Page 358 - Modern Analytical Chemistry

1400-CH09 9/9/99 2:13 PM Page 341

Chapter 9 Titrimetric Methods of Analysis 341
9D.4 Quantitative Applications

As with acid–base and complexation titrations, redox titrations are not frequently
used in modern analytical laboratories. Nevertheless, several important applications
continue to find favor in environmental, pharmaceutical, and industrial laborato-
ries. In this section we review the general application of redox titrimetry. We begin, Aqueous
however, with a brief discussion of selecting and characterizing redox titrants, and sample
methods for controlling the analyte’s oxidation state.

Solid
Adjusting the Analyte’s Oxidation State If a redox titration is to be used in a reductant
quantitative analysis, the analyte must initially be present in a single oxidation state.
For example, the iron content of a sample can be determined by a redox titration in
3+
which Ce 4+ oxidizes Fe 2+ to Fe . The process of preparing the sample for analysis
Porous
2+
must ensure that all iron is present as Fe . Depending on the sample and the
plug
method of sample preparation, however, the iron may initially be present in both
the +2 and +3 oxidation states. Before titrating, any Fe 3+ that is present must be re-
2+
duced to Fe . This type of pretreatment can be accomplished with an auxiliary re-
ducing or oxidizing agent.
Metals that are easily oxidized, such as Zn, Al, and Ag, can serve as auxiliary re-
ducing agents. The metal, as a coiled wire or powder, is placed directly in the solu- Figure 9.39
Schematic diagram of a reductor column.
tion where it reduces the analyte. Of course any unreacted auxiliary reducing agent
will interfere with the analysis by reacting with the titrant. The residual auxiliary re-
ducing agent, therefore, must be removed once the analyte is completely reduced. auxiliary reducing agent
This can be accomplished by simply removing the coiled wire or by filtering. A reagent used to reduce the analyte
An alternative approach to using an auxiliary reducing agent is to immobilize it before its analysis by a redox titration.
in a column. To prepare a reduction column, an aqueous slurry of the finely divided
metal is packed in a glass tube equipped with a porous plug at the bottom (Figure
9.39). The sample is placed at the top of the column and moves through the column
under the influence of gravity or vacuum suction. The length of the reduction col-
umn and the flow rate are selected to ensure the analyte’s complete reduction.
Two common reduction columns are used. In the Jones reductor the column Jones reductor
is filled with amalgamated Zn prepared by briefly placing Zn granules in a solution A reduction column using a Zn amalgam
of HgCl 2 to form Zn(Hg). Oxidation of the amalgamated Zn as a reducing agent.
2+
Zn(Hg)(s) t Zn (aq) + Hg(l)+2e –
provides the electrons for reducing the analyte. In the Walden reductor the column Walden reductor
is filled with granular Ag metal. The solution containing the analyte is acidified with A reduction column using granular Ag as
HCl and passed through the column where the oxidation of Ag a reducing agent.
–
Ag(s)+Cl (aq) t AgCl(s)+ e –
provides the necessary electrons for reducing the analyte. Examples of both reduc-
tion columns are shown in Table 9.19.
Several reagents are commonly used as auxiliary oxidizing agents, including auxiliary oxidizing agent
ammonium peroxydisulfate, (NH 4 ) 2 S 2 O 8 , and hydrogen peroxide, H 2 O 2 . Ammo- A reagent used to oxidize the analyte
nium peroxydisulfate is a powerful oxidizing agent before its analysis by a redox titration.
–
2–
2–
S 2 O 8 (aq)+2e t 2SO 4 (aq)
2–
4+
–
capable of oxidizing Mn 2+ to MnO 4 , Cr 3+ to Cr 2 O 7 , and Ce 3+ to Ce . Excess per-
oxydisulfate is easily destroyed by briefly boiling the solution. The reduction of hy-
drogen peroxide in acidic solution
+
–
H 2 O 2 (aq)+2H 3 O (aq)+2e t 4H 2 O(l)