Page 118 - Analytical Electrochemistry 2d Ed - Jospeh Wang

4-3 OXYGEN REMOVAL 103

amounts of naturally occurring electrolyte). The inert supporting electrolyte may be
an inorganic salt, a mineral acid, or a buffer. While potassium chloride or nitrate,
ammonium chloride, sodium hydroxide, or hydrochloric acid are widely used when
using water as a solvent, tetraalkylammonium salts are often employed in organic
media. Buffer systems (such as acetate, phosphate, or citrate) are used when a pH
control is essential. The composition of the electrolyte may affect the selectivity of
voltammetric measurements. For example, the tendency of most electrolytes to
complex metal ions can bene®t the analysis of mixtures of metals. In addition,
masking agents [such as ethylenediaminetetraacetic acid (EDTA)] may be added to
``remove'' undesired interferences. The supporting electrolyte should be prepared
from highly puri®ed reagents, and should not be easily oxidized or reduced. The
usual electrolyte concentration range is 0.1±1.0 M, in other words, in large excess of
the concentration of all electroactive species. Signi®cantly lower levels can be
employed in connection with ultramicro working electrodes (see Section 4-5.4).

4-3 OXYGEN REMOVAL

The electrochemical reduction of oxygen usually proceeds via two well-separated
two-electron steps. The ®rst step corresponds to the formation of hydrogen peroxide:

O ‡ 2H ‡ 2e ! H O 2 4-1†
‡

2
2
and the second step corresponds to the reduction of the peroxide:
H O ‡ 2H ‡ 2e ! 2H O 4-2†
‡

2
2
2
The half-wave potentials of these steps are approximately 0.1 and 0.9 V (versus the
saturated calomel electrode). The exact stoichiometry of these steps is dependent on the
medium. The large background current accruing from this stepwise oxygen reduction
interferes with the measurement of many reducible analytes. In addition, the products
of the oxygen reduction may affect the electrochemical process under investigation.
Avariety of methods have been used for the removal of dissolved oxygen (4). The
most common method has been purging with an inert gas (usually puri®ed nitrogen)
for 4±8 min prior to recording of the voltammogram. Longer purge times may be
required for large sample volumes or for trace measurements. To prevent oxygen
from reentering, the cell should be blanketed with the gas while the voltammogram
is being recorded. Passage of the gas through a water-containing presaturator is
desirable to avoid evaporation. The deaeration step, while time consuming, is quite
effective and suitable for batch analysis. (The only exception is work with micro-
samples, where deoxygenation may lead to errors caused by the evaporation of
solvent or loss of volatile compounds.)
Other methods have been developed for the removal of oxygen (particularly from
¯owing streams). These include the use of electrochemical or chemical (zinc)
scrubbers, nitrogen-activated nebulizers, and chemical reduction (by addition of
sodium sul®te or ascorbic acid). Alternately, it may be useful to employ voltam-