Page 268 - Modern Analytical Chemistry

1400-CH08 9/9/99 2:18 PM Page 251

Chapter 8 Gravimetric Methods of Analysis 251

EXAMPLE 8.1
An ore containing magnetite, Fe 3 O 4 , was analyzed by dissolving a 1.5419-g
3+
sample in concentrated HCl, giving a mixture of Fe 2+ and Fe . After adding
3+
HNO 3 to oxidize any Fe 2+ to Fe , the resulting solution was diluted with water
and the Fe 3+ precipitated as Fe(OH) 3 by adding NH 3 . After filtering and
rinsing, the residue was ignited, giving 0.8525 g of pure Fe 2 O 3 . Calculate the
%w/w Fe 3 O 4 in the sample.
SOLUTION
This is an example of a direct analysis since the iron in the analyte, Fe 3 O 4 , is
part of the isolated precipitate, Fe 2 O 3 . Applying a conservation of mass to Fe,
we write

3 ´moles Fe 3 O 4 =2 ´moles Fe 2 O 3
Using formula weights, FW, to convert from moles to grams in the preceding
equation leaves us with

3
2
3 ´g Fe O 4 2 ´g Fe O 3
=
3
2
FW Fe O 4 FW Fe O 3
which can be solved for grams of Fe 3 O 4 and %w/w Fe 3 O 4 in the sample.
2 ´ 0 8525 g ´231.54 g/mol
.
2 ´g Fe O 3 ´FW Fe O 4
3
2
= = . 3
0 82405 g Fe O 4
2
3 ´FW Fe O 3 3 ´159.69 g/mol
.
0 82405 g
3
g Fe O 4 ´100 = ´100 =53 44.% w/w Fe
g sample 1.5419 g
As discussed earlier, the simultaneous analysis of samples containing two analytes
requires the isolation of two precipitates. As shown in Example 8.2, conservation of
mass can be used to write separate stoichiometric equations for each precipitate.
These equations can then be solved simultaneously for both analytes.
EXAMPLE 8.2
A 0.6113-g sample of Dow metal, containing aluminum, magnesium, and other
metals, was dissolved and treated to prevent interferences by the other metals. The
aluminum and magnesium were precipitated with 8-hydroxyquinoline. After
filtering and drying, the mixture of Al(C 9 H 6 NO) 3 and Mg(C 9 H 6 NO) 2 was found
to weigh 7.8154 g. The mixture of dried precipitates was then ignited, converting
the precipitate to a mixture of Al 2 O 3 and MgO. The weight of this mixed solid was
found to be 1.0022 g. Calculate the %w/w Al and %w/w Mg in the alloy.

SOLUTION
This is an example of a direct analysis in which the two analytes are determined
without a prior separation. The weight of the original precipitate and the
ignited precipitate are given by the following two equations
g Al(C 9 H 6 NO) 3 + g Mg(C 9 H 6 NO) 2 = 7.8154

g Al 2 O 3 + g MgO = 1.0022