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Chapter 7 Obtaining and Preparing Samples for Analysis 211

pH-dependent solubility of metal oxides and hydroxides, and the solubility of
metal sulfides.
Separations based on the pH-dependent solubility of oxides and hydroxides are
usually accomplished using strong acids, strong bases, or NH 3 /NH 4 Cl buffers. Most
metal oxides and hydroxides are soluble in hot concentrated HNO 3 , although a few
oxides, such as WO 3 , SiO 2 , and SnO 2 remain insoluble even under these harsh con-
ditions. In determining the amount of Cu in brass, for example, an interference
from Sn is avoided by dissolving the sample with a strong acid. An insoluble residue
of SnO 2 remains that can then be removed by filtration.
Most metals will precipitate as the hydroxide in the presence of concentrated
NaOH. Metals forming amphoteric hydroxides, however, remain soluble in concen-
trated NaOH due to the formation of higher-order hydroxo-complexes. For exam-
ple, Zn 2 + and Al 3 + will not precipitate in concentrated NaOH due to the formation
–
–
of Zn(OH) 3 and Al(OH) 4 . The solubility of Al 3 + in concentrated NaOH is used to
isolate aluminum from impure bauxite, an ore of Al 2O 3. The ore is powdered and
placed in a solution of concentrated NaOH where the Al 2 O 3 dissolves to form
–
Al(OH) 4 . Other oxides that may be present in the ore, such as Fe 2 O 3 and SiO 2 , re-
main insoluble. After filtering, the filtrate is acidified to recover the aluminum as a
precipitate of Al(OH) 3 .
The pH of an NH 3 /NH 4 Cl buffer (pK a = 9.24) is sufficient to ensure the precip-
itation of most metals as the hydroxide. The alkaline earths and alkaline metals,
however, will not precipitate at this pH. In addition, metal ions that form soluble
2+
2+
2+
2+
complexes with NH 3 , such as Cu , Zn , Ni , and Co , also will not precipitate
under these conditions.
2–
Historically, the use of S as a precipitating reagent is one of the earliest exam-
ples of a separation technique. In Fresenius’s 1881 text, A System of Instruction in
15
Quantitative Chemical Analysis, sulfide is frequently used as a means for separat-
ing metal ions from the remainder of the sample matrix. The importance of sulfide
as a precipitating reagent for separations is due to two factors: most metal ions, ex-
cept for the alkaline earths and alkaline metals, form insoluble sulfides; and the sol-
ubilities of these metal sulfides show a substantial variation. Since the concentration
of S 2– is pH-dependent, control of pH was used to determine which metal ions
would precipitate. For example, in Fresenius’s gravimetric procedure for the deter-
mination of Ni in ore samples (see Figure 1.1 in Chapter 1 for a schematic diagram
of this procedure), sulfide is used three times as a means of separating Co 2+ and
2+
2+
2+
Ni from Cu and, to a lesser extent from Pb .
7 5 Separations Based on a Partitioning Between Phases
F.
The most important class of separation techniques is based on the selective parti-
tioning of the analyte or interferent between two immiscible phases. When a phase
containing a solute, S, is brought into contact with a second phase, the solute parti-
tions itself between the two phases.
7.18
S phase 1t S phase 2
The equilibrium constant for reaction 7.18

[ S phase 2 ]
K D = partition coefficient
[ S phase 1 ]
An equilibrium constant describing the
distribution of a solute between two
is called the distribution constant, or partition coefficient. If K D is sufficiently large, phases; only one form of the solute is used
then the solute will move from phase 1 to phase 2. The solute will remain in phase 1, in defining the partition coefficient (K D ).