Anions in non saline waters - Determination of Anions in Natural and Treated Waters - page 192

Jones [389] found that when the reduction solution was buffered above pH8 and
optimum cadmium contact was used, reduction of 20µM nitrate to nitrite was more than
99% complete, even in the presence of sodium chloride concentration corresponding to
that of seawater.
Certain metals and phosphate may interfere with nitrate reduction. Precipitation of
metal salts onto the cadmium surface may be prevented by addition of EDTA to the
buffer solution.
Davison and Woof [390] compared different forms of cadmium as a reducing agent for
the batch determination of nitrate. These workers have also carried out interference
studies on the batch determination of nitrate in non saline waters by cadmium filings and
found that reduction with spongy cadmium is less prone to interference that with
cadmium filings. The optimum reduction time or the amount of cadmium filings for
mixing reduction efficiency for natural samples or solutions containing interferences was
obtained for standard solutions. Possible interfering ions in freshwaters were shown to be
phosphate, silicate, hydrogen carbonate, sulphide and some organic compounds.
Earlier work by Davison and Woof [391] had shown that good recoveries on some
natural freshwater samples could only be achieved with longer reduction times than had
been found to be optimum for standard solutions. Many workers [391-395] have
investigated the time dependence of the reduction step and observed that, for the more
vigorously reducing forms of cadmium, the efficiency of the reduction attains a
maximum and then declines. With less active cadmium preparations, the reduction
efficiency slowly approaches a plateau. However, most of these experiments were
performed on standard solutions. Conditions have been optimised for studying the time
dependence of the reduction efficiency, as reflected by different column lengths at a
given flow-rate [396]. As neither standards nor different water samples were studied in
the same way, any change in reduction efficiency attributable to the sample water could
not be assessed.
The mechanism of reduction must involve the oxidation fo cadmium metal as given in
the redox reactions (1) and (2):
(1)
(2)
Anything that can change the ultimate rate of electron transfer of the redox potential of
reaction (1) can possibly interfere with the analytical method. Other metal ions and
ligands can change the redox potential of reaction (2) and hence possibly decrease or
increase the electron availability, which could show up as a positive or negative
interference in the method. Alternatively, inorganic or organic complexing agents can
interfere by associating with cadmium ions formed at the metal surface so providing a
block to the most active reducing sites.
Two types of cadmium were used in this study, filings and a sponge prepared by
electrolytic deposition onto zinc rods [390].