Page 6 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

Analytical Chemistry 545

cations. As one example let us consider the silver group. and therefore the error, on reagent precipitation. The fa-
Treatment of the white precipitate of Hg 2 Cl 2 , AgCl, and cility with which a precipitate is removed from solution
PbCl 2 with hot water results in solubilization of the lead is related to the particle size of the solid phase, which in
compound to yield a colorless solution. The presence of turn is governed by the conditions that exist at the time
Pb 2+ is confirmed by the addition of K 2 CrO 4 , which gives of formation of the precipitate. Thought to be important
a yellow precipitate of PbCrO 4 . Addition of NH 4 OH to are the solubility of the precipitate, temperature, reactant
Hg 2 Cl 2 and AgCl results in either a gray mixture of mer- concentration, and mixing phenomena. These parameters
2+
cury and HgNH 2 Cl, confirming Hg , and/or colorless control the supersaturation that exists in solution at a par-
2
+
−
solutions of Ag(NH 3 ) Cl . Acidification of the latter re- ticular time. Although the usual aim is to achieve an eas-
2
precipitates AgCl, identifying the presence of Ag . ily manipulated crystalline precipitate, colloidal particles
+
The detection of anions in the original sample does not (10 −6 to 10 −4 mm in size) are sometimes obtained. In
proceed in the same way in that the material is usually sub- these cases, the individual particles must be coagulated
jected to a series of preliminary tests. Moreover, the cation by an experimental procedure such as heating, stirring,
analysis described above can often be assessed to infer the and the addition of auxiliary electrolyte. The mechanism
presence of certain anions. A sample solution is separately of this process lies in the reduction of electrical repulsive
treated with AgNO 3 and BaCl 2 , and the various precipi- forces on the particles. Purer, more dense precipitates can
tates of silver and barium are used to verify the presence of often be produced by precipitation from homogeneous so-
anionic components. This procedure is then followed by lution, where the reagent is generated in solution. Finally,
the addition of concentrated cold H 2 SO 4 to the unknown after filtration a gravimetric precipitate is heated until its
solid, which results in the liberation of characteristic gases weight becomes constant. A wide range of temperatures
(e.g., I gives the odor of H 2 S and violet fumes of I 2 ), and (110–1200 C) are used for this purpose, the most impor-
◦
−
by a series of confirmatory tests for each anion. tant requirement being weight constancy and unequivocal
On a practical level the whole analysis is performed knowledge of the composition of the precipitate at a par-
at the semimicro level. This means that sample sizes are ticular temperature.
in the range of 10 mg and solutions are of the order of Some examples of inorganic and organic precipitation
1–2 ml in volume. Special equipment is required to han- agents for inorganic analytes are given in Table I.
dle low volumes of reagent solutions and to avoid serious There are also methods available for the precipitation
losses in separative and transfer procedures. The sepa- of organic compounds through the reaction of a partic-
ration of precipitates is performed by centrifugation and ular functional group. For example, compounds with
decantation, and heating is achieved in specially designed carbonyl functional groups can be precipitated with
test tube heating blocks. 2,4-dinitrophenylhydrazine according to the following
reaction:
B. Gravimetric Analysis
RCHO + H 2 NNHC 6 H 3 (NO 2 ) 2 →
Here, quantitative analysis is based on the measurement of R—CH NNHC 6 H 3 (NO 2 ) 2 ↓+ H 2 O.
the weight of a substance of precisely known composition
that is chemically related to the analyte. Most often the In summary, the gravimetric method does not require
unknown is precipitated from solution by a reagent and, calibration, as is the case with many other analytical
after separation and drying, is weighed. Less frequently
the species being determined is volatilized, and the weight TABLE I Typical Inorganic and Organic Precipitat-
of the condensed gas or residual solid serves to complete ing Agents
the analysis. The precipitate must be insoluble in water Element precipitated
(or other solvent) in order to minimize obvious losses, be Agent (weighed form)
easily filtered and washed, and be stable after drying or ig-
NH 3 Al (Al 2 O 3 ), Fe (Fe 2 O 3 )
nition procedures. With respect to the first requirement, an
H 2 S Zn (ZnO), Ge (GeO 2 )
important quantitative parameter is the solubility product
of the compound produced from the analyte. For exam- H 2 SO 4 Pb (PbSO 4 ), Ba (BaSO 4 )
HCl Ag (AgCl)
ple, for the precipitation of SO 2− with Ba , the solubility
2+
4 Bi (Bi 2 O 3 )
product (K sp ) for BaSO 4 is defined by (NH 4 ) 2 CO 3
BaCl 2 SO 2− (BaSO 4 )
4
−10
K sp = [Ba ] SO 2− = 1.3 × 10 MgCl 2 ,NH 4 Cl PO 3− (Mg 2 P 2 O 7 )
2+
4 4
8-Hydroxyquinoline (HQ) Al (AlQ 3 )
for a saturated solution. Clearly, this value can be used
Dimethylglyoxime (DMG) Ni (NiDMG 2 )
to compute the loss of analyte (remaining in solution),