Chemistry Reference
In-Depth Information
14.4
Electrochemical detection in flowing solutions
14.4.1 Injection techniques
Flow injection analysis (FIA) has been recognized as an attractive tool for the development of green
methodologies. In particular, methods based on flow injection coupled with electrochemical detection have
proved their analytical utility, mainly because of their high sensitivity independently of the cell miniaturization,
possibility of use turbid samples and low cost. FIA has demonstrated to be a useful technique for the rapid
acquisition of preliminary chemical information about the occurrence of organic compounds in environmental
samples. Very recently, the use of FIA methodologies to determine chemical oxygen demand (COD) or total
concentration indices of halogenated organic compounds or phenols in water samples, have been reviewed
[82]. Electrochemical detection is probably the most environmentally friendly alternative for COD
measurements. On the contrary of the classical determination, which implies the use and generation of big
amounts of extremely hazardous wastes, some recent electrochemical methods for direct COD determination
have been proposed which do not need sample pre-treatment and are characterized by a limited reagent
consumption, easy automation and low environmental impact. Methods based on the use of UV/TiO
2
packed
bead reactors for sample digestion followed by the amperometric measurement of the O
2
concentration
changes have been proposed [83]. Other improvements in the COD determination using EC detection are
based on F-PbO2 modified electrode, and are useful within a linear range between 100 and 1200 mg l
−1
[84].
Another electrochemical alternative involving a Ti/TiO
2
/PbO
2
electrode for the flow injection with
amperometric detection allows us to determine COD by applying a detection potential of
1.35 V [85].
Total organic fluorine (TOF) determination was also performed as an amount of inorganic fluoride obtained
after defluorination reaction and analysis of recovered fluoride by using flow-injection system with
potentiometric detection [86]. Defluorination reactions took place in a reaction coil in conditions of constant
flow using sodium biphenyl. Figure 14.4 shows a scheme of the used manifold. An in-flow phase separator
of small volume was needed for the well functioning of the potentiometric detector, which is based on a
fluoride ion selective electrode incompatible with organic solvents. This system was simpler and easier to
optimize than that based on fluorimetric detection and allows minimization of manual operations, thus
providing improvement in precision, elimination of aggressive chemical handling as well as reduction of
possible sources of contamination.
+
25 mm
Valve for injection of
ether sample solution
0.5 mm
Ether/
SBP
Ether
& gas
Aqueous
standard F
−
Pump
Buffer
Separator
Aqueous
waste
Detector
Figure 14.4
FIA system design with the use of an on-line defluorination reaction with sodium biphenyl, phase
separator and potentiometric detection. Reproduced from [86] with permission from Elsevier, © 2007.
