Mg(PO 3 ) 2
weigh 364.5 g. Any replacement of Mg 2 P 2 O 7 with Mg(PO 3 ) 2 must increase the
precipitate’s mass.
5. What additional steps in the procedure, beyond those discussed in questions 2
and 3, are taken to improve the precipitate’s purity?
Two additional steps in the procedure help form a precipitate that is free of
impurities: digestion and reprecipitation.
6. Why is the precipitate rinsed with a solution of 5% v/v NH 3 ?

This is done for the same reason that precipitation is carried out in an
ammonical solution; using dilute ammonia minimizes solubility losses when
rinsing the precipitate.

8B.2 Quantitative Applications
Although not in common use, precipitation gravimetry still provides a reliable
means for assessing the accuracy of other methods of analysis or for verifying the
composition of standard reference materials. In this section we review the general
application of precipitation gravimetry to the analysis of inorganic and organic
compounds.

Inorganic Analysis The most important precipitants for inorganic cations are
chromate, the halides, hydroxide, oxalate, sulfate, sulfide, and phosphate. A sum-
mary of selected methods, grouped by precipitant, is shown in Table 8.1. Many in-
organic anions can be determined using the same reactions by reversing the analyte