Anions in seawater - Determination of Anions in Natural and Treated Waters - page 370

Ke and Regier [74] have described a direct potentiometric determination of fluoride in
seawater after extraction with 8-hydroxyquinoline. This procedure was applied to
samples of seawater, fluoridated tap-water, well-water and effluent from a phosphate
reduction plant. Interfering metals, eg calcium, magnesium, iron and aluminium were
removed by extraction into a solution of 8-hydroxyquinoline in 2-butoxyethanol-
chloroform after addition of glycine-sodium hydroxide buffer solution (pH 10.5 to 10.8).
A buffer solution (sodium nitrate-1,2-diamino-cyclohexane-NNN ′ N ′ -tetra-acetic acid-
acetic acid) (pH 5.5) was then added to adjust the total ionic strength and the fluoride
ions were determined by means of a solid membrane fluoride-selective electrode (Orion,
model 94-09). Results were in close agreement with, and more reproducible than those
obtained after distillation [75]. Omission of the extraction led to lower results. Four
determinations can be made in 1h.
3.15 Formate
There is considerable interest in the role of formic acid and other volatile fatty acids in
the early diagenesis of organic matter in lacustrine and marine sediments. Formic acid is
an important fermentation product or substrate for many aerobic and anaerobic bacteria
and for some yeasts. In the atmosphere, formic acid is an important product in the
photochemical oxidation of organic matter.
Despite its potential importance, formic acid has proven difficult to quantify at
submicromolar levels in non saline water samples. Formidable analytical difficulties are
associated with its detection in highly saline samples. Ion exclusion, anion exchange, and
reversed-phase high performance liquid chromatography techniques based on the direct
detection of formic acid in aqueous samples are prone to interferences (especially from
inorganic salts) that ultimately limit the sensitivity of these methods.
A potentially more sensitive and selective approach involves reaction of formic acid
with a reagent to form a chromophore or fluorophore, followed by chromatographic
analysis. A wide variety of alkylating and silylating reagents have been used for this
purpose. Two serious drawbacks to this approach are that inorganic salts and/or water
interfere with the derivatisation reaction, and these reactions are generally not specific for
formic acid or other carboxylic acids. These techniques are prone to errors from
adsorption losses, contamination and decomposition of the components of interest.
Enzymatic techniques, in contrast, are ideal for the analysis of non saline water samples,
since they are compatible with aqueous media and involve little or no chemical or
physical alterations of this sample (eg pH, temperature).
3.15.1 High performance liquid chromatography
As a consequence of the above considerations Kieber et al. [76] have developed an
enzymatic method to quantify formic acid in non saline water samples at sub-micromolar
concentrations. The method is based on the oxidation of formate by formate
dehyrogenase with corresponding reduction of β-nicotinamide adenine dinucleotide (β-
NAD + ) to reduced β-NAD + (β-NADH); β-NADH is quantified by re versed-phase high